Transcriber's Note:
Minor typographical errors and inconsistencies have been corrected. Some
words had inconsistent hyphenation throughout the book; these have been
made consistent.
In the mathematical and chemical expressions, the caret character
represents "to the power of", e.g. e^2 means e-squared. Curly braces are
used to represent underscores, e.g. n{1} means n with a subscript of 1.
Equal signs are used to represent =bold words=, and underscores are used
to represent italics.
On page 152 NaCO{23} has been corrected to Na{2}CO{3}. On page 212,
the variable n has been replaced with the correctly subscripted forms
n{1} and n{2}.
The index entry for Hemlock bark had no page number in the original
text, so the correct page number, 34, has been supplied.
Ions are shown as Fe+++, instead of using superscripts.
There is some inconsistency in the notation used in the original text
for chemical formulae such as Na{2}Cr{2}O{7}(2H{2}O). These have
been regularized to use the modern mid-dot, for example,
Na{2}Cr{2}O{7}·2H{2}O.
Greek letters in the original have been represented in this version by
the name of the letter enclosed in square brackets, e.g. [alpha].
ANIMAL PROTEINS
BY
HUGH GARNER BENNETT, M.Sc. (LEEDS)
MEMBER OF THE SOCIETY OF LEATHER TRADES' CHEMISTS; FORMERLY ASSISTANT
LECTURER AND DEMONSTRATOR AT THE LEATHER INDUSTRIES DEPARTMENT OF THE
UNIVERSITY OF LEEDS
AUTHOR OF "THE MANUFACTURE OF LEATHER"
LONDON
BAILLIÈRE, TINDALL AND COX 8, HENRIETTA STREET, COVENT GARDEN
1921
(_All rights reserved_)
GENERAL PREFACE
The rapid development of Applied Chemistry in recent years has brought
about a revolution in all branches of technology. This growth has been
accelerated during the war, and the British Empire has now an
opportunity of increasing its industrial output by the application of
this knowledge to the raw materials available in the different parts of
the world. The subject in this series of handbooks will be treated from
the chemical rather than the engineering standpoint. The industrial
aspect will also be more prominent than that of the laboratory. Each
volume will be complete in itself, and will give a general survey of the
industry, showing how chemical principles have been applied and have
affected manufacture. The influence of new inventions on the development
of the industry will be shown, as also the effect of industrial
requirements in stimulating invention. Historical notes will be a
feature in dealing with the different branches of the subject, but they
will be kept within moderate limits. Present tendencies and possible
future developments will have attention, and some space will be devoted
to a comparison of industrial methods and progress in the chief
producing countries. There will be a general bibliography, and also a
select bibliography to follow each section. Statistical information will
only be introduced in so far as it serves to illustrate the line of
argument.
Each book will be divided into sections instead of chapters, and the
sections will deal with separate branches of the subject in the manner
of a special article or monograph. An attempt will, in fact, be made to
get away from the orthodox textbook manner, not only to make the
treatment original, but also to appeal to the very large class of
readers already possessing good textbooks, of which there are quite
sufficient. The books should also be found useful by men of affairs
having no special technical knowledge, but who may require from time
to time to refer to technical matters in a book of moderate compass,
with references to the large standard works for fuller details on
special points if required.
To the advanced student the books should be especially valuable. His
mind is often crammed with the hard facts and details of his subject
which crowd out the power of realizing the industry as a whole. These
books are intended to remedy such a state of affairs. While
recapitulating the essential basic facts, they will aim at presenting
the reality of the living industry. It has long been a drawback of our
technical education that the college graduate, on commencing his
industrial career, is positively handicapped by his academic knowledge
because of his lack of information on current industrial conditions. A
book giving a comprehensive survey of the industry can be of very
material assistance to the student as an adjunct to his ordinary
textbooks, and this is one of the chief objects of the present series.
Those actually engaged in the industry who have specialized in rather
narrow limits will probably find these books more readable than the
larger textbooks when they wish to refresh their memories in regard to
branches of the subject with which they are not immediately concerned.
The volume will also serve as a guide to the standard literature of the
subject, and prove of value to the consultant, so that, having obtained
a comprehensive view of the whole industry, he can go at once to the
proper authorities for more elaborate information on special points, and
thus save a couple of days spent in hunting through the libraries of
scientific societies.
As far as this country is concerned, it is believed that the general
scheme of this series of handbooks is unique, and it is confidently
hoped that it will supply mental munitions for the coming industrial
war. I have been fortunate in securing writers for the different
volumes who are specially connected with the several departments of
Industrial Chemistry, and trust that the whole series will contribute
to the further development of applied chemistry throughout the Empire.
SAMUEL RIDEAL.
AUTHOR'S PREFACE
It has been the author's chief concern that this volume should fulfil
its own part in the programme set forth in Dr. Rideal's General Preface.
The leather, glue, and kindred trades have been for many years
recognized as chemical industries, but the great development of colloid
chemistry in the last few years has given these trades a more definite
status as such, and they can now be placed in the category of applied
physical chemistry. The time is probably not far distant when some
knowledge of pure physical chemistry will be a first essential to
students, chemists, chemical engineers, and to all engaged in these
industries in supervision, administration, or control. It is hoped that
this volume will stimulate the study of these industries from that
standpoint.
As the author has previously written upon one of the industries involved
herein ("The Manufacture of Leather": Constable & Co.), he has, rather
inevitably, found it difficult to avoid altogether his own phraseology.
The changes of a decade, however, together with the wider field and
newer view-point, have made possible a radical difference of treatment.
The author desires to acknowledge the help he has received from the many
books, essays, and researches which are mentioned in the references at
the end of each section, especially to Procter's "Principles of Leather
Manufacture," and also to thank Dr. Rideal for many useful suggestions.
The author would like also to acknowledge here his indebtedness (as well
as that of the trade generally) to the work of Dr. J. Gordon Parker, who,
through his researches, lectures, and teaching work, has done more than
any other man to disseminate a knowledge of practical methods of
tanning.
The author's thanks are also due to his brother, Mr. W. Gordon Bennett,
M.Sc., A.I.C., M.C., for assistance in proof revision, and to his
father, Rev. John Bennett, for some literary criticism.
H. GARNER BENNETT.
BEVERLY, _June_, 1921.
CONTENTS
GENERAL PREFACE
AUTHOR'S PREFACE
CONTENTS
INTRODUCTION
* * * * *
PART I.
HIDES FOR HEAVY LEATHERS.
SECTION
1. THE RAW MATERIAL OF HEAVY LEATHERS
2. THE PREPARATION OF PELT
3. VEGETABLE TANNAGE
4. FINISHING PROCESS
5. SOLE LEATHER
6. BELTING LEATHER
7. HARNESS LEATHER
8. UPPER LEATHERS
9. BAG LEATHER
10. PICKING BAND BUTTS
* * * * *
PART II.
SKINS FOR LIGHT LEATHERS.
1. PRINCIPLES AND GENERAL METHODS OF
LIGHT LEATHER MANUFACTURE
2. GOATSKINS
3. SEALSKINS
4. SHEEPSKINS
5. CALFSKINS
6. JAPANNED AND ENAMELLED LEATHERS
* * * * *
PART III.
CHROME LEATHERS.
1. THE NATURE OF CHROME LEATHERS
2. GENERAL METHODS OF CHROME LEATHER
MANUFACTURE
3. CHROME CALF
4. CHROME GOAT AND SHEEP
5. HEAVY CHROME LEATHERS
* * * * *
PART IV.
MISCELLANEOUS TANNAGES.
1. ALUM TANNAGES
2. FAT TANNAGES
3. OIL TANNAGES
4. FORMALDEHYDE TANNAGE
5. SYNTHETIC TANNING MATERIALS
6. COMBINATION TANNAGES
7. THE EVOLUTION OF THE LEATHER INDUSTRY
* * * * *
PART V.
GELATINE AND GLUE.
1. PROPERTIES OF GELATINE AND GLUE
2. RAW MATERIALS AND PRELIMINARY
TREATMENT
3. EXTRACTION
4. CLARIFICATION AND DECOLORIZATION
5. BLEACHING
6. EVAPORATION
7. COOLING AND DRYING
8. USES OF GELATINE AND GLUE
9. THE EVOLUTION OF THE GELATINE AND
GLUE INDUSTRY
* * * * *
PART VI.
MISCELLANEOUS PROTEINS AND BYE-PRODUCTS.
1. BYE-PRODUCTS OF THE LEATHER TRADES
2. BYE-PRODUCTS OF THE GELATINE AND
GLUE TRADES
3. FOOD PROTEINS
4. MISCELLANEOUS ANIMAL PROTEINS
INDEX
ANIMAL PROTEINS
INTRODUCTION
Proteins are organic compounds of natural origin, being found in plants
and in animals, though much more plentifully in the latter. They are
compounds of great complexity of composition, and of very high molecular
weight. The constitution of none of them is fully understood, but
although there are a great number of different individual proteids, they
present typical resemblances and divergences which serve to
differentiate them from other groups of organic bodies, and also from
one another.
Proteins resemble one another in both proximate and ultimate analysis.
They contain the usual elements in organic compounds, but in proportions
which do not vary over very wide limits. This range of variation is
given approximately below:--
Element. Per cent.
Carbon 49 to 55
Hydrogen 6.4 to 7.3
Oxygen 17 to 26
Nitrogen 13 to 19
Sulphur 0.3 to 3.0
The most characteristic feature of the protein group is the amount of
nitrogen usually present. This is generally nearer the higher limit,
seldom falling below 15 per cent. This range for the nitrogen content is
determined largely by the nature of constituent groups which go to form
the proteid molecule. Roughly speaking, proteins consist of chains of
amido-acids and acid amides with smaller proportions of aromatic groups,
carbohydrate groups and thio compounds attached. In these chains an acid
radical may combine with the amido group of another amido acid, the acid
group of the latter combining with an amido group of another amido acid,
and so on. Hydrogen may be substituted in these chains by alkyl or
aromatic groups. There is obviously infinite possibility of variation in
constitution for compounds of this character, the general nature of
which varies very little. Practically all of the proteins are found in
the colloid state, and this makes them very difficult to purify and
renders the ultimate analysis in many cases doubtful. It is, for
example, often difficult to ascertain their moisture content, for many
are easily hydrolyzed with water only, and many part easily with the
elements of water, whilst on the other hand many are lyophile colloids
and practically cannot be dehydrated or dried. A few, such as gelatin
and some albumins, have been crystallized.
The constituent groups have been investigated chiefly by hydrolytic
methods. The chains of amido acids are split up during hydrolysis, and
individual amido acids may thus be separated. The hydrolysis may be
assisted either by acids, alkalies or ferments, but follows a different
course according to the nature of the assistant. Under approximately
constant conditions of hydrolysis, the products obtained are in
approximately constant proportions, and this fact has been utilized by
Van Slyke in devising a method of proximate analysis. It is not possible
in this volume to enter deeply into the constitution of the different
proteids. Reference must be made to works on pure chemistry, especially
to those on advanced organic chemistry. It will be interesting, however,
to mention some of the amido acids and groups commonly occurring in
proteids. These comprise ornithine (1:4 diamido valeric acid), lysine
(1:5 diamido-caproic acid), arginine (1 amido, 4 guanidine valeric
acid), histidine, glycine (amidoacetic acid), alanine (amido propionic
acid), amido-valeric acid (amido-iso-caproic acid), liacine, pyrollidine
carboxylic acid, aspartic acid, glutamic acid (amido-glutaric acid),
phenyl-alanine, serine (hydroxy-amido propionic acid), purine
derivatives (_e.g._ guanine), indol derivatives (_e.g._ tryptophane
and skatol acetic acid), cystine (a thioserine anhydride),
glucosamine, and urea.
There are a few general reactions which are typical of all proteins, and
which can usually be traced to definite groupings in the molecule.
Amongst these is the biuret reaction: a pink colour obtained by adding a
trace of copper sulphate and an excess of caustic soda. This is caused
by the biuret, NH(CONH{2}){2} radical or by similar diacidamide
groups, _e.g._ malonamide, oxamide, glycine amide. Another general
reaction is with "Millon's reagent," a solution of mercuric nitrate
containing nitrous fumes. On warming the proteid with this reagent, a
curdy pink precipitate or a red colour is obtained. This reaction is
caused by the tyrosine group (p. oxy [alpha] amido phenyl-propionic
acid). Another general reaction is to boil the protein with 1:2 nitric
acid for some days. A yellow flocculent precipitate of "xanthoproteic
acid" is obtained, and this dissolves in ammonia and caustic alkalies
with a brown or orange-red colour. Another characteristic of proteins is
that on dry distillation they yield mixtures of pyridine C{5}H{5}N,
pyrrol C{4}H{5}N, and their derivatives.
On the subdivision, classification and nomenclature of the proteins much
ink has been spilled, and it is impossible in this volume to go into the
various systems which have been suggested. It should be noted, however,
that some writers habitually use the terms "proteid" or "albuminoid" as
synonyms for protein. The classification of proteins adopted in this
work is used because it is the most suitable for a volume on industrial
chemistry and has the additional merits that it is simple and is already
used in several standard works on industrial chemistry. It is based upon
the behaviour of the proteins towards water, a matter of obvious moment
in manufacturing processes. On this basis proteins may be divided into
albumins, keratins and gelatins.
Cold water dissolves the albumins, does not affect the keratins, and
only swells the gelatins. The behaviour in hot water confirms and
elaborates the classification. When heated in water, the albumins
coagulate at temperatures of 70°-75° C., the gelatins (if swollen)
dissolve readily, whilst the keratins only dissolve at temperatures
above 100° C. Albumins and keratins may be distinguished also from
gelatins by adding acetic acid and potassium ferrocyanide to their
aqueous solutions. Albumins and keratins give a precipitate, gelatins do
not. Another distinguishing reaction is to boil with alcohol, wash with
ether, and heat with hydrochloric acid (S.G. 1.2). Albumins give a
violet colour, keratins and gelatins do not.
=Albumins= may be first discussed. They are typified by the casein of
milk and by white of egg. Their solutions in water are faintly
alkaline, optically active, and lævorotatory. They are coagulated by
heat and also by mineral acids, alcohol, and by many poisons. The
temperature of coagulation (usually about 72° C.) is affected by
mineral salts, the effect being in lyotrope order (see Part V.,
Section I.). The coagulated albumin behaves in most respects like a
keratin. Some of the albumins (globulins) are, strictly speaking, not
soluble in cold water, but readily dissolve in weak solutions of salt.
The albumins are coagulated from these solutions, as usual, when
heated. Into this special class fall myosin (of the muscles),
fibrinogen (of the blood) and vitellin (of egg yolk). By a gentle or
limited hydrolysis of the albumins with dilute acids in the cold, a
group of compounds called albuminates are obtained. They dissolve in
either acids or alkalies, and are precipitated by exact
neutralization. They may also be "salted" out by adding sodium
chloride or magnesium sulphate. They are not coagulated by heat. After
further hydrolysis with either acids, alkalies or ferments, very
soluble compounds are obtained called albumin peptones or albumoses.
These are soluble in alkalies, acids and water, and are readily
hydrolyzed further into amido acids and acid amides. They are very
similar to the peptones obtained from keratins and gelatins. They are
not coagulated by heat.
=Keratins= are typified by the hair of animals. They soften somewhat
in cold water and even more in hot water, but are not dissolved until
digested for some time at temperatures exceeding 100° C. With some
keratins, however, the cystine group is to some extent easily split
off by warm water, and on boiling with water hydrogen sulphide is
evolved. The sulphur content of keratins is often greater than the
average for proteids. All keratins are dissolved with great readiness
by solutions containing sulphydrates and hydrates, _e.g._ a solution
of sodium sulphide. In solutions of the hydrates of the alkali and
alkaline earth metals, keratins behave differently. Some dissolve with
great ease, some with difficulty, some only on heating and some not
even if digested with hot caustic soda. They are dissolved (with
hydrolysis) by heating with mineral acids, yielding peptones and
eventually amido acids, acid amides, etc. Many keratins have a
comparatively low content of nitrogen.
=Gelatins= are very difficult to distinguish from one another, their
behaviour being closely similar to reagents. They are also very
readily hydrolyzed even with water, and the products of hydrolysis are
even more similar. The gelatins are known together, commercially,
under the general name of gelatine. Gelatins of different origin,
however, have undoubtedly a different composition, the nitrogen
content being variable. If the gelatins are not bleached whilst they
are being manufactured into commercial gelatine, they are called
"glue." Gelatine is colourless, transparent, devoid of taste and
smell. It is usually brittle. Its S.G. is about 1.42, and it melts at
140° C. and decomposes. It is insoluble in organic solvents. When
swelling in cold water it may absorb up to 12 times its own weight of
water. The swollen product is called a "jelly." Jellies easily melt on
heating and a colloidal solution of gelatine is obtained. This "sets"
again to a jelly on cooling, even if only 1 per cent. gelatin (or
less) be present. The solution is optically active and lævorotatory,
but with very variable specific rotation. Some observers have thought
that the different gelatins have different specific rotations and may
so be distinguished. Gelatins are precipitated from solutions by many
reagents, such as alcohol, formalin, quinone, metaphosphoric acid,
tannins, and many salt solutions, _e.g._ those of aluminium, chromium
and iron, and of mercuric chloride, zinc sulphate, ammonium sulphate,
potassium carbonate, acidified brine. Many of these precipitations
have analogies in leather manufacture (see Parts I. to IV.). The
gelatin peptones or gelatoses are formed by hydrolysis with acids,
alkalies, ferment or even by digestion with hot water only. A more
detailed description of the properties of gelatine is given in Part
V., Section I. Gelatine is sometimes called "glutin" and "ossein."
Animals are much the most important source of proteins, especially of
those which are of importance in industrial chemistry. Proteins occur
in nearly every part of all animals, and the "protoplasm" of the
living cell is itself a protein. The keratins include the horny
tissues of animals: the epidermis proper, the hair, horns, hoofs,
nails, claws, the sebaceous and sudoriferous glands and ducts, and
also the elastic fibres. The gelatins are obtained from the collagen
of the skin fibres, the bones, tendons, ligaments, cartilages, etc.
Fish bladders yield a strong gelatin. The albumins are obtained from
the ova, blood, lymph, muscles and other internal organs of animals.
The classification of proteins herein adopted fits in well with the
scope and purpose of this volume. The keratins are of little
importance in chemical industry, but are of immense importance in
mechanical industry, _e.g._ the woollen trade, which is based upon the
keratin comprised by sheep wool. The collagen of the hide and skin
fibres is of vast importance to chemical industry, and is the basis of
the extensive leather trades discussed in Parts I. to IV. The waste
pieces of these trades, together with bones, form the raw material of
the manufacture of gelatin and glue, as discussed in Part V. The
proteids of animals' flesh and blood, milk and eggs form the source of
the food proteins discussed in Part VI. The food proteins embrace
chiefly albumins, but gelatins and even keratins are involved to some
extent.
PART I.--HIDES FOR HEAVY LEATHERS
Section I.--THE RAW MATERIAL OF HEAVY LEATHERS
The term "hide" possesses several shades of meaning. In its widest sense
it applies to the external covering of all animals, and is sometimes
used derogatively for human skin. In this wide sense, it is almost
synonymous with the term "skin." The term "hide," however, has a
narrower meaning, in which it applies only to the outer covering of the
larger animals, and in this sense is used rather in contrast with the
term "skin." Thus we speak of horse hides, cow hides, camel hides, and
buffalo hides. It is used in this sense in the title of Part I. of this
volume. As such hides are from large animals, the leather which is
manufactured therefrom is thick and in large pieces, and is therefore
commercially designated as "heavy leather." From the standpoint of
chemical industry hides are amongst the most important of animal
proteins, and their transformation into leather for boots, shoes,
belting, straps, harness, and bags comprises the "heavy leather trade,"
which is one of the largest and most vital industries of the country.
The heavy leather trade predominates over other branches of leather
manufacture, not only because of the comparatively large weight and
value of the material handled, but also because the resulting products
have a more essential utility. There is also a still narrower use of the
term "hide," in which it applies only to the domesticated cattle--the
ox, heifer, bull and cow--which use arises from the fact that the hides
of these are both the largest and most valuable portion of the raw
material of the heavy leather industries. In a very narrow sense the
term is also sometimes applied only to ox hides, which for most heavy
leathers are the ideal raw material.
=The Home Supply= of hides forms a large important proportion of
the total raw material. Its importance, moreover, is rapidly increasing,
for the excellence and abundance of the home supply determines the
extent to which it is necessary for the industry to purchase its raw
material abroad. The position of our national finances makes this an
increasingly serious matter, for hides are comparatively a very
expensive material.
The quality of our home supply of hides is very valuable, being
determined by the conditions of the animal's life, its precise breed,
and by other factors such as age and sex. The best hides are usually
obtained from animals which have been most exposed to extremes of wind
and cold, as such conditions tend naturally to develop a thicker and
more compact covering. Broadly speaking, these include the hides from
cattle of the northern and hilly districts. The age of the animal when
killed is also a dominating factor. Calf skins are very soft, fine
grained and compact, the state of rapid growth favouring the existence
of much interfibrillar substance. The youngest animals supply suitable
raw material for various light leathers (see Part II., Section V.), and
are also very suitable for chrome work (see Part III., Section III.).
Bull and cow hides, on the other hand, are from animals whose growth is
complete, and show in consequence a lack of interfibrillar substance,
coarse fibres and a rough and often wrinkled grain. The resulting
leather tends consequently to be spongy, thin, empty and non-waterproof.
Intermediate between these extremes are the hides of the ox and heifer,
large, yet of good texture, and well supplied with interfibrillar
substance. These hides are much the best for sole leather, a firm,
smooth-grained and well-filled leather being needed. The term "kip" is
often applied to small hides and to hides from large calves. In the
trade, however, "kip" is sometimes used also for larger hides, as a
verbal enhancement of value; just as a man with a few old fowls is said
to keep "chickens." Cow hides tend to be "spready," _i.e._ to have a
large area per unit weight, and are therefore more suitable for dressing
leather. Bull hides are thicker in the neck and belly, and thinner in
the back, which characteristics reduce their commercial value.
Market hides are sold by weight, and are therefore classified chiefly by
their weight, which is marked on near the tail by a system of
knife-cuts. The animals are flayed after cutting the hide down the belly
and on the inside of the legs.
Of the various breeds, "Shorthorns" yield a large supply of useful
hides. The name, however, covers a variety of similar breeds, and the
hides therefrom are rather variable in texture and quality. They tend to
be greasy owing to high feeding. The "Herefords," obtained from Midland
markets, are generally excellent hides for sole and harness leathers.
They give a good yield of butt pelt, a stout and smooth shoulder, and
are not often greasy. "Devons" yield a good-textured and well-grown
hide, but are often badly warbled. The "Sussex" cross breeds yield
somewhat larger hides. "Suffolk Red Polls," common in East Anglia, yield
a good butt, and the cow hides make good dressing leather. "Channel
Island" cattle yield very thin hides, but with a fine undamaged grain.
Scotch hides possess deservedly the very highest reputation. The
climatic conditions favour the production of a hardy race of cattle with
thick well-grown hides, yielding a large proportion of butt. These hides
are amongst the best obtainable for heavy leather, and particularly for
sole leather. "Highlanders," "Aberdeen Angus," "Galloways" are typical
breeds, with short neck, legs and straight backs. Cross breeds are also
excellent (_e.g._ "Scotch Shorthorns"). The natural value of these hides
is further enhanced by the usual care in flaying. "Ayrshires" yield good
milch cows and consequently yield often a more spready hide. The Welsh
breeds for rather similar reasons also yield valuable hides. The Irish
"Kerrys" are small but stout, and yield hides suitable for light sole
leather. Irish cross-breeds, Shorthorns, have a rather bad reputation,
and are often ill flayed.
All the varieties of the home supply are subject to various defects,
which influence seriously their commercial value. One of these defects
is warble holes or marks, caused by the Ox Warble fly (_Hypoderma
bovis_). This is a two-winged fly about half an inch long. The larva of
this fly, the "Warble maggot," lives and thrives in the skin of cattle,
and causes a sore and swelling. The life-history of this insect is still
in dispute, but it is generally thought that the eggs are laid in the
hair on the animal's back, and the young larva eats its way through the
hide until just below the dermis, and there feeds until mature. It then
creeps out of this "warble hole," falls to the ground, pupates for a
month, after which the imago or perfect insect emerges from the
chrysalis. Hides which have been thus infected have, in consequence,
often quite a number of holes through the most valuable part of the
hide, thereby rendering it unsuitable for many kinds of leather. Even
old "warbles" which have more or less healed up are a weakness, and
warbled hides and leather fetch a decidedly lower price than undamaged.
Another of these defects is bad flaying. Clearly the hide should be as
little cut as possible, but many of our market hides are abominably
gashed and often cut right through. This, of course, often reduces
seriously the commercial value of the hide. Careless treatment after
flaying also results in another common defect, viz. taint. As the term
implies, the hide is partly putrefied, sometimes only in patches, but
sometimes also so extensively as to render the hide quite rotten and
quite incapable of being made into leather at all. Hides are of course
putrescible, and dirt, blood, dung and warm weather encourage rapid
putrefaction. As market hides are usually uncured, this defect is
constantly appearing, and is a cause of considerable loss. Other defects
are due to injuries to the animal before it is killed, _e.g._ brands,
scratches due to hedges and barbed wire, old scabs, goad and tar marks.
All these reduce the value of the hide.
All the defects in hides involve a very serious loss to the community,
and the time is rapidly approaching when their continuance is
insufferable. The loss is not usually very considerable to any
individual, though very large in the aggregate. The hide is a minor part
of the beast's value, and a somewhat damaged hide does not involve a
very serious loss to the farmer. Some with typical stupidity regard a
few warbles as "the sign of a healthy beast." These defects involve
practically no loss to the hide merchant, tanner or currier, as each
pays less for damaged material. The loss falls upon the community, and
the time is ripe for the community to insist upon the elimination of
these defects. The national resources will be for some years strained to
their uttermost, and preventable damage must be considered intolerable.
The principal defects in hides are preventable, and ought to be
prevented. The warble fly could, by a united effort, be rendered before
long practically extinct, a task which is facilitated by the fact that
it is not migrative. Bad flaying and careless treatment of hides
resulting in putrefaction are still more easily remedied. The communal
slaughter-house is long overdue from the standpoint of public health,
and would, under conditions of cleanliness and skilled workmanship and
oversight, also solve the problem of ill-flayed and tainted hides.
The question of the raw material is of first importance to the leather
trades. There was, before the commencement of the European War, a
steadily increasing scarcity of hides, causing a constant increase in
their price. This was due partly to the fact that cattle were increasing
at a less rate than the population, partly to the growth of
civilization, and more extensive use of leather in proportion to the
world's population, and partly to the constant discovery of new uses for
leather, _e.g._ for motor cars, aeronautics, etc. The question of raw
material was under these conditions serious enough. The terrific
slaughter, necessary at the same time to provide the belligerents with
food and the army with leather, is bound to result in a serious crisis
for the leather industries; and in conjunction with the country's
financial condition, will make it absolutely necessary that all care
should be taken with the raw material of one of our most important
industries. The farmer who pays no heed to the warble fly, the man who
gashes the hide in flaying and who allows the hide to putrefy, are
equally criminal with the man who throws bread crusts into the dustbin.
It is impossible to foresee, as yet, anything in the nature of a
satisfactory solution to the problem of raw material, especially in
respect to heavy leather production, for the food question will rank
first in the popular mind, and the earlier slaughter enjoined for the
more economical production of meat will scarcely tend to increase the
proportion of heavy hides.
=The Foreign Supply= of hides is also of great importance and value. In
the case of imported hides precautions to prevent putrefaction are
essential, and some method of "curing" is always used.
=Salting= the hides is one of the most satisfactory methods for
temporary preservation. The action of salt is hygroscopic, and mildly
antiseptic. Moisture is withdrawn from the hides, which are then under
conditions no longer favouring the growth of bacteria. Well-salted
hides will keep for years, especially if quite clean. A light salting
is also useful for a short preservation, and is becoming common in
hide markets and tanneries during the summer and autumn months.
Salting is a method used extensively in the United States. The "packer
hides" of the stockyards are carefully and systematically salted with
about 25 per cent. of salt and stored in cool cellars. The hides are
so piled up in heaps, that brine easily drains away. The great
disadvantage of salting is the so-called "salt stains." These stains
have been ascribed to the iron in the salt, to the iron in the blood,
to calcium sulphate in the salt, and also to chromogenic bacteria,
whose development is favoured by salting. The relative importance of
these factors is not yet satisfactorily determined, but cleanliness
and pure salt tend to eliminate the trouble.
=Drying= the hides is a less satisfactory cure. The principle is
similar, viz. removal of moisture. Dried hides are, however, much
drier than salted, and are quite hard and horny, hence the name "flint
hides." The hides also lose much weight, a considerable advantage in
reducing freight. Tropical hides are often flint-dry, and where
preservatives are expensive or unprocurable, it is often the only
practicable method of cure. Nevertheless, the method has many serious
disadvantages, and is difficult to execute. If dried too slowly the
hides putrefy partially; if too quickly they dry on the outside, and
the interior is left to putrefy. The fact that hides are of uneven
thickness, and the climate often hot, increases the difficulty, and
often results in partial destruction of the fibrous structure of the
hide. When dried, moreover, the hides are still subject to the attacks
of insect larvæ, for the prevention of which the usual sprinkling of
naphthalene or arsenic is only an imperfect remedy. This method of
cure is also a nuisance to the tanner, who has to employ labour, pits
and time in attempting to restore the hides to their original
condition, and often loses up to ten per cent. of the goods in so
doing. Dried hides are also subject to the presence of anthrax.
=Dry Salting= the hides is an excellent method of curing. As the name
implies, it combines methods of drying and salting which are used
alternatively. The method is used extensively in South America. A
modified form of it is also used for preserving the "E.I. kips," which
are cured, however, not with common salt, but with earth containing up
to 70 per cent. of sodium sulphate. Dry-salted hides are largely free
from the defects of dried hides, but of course are more trouble to the
tanner in the process of soaking (see Section II.) than the wet-salted
goods.
=Freezing= the hides is now a commercial process. On the whole the
process is satisfactory, but the expansion of water after freezing may
tend to damage the hide fibres.
Sterilizing the hides has been frequently suggested, but no method has
yet been advocated which does not interfere either with the tanning
processes or with the quality of the finished leather.
Hides from the European Continent, usually wet salted and well flayed,
exhibit much the same variable quality as the home supply, those from
highland districts tending to be thick, yet even, well grown, tight
textured and smooth grained, whilst those from lowland regions are
less satisfactory. Thus hides from the Swiss Alps and Scandinavia have
ranked high, whilst the spready Dutch cows are typical of a lowland
hide. In the hides which once came from Germany the same features
appear. Bavarian highland hides had an excellent reputation, whilst
those from Berlin, Cologne, etc., tended to be long in shank and not
well grown. French hides are often ill flayed, and Spanish and
Portuguese are often subject to scratches. Italian hides have a very
good name, being small but stout in butt.
The American supply is important. South America yields an excellent
class of hide, salted or dry-salted. They are from an excellent breed
of animals, slaughtered and flayed with every care, and efficiently
cured. A most serious defect in this class of hide is the "brand,"
which is both deep and large and in the most valuable part of the
hide. One side, however, is usually unbranded, so that each hide
yields one good "bend." These hides, _e.g._ "Frigorifics," have
recently been much more extensively tanned in Britain because of the
shortage in the home supply of market hides caused by the European
War. South America also yields good horse hides. North American hides
are usually wet-salted (_e.g._ packer hides). They are usually good.
Central America yields mostly dried hides exhibiting usual defects.
The Asiatic supply comprises the frozen China hides, which are clean
but small, with flaying of uncertain quality. There are the buffalo
hides from Asia and East Europe, which are suitable for cheap and sole
and strap leather, and also the dry-salted "E.I. kips," obtained from
a small breed of Indian cattle, and extensively made into upper
leather. The Asiatic humped cattle also provide a limited supply. The
African supply is of increasing importance. The tropical parts yield
dried hides of uncertain quality, but the more temperate parts of
South Africa yield a growing supply of good quality.
REFERENCES.
"The Manufacture of Leather" (Bennett), pp. 27-37.
"Principles of Leather Manufacture" (Procter), pp. 33-56.
"The Ox Warble or Bot Fly" (E. Ormerod).
"The Making of Leather" (Procter), pp. 2-22.
Section II.--THE PREPARATION OF PELT
Before hides are tanned it is necessary for them to pass through a
series of preparatory processes. The object of these processes is to
obtain from the hide the true hide substance in a pure and suitable
condition. Each class of leather has its own appropriate processes, the
adjustment of which largely determines the quality of the finished
article. So prominent is the influence of these preparatory methods that
the paradox "good leather is made before tanning" is in trade circles
almost a platitude. These processes, sometimes lumped together under the
general name of "Wetwork," comprise soaking, liming, beam house work and
deliming. These will be discussed in turn.
The term applied to the hide after these processes, but before tannage,
is "pelt."
=Soaking= has for its object the cleansing and softening of the hides,
chiefly by means of water. It aims at the removal of dirt, blood, dung,
and curing materials by washing. The process is usually simple, and is
much the same for all classes of leather. The ideal to be aimed at is to
restore the hide to its condition when it left the animal's back.
Cleanliness in leather manufacture is as essential at the commencement
as anywhere, for the hide is in its most putrescible state. The soluble
proteids (blood, lymph, part of dung, etc.) which always adhere to hides
encourage the rapid growth of putrefactive bacteria, and cannot be
washed away too soon. Dung is often difficult to remove, being caked on
the butt end amongst the hair. Soaking only softens it, and mechanical
removal is usually necessary. If such substances are not removed, they
go forward with the goods into the lime liquors, causing stains,
loss of hide substance, and counteracting plumping.
The detailed method and time of soaking are determined mainly by the
nature of the cure. One of the purposes of the soak liquors is to
dissolve the salt used in curing hides and to rehydrate the hide and
make it again soft and pliable. As a 10-per-cent. salt solution exerts a
solvent effect on hide substance, it is necessary soon to change the
first soak liquor of salted goods.
Market hides, which are uncured, require the least soaking, the
cleansing effect being most needed. The hides are inserted into pits
("water dykes") of water for a few hours, and the water changed once or
twice. The soaking should not be prolonged as the hides are so
putrescible, and where it is customary to leave the goods in a soak
liquor overnight, it is advantageous to add a little slaked lime to the
water before inserting the goods. This not only softens hard water, but
is mildly antiseptic and plumping, and forms a suitable introduction to
the liming proper. Each pit contains a "pack" of 30-50 hides, according
to its capacity, which varies in different tanneries from 1000 to 2000
gallons. Tainted goods, which are indicated by a characteristic white
colour on the flesh side and by loose hair, need a preliminary washing
either in a "drum," "tumbler" or in a "paddle." This ensures a rapid
change of liquor and the removal of most of the putrefactive agencies.
Bad cases may need the application of antiseptics, such as immersion in
0.1 per cent. carbolic acid; but if possible these should be avoided, as
they lengthen the time required for liming. After drumming or paddling,
tainted goods should be placed directly into a lime liquor.
Salted hides need very similar treatment to uncured hides, but the
soaking is longer, because of the dehydration caused by salting. Hence
they receive also a greater number of changes of water, three or four
usually, but often more. As much loose salt as possible should be shaken
from the hides before insertion into any liquor. The employment of drum
or paddle before pit soaking is extremely useful to effect the rapid
removal of superficial salt, and is also useful after pit soaking to
remove the last traces.
Dried and dry-salted goods need a soaking still more prolonged, up to
one week if water alone be used. With the assistance of caustic soda,
however, the process can be shortened to about two days. The first soak
liquor should consist of a 0.1 per cent. solution of caustic soda, and
after the goods have been inserted twenty-four hours, they will be
materially improved by a few hours' drumming or paddling. Another
caustic soda soak will complete the process. Sodium sulphide crystals
may replace caustic soda, but about three times the weight will be
needed. Carbonate of soda and caustic lime also are a convenient
commercial substitute for caustic soda. For 10 lbs. caustic soda, use 36
lbs. carbonate and 7 lbs. lime. Extra lime should be added in all cases
when the water is hard. Acid liquors will also soften dried and
dry-salted goods, but such processes do not fit in so well with the
subsequent liming. The use of putrid soaks and stocks may be now
considered out of date.
=Liming= follows soaking, and consists essentially in immersing the
hides for 7-10 days in milk of lime. The chief object in view is to
loosen the hair and prepare for its mechanical removal. Liming takes
place in pits, the tops of which are level with the limeyard floor.
The lime is slaked completely and mixed well with water in the pit,
being particularly well plunged just before the insertion of a pack of
goods. Saturated limewater is only a 0.13-per-cent. solution. The
goods are occasionally "handled" _i.e._ hauled out of the pit and
reinserted after plunging ("hauling" and "setting"). This is necessary
to keep the liquor saturated with lime. The hides are inserted one by
one, each being "poked down" to ensure its contact with the liquor.
The goods are invariably immersed first in a previously used lime
liquor. Most tanneries now carry this out in a systematic way, so as
to ensure regularity in the process. As the goods are large and heavy
it is less laborious to carry out the whole process in one pit. In
this "one-pit system" the goods are inserted for (say) four days in an
old used lime liquor, with occasional handling; this liquor is then
run to the drain and a new liquor made up in the same pit, into which
the goods are inserted for (say) five days. They are then hauled and
sent to the unhairers. Each pack thus gets two liquors, old and new.
A better method is the "three-pit system." In this case each pack
receives three liquors and has (say) three days in each, first an "old
lime," then a "medium lime," and finally a "new lime." This system
ensures a greater regularity of treatment, and is deservedly the most
popular method for liming hides for sole leather. After being used once
as a "new lime," a liquor then becomes a "medium lime," and after being
thus used becomes the "old lime" which receives the green hides from the
soaks. The system involves the goods being shifted twice to another pit,
which is more laborious than reinsertion into the old pit, but if the
limeyard be arranged in "sets" or "rounds" of three pits, the shift is
usually only to the adjacent pit. One special advantage of this system
is that the top hides in one pit become the bottom hides in the next
pit, and _vice versâ_. Rounds of more than three pits are sometimes
used.
Many factories have now adopted systems in which there is no handling at
all. The hides are suspended in lime liquors which are agitated by
mechanical contrivances (_e.g._ Tilston-Melbourne process), or by jets
of compressed air (_e.g._ Forsare process). The goods are soaked and
limed "mellow to fresh" by changing the liquors by means of pumps, air
ejectors, etc. Thus the hides need no labour from first being inserted
until drawn for depilation.
In liming, the whole of the epidermis as well as the hair is loosened,
and is subsequently removed in depilation. The corium or true hide
substance becomes much more swollen by imbibation of water, and when
taken out of the new lime is "plumped" to very firm jelly. This plumping
is a matter of prime importance to the tanner. The coarser fibres are
thereby split up into the finer constituent fibrils, which fact assists
very materially in obtaining a quick and complete tannage, good weight,
and a firm leather. During the liming, the natural grease of the hide is
saponified or emulsified, which prepares for its removal in scudding.
Liming is thus a complex process: the hair is loosened, the hide is
plumped, and the grease is "killed." All these results may be hastened
by the use of other alkalies in addition, and most heavy leather yards
assist the liming by adding also sodium sulphide or caustic soda or
both. Sodium sulphide is a powerful depilatant, and will alone unhair
hides easily in strong solutions even in a few hours. As in solution it
forms caustic soda by hydrolysis, it possesses also the powerful
plumping and saponifying powers characteristic of the latter. The
addition of arsenic sulphide (As{2}S{2}) (realgar) to the lime when
slaking causes the presence of calcium sulphydrate in the lime liquors
thus made. This is also a powerful depilatant, but not much used for
heavy leather.
The function of the lime in depilating is complex and has occasioned
much discussion. Its main purpose, however, is that of a partial
antiseptic. When hides putrefy, one of the first results is that the
hair is loosened. In America depilation by "sweating" is carried out
commercially by such a mild putrefaction, the lime liquor permits a
similar fermentation at a slower rate, and all tannery lime liquors are
swarming with putrefactive bacteria. Liming is thus a safer method than
sweating, which may be easily carried too far. Various workers have
isolated specific organisms--Wood a _bacillus_, Schmitz-Dumont a
_streptococcus_--but it seems highly probable that the limeyard bacteria
are just the common organisms of putrefaction sorted out or selected by
the exact nature of the liquor and the method of working the limes. Many
putrefactive bacteria are very adaptable and could easily accommodate
themselves in this way. It is known that the exact nature of the
culture medium has a great influence on the rate of development of such
organisms, and which particular species thrive and obtain predominance
in any limeyard will depend upon the amount and nature of the dissolved
organic matter available as food, and upon the exact alkalinity and the
concentration of other apparently inert substances, such as common salt
and sodium, calcium and arsenic salts. Hence no two lime liquors operate
alike, and approximate regularity is only assured by systematic method.
In handling and shifting, the organisms are subjected to further
selection, and the most adaptable survive. It is probable that different
species may act symbiotically. The depilating organisms of lime liquors
are probably mostly anærobes, but some may be anærobic by adaptation. It
is probable that ærobic ferments commence the depilation, but this will
be done before the goods are put into work, or at any rate before they
reach the limes. More strictly, it is the enzymes secreted by bacteria
which are directly responsible for the hydrolytic work; these enzymes
are chiefly proteolytic (proteid splitting), but the lipolytic (fat
splitting) enzymes have also a place.
The lime, however, not only limits and selects the course of the
putrefaction, but also affords more positive assistance. Lime plays its
own hydrolytic part and assists the depilation by purely chemical
action. Lime will unhair without the assistance of bacteria, but its
action is slow and forms a minor part of the operation in the average
limeyard. This action is due chiefly to its progressive formation of
calcium sulphydrate from the cystine group of the softer keratins. Lime
also plays an essential part in assisting the putrefactive fermentation.
It softens the keratins and thus assists the bacterial attack, it
hydrolyzes other proteids and provides the bacteria with food in
solution, the calcium ion increases the proteolytic action of certain
enzymes, and finally the apparently inert excess of undissolved lime has
an accelerating effect on the bacterial activity.
In the average limeyard these various functions are inextricably mixed
up, and it is impossible to assign any definite proportion of the total
depilatory effect to any of the factors at work. Lime alone will unhair,
bacteria alone will unhair, and sulphides will also unhair without lime
or bacteria, but in the limeyard all three agencies are at work.
Putrefactive fermentation, however, obtains a good start. Ærobic
fermentation commences with the slaughter of the animal, and the
anærobic organisms soon commence their part, and are at work in the hide
house and soaks. On entering the limes, the purely chemical hydrolytic
action of lime is added to that of the bacterial enzymes as well as the
action of lime as bacterial assistant, and the three continue to operate
side by side. Each gives rise to the formation of calcium sulphydrate,
whose own special solvent effect is superadded. If sulphydrates be
deliberately added to the liquors there is yet another factor assisting.
Speaking broadly, the bacterial enzymes have their maximum activity in
the old limes, and the chemical action of sulphydrate formed from the
keratin cystine is also at a maximum in these liquors. The chemical
action of added sulphide, and the simple hydrolytic action of calcium
hydrate have their maximum activity in the new limes. Most observers
would agree that in practice the bacteria shoulder the greater part of
the work.
From the limeyard is taken about the only waste bye-products of the
tannery, viz. the residues from the soak and lime pits. These consist
mainly of lime and chalk, with some hair and dung, and possibly a little
sulphide. The sludge possesses some value as a manure, especially if
from the soak pits on account of the greater nitrogen content. (Part
VI., Section I.)
=The Beam House Work= consists in the mechanical removal of those
parts of the hide not wanted for leather manufacture. _Unhairing_
removes the hair and the epidermis made loose in liming. The hides are
placed over a sloping "beam" with a convex surface, and the hair scraped
off with a blunt concave and double-handled knife. The hides are then
thrown into a pit of water. The hair is carefully collected, washed well
with water, preferably centrifuged, and then dried out by a current of
warm air. It forms a valuable bye-product. White hair is usually kept
separate and fetches a higher price. _Fleshing_ is the next process. The
hides are again placed over a beam, with the flesh side (_i.e._ the side
nearest the flesh) uppermost. Skilled workmen then cut off, with a sharp
convex knife, the fat, flesh and connective tissue left in flaying.
_Rounding_ is usually the next process. The unhaired and fleshed hide is
spread out flat and cut up into butt, shoulder and a pair of bellies.
These parts have different commercial values, and may afterwards be
tanned by different methods for very different purposes--for dressing
leather, and sometimes even for sole leather. _Scudding_ is the last
piece of beam work. The fleshed hides (whether rounded or not) are
washed, or at least rinsed, with water, and again placed on the beam
grain side up. They are then scraped with a rather sharp concave knife,
to remove "scud," which consists of hair roots and sheaths, lime soaps,
fat, pigment and other dirt. Short hair is shaved off by a very sharp
hand knife.
The beam work demands a certain amount of skill from the workmen,
especially from the flesher, whose sharp knife may prove very wasteful
in incompetent hands. Hand labour was slowly but surely being replaced
by machinery before the war, and war-time conditions have greatly
accelerated the rate of transition. Beam house machinery is rapidly
becoming universal. The machines are cumbrous and expensive in cost and
in power, but machine work is quicker, less laborious, and needs much
fewer workmen. Many types of machine have been suggested, but the most
useful are those in which the hides pass over rollers and are
simultaneously acted upon by a rapidly revolving cylindrical knife with
spiral blades, one half being a left-handed and the other a right-handed
spiral, so that the hide is scraped outwards as well as in the direction
of motion. The part of the hide being acted upon rests on a pneumatic
roller. By changing the type of spiral knife cylinder the machine will
unhair, flesh or scud.
=Deliming= is a general name covering a number of similar operations
whose primary object is the neutralization and removal of the caustic
lime and soda in the plumped pelt, or at any rate on the surface of the
hide. This is a preparation for the tan liquors. All the tannins and
many associated substances darken rapidly with oxidation when in
alkaline solution, so that to place the fully limed hide in a tan liquor
would give a dark-coloured leather. A short insertion in a bath of weak
acid would secure the elimination of surface lime and the disappearance
of this difficulty, but there are other purposes in deliming. The more
completely lime is removed the more the plumped pelt "falls" into a
soft, pliable, unswollen and relaxed condition, and this change assists
very materially in the production of a soft dressing leather, suitable
for boot uppers, bags, etc. For such leathers, therefore, the deliming
must be much more complete than for sole leather, in which the object is
to obtain a firm and plump leather.
In the case of the softer dressing leathers, experience indicates the
advisability of allowing some further bacterial action on the
interfibrillar substance in order to produce the requisite pliability
and softness. This is secured by "bating" the hides. This process
consists in immersing the goods into a cold fermenting infusion of hen
or pigeon dung. The infusion is made in a special tub or pit with warm
water and allowed to stand for a day or two until the fermentation has
commenced, and then run into the bating pit through a coarse filter such
as sacking. The hides are immersed for some days, but are handled
frequently to ensure an even effect. The bate is always slightly
alkaline. The caustic alkalinity increases rapidly at first owing to the
diffusion of caustic lime, then at a slower rate, afterwards slowly
declining. This is explained by the production of organic acids, and
their salts with weak bases from the dung infusion by the action of
bacteria. The total alkalinity of the bate liquor increases rapidly at
first owing to the diffusion of lime and its liberation of organic
bases, then very slowly, but towards the end of the operation the total
alkalinity increases very rapidly indeed, owing probably to the
commencement of a violent anærobic fermentation which produces ammonia
and other organic bases, and which heralds the approach of a
putrefactive action, which if allowed to continue for even a short time
will ruin the hides. Bating is consequently a risky process, and needs
experienced oversight. For goods which need only a mild bating, there is
the alternative of giving a longer liming in older limes. This of course
involves more bacterial hydrolysis, and perhaps does it in a safer, more
economical and certainly in a less offensive manner. Bating is often
followed by a further deliming by acids. Boric, lactic, acetic, formic
and butyric acids are all used, and with care even hydrochloric and
sulphuric acids may be employed. Innumerable "artificial" bates have
been put on the market, but most are merely weak acids, acid salts or
salts of strong acids with weak bases. An American "bacterial bate"
consists of a lactic fermentation of glucose in the presence of glue.
Closely similar to bating is "puering," investigated by Wood.
Drenching is another fermentive deliming process. In this the goods are
inserted into an infusion of bran. This is made by scalding the bran
with hot water, and allowing it to stand until it is about 70°-90° F.
The infusion is then "inoculated" with a few gallons of old drench
liquor, and the goods are immersed. This fermentation has been examined
carefully by J. T. Wood. First the enzyme cerealin converts bran starch
into glucose, which is then fermented by the drench bacteria with the
production of lactic acid, some acetic acid and small amounts of formic
and butyric acids. The butyric fermentation is liable to become too
violent. These acids, as they are formed, neutralize the lime in the
hides and plump the pelt slightly.
Various gases (carbon dioxide, hydrogen, nitrogen, methane and
sulphuretted hydrogen) are involved, and the proportion produced in the
pelt itself has a peculiar opening effect on the hide fibres. The
activity of the drench can be decreased by dilution and by using a less
starchy bran, and can be increased by adding pea meal or rye meal.
Drenching usually follows bating. Scudding sometimes follows deliming.
The theory of the volume and elasticity changes of pelt during
preparation will be better understood after considering the behaviour of
gelatine gels. The determining factors are the _nett_
charge of hydroxyl ions on the disperse phase, resulting from ionic
adsorptions, and the lyotrope influence of dissolved substances on the
continuous phase.
In softening dried hides the swelling may be due to either influence,
but the latter tends to loss of hide substance and the production of
soft leather.
In liming, the nett adsorption of hydroxyl ions is the principal factor,
but the lyotrope influence of the alkali cations and of the impurities
is important. Plump pelts are those in which the contained water is in a
relatively greater average state of compression. Few substances can
assist plumping, but many can hinder it. In plumping all lyotrope
influence is objectionable, and "sharp" (pure) alkali solutions are
required. Mellow limes reduce elasticity and plumpness by lyotrope
influence.
In bating and puering the essential change is that before the process
the swelling is due chiefly to adsorption of hydroxyl ions, whereas
afterwards it is due chiefly to a composite lyotrope influence.
REFERENCES.
"Principles of Leather Manufacture," Procter, pp. 108-184.
"The Manufacture of Leather," Bennett, pp. 49-113.
"Lyotrope Influence and Adsorption in the Theory of Wetwork,"
Bennett, _J.S.L.T.C._, 1920, pp. 75-86.
"Analytical Examination of Bating," Bennett, _Leather Trades
Review_, 1911, p. 972, and 1912, p. 28.
"The Bating, Puering and Drenching of Skins," by J. T. Wood.
SECTION III.--VEGETABLE TANNAGE
All tannages have for their object the conversion of the readily
putrescible hide tissue of the corium (the pelt) into an
imputrescible, insoluble and permanent material called "leather"
which, possessing considerable strength and pliability, is capable of
application to a variety of useful purposes. The conditions necessary
for this transformation have been clearly stated by Procter.[1] For
the production of leather from pelt "it is not only necessary to dry
the fibres in a separate and non-adherent condition, but so to coat
them or alter their chemical character that they are no longer capable
of being swelled or rendered sticky by water." Whatever substance will
secure this permanent dehydration of the hide fibres in a separate
condition is called a "tanning material." The change from pelt to
leather is known as "tannage," the process is termed "tanning," and
those who undertake it are "tanners."
[Footnote 1: "Principles of Leather Manufacture," p. 184.]
In "vegetable tannage" the tanning materials are of vegetable origin,
and contain a group of organic compounds called "tannins" which are
extracted by the infusion of these materials with water. Pelt, when
immersed in these infusions, is converted into leather, rather slowly;
but a gelatin solution gives an immediate precipitate of "amorphous
leather," even if the tannin infusion be exceedingly dilute. The
tannins are aromatic compounds of phenolic character, and contain
carbon, hydrogen and oxygen only, but our knowledge of their chemical
constitution is exceedingly small owing to their instability and
colloid nature, which make impossible their preparation in a pure
state. They are all, however, derived from either catechol or
pyrogallol, and yield these substances if carefully heated to about
200° C. The tannins are soluble in water, alcohol, acetone, ethyl
acetate and acetic acid, but insoluble in benzene, chloroform, carbon
disulphide, petroleum ether,
dilute sulphuric acid and _pure_ ethyl ether. The aqueous infusions of
the tannins are in reality colloidal solutions; _i.e._ heterogeneous
systems of two phases. The systems are lyophile, or, more particularly,
hydrophile, _i.e._ there is an affinity between the two phases. As usual
with lyophile systems the two phases may be considered as both liquid,
and an aqueous infusion of tannin forms an emulsoid sol, which therefore
is subject to the phenomenon of adsorption. The tannins are all
precipitated by solutions of basic lead acetate and copper acetate, and
many of them with varying completeness by solutions of many other
metallic salts and hydroxides, of basic dyestuffs and of alkaloids. They
give dark colorations with ferric salts.
The tannins are widely distributed in plant-life, but only in a limited
number of cases do the plants contain sufficient tannin to render them
of commercial importance. Tannin is found in all parts of plants, but
usually in greatest amount in the bark or fruit. The tannins are
classified into "pyrogallol tans" and "catechol tans," according to the
parent phenol. This classification is confirmed by their chemical,
analytical and practical behaviour, and the vegetable tanning materials
may be classified into the same two groups, for, although even the same
plant contains both pyrogallol and catechol tans, it is usual to find in
any one part of the plant that one group is predominant.
=Pyrogallol tans=, which are oftenest obtained from fruit or leaves,
contain usually about 52 per cent. of carbon. Used alone they produce a
rather soft and porous leather. Associated with them--in many cases
probably as decomposition products--are certain other substances of
well-known properties and constitution. These substances are not only
typical of the group, but also form the most valuable clue to the
chemical constitution of the group and the key to their chemical
behaviour. One of these substances is gallic acid (3:4:5
trihydroxy-benzoic acid C{6}H{2}(OH){3}COOH), which possesses
properties very similar to the tannins, but does not precipitate gelatin
and will not itself make leather. Another of these substances is ellagic
acid C{14}H{6}O{8}, a double lactone of a
hexa-hydroxy-diphenyldicarboxylic acid. This is deposited as an
insoluble yellow powder from infusions of many pyrogallol tans, by
boiling with dilute acids only, allowing them to stand for a few days.
In practice the deposit is found as mud at the bottom of the tan pits,
and also upon the leather, to which it strongly adheres. It is
technically known as "bloom." It is insoluble in acids and cold alcohol,
but soluble in alkalies. It is a feeble dye-stuff. The pyrogallol tans
yield very different amounts of bloom. Other associated substances are
the sugars. In practice these sugars ferment to lactic, acetic, and
other acids which cause "sour" liquors. Such liquors plump the hides and
tend to give firm, thick leather. These acids also probably cause
increase of adsorption of tannin by the hide and therefore assist in
giving "good weight." Solutions of pyrogallol tans all give a blue-black
colour with a dilute solution of ferric alum. If a solution of sodium
arsenate be added to an infusion of pyrogallol tan diluted until no
longer distinctly coloured, and the mixture allowed to stand for about
two hours, a green colour develops at the surface of the liquid. The
reaction is due to gallic acid or a similar grouping, and is, in the
author's experience, the most satisfactory qualitative test for the
group. Another test is to mix equal volumes of a 0.4 per cent. infusion
of tan and a 10 per cent. solution of sodium bisulphite; a few drops of
10 per cent. potassium chromate are added, and either a transient
blood-red colour or a more permanent deep purple is obtained. The former
colour is due to gallic acid. If a tannin infusion be largely diluted
with hard water and a little iodine solution added, the pyrogallol tans
yield either a purple-red or a dark blue colour, the former being a
reaction of gallic acid. Pyrogallol tans yield no precipitate with
bromine water. They yield a yellow or brown colour when one drop of
infusion is added to concentrated sulphuric acid.
Myrabolans is one of the most important of the pyrogallol tanning
materials. It is a name given to the dried fruit of _Terminalia chebula_
and other species of Indian trees. The nuts resemble an elongated
walnut. They are dried and exported from many parts of India to all
parts of the world, but largely to this country. The varieties of
commerce are named according to origin and quality: thus we have "J1's,"
_i.e._ Jubbelpore, No. 1 quality, "R1's" (Rajpore, No. 1),
"B1's" (Bhimley, No. 1), etc. The little difference in tannin strength
(about 32 per cent.) in these varieties is usually compensated by
corresponding differences in price. The quality of myrabolans cannot be
safely judged by appearance. Much bloom is deposited by myrabolans
liquors, especially by "J's." Myrabolans are amongst the most sugary of
tanning materials, containing up to 5-1/2 per cent. It is therefore one
of the best materials for giving a plump leather. Broadly speaking,
those varieties which yield most sugar yield least bloom, and _vice
versâ_. Myrabolans tannin has a small affinity for hide substance and
penetrates the hide very slowly. It gives a "mellow" tannage, but a
bright, good colour, which characteristics are imparted to the leather
when the material is blended with other materials containing dark or
astringent tannins. When used alone it yields a rather spongy leather,
and it is not considered a good weight-giving material, though its
acid-producing powers are very helpful to other more astringent tannins.
Valonia has been the other staple tanning material of the heavy leather
trade. It is the acorn cup of oaks common in Asia Minor and Greece,
chiefly the Turkish oak (_Quercus ægilops_). The fruit is gathered when
ripe and dried in layers of about one foot deep until the acorn drops
out, Smyrna is the great export centre. Greek valonia is obtained from
many parts of the Archipelago and mainland. It is gathered in a more
immature condition and includes the acorn. It is considered slightly
inferior in strength and colour to the Smyrna valonia. The exterior of
the acorn cup is covered with rather scaly protuberances known as
"beard," which contains usually about 40 per cent. of tannin. The cup
alone contains usually about 25 per cent. tannin, and the whole about 30
per cent. The valonia tannin has been thought to contain two chemical
individuals, only one of which produces bloom. Parker and Leach[2] found
that the tannin of the cup produces more bloom than that of the beard,
and that Smyrna valonia yields more bloom than Greek. The more bloom is
deposited, the less acid will be produced. Under all conditions the
yield of bloom is large, and its deposition in and on the leather
assists materially in giving the weight and water-resisting powers
associated with sole leather which has been largely tanned with valonia.
The valonia tannins have only a moderate affinity for hide, which, like
myrabolans, they penetrate very slowly. When used alone the leather is
less yellow than that from myrabolans, and is also duller. After most of
its bloom has been deposited valonia makes a very suitable tannage for
dressing leather, and in conjunction with gambier has been largely thus
used. Since the outbreak of war the Turkish product has, of course, not
been available for importation.
[Footnote 2: _J.S.S.I._, 1903, 1184.]
Sumach[3] is the other pyrogallol tan of commercial importance. It
consists of the leaves and small twigs of the Sicilian sumach (_Rhus
coriaria_) cultivated in Italy extensively for export. The leaves are
hand picked, dried and often ground to powder. It contains 26-28 per
cent. of a tannin which yields little or no bloom, but much gallic acid.
It is an unstable tannin, and its infusion rapidly ferments. Sumach is a
very valuable tanning material, and when used alone gives an exceedingly
durable leather of excellent light colour. It gives a soft mellow
tannage, and is therefore most suitable for light leather tanning, and
is extensively used for this purpose. It is used, nevertheless, in large
quantities by the heavy leather tanners for finishing purposes, for it
contains some organic reducing agent which exerts a powerful bleaching
action on other tannages, and which assists to brighten as well as
lighten the rather dull appearance of leathers largely tanned with
valonia. It is rather an expensive tannin, but most manufacturers find
that its results are worth its cost.
[Footnote 3: Also spelt Sumac and Shumac, and always pronounced like the
latter.]
Other pyrogallol tans are also used to a limited extent. Algarobilla and
divi-divi are the fruit pods of several species of American _cæsalpina_.
They are strong in tan (45 per cent.) and yield a light-coloured and
bright leather, but are unstable tans, yielding much bloom. Babla is a
small pod yielding a mellow tannage and much gallic acid. Celavinia is
another pod containing no colouring matter and giving an almost white
leather. The tannin is closely similar to that of oak galls. These last
were once extensively used for tanning in Austria. Willow bark is used
for tanning in Russia and Denmark. Valuable pyrogallol tannins are
obtained from oak wood and chestnut wood, but the woods are not used in
tanning as the percentage of tan is so small.
=Catechol tans=, often obtained from barks, contain usually about 60 per
cent. of carbon. They are seldom used alone, for they usually have
little or no sugar associated, and hence their liquors do not either
"sour" or "plump." They can be used alone if artificially acidified, but
without acidifying or blending would give a rather flat leather, though
possibly firm. They yield no bloom or gallic acid, but have associated
with this other characteristic substances. Of these the catechins are
the most typical, and have been considered as the parent substances of
the catechol tans.
The catechins are white crystalline substances, apparently isomers with
the general formula C{15}H{14}O{6}. They have different
melting-points, and varying amounts of water of crystallization, but are
otherwise exceedingly similar in properties. They are sparingly soluble
in cold water, but freely in hot, and in alcohol and ether. They are
precipitated by lead acetate, mercuric chloride and albumin, but not by
gelatin, tartar emetic or alkaloids. In gambier liquors they are
especially strong, and sometimes crystallize on the side of the pits,
being thus known as "whites." The phlobaphenes or "reds" are also
typical of catechol tans from which grow catechins; they can be formed
by boiling with dilute mineral acids. They are considered to be
anhydrides of the catechol tans. They are difficultly soluble in cold
water, but freely in hot, and in cold alcohol and dilute alkalies. They
are true tannins and alone are capable of making a red leather, but in
practice are often found as mud in the tan liquors owing to their
limited solubility. They naturally influence the colour of leather made
with catechol tans, which is usually distinctly redder than the leather
made from pyrogallol tans. Infusions of catechol (_cp._ catechin) give a
green-black colour with iron alum. The sodium arsenate test gives a red
colour due to catechin. The chromate and iodine tests mentioned for
pyrogallol tans give negative results with the catechol tans, but
bromine water gives a precipitate, and sulphuric acid a crimson colour.
Mimosa bark is one of the most important catechol tans. It is usually
obtained in this country from Natal ("Natal bark"); but the tree (Sydney
green wattle, _Acacia mollissima_) is a native of Australia. It is being
cultivated now extensively in South Africa, and forms a most valuable
portion of the Empire's stock of tanning material. Its more extensive
use has been long recommended by the author,[4] but its gradually
increasing employment in British tanneries has been greatly accelerated
by the war, which has prevented its delivery in Germany and has cut off
Turkish valonia from Britain. It yields about 30 per cent. of a stable
and excellent tannin, and will produce a firm, durable leather, with a
colour much less red than that obtained from many other catechol tans.
It is an astringent tan, and if carelessly used yields a harsh or even
"drawn" grain. Most of the tannin is easily extracted, yielding a clear
infusion which penetrates fairly quickly and gives good weight. It
contains less than 1 per cent. of sugar, which unfortunately rapidly
ferments to carbonic acid, so that it is not a good plumping material.
It makes in all respects an excellent blend with myrabolans. Like all
catechol tans, the resulting leather darkens on exposure to sunlight.
[Footnote 4: J.S.C.I., 1908, 1193.]
Oak bark, from _Quercus robur_, is the ancient tanning material of
Britain, and is still used to a limited extent. It contains about 13 per
cent. of tannin and is mainly a catechol tan, but also contains a
pyrogallol derivative. It yields catechin, and gives a red colour with
the sodium arsenate test, but also will yield some bloom and gallic
acid, and gives a blue-black with ferric salts. The tannin itself is
exceedingly similar to that of mimosa bark, but the material contains
about 2-1/2 per cent. of sugar, which makes it possible to employ oak
bark alone for making sole leather. It is noted for yielding a sound,
durable leather of good typical tan colour. Its tannin combines well
with hide and penetrates quickly. The fatal disadvantage of oak bark is
its weakness in tannin strength compared with other materials. This
results in heavy freight and heavy cost per unit tannin, bulky storage,
expensive handling in the factory, comparatively large bulk of spent
tan, after relatively greater trouble in extracting, and the
impossibility of making the strong liquors so necessary in these days to
produce good weight in a short time. No satisfactory extract has yet
been made from it.
Pine bark, from _Pinas abies_, is one of the staple materials of the
Continent. It contains up to 14 per cent. of a catechol tan, and, unlike
most of this group, contains a high proportion of sugar and will give
good results alone. Hemlock bark has been the staple tanning material of
North America. It is obtained from the hemlock, or _Pinus canadensis_.
It contains up to 11 per cent. of tan and much phlobaphene, and yields a
characteristic red leather of good quality, but which rapidly darkens
with sunlight. It contains some sugar, but is usually employed in
conjunction with sulphuric acid or with sugary materials. Mallet bark
yields another catechol tan similar to that of mimosa, but somewhat less
astringent and more yellow in colour. Quebracho wood and mangrove bark
have been used, but are now made into extracts.
=Leaching.=--Whatever class of leather is being made, and whatever blend
of tanning materials is being employed, the tannins must be efficiently
extracted by water in order to make the tanning liquors. This process is
called "leaching." The tanning materials, after being ground, crushed or
shredded, are placed in large pits arranged in "rounds," "sets," or
"batteries" of 6, 8 or 10 units, through which water is percolated
systematically, so as to secure a continuous extraction. Water itself is
added to only one of the pits of material. The liquor produced is passed
on to the next pit, and then to the next, and is continually gathering
strength. After passing thus through the series, the liquor becomes the
source of the strong extracted tan liquors which are used in the tannery
proper. With this system the stronger leach liquors are being acted upon
by fresh material, and the nearly "spent" material is being acted on by
the weakest liquors, and finally by water, thus ensuring a complete
extraction. In the press leach system, which is now practically
universal, the bottom of one pit communicates with the top of the next,
and the liquor presses round by gravity flow caused by a few inches
"fall." Liquor is thus constantly percolating downward through the
material in each pit. The "head leach" and "tail leach" are always
adjacent in a double row of pits, and when the material in the latter is
quite spent, it is "cast," and the pit is filled with fresh material.
The liquor is then pressed round into this pit by adding water to the
tail leach. Hot water is used to secure better diffusion. At least two
such sets of leaches ("taps" and "spenders") are necessary to spend the
material of the average tannery and to obtain liquors of the necessary
strength.
=The Manufacture of Extracts.=--In addition to the use of the natural
tanning materials described above, modern leather manufacturers employ
also a variety of "tanning extracts," _i.e._ vegetable tanning materials
in which the tannin has been already extracted, and which are supplied
in form of a solid or concentrated liquid. Such extracts only need to be
dissolved in warm water in order to make a tan liquor, and the cost and
trouble of leaching is avoided. They are a great convenience as making
strong liquors of definite strength. Many vegetable tanning materials
are too weak in tan for the tanner to leach, and indeed to justify the
cost of importation have been made available by manufacturing an extract
at the source of the material. With such weak materials the extract
manufacturer has had to secure a much more complete extraction than in
ordinary leaching, and to concentrate his infusions by means of
steam-heated vacuum pans. With such experience he has naturally begun to
make extracts also from the stronger materials, such as myrabolans and
mimosa bark, and it is now possible to have a tannery without any
leaches at all. Tanners also have begun to realize the advantages not
only of more rapid and complete extraction, but also of doing the work
for themselves, and extract factories are beginning to appear as an
adjunct to the larger tanneries. The more complete extraction of tan
also involves a greater extraction of unwanted colouring matters, hence
decolorization is a feature of extract manufacture.
=Chestnut Extract= is from the wood of the Spanish chestnut (_Castanea
vesca_), which contains 3-6 per cent. of a valuable pyrogallol tan very
similar to that of valonia. Its weight-giving and water-resisting powers
are as good as valonia, and its penetrating power is even better, so
that it forms an exceedingly suitable material for the modern short
tannage, and also for drum tannages. The extract is manufactured
extensively in France. The wood is stripped of bark and usually piled
for some months to dry and to allow the resins to become insoluble. Some
factories, however, use the green wood direct. There are two methods of
extraction, viz. in open vats and in closed vats under pressure. The two
methods yield extracts which differ in composition and properties. In
either case the vats have a capacity of up to 3,000 gallons, and hold up
to 6-1/2 tons of wood. They are arranged in series, as in leaching, and
the liquor passes in succession through all the vats over wood less and
less spent. The temperature is highest in the vat containing the fresh
water and nearly spent wood. In open vats of wood or copper the
temperature is near boiling-point, whilst in the closed autoclaves
(copper or bronze) the pressure reaches about two atmospheres and the
temperature about 130° C. (266° F.). The series may contain 5, 7, 9 or
even 12 vats, and the liquor obtained has a strength of 3° to 4-1/2°
Beaumé (22° to 33° Bkr.).
After extraction the liquor is allowed to stand, and much insoluble
matter settles out--resins, wood, fibre, etc. The clarified and settled
liquor is then passed through a cooler up to about 55° C., and then run
into the decolorizing plant, a deep vat fitted with a copper steam coil
and mechanical stirrer attached to power. The best decolorizer is
bullock's blood, which is run into the vat and well mixed. The
temperature is next raised to about 70° C., causing the blood albumin to
coagulate. It carries down with it a little tannin, but much colouring
matter. After standing a few hours the settled liquor is run off direct
to the evaporator. A multiple-effect evaporator is usually employed, and
the concentrated liquor, which has a strength of about 25° Beaumé, is
run into suitable oak casks. The extracts contain 27-32 per cent.
tannin. An extract made with open vats has about 7 per cent. soluble
non-tanning matters, whilst a "pressure extract" may contain up to 12
per cent. of these "non-tans." Pressure extracts obtain also a better
yield of tannin, which more than compensates for the slightly lower
price. Open extraction yields, however, the purer product and an extract
with better penetrating powers, and is consequently the more suitable
for drum tannages. Chestnut extract is extensively used by the heavy
leather tanners.
=Oakwood Extract= is manufactured from the wood of the common oak
(_Quercus robur_). The centre of the industry has been the oak forest of
Slavonia. The wood contains 2-4 per cent. of a tannin very similar to
that of chestnut wood, but somewhat more astringent.
The manufacture is also similar to that of chestnut extract, but
decolorization is often omitted, and greater care has to be taken and in
other ways to keep the colour within limits. One of these is to strip
the wood more completely of bark. Another is to operate at as low a
temperature as possible, about 110° C. The extraction is made in large
circular vats about 14 feet high and holding about two tons material. A
battery is composed of about eight vats or extractors. Open extraction
is used, and the liquor is passed forward after 2-3 hours' boiling, so
that the material is spent in about 24 hours. A liquor of about 5° Bé
(36 Bkr.) is obtained, and the strength of the material reduced from 4
to 1/4 per cent. of tannin. Getting rid of insoluble matter is a
difficulty, and is attained by settling, by rapidly cooling, and then
passing through a filter press of wood. For evaporation a double-effect
vacuum pan is preferred, which operates first at about 113° F., and
afterwards at 140° F. with a higher vacuum. The liquor is concentrated
from 5° to 25° Beaumé (s.g. 1.036 and 1.210 respectively).
The extract has a much higher colour than chestnut, and is not used now
as much as some years ago. As the principal supply was German, it has
been unavailable.
=Quebracho Extract= is made from the wood of the South American tree
_Loxopteryngium Lorenzii_, which contains about 20 per cent. of a
typical catechol tan. It is associated with a little catechin, much
phlobaphene, but practically no sugar. The tannin is very astringent,
penetrates quickly and gives a firm red leather which darkens on
exposure to light. It is not noted for weight-giving powers. The wood
itself, as chips or shavings, has been used in British tanneries, to a
limited extent, but the great bulk of the material is made into extract
chiefly in South America. The crude "extract," made by evaporating
aqueous infusions of the wood, is largely exported for refinement in
Europe. It is also refined on the spot to a large extent and converted
into solid extract containing 60 per cent. of tannin.
The great difficulty with quebracho has been the disposal of the
phlobaphenes, and a great variety of quebracho extracts are now
available which deal with this problem in different ways. In some the
more soluble reds are simply left in the extract under the idea that
they are really tannins and may be of some use in some part of the
tanning process; in others they have been removed by settling and
filtration at appropriate temperatures and concentrations; in most,
however, they have been solubilized by treatment with alkalies, in the
presence of reducing agents, notably by heating with sodium bisulphite
in closed vats. The base combines with the phlobaphenes, which are made
completely soluble and available for tanning. Sulphurous acid is
evolved, and its reducing powers assist materially in retaining and
promoting a good colour in the product. Such "sulphited extracts" are
now extensively manufactured in this country from the imported "crude"
extract, and sold as liquid extracts containing 30, 35 or 40 per cent.
of tan according to the requirements of the buyer; "mixed extracts"
which are solubilized quebracho blended with about 15 per cent. of
myrabolans, are also used.
By solubilizing quebracho with excess of bisulphite an extract is
obtained which possesses considerable bleaching powers, and such
extracts are also extensively manufactured for the "vatting" or
bleaching of heavy leather after tannage. The excess of sulphurous acid
not only bleaches the leather, but also swells it up and thus permits a
further absorption of strong tan liquor, which is conducive to good
weight. These bleaching extracts are usually of 36-38 per cent. strength
in tan.
=Gambier= is an extract of the leaves and twigs of the eastern shrub
_Nauclea gambir_. It is a catechol tan of peculiarly mellow quality and
great practical value. It contains much catechin, but little
phlobaphene, and yields a beautifully soft leather, but without weight.
It is an exceedingly suitable material for the early stages of tanning,
and is much liked for tanning leathers that have to be curried, and is
widely used in the manufacture of upper leather. It is, however, an
exceedingly expensive tannin, and the extract is made in a very crude
way by Chinese and Malays without much supervision. Hence its strength
in tan and general quality is extremely variable. The plant is
cultivated for the purpose of extract manufacture, and prunings are
taken in the plant's third year. They are bruised and boiled with water
in the open. The infusion is strained, concentrated, and poured into
cooling vessels in which it sets to a paste. Two varieties of gambier
are well known, "cube gambier" and "block gambier." In the latter the
extract remains as a paste containing 25 to 40 per cent. of tannin. It
is sold in oblong blocks of 1 or 2 cwt., either wrapped in cocoanut
matting or in wooden boxes. Cube gambier is made by running the
concentrated syrup into trays 2 inches deep and drying in the sun. When
partly dry, it is cut up into 1-1/2-in. cubes and dried further on
cocoanut matting. The rough "cubes" as imported contain 40-50 per cent.
of tannin.
=Myrabolans Extract= is now largely manufactured in this country. A
liquid extract of 25, 30 or even 35 per cent. strength is made for
home consumption, and a solid extract for export. The light colour,
high strength and easy extraction of the natural material have all
facilitated the task of the manufacturer.
The material is extracted in open vats or stills of copper, which take
one ton or more of nuts. A battery of 4, 6 or 8 of such stills is
usually employed, and the temperature is kept well below boiling-point
except in the vats containing the nearly spent material. The liquors
move forward quickly, and the material is quite spent in 24 hours. The
material when cast contains less than 1/2 per cent. of tannin. The
liquor obtained is 40°-50° Bkr. (6 - 7-1/2 per cent. tan), and after
settling is concentrated at 40°-50° F. in a single effect vacuum pan,
which though more costly in steam is quicker than the multiple effects,
and gives the low temperature required. For solid extract the more
concentrated liquor is run direct into tarred bags, in which it soon
solidifies.
=Hemlock Extract= is manufactured from the North American pines and
imported into this country to some extent. It gives a very red colour.
=Mangrove Extract= is made from the bark of _Rhizophona Mangle_ and
other species of mangrove which grow freely in the tropical swamps of
West Africa, Borneo, etc. Much solid and liquid extract has been made
from this material, but is not very popular on account of its harsh
tannage and dark red colour.
=Pine Bark Extract= (_Larch extract_) is made in Sweden from the Norway
spruce (_Pinus abies_). It is slightly sulphited and gives a good
colour. It is a liquid extract of about 30 per cent. strength, and is
sometimes used as a chestnut substitute. It should not be confused with
the so-called "spruce" or "pine wood" extract, which is a paper trade
bye-product and contains ligneous matters rather than tannin.
=American Chestnut Extract=, made from the chestnut oak, is either a
liquid or a solid extract in powder form. It gives a wretched
brown-black colour, which is quite unsuited to the usual British needs.
THEORY OF VEGETABLE TANNAGE.
Vegetable tannage is a phenomenon of colloid chemistry. The old
arguments as to whether tanning was a chemical or a physical process
have been rendered obsolete by the advent of a new set of explanations,
which, though shedding light on many obscure points, have enormously
increased the complexity of the problem. In vegetable tannage an
emulsoid gel (pelt) is immersed in a complex emulsoid sol (tan liquor),
which immersion results, not in simple reaction or change, but in a
series of changes.
One of these changes is _adsorption_. Pelt is a gel which possesses a
great development of surface. It not only exhibits like gelatine the
phenomenon of imbibition and dehydration to a very marked extent, but
also possesses a very fine fibrous structure due to its organic origin;
thus pelt possesses an enormous specific surface, further intensified by
the preparation processes previously discussed, which split up the hide
fibres into smaller bundles and into much finer constituent fibrils.
Tannins, on the other hand, are hydrophile colloids which in water form
emulsoid sols, and which may thus be expected to exhibit the phenomenon
of adsorption. A tan liquor usually contains several tannins in addition
to other closely similar substances, also in colloidal solution, and is
therefore a sol of considerable complexity. The immersion of pelt into a
tan liquor results in an adsorption, which consists essentially in an
inequality of concentration in the sol, the greater concentration being
at the interface. This inequality between the surface concentration and
the volume concentration of the sol, is due primarily to considerations
of surface tension and surface energy, and exists before the immersion
of the pelt. The surface layer having excess over the volume
concentration, any considerable extension of surface in a fixed volume
of sol must produce a very considerable decrease in the volume
concentration. This is what occurs when pelt is immersed in a tan
liquor, the immersion being the considerable extension of surface. It
should be especially remembered that the inequality of concentration is
in the sol, on the liquid side of the interface. In adsorption, the
substance adsorbed, _i.e._ the excess at the surface, is too frequently
regarded as bound to the solid immersed. This is because the excess is
in the layer which wets the solid and remains wetting it when the solid
is removed. Thus the immersion of pelt produces primarily only a change
in the distribution of the tannins in the liquor. It follows from this
that the adsorption is an equilibrium, and that if the sol be diluted,
the equilibrium will become the same as it would have been by immersing
the pelt directly into the dilute solution. Thus, if pelt be first
immersed in one tan liquor and then into a weaker one it will yield tan
to the latter solution.
The chief object in heavy leather tanning is to obtain the maximum
possible adsorption in the minimum possible time, or in other words, to
obtain good weight quickly. The amount adsorbed is proportional to the
actual extension of surface, _i.e._ the adsorption is a function of the
specific surface of the adsorbent. Hence, to obtain good weight it is
necessary to develop in the pelt its maximum possible specific surface.
This is one of the objects of "plumping," which splits up the fibres. It
is attained also by the solution of interfibrillar substance in limes
and bates.
The amount adsorbed is also a function of the volume concentration in
the sol after equilibrium is reached. Hence the better weights are
obtained with stronger liquors.
The adsorption law is
y/m = ac^(1/n)
where y is weight adsorbed by the weight m of adsorbent, and c the
volume concentration after adsorption; a and n are numeral constants.
Hence weight is determined by the strength of the liquor which the goods
finally leave. The commencement of tannage is necessarily in weak
infusions, in order to secure the maximum diffusion into interior of the
fibres before they become heavily coated on the exterior. As the
equilibrium is being established in such liquors the volume
concentration diminishes, and thus makes it less likely that good weight
will be attained; hence it is necessary in practice to move the goods
constantly into fresh liquors of gradually increasing strength, and so
maintain the rate of adsorption and save time. A further consequence of
the adsorption isotherm is that as y varies as c^(1/n) and n is > 1, y
is increased appreciably only by a relatively large increase in c.
Hence, though stronger liquors give better weight, there is a limit
beyond which any further gain in weight is not justified by the enormous
increase in the concentration necessary to attain it. Such great
increase in c is impracticable not only on the ground of expense, but
also on account of the great viscosity of the sol.
The amount of adsorption depends also upon the exact nature of the sol.
It has been previously pointed out that the tannins differ largely in
their penetrating and weight-giving powers. Some are readily adsorbable
and are deposited in great concentration at the surface of the fibre,
but for good weight it is necessary to use also the less adsorbable and
more diffusible tans, which penetrate the fibre itself. Hence it is
necessary for good weight to use a blend of materials, and so supply
many grades of liability to adsorption. It is particularly advantageous
to blend judiciously the two main types of material, the pyrogallol and
catechol tans. It is also necessary for good weight to present to the
pelt the more diffusible and less adsorbable tannins first, in order to
secure the maximum diffusion into the interior of the fibre before the
exterior of the fibre is heavily coated with the heavily adsorbable and
astringent tans. The least adsorbable materials are therefore used in
the early stages of tanning, and the most adsorbable materials at the
end of the tanning process. Thus gambier is added to the early liquors
(suspenders), solubilized quebracho to the later liquors (handlers), and
mimosa bark extract to the final liquors (layers). There is also another
excellent way of ensuring this progressive astringency of the liquors;
this consists in leaching the required blend of materials together (or
mixing them in the case of extracts) and presenting the mixed infusion
to the nearly tanned goods, which adsorb chiefly the more astringent
tannins. The liquor is then used for goods at a less advanced stage of
tanning, which again take the most adsorbable constituents. This is
repeated until the stage is reached when the fresh pelt is inserted into
the nearly exhausted liquor, which naturally contains only the least
adsorbable substances. This system is almost universal, and in practice
is known as "working the liquors down the yard." It has the additional
advantage of being a systematic method of economically exhausting
("spending") the tan liquors. When free acid is present in the tan
liquors, it tends to distend the fibres composing the pelt by a strong
and rapid adsorption. Thus distended or plumped the fibres present a
still greater surface for adsorptive operation, but the distension
naturally leaves less space between the fibres for the diffusion of the
sol. Hence acid or "sour" tan liquors give in the long run more weight,
but tan more slowly. Pelt tanned whilst thus plumped forms naturally a
thicker and less pliable leather. This occurs in tanning sole leather,
to a less extent with heavy dressing leather, and to a very small extent
in the case of softer dressing leathers.
In addition to adsorption, there is another phenomenon of colloid
chemistry in operation, viz. the _mutual precipitation_ of the sols in
the liquid by the gels in the hide. In most sols the disperse phase is
electrically charged. The sol therefore possesses electric conductivity,
and migration occurs in the electric field to the cathode or anode
according to the nature of the charge. Oppositely charged sols
precipitate one another, the precipitate containing both colloids. The
maximum precipitation occurs when the + charge of one sol exactly equals
and neutralizes the - charge of the other. There is thus an electrical
equivalence; an amount of sol which is equivalent to a given amount of
the other. This is not a chemical equivalence, however, and the
precipitate is not a chemical compound in spite of its fairly constant
composition. The composition of the precipitate, indeed, is not quite
constant, for the optimum precipitation may not correspond exactly with
the electrical equivalence, being influenced by the number of particles
required, their size (dispersity), the rate of mixture, and the relative
concentrations of the sols. This mutual precipitation is exhibited by
emulsoids as well as suspensoids, but the charge (+ or -) on an emulsoid
is in many instances largely an accidental matter, being determined by
the medium in which it happens to be, its normal condition being
electrical neutrality. Gelatin and pelt are such emulsoids, and a
positively charged gelatin sol has been observed to precipitate a
negatively charged gelatin sol. It is thought, however, that gelatin is
primarily a positive sol. Pelt (whether delimed or not) is rapidly
acidified by the quickly penetrating and strongly adsorbed organic acids
of the old tan liquors and becomes positively charged before the tannins
are adsorbed. The positive charge increases with the acidity of the
liquor. Other emulsoids are not electrically neutral, but are
electrically charged and exhibit considerable conductivity. Into this
class fall the tannins, and in tanning it is thought that there is a
mutual precipitation of the negative tannin sol with the positive hide
gel, the precipitation of the negative sol being favoured by the acid
condition of the liquor. The effect of increasing acidity soon falls
off, however, as a saturation limit is soon reached. This mutual
precipitation of colloids in tanning is in reality but an extension of
the adsorption theory, which explains the predominant effect of H+,
and OH- on the electric charge by stating that these ions are more
readily adsorbed than other ions, and that as OH- is more readily
adsorbed than H+ most sols are negative to water.
In addition to the adsorption phenomena described, there are in
vegetable tannage _secondary changes_ which are slow and "irreversible."
These changes are obscure and are difficult to investigate. Oxidation,
dehydration and polymerization have all been suggested, but there is
little direct evidence. Certain it is, however, that time renders the
tannage more permanent. It perhaps should be pointed out that in the
very strongest tan liquors the viscosity of the tannin sol is so great
that adhesion would be a better term than adsorption. There is no abrupt
division between the two phenomena.
In the theory of vegetable tannage there is another factor the
importance of which has been strongly emphasized by the author, viz.,
_lyotrope influence_. This has been most conveniently discussed in
connection with gelatin gels, but its effect on hide gels is analogous.
It has also an effect upon the diffusion and gelation of the tannin and
non-tannin sols.
=Mechanical Operations.=--In the tanyard the liquors are almost
invariably divided up into sections, called "rounds" or "sets," in
which the mechanical operations are different in aim and method. In
the first pits entered by the goods there is rapid adsorption in spite
of the low concentration and small astringency, and the great aim is
to obtain evenness of action and a good level colour. It is also
necessary to maintain the rate of adsorption. All the aims are
attained by frequently moving the goods. Heavy leather is suspended
vertically in the pits of tan liquor and handled up and down as well
as forward from pit to pit. Such pits are termed "suspenders." In the
earliest suspenders it is indeed advantageous to have the goods in
constant motion. This is done by suspending on wooden frames which are
rocked gently by mechanical power; such pits are termed "rockers." For
dressing leather in which firmness and smooth grain are not so
essential, the goods may be paddled in the first liquors. This is
occasionally done with stronger liquors for the express purpose of
working up the "grain" pattern. The goods after passing through the
suspenders are usually passed to "handler" rounds, in which they are
moved less frequently. In these pits the goods are laid horizontally
one above the other. One advantage of handlers is that the goods
flatten thoroughly and straighten one another by their own weight;
another is that more goods can be placed in one pit than in
suspenders. They are not so convenient to work, however, as
suspenders, and the goods do not feed so rapidly. Hence the tendency
is now to tan more in suspension, and to economize labour by an
extension of the rockers. The handling of the goods is also saved by
pumping the liquors and by working rounds of suspenders or rockers
like the press leach system, with the difference that the stronger
liquor is pumped in to the head pit, and the liquor passes upwards
through the goods.
Finally the goods are placed in "layers" or "layaways," in which they
remain undisturbed for a decidedly longer time. These pits contain the
strongest liquors of the yard, and their principal function is to
complete the tannage and give weight and firmness by the adsorption of
bloom, reds, etc., in the interior of the hide. The goods are placed in
horizontally, and are dusted in between with fresh tanning material
which maintains the local strength of the liquor and keeps the goods
somewhat apart. Drum tanning attains a more rapid penetration of the
pelt by giving constant motion in stronger infusions. It is of course
liable to result in an under-tannage of the interior of the fibre. After
the goods have been "struck through" in the ordinary way, however,
drumming in extract is increasingly used as a substitute for much labour
in handling, and also to save the time spent in the early layers.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 220-350.
Bennett, "Manufacture of Leather," pp. 113-179.
Bennett, "Celavima and Babla," L.T.R., 1914, 122.
Dumesny and Noyer, "Manufacture of Tanning Extracts."
_Theory_:--
Meunier and Seyewetz, Collegium, 1908, 195.
Stiasny, Collegium, 1908, 117-159, 289, 294, 337.
Procter and Wilson, Collegium (London), 1917, 3.
Wilson, Collegium (London), 1917, 97, 100, 105.
Moeller, Collegium (London), 1917, 13, 38, 46, 103; and
_J.S.L.T.C._, 1917, 22, 56, 92.
Bennett, _J.S.L.T.C._, 1917, 130-133, 169-182; 1918, 40; 1920,
75-86; _S.L.R._, 1916, March.
SECTION IV.--FINISHING PROCESSES
After the tannage is complete, leather is hung up to dry. In the case of
heavy leather this drying must be very carefully carried out in order to
obtain a product of satisfactory appearance and saleable qualities.
Associated with the drying are many mechanical operations (scouring and
rolling) which assist very materially in imparting the desired
qualities. After tanning, however, the quality of the final product is
most strongly influenced by the amount of grease added in finishing.
Some grease is always used in finishing, partly because even sole
leather requires some measure of pliability and partly because a coating
of oil over the leather during drying prevents the loose tannin from
being drawn to the surface of the leather by capillarity, thereby
causing dark and uneven patches and a "cracky" grain. The added grease
is also a contribution to the "weight" of the finished article--a
primary consideration for heavy leather, which is usually sold by
weight. The finishing processes, indeed, tend to be dominated by this
consideration, and become a series of efforts to retain as much tannin
and add as much grease as are consistent with the requirements of the
class of leather being manufactured. Sole leather does not contain more
than about 2 per cent. grease, or its firmness is impaired. Belting
leather, in which considerable pliability is needed, may contain about 9
per cent., whilst harness leather, which must be exceedingly tough and
durable, may contain up to 13 per cent. of fatty matters. Upper
leathers, which need to be soft and pliable as well as waterproof and
durable, are very heavily "stuffed" and often contain up to 30-40 per
cent. of grease. Sole leather is thus rather distinct from the rest,
which are called "curried," "stuffed," or "dressed" leathers. The actual
drying out before, after and between the various mechanical operations,
each have an appropriate degree of wetness. In this country the drying
is usually under the prevailing atmospheric conditions and is known as
"weather drying." The goods are suspended by hooks or strings or by
laying over poles in special sheds fitted with louvre boards by which
the rate of drying can be roughly controlled. Weather drying is cheap,
but exceedingly slow, and in unfavourable weather is very unreliable.
The goods, moreover, need constant attention to obtain an even result.
Steam pipes are usually laid along the shed floors, and are used in
winter and damp weather to accelerate the drying, and also in the final
shed stove to remove the last traces of moisture. Wet weather, however,
will not stand a high temperature, and steam drying is better avoided
when possible. Air-dried leather still contains about 14 per cent. of
moisture. Many systems of shed ventilation have been suggested to hasten
the drying and to secure a better control of the process. In one system
a screw fan is fitted at one end of a shed (without louvre boards) and
sucks air through the goods from an inlet at the other end. The air can
be heated by a steam coil near the inlet. In another system a
centrifugal fan blows air through an arrangement of pipes which
distributes it to the drying sheds, and discharges it close to the floor
by various branch pipes. The outlets are near the roof. A system of
dampers permits hot air, warm air and the used wet air to be blended in
the desired proportions. In America turret drying has been used. The
sheds are vertically above one another and have latticed floors. Heated
air is admitted at the bottom and rises through the goods up the
building just as in a chimney. For many of the finishing operations it
is important to obtain the leather in a uniformly half dry or "sammed"
condition. This may be done by careful drying, and wetting back the
parts that have become too dry with tepid water or weak sumac liquor,
and then leaving the goods "in pile" until of uniform humidity. It may
also be done by "wetting back" leather which has been completely dried
out. There are also "samming machines" which by means of rollers squeeze
out the excess liquor. Sole leather is dried out and finished
immediately after tanning, but dressing leather is often "rough dried"
out of tan liquors and wet back for finishing when required. Dressing
leather is often treated in different factories; tanners selling it as
rough leather and "curriers" finishing it.
Scouring is one of the first operations in finishing leather. The grain
side is wet and worked with brushes and stones until the bloom and loose
tannin are removed. This process aims at producing a good even colour
and level surface, but is liable to cause a loss of weight. Dressing
leather is often scoured on both grain and flesh, and weak soap or borax
solutions are used to assist the process. In this operation hand labour
has been now quite superseded by machine work. A great variety of
machines have been devised. The mechanical working of leather takes
place in various parts of finishing. These operations, known as
"striking," "setting," "pinning," "jacking," may be carried out often by
the same machine as used for scouring, but with a change of tool. The
object of these operations is to get rid of wrinkles and creases, to
produce softness, pliability and area, and to remove superfluous
moisture, grease, dirt. The tools are of steel, brass, slate or
vulcanite. Scouring is often effected by putting the goods into rotating
drums together with extract and sumach. The bloom is removed by
friction, the colour is improved by the sumach, whilst the extract keeps
up the weight.
In finishing sole leather firmness is enhanced by "rolling." A brass
roller passes to and fro over the goods with the exertion of
considerable pressure. The operation is carried out by machinery.
Shaving is an important operation in the case of many dressing leathers.
Its object is to produce a uniform thickness of the leather and an even
surface on the flesh side. The sammed goods are laid over suitable beams
and shaved with special sharp knives which possess a turned edge. This
hand process, which demanded considerable skill, is fast becoming
extinct, and machine shaving is already almost universal on account of
its greater speed. The machines consist essentially of two rollers, one
of which is smooth, whilst the other is a spiral knife-blade cylinder
(cp. Section II.). The sammed goods are held in the hands and placed
over the smooth roller, which is raised to the cutting roller by a foot
treadle. A number of similar operations ("flatting," "whitening,"
"buffing") are carried out by a suitable change of tool. In all these
operations good samming is important.
Splitting is another important operation on tanned leather. In this
process the leather is cut parallel to its grain surface, thus yielding
two pieces with the same area as the original, the "grain" and the
"flesh split." It is essentially a machine operation, and is carried out
by presenting the carefully sammed leather to a sharp knife-edge,
towards which it must be constantly pressed. The "band knife" machine is
the most popular arrangement. The knife is an endless belt, which
continually revolves round two pulley wheels of equal size. In between
these the knife is horizontal, and is then used for splitting. The
sammed leather is pushed towards the blade by two feed rollers, and the
grain passes above the knife on to a small platform, whilst the flesh or
"split" passes below and falls to the ground. Emery grinders and thick
felt cleaners in the lower part of the machine keep the knife in good
condition. The adjustment of the machine is delicate and requires
considerable experience. With care splits may be obtained down to 1/16"
thick, and sometimes as many as 6 or 7 splits are obtained from one
hide.
Oiling is still usually done by hand, and cod oil is still preferred for
many classes of goods. Of recent years there has been a great extension
of the use of sulphonated oils, which have the valuable property of
forming an emulsion with water or tan liquor. With these materials it is
easier to ensure the goods being completely covered with oil. The
penetration of the oil into the leather is also quicker and more
complete. These oils have often the disadvantage of leaving solid fats
on the exterior of the leather, which gives it an ugly smeared
appearance.
Stuffing the dressing leathers is carried out in a variety of ways and
with a variety of materials. The old process of hand stuffing employs a
mixture of tallow and cod oil called "dubbin." This is made by melting
the ingredients together and allowing them to cool with constant
stirring to a nearly homogeneous salve. The dubbin is brushed thickly on
to the flesh side of the sammed leather, which is then hung up to dry.
As the moisture dries out the oils and soft fats penetrate the leather
and leave the more solid fats on the outside. The proportions of tallow
and oil are varied with the time of year and with the method of drying,
for if the dubbin be too soft it will run off the leather, and if too
hard will not penetrate it so well.
Drum stuffing is a more modern development in which a higher temperature
is employed, about 140° F. The drum is heated up by steam or by hot air,
and the sammed goods are then inserted and drummed for a few minutes
until they are warmed. The drum is fitted with a heated funnel
containing the melted grease, which is run in through the hollow axle.
After a half to three-quarters of an hour's drumming the grease is
completely absorbed by the leather. The drumming is continued for a
while until the goods have cooled. Whilst still warm they are "set out"
to remove creases and superfluous grease. Drum stuffing is not only
quicker than hand stuffing, but also makes it possible to use the hard
fats, and so make a leather which carries more grease without appearing
greasy. Thus in drum stuffing, paraffin wax and wool fat are used, and
their penetration assisted by small proportions of cod oil or dégras. If
the leather be too wet the grease is not absorbed, whilst if it be drier
than usual the leather will take more grease, but the resulting colour
is not so good. There is also another method of stuffing which
originates from the Continent. It is known as "burning in" and involves
the use of still higher temperatures (195° to 212° F.). Wet leather
will, of course, not stand this temperature, so that it is first
necessary to make the leather absolutely dry. This is effected by drying
in stoves at temperatures up to 110°-115° F. There are two ways in which
the grease is applied. In one method the melted grease is poured by a
ladle on to the flesh side and brushed over until evenly distributed. A
second application of grease is made to the thicker parts. The hides are
then put into warm water (120° F.) for about a quarter of an hour, and
then drummed for half an hour. In the other method the goods are
completely immersed in the melted fats for a few minutes in a
steam-jacketer tank at a temperature of 195° F. After softening in water
at 120° F. the goods are drummed. "Burning in" is used for the heavier
dressing leathers such as belting and harness. It does not give good
colour, but permits the employment of still more hard fats.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 223, 378.
Bennett, "Manufacture of Leather," pp. 251-312.
Bennett, "Principles of Leather Stuffing," _Leather Trades Review_,
1911, 186.
SECTION V.--SOLE LEATHER
Leather for the soles of boots and shoes is a matter of essential
interest to all, and forms one of the best appreciated applications of
animal proteids to useful purposes. Methods for its manufacture are as
numerous as the factories producing it, hence all that can be done is to
describe broadly the general method which is typical of our time, to
classify the many varieties into types, and to indicate the recent
changes and present tendencies.
Sole leather is mainly manufactured from butt pelt, and the great aim is
to produce a firm, thick, waterproof and smooth grained leather which
will bend without cracking. It must have a light tan colour to be
saleable, and contain as much weight as possible to be profitable.
The modern mixed tannage of "sole butts" or "scoured bends" generally
utilizes ox-hides of the Scotch and English markets, though salted
Continentals and South Americans are also employed. After the usual
soaking a short and sharp liming is given. The special aim in liming
sole hides is to obtain the maximum plumping effect with the minimum
loss of hide substance. Both these achievements are necessary to obtain
good weight. The limes should be kept as clean as possible, which is
best obtained by putting clean hides into work. This reduces bacterial
activity and loss of hide substance. The "shortness" of the process is
attained by the use of sodium sulphide (from 2 to 16 ozs. per hide of
sulphide crystals), by which depilation may be accomplished easily in
about nine days. The amount of sulphide should be increased somewhat in
the short-hair season and in cold weather. Some factories take up to
about 12 days using less sulphide, whilst others will lime in about a
week by using the larger quantities. The amount of lime used varies
enormously, and is invariably in great excess of the actual
requirements. "Probably 2-3 per cent. on the green weight of the hides
is all that can be really utilized, the remainder being wasted."[5] This
amounts to about 2-1/2 lbs. lime per hide, but in practice it is more
frequent to find 7, 8, 9 or even 10 lbs. per hide being used. The excess
is innocuous, owing to the limited solubility of lime. Some excess is
desirable, to replace in the liquor the lime adsorbed by the goods in
plumping, to assist bacterial activity, and also because in
sharp lime liquors the undissolved portions do not remain so long in
suspension. The use of sulphide and other alkalies does not "make it
possible" to reduce the amount of lime used, it merely renders the
excess more superfluous. The use of sulphide not only shortens the
process, but also sharpens it, on account of the caustic soda produced
by hydrolysis. Usually for sole leather, however, it hardly sharpens it
sufficiently, and it is very common to add also caustic soda (or
carbonate of soda) to the limes. About 2 ozs. caustic soda (or its
equivalent in carbonate) is used per hide. The hides are limed generally
by the three-pit system, giving about three days in each pit. They
should be handled each day in the first pit (old lime) and once in the
other pits.
[Footnote 5: Procter, "Principles of Leather Manufacture," p. 129.]
Unhairing and fleshing by hand labour is still common, in order to avoid
great pressure on the plumped hide. Scudding should be very light, and
in some yards is entirely omitted. Only the lime on the surface of the
hide should be removed by deliming, and this immediately prior to the
insertion of the butts into the tan liquor. This is to ensure good
colour and yet keep the butts plump. Boric acid is the best for this
purpose, using 10-15 lbs. per 100 butts. The goods are inserted (and
preferably rocked) in a dilute solution for a few hours only. About the
same quantity of commercial lactic acid may be substituted for the
boracic. This deliming can also be accomplished by adding the acid to
the worst suspender in the tanyard.
To obtain firmness and plumping it is necessary that the early liquors
in tanning should be more acid than for other leathers. With old methods
of tanning one could trust to the natural sourness of the liquors to
complete the deliming and replump the goods with acid. In such cases any
deliming was also unnecessary. In the modern yard, however, we get
"sweet" liquors coming down the yard, partly on account of the greater
proportion of extract used and partly because the liquors themselves are
not so old. Hence it is now practically always necessary to acidify
artificially the tan liquors. This may be done by adding a few gallons
of lactic, acetic, formic, or butyric acid to the handlers and
suspenders, especially in the winter and spring. It is now increasingly
common to place sole butts in a special acid bath after they have been
in tan liquor for about a week. This bath is often made from sulphuric
acid, and may be 1 or 2 or even 4 per cent. in strength.
The actual tanning of sole butts lasts three to four months, and just
prior to the war the tannage consisted often of about one-third myrabs,
one-third valonia, and one-third extract. The myrabs and valonia were
leached together, and the extract added to the best leach to make layer
liquors of the required strength. Some mimosa bark was generally used
also, and now it is extensively employed to replace the valonia. The
most widely preferred extract is chestnut, but quebracho, myrabs extract
and mixtures have also a prominent place, and mimosa bark extract an
increasing importance. It is recognized that this tannage is if anything
too mellow, and that if only a smooth grain and plump butt can be
ensured in the first weeks of tanning, it is much better for sole
leather to employ the most astringent tans possible and the sharpest
liquors (_i.e._ liquors with a small relative proportion of soluble
non-tannin matters). Hence there is the tendency in sole-leather tanning
to employ fresh clear liquors for the butts and use up the more mellow
liquors on the "offal" (shoulders and bellies).
Four types of sole butt tannage will now be described, all of which
illustrate the methods employed in a modern mixed tannage.
=1.= The first type consists in a four-months tannage, in which the
liquors are worked down the yard.
The butts pass first through the suspenders (20°-40° Bkr.) in about a
week, and are rocked in the first liquors. They next enter the handlers
(40°-55°) rounds of eight pits, six floaters and two dusters. Myrabs, or
a mixture with algarobilla is used as dusting material. The goods remain
in this set for two weeks, and should then be struck through. The
suspender handlers (55°-65°) are next entered, in which they remain up
to three weeks in suspension, being shifted forward on alternate days.
The goods now enter the layers, of which four are given: first 70° for
one week; second 75° for two weeks; third 80° for three weeks; and
fourth 90° for a month. The goods thus take sixteen weeks to tan, of
which ten weeks (62-1/2 per cent.) are in layers.
The system of working the liquors is expensive, and is only possible if
the butt liquors can be spent out by the offal. The best or fourth
layer, 90°, is made from the best leach liquor, 65°, and extract
(chestnut with some oakwood or mimosa bark). After use it becomes the
second layer, 75°. The third layer, 80°, is also made from fresh leach
liquor and extract (chestnut with some myrabs or mixed extract). After
being used thus it is used for the first layer, 70°. The used first and
second layers are mixed together and used partly to form the belly
layers, and partly to make a sharp liquor for the handlers (55°-40°) by
diluting with 40° leach liquor and adding quebracho extract. The old
handler liquor is run to the suspenders (40°-20°), and finally used for
colouring off the offal in drum or paddle 18°. The suspender handlers
(65°-55°) are made from fresh leach liquor and chestnut extract. They
are afterwards used to make shoulder layers. The course of the liquors
is shown in the diagram below.
It will be seen that fresh leach liquor and fresh material are used to
each set except the suspenders, which must have some mellowness to
ensure plumping and smooth grain. Layer liquors are used twice only, and
then (when only five weeks old) pass to the handlers. These are further
sharpened by fresh leach liquor and fresh extract and dry materials.
The forward handlers are fresh liquors with fresh extract. This tannage
is fairly typical of high-class sole leather, in which the liquors are
worked down the yard, but worked towards the offal, which thus receives
liquors with relatively greater proportions of mellow tans and soluble
non-tans.
----------Leach liquor-------------------
| / \ |
| V \ |
| 4th layer, 90° \ |
| | V |
| V 3rd layer, 80° |
| 2nd layer, 75° | |
| \ V |
| \ 1st layer, 70° |
\ \ / |
\ \ / |
\ V V |
\ (mixture)[-->bellies] |
\ / V
V V Suspender handlers-->(65°-55°)(shoulders)
Handlers, 55°-40°
|
V
Suspenders [-->offal]
=2.= The second type consists in a tannage of about four months, in
which the liquors are not worked down the butt yard. In this method also
there is an attempt to save much of the labour in handling, first by
shortening the time in the handlers by one week (as compared with the
above), and second by fusing the two progressive handler sets into two
sets of equal strength, through which the goods pass more slowly and
with less disturbance.
The goods go through the suspenders (10°-25°) in about a week, rocking
in the early liquors, and then into large rounds of handlers (30°-45°)
for one month. The handlers consist of floaters and several dusters, in
which the butts are laid away with 1-3 cwt. myrabs. The goods next enter
the layers, of the same strength as in Type 1, and in which they remain
the same time. The total tannage is thus 15 weeks, of which 10 weeks
(nearly 67 per cent.) are in layers.
The best or fourth layer is made up from leach liquor and extract, and
is then used successively as a third, second and first layer, and then
passes to the offal layers. The handler liquor is made entirely from
fresh leach liquor and quebracho extract, and is a sharp liquor of
greater strength than its Bkr. strength would indicate. The old handler
liquor is run to the butt suspenders. The course is represented thus:--
Leach liquor
/ |
/ 4th layer, 90°
/ |
/ 3rd layer, 80°
/ |
/ 2nd layer, 75°
/ |
/ 1st layer, 70°-->[offal layers]
V
Handlers (45°-30°)
|
V
Suspenders (25°-10°)
=3.= The third type consists of a short three-month's tannage in which
the liquors are worked straight down the yard. To compensate for the
short time it is necessary to have stronger layer liquors in which the
goods spend a still greater proportion of their total time. The stronger
liquors involve a greater proportion of extract, particularly of
quebracho, which fact causes the whole of the liquors to be sharper than
their Bkr. strength indicates, and justifies them being worked straight
down the yard.
The goods go through suspenders (20°-40°) as usual one week, and then
pass into suspender-handlers (40°-60°) for two weeks, and thence to the
layers. In the first two of these (65° and 70°) they are actually in
suspension, a week in each liquor. They are then dusted down for ten to
eleven days, first in 85° and then in a 95° liquor, and finally for a
month in a liquor of 110°. The total tannage is thus twelve weeks, of
which nine weeks (75 per cent.) are in layers. There is considerably
less handling than in Type 2, and it is more convenient, the goods being
in suspension.
=4.= The fourth type is also a three-month's tannage. In this it is
attempted to obtain even greater weight with still less labour. The
layer liquors are kept much stronger by the more extensive use of
extract, and this makes it impracticable as well as too costly to run
these liquors down the yard. They are therefore repeatedly strengthened
with extract and used again.
The goods go through suspenders (20°-40°) as usual one week, and then
through a round of suspender-handlers (40°-55°) consisting of fresh
sharp liquor from the leaches together with quebracho extract. They are
in this set two weeks, and then are laid away. They receive three
layers: first, 105° for 2 weeks; second, 110° for three weeks; and
finally, 120° for a month. Of the twelve weeks, therefore, nine weeks
(75 per cent.) are spent in layers. In this method the goods are
immersed in 3 per cent. sulphuric acid after passing through the
suspenders.
There is possible, of course, a tremendous number of variants of the
above types. The number of handler rounds is determined by the number of
butts being dealt with. With a large number it is more easily possible
to arrange for them to be in progressive strength as in Type 1. There
are also many systems of working the layers, of which the most notable
is to make the second or third layer from fresh leach liquor and
extract, and strengthen it with extract for the succeeding layers. It is
then used as a first layer and worked down the yard.
The bellies and shoulders often go through separate sets of liquors, but
it is common to put them through suspenders, and even handlers together.
They receive, of course, a distinctly shorter tannage, and are often
drummed with extract before laying away or after the first layer. By way
of illustration, the course of the offal and their liquors may be given
in the case of Type 1. The shoulders and bellies are coloured off in a
paddle or drum with old butt suspender liquor, which is then quite
exhausted. They then pass through suspenders (18°-40°) together in 4-5
days, and go through a handler round (40°-55°) for 3 weeks, including
one duster. The bellies are removed after 2 weeks, and given three
layers (60°, 70°, 80°) of a week each. They receive, therefore, nearly 6
weeks in all. The shoulders also have three layers (60°, 65° and 80°)
of 2, 3 and 4 weeks respectively.
The course of the liquors is shown thus:--
Butt layers Butt suspender Butt suspenders
(and + extract) Handlers (+ extract) |
| | |
V V |
Belly layers Shoulder layers |
(80°-60°) (80°-60°) |
\ / |
V V |
Offal handlers (55°-40°) |
| V
V Offal drums
Offal suspenders (40°-18°) (18°)
\ /
\ /
\ /
\ /
\ /
\ /
V V
Drain
The tanned butts are piled for 2-3 days, sometimes rinsed to remove
dusting material, and then scoured either by machine or by drumming with
sumac and extract. This removes bloom, but causes some loss of weight.
"Vatting" or "bleaching" now follows, in which it is attempted not only
to bleach the colour of the leather, but also to impart as much weight
as possible. The vat liquor is made several degrees stronger than the
last layer by means of quebracho bleaching extract and good coloured
chestnut or myrabs extract. The liquor is kept warm by a steam coil, at
about 100° F., but not much more without risk. The goods remain in the
bleach liquor 2-3 days and are then horsed or suspended to drain. Sumach
is sometimes used in the vats. A new vat liquor must be made up after
some weeks' use. The goods are sometimes rinsed in weak sumac liquor
before vatting to get good penetration, and sometimes after to ensure
good colour.
The butts are next oiled and hung up in a dark shed and allowed to dry
slowly and evenly to an "india-rubbery" consistency and rather slimy
feel. They are then "struck out" by machine, wiped, re-oiled and again
hung up to dry, preferably with sulphonated oil. After a short drying
to a suitable and even condition they are "rolled on," and, possibly
after further drying, "rolled off" with greater pressure, and then dried
for a day or two with the help of a little steam. Finally they are
machine-brushed and sent to the warehouse, where they are weighed and
classified.
The offal is often drum oiled. It needs more striking and is more
difficult to obtain in suitable condition for striking, rolling. It is
treated similarly to butts, but often also goes for dressing leather,
and may be split. It is of some interest to compare the above processes
with that once very popular manufacture of "bloomed butts" in the West
of England from South American salted hides. These receive a liming from
12-14 days, using 12-16 lbs. of lime per hide. They receive then a
tannage of about 9 months, comprising 3 weeks in suspenders
(20°-40°)--very sour and mellow liquors--4 weeks in handlers (40°-55°),
4 weeks in dusters (60°), 4 weeks in round made from hemlock extract
(60°), and 20 weeks in six layers (60°-90°) in which they were dusted
heavily with valonia. Oakwood extract was used for the layers, which
took 57 per cent. of the total time. The butts were scoured in a
much-dried condition, so that only the loose and surface bloom was
removed. No bleaching was given in the modern sense.
In the old oak-bark tannage of sole leather up to 12 months were taken
for tanning, two-thirds to four-fifths of which time the goods were in
layers. The strongest liquor rarely exceeded 50° even where valonia and
gambier were also used, and rather more than 30° if not.
It will be understood from the above that the tendency for many years
has been to shorten the time and the labour required for tanning. Drum
tanning is obviously the next stage in shortening the time. In one such
process the butts are put through suspenders (25°-40°) for 2 weeks,
drummed for 12 hours in an 80° extract liquor, and finally in a neat
extract 200° for 36 hours. Drum tanned sole leather, however, is not as
yet of good quality; the grain is not smooth, and the heavy weight
finish (striking and rolling) needed to counteract this tendency is
liable to cause poor "substance." The leather, too, readily wets and
goes out of shape. Possibly some drumming may be adopted to save time in
the early layers, but the most serious rival to the 3 months' tannage is
the waterproof chrome sole leather (Part III., Section V.).
REFERENCES.
Parker, _J.S.C.I._, 1902, 839.
Procter, "Principles of Leather Manufacture," p. 220.
Bennett, "Manufacture of Leather," pp. 179, 259.
Bennett, _J.S.C.I._, 1909, 1193.
SECTION VI.--BELTING LEATHER
The manufacture of belting leather is well illustrated by the tanning
and finishing of "strap butts." In general, the tannage presents many
points of great similarity with the tannage of sole leather; indeed, the
resemblance is so close that in some factories there is little
difference observed, and the currying and finishing operations are
relied on to produce the desired difference in final results.
Nevertheless, there is considerable difference in the type and ideal of
the two leathers, which may be expressed in trade parlance as a greater
"mellowness" for the belting leather, and in the best methods of
manufacture this fact is in evidence throughout the whole process of
manufacture.
In liming, there need be little difference between sole and belting
hides, and a sharp treatment of 9-10 days, by the three-pit system, with
a day or two extra in the coldest weather, would meet ordinary needs.
For the conservation of hide substance and for the saving of time a
shorter liming is sometimes given, in which more sulphide is employed
than is usual for sole leather. Even the very short processes of liming,
1 to 3 days, which involve the use of strong solutions of sodium
sulphide, have been successfully employed for belting leather. The
tendency to harsh grain with such processes is not so serious a defect
with belting as with sole leather, and can be minimized by careful
deliming. American and Continental factories tend to favour the use of
those quick processes which employ warm water in addition to sulphide.
The hides after a short liming in sulphide limes are immersed in warm
water, which greatly accelerates both the chemical and bacterial
actions. For example, after about 3 days' liming, in which both old and
new limes are used as usual, the hides may be thrown into water from
100°-105° F., and will be ready for depilation in 7 or 8 hours.
Even a stronger liming may be given, especially if the soaking is
unusually prolonged. Such processes undoubtedly save hide substance, and
the pelt is obtained more free from lime, but they have the disadvantage
that the natural grease of the hide is only imperfectly "killed" (_i.e._
saponified or emulsified), and may interfere with the normal course of
the tannage. The plumping is also apt to be insufficient. On the other
hand, liming processes are also used in which a mellower liming or a
longer liming is preferred in order to produce the desired degree of
softness and pliability in the finished leather. Belting must not be too
soft, of course, and it will be clear that the required difference from
sole leather can be produced either in liming or tanning or partly in
both. These considerations also decide whether bating is to be omitted
or not. A hard astringent tannage in sour liquors after a sharp liming
might make bating essential, but in these days it is usual to avoid it
and produce the effect in other ways. A light bating of a few hours is
sometimes given, but it is more unusual to delime the grain thoroughly
with boric acid, using up to 20 lbs. per 100 butts. Crackiness is a
fatal defect in strap butts, so that a sound grain must always be
obtained. Generally speaking, therefore, strap butts receive more
washing in water, and rather more deliming than sole leather, even when
they are not bated. It is also usual to scud much more thoroughly, and
to round a larger proportion of butt, especially in length.
The tannage is usually carried out with a blend which includes a much
greater proportion of the fruit tans, and correspondingly less of
extract.
Distinctly more myrabs are used than in sole leather tannages, in the
dry material, and amongst the extracts chestnut is preferred to
quebracho, and myrabs to mimosa bark, though all these may be used in
some degree. In the past the most favoured extract has been undoubtedly
gambier, which gives a tannage which is easily curried and imparts the
required mellowness to the uncurried leather. The great expense of this
material, however, together with the advent of drum stuffing and shorter
tannages in stronger liquors, have tended to cause a considerable
reduction in the proportion used for strap butts, and to limit its
employment to the earlier stages of tanning.
The same tendencies for reducing the time taken to tan, employing
stronger liquors, and securing economy of labour in handling, have been
evidenced in the tannage of strap butts as in sole butts. It is
nevertheless true that, broadly speaking, strap butts receive rather
more handling and rather weaker liquors than sole butts. A greater
amount of mechanical assistance is also employed with early stages
(paddling, drumming, rocking). This is less objectionable for curried
leather than for sole butts. The handling is more usually in suspension.
The liquors are usually worked straight down the yard as a greater
mellowness is needed in the early liquors than for sole butts. The offal
is given a separate tannage and often used for different purposes,
_e.g._ the shoulders for welting and the bellies for fancy goods.
Plumping with sulphuric acid is generally considered inadmissible for
strap butts. It has been shown that leather containing sulphuric acid
tends to perish after the lapse of a number of years. Sole leather will
be worn up before this effect is observed, but belting is an article
which is intended to last much longer, and the use of sulphuric acid is
consequently inadvisable. Plumping must be obtained, to a considerable
extent, but must be achieved by the organic acids (lactic, acetic,
formic and butyric acids). A few gallons of such acids are consequently
added to the handlers, especially in the winter and spring. Less may be
used in the autumn, when the layer liquors which fermented in the summer
months have worked down to the suspenders. A mixture of these acids is
usually better than any one alone, for they not only differ very
considerably in price, but also have different powers of neutralizing
lime and plumping the goods. Lactic acid (M.W. 90), Acetic acid (M.W.
60), and formic acid (M.W. 46) are each monobasic acids; consequently 3
lbs. formic will neutralize as much lime as 4 lbs. acetic or 6 lbs.
lactic. Their plumping powers are somewhat influenced by the anion. In
determining what quantities to take, the commercial strength of the
acids must also be considered. Formic is often 80-90 per cent. pure,
acetic 60-80 per cent., and lactic 40-60, but may be as low as 25 per
cent. The blend must be adjusted accordingly. As strap butts do not need
the firmness of sole leather, less of these acids may be used than for
sole butts.
The exact nature of the tannage and the strength of the liquors is
largely influenced by commercial considerations. If the manufacturer is
both tanner and currier, he need not go to such great expense in strong
liquors and in time in layers, for he can obtain some of this weight in
currying. If, however, the tanner sells the butts rough dried, he must
naturally aim at obtaining greater weight in tanning.
The actual details of the tanning processes are as usual very varied,
but may be classified according to type, just as in the case of sole
butts.
Illustrations will now be given.
=Type 1=, which may be compared with Type 1 for sole butts, is a tannage
of about 5 months. The goods pass through suspenders (8°-30°) in 2-1/2
weeks, and then pass to the handlers (30°-50°), in which they remain a
month; they are then put into suspension again and pass through the
suspender handlers (40°-55°), which takes 2-1/2 weeks. In this round
much gambier is added, and the goods are frequently handled. Four layers
are usually given, viz. first layer 55°, one week; second layer 60°, two
weeks; third layer 65°, four weeks; and fourth layer 75°, four weeks.
The tannage is thus 20 weeks, of which 11 weeks (55 per cent.) are in
layers. Extra layers may be given to heavier goods, using stronger
liquors made up with extract. All liquors work straight down the yard.
The tannage consists of 35 per cent. myrabs, 35 per cent. valonia, 10
per cent. Natal bark, and 20 per cent. extract, chiefly gambier, though
some chestnut and quebracho are used.
=Type 2= represents the modern tendency to use stronger liquors and a
shorter time. The strap butts pass through the suspenders (22°-50°) in
1-1/2 weeks, during about a third of which time they are rocked. They
next pass through two sets of suspender-handlers (50°-67° and 67°-80°),
which takes a month, and thence to the layers. Three layers are given
(85°, 90° and 100°), in which the goods remain one, three and four weeks
respectively. The tannage is thus 13-1/2 weeks, of which 8 weeks (nearly
60 per cent.) are in layers. The liquors work down the yard. Longer time
may be given to heavier goods. The tannage consists of 40 per cent.
myrabs, 35 per cent. valonia or Natal bark, and 25 per cent. extract,
chiefly chestnut, though some gambier may be added to the suspenders.
However tanned, strap butts are first dried out rough over poles. This
assists in making the tannage permanent, on account of secondary changes
discussed in Section III. They are next wet back for currying by
soaking in water or sumach liquor for a few hours and piling to become
soft and even. The first operation is "skiving," which is a light
shaving on the flesh side, carried out by a sharp slicker with a turned
edge. The butts are next scoured thoroughly by machine on both flesh and
grain, and sumached in a vat for several hours at 100° F., after which
they are slicked out and hung up in a cool shed to samm for stuffing.
Hand stuffing is often still preferred, with tallow and cod oil. The
butts are next set out, and it is important that this should be
thoroughly done. Machines are now generally used, and the goods are
often reset after further drying. After drying out completely they are
given a light coating of tallow and laid away till wanted for cutting up
into straps, which is now done by machinery.
A Continental method for making belting leather is to give 6 weeks in a
suspender set (70°-24°) of twelve pits arranged on the press system,
running two fresh liquors a week, and to give them two layers (24° and
28°) of 6 and 8 weeks. The material is chiefly pine bark, but some oak
bark, valonia, myrabs and quebracho are also used. The goods are stuffed
by "burning in," molten fat being poured on the flesh side.
REFERENCE.
Bennett, "Manufacture of Leather," pp 194, 295.
SECTION VII.--HARNESS LEATHER
When discussing the question of oak bark (Section III.), reasons were
advanced for its decreased use and popularity. These were quickly
appreciated in the sole leather trade, but the obsolescence of oak bark
in the dressing-leather section was much more prolonged, partly because
there was less pressing need to obtain good weight in the actual
tanning, and partly because in some branches of dressing leather, such
as belting and harness, a leather was required of great durability and
toughness, for which qualities oak bark tannage had a deservedly high
reputation. Hence harness leather manufacture affords a good
illustration of the transition between the methods of the late
nineteenth and those of the twentieth century. With the use of oak bark
lingered the old methods of liming, bating and tanning in weak liquors
for a long time with plenty of gambier. Hence in this section it will be
necessary to observe a gradual transition of method, both in wet work
and tanning. It should be pointed out that this transition has not been
and is not going on in all factories at the same rate. Many factories
remain in which the old methods are still preferred at some stages of
the manufacture, and some remain in which many of the changes indicated
below have not taken place at all. The leather trade has always been
considered conservative in its methods, but it should be realized that
much of the prejudice in favour of old methods is due to the public, and
that after all tanners and curriers, like other business men, have to
suit their customers. The march of industry is not like a regiment in
line; it is rather more like nature, a survival of the most adaptable.
Hides for harness leather are limed in various ways, of which the
following are types.
1. A rather mellow liming of 10-15 days (longer than for sole leather),
in which nothing but lime is used, and a certain amount of old liquor
used in making up the new limes. The liming was carried out by the
one-pit system, but the goods and liquors were kept clean by a good
soaking process. Hence the loss of hide substances was not very great;
goods so treated were bated before tanning.
2. A shorter liming than the above by the three-pit system. This saved
time (taking 9-10 days), saved hide substance, and ensured greater
regularity of treatment. The limes were about as mellow, but a little
sulphide (2-4 ozs. per hide) was used to assist the depilation,
especially during the short-hair season. These goods were also bated.
3. A distinctly longer liming, 15-16 days, in mellower limes. This
differed from Type 1 also in the respect that greater regularity was
ensured by the three-pit system; a foot or two of old liquor was used in
making up the new lime. More hide substance was lost than in either of
the above processes, but this was deliberate, the object being to
dispense with bating, which is always light for harness hides. Thus a
longer and mellower but systematic liming was used as a substitute for
shorter liming and bating. No sulphide was used in this process.
4. A short liming of 6-7 days, using up to 12 ozs. of sulphide per hide.
The object here is to save time and hide substance. The three-pit system
is preferred. Bating again becomes necessary, but the pigeon-dung bate
is replaced by artificial bates, less objectionable, quicker, and more
scientific in management.
5. A still shorter process of about five days, using still more sulphide
(about 16-20 ozs. per hide), together with some calcium chloride to
reduce harshness. In such a method there is a tendency to revert to the
one-pit system, which involves rather less labour. The three-pit system
shows to a great advantage in the longer processes of liming when the
process is reduced to five days; there is little difference between the
two, for a one-pit system is a two-liquor method. Hence again an
artificial bate is used.
The various methods of liming, together with analogous variations in
tannage, have resulted in great variety in bating. Sometimes up to three
days' bating has been given at 70° F., but more often the goods are
merely immersed overnight, and then delimed with boric acid, but with
sulphide processes it is an advantage to use some of the commercial
bates of the ammonium chloride type, and finish off with boric acid.
Scudding is always more thorough than for sole or belting, the hides are
rounded into long butts which include most of the shoulder "harness
backs." The goods are sometimes bate shaved.
A few tannages will now be outlined, in order of historic type.
=Type 1= may be taken to represent the so-called "high-class" process in
which oak bark myrabs and valonia are the staple materials. A good deal
of gambier is also used, and a little myrabs and chestnut extract are
helpful in attaining the desired strength of liquor. The "backs" go
first through suspenders (8°-30°), which takes up to three weeks, and
then in to handlers (30°-40°) for four weeks, consisting of rounds of
clear liquor. They next go through a duster round, in which they are put
for a week with oak bark and myrabs into a liquor of 45°. Four layers
are given (50°, 55°, 60° and 65°), in which the goods remain for two,
three, four and five weeks respectively, oak bark being the chief
dusting material. The tannage is thus for twenty weeks. Light backs
receive less time in the layers (only 11 weeks). If the tanner is also
the currier, the fourth layers are omitted. He then saves five weeks and
gets the weight in the stuffing.
=Type 2= is a tannage in which oak bark and valonia are replaced by
myrabs, mimosa bark and chestnut extract. It is therefore considerably
cheaper and probably no less durable. Expense is also curtailed in
handling. The harness backs go through suspenders (16°-30°) in two
weeks, handlers (30°-45°) in four weeks, and then receive four layers of
the same strength as in Type 1, but only one, two, three and four weeks
respectively. The last layer is omitted for light harness, and an extra
layer of 75° is given if the tanner is not the currier also. Thus the
usual tannage is 16-20 weeks, of which 10-14 weeks (63-73 per cent.) are
in layers.
=Type 3= is a tannage which may consist of myrabs (55 per cent.),
valonia or mimosa bark 25 per cent., and extract (26 per cent.). The
extract is chiefly quebracho, though some chestnut may be used. More
valonia and less myrabs may be used if desired (and when possible), and
myrabs extract will then replace quebracho and chestnut. The goods are
coloured off in drums or paddles, and then pass through two sets of
suspenders handlers (20°-55° and 55°-75°). They are handled up and down
very frequently in the first set and rapidly pass into stronger liquors.
The backs then receive three floaters at 80°, in each of which they
remain one week. The tannage is completed by three layers: first, 85°
for one week; second, 90° for one week; third, 95° for two weeks. The
tannage is thus 11 weeks, of which 7 weeks involve little labour. If the
tanner is not the currier, still stronger liquors may be used.
In all these tannages little or no acid is used for plumping, as the
natural acids of the liquors are sufficient to ensure what is necessary
in this direction for this class of leather. A little organic acid or
even boric acid may be used in the earliest liquors for deliming
purposes, when necessary. After tanning the goods are dried out and
sorted in the rough state. Harness is a somewhat broad term, and there
is scope for considerable variety in classification. The hides are
sometimes not rounded until after tanning. The finished article may be
any grade between heavy harness for artillery and leather for ordinary
bridles.
In currying heavy black harness, the backs are soaked and sammed for
shaving. Lighter goods may be machine-shaved, but the heaviest are
shaved lightly by hand over the beam or merely "skived" with the shaving
slickers. The neck needs most attention, and it is often advisable to
stone by machine and split. The scouring should be thorough, on flesh
and grain. This is done by machine, and not only cleans the goods from
bloom, dirt and superfluous tan, but also assists in setting out.
Sumaching may be for several days, merely overnight or even only for a
few hours, being stoned after wetting back to temper. Hand-stuffed goods
get a coat of cod oil first, and during the drying are often well set
out. Drum-stuffed goods are well set out by machine, and after some
drying, stoned and reset by hand. It is now usual to buff the grain,
_i.e._ remove the coarser parts by light shaving. This prevents cracking
in the finished article. The goods are blacked with logwood, iron and
ammonia, thinly dubbined again, again well set out and tallowed. Setting
out, indeed, may be done at any convenient opportunity. The superfluous
grease is removed by slicking, scraping, brushing with a stiff brush,
and finally with a soft brush.
For brown harness the goods are more carefully selected, more thoroughly
scoured and sumached, and bleached frequently with oxalic acid. They are
hand stuffed, stained twice, and after the usual setting out, glassing
and brushing, are finally rubbed with flannel.
For bridle leather the goods are carefully shaved but are not stuffed,
being merely oiled with cod oil on flesh and grain. They are dried out
before scouring, and then sized, set out, stained and resized. The goods
are heavily glassed during the finishing.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 195, 297.
Section VIII.--UPPER LEATHERS
The manufacture of leather for the uppers of boots and shoes embraces a
bewildering variety of goods, suitable for anything between a baby's
shoe and a man's shooting boot. Almost all degrees of lightness,
softness, and waterproofness are in demand. A great variety of finish is
also involved, determined by the ingenuity of the currier and the
ever-changing fancy of the public. Even greater is the variety of
methods by which all these results are obtained by methods which
superficially seem quite different; the desired qualities being imparted
in one case largely by the tannage and in another case almost entirely
by the currying. Under such circumstances the selection of types becomes
a problem.
The variety, moreover, commences from the earliest stages, the selection
of the raw material. Upper leather may be made from light calfskins,
heavy calfskins, kips (home and foreign), light dressing hides and heavy
dressing hides, which last may replace any of the former after splitting
to the required substance. In this section it will be necessary to take
kips as typical of the rest, and to use it in a rather broad sense,
including heavy calf and light dressing hides.
Speaking quite generally, kips for upper leather receive usually a long
and mellow liming, a thorough bating and a sweet and very mellow tannage
in weak liquors. In currying they are well scoured and set out, heavily
stuffed and stained black, being sometimes finished on the grain and
sometimes on the flesh. These outstanding features of upper-leather
methods will be further illustrated by a brief account of the tanning
of kips (light hide and heavy calf), and outlining the best known types
of finish for butt, shoulder and belly.
The goods receive usually a long and mellow liming of 14-16 days, using
only lime as a rule. In some factories lime liquors are used repeatedly
for successive packs to an almost indefinite extent. Dissolved hide
substance, ammonia, mud and dust, and bacteria accumulate for months and
sometimes for years. It is obvious that in such liquors "putrefaction"
is a more correct term than "liming" for the depilation. Such methods
have been used even in recent years, but there has now been a tendency
for some time to make the liming more methodical. Such old limes make a
leather which is empty, loose, and dull grained, but the defects are
minimized by the system of stuffing heavily and finishing the flesh, and
hence the ancient lime remained with surprising tenacity. Even so late
as 1903 we find that Procter with characteristic caution could write,
"Probably no lime ought to be allowed to go for more than three months
at the outside limit without at least a partial change of liquor." It is
within the writer's experience to find an upper leather factory with
limes which had never been emptied for over three years. In other
factories, however, there has been a revulsion of feeling with regard to
such processes, and it has been found advantageous to adopt a more
scientific routine, in which the lime pits are cleaned out at regular
intervals. There is little doubt that a mellow liming is desirable, but
this can be secured by blending some old lime liquors with fresh lime
liquor in a systematic manner. Similar considerations apply to the
question of working the various packs through the limes. It is clear
that with a mellow liming a one-pit system is quite possibly
satisfactory, but the revulsion of feeling against a lack of method
produced a method of liming more elaborate than usual, and it is now not
uncommon to find kips limed in a "round" of 6-8 pits, the goods passing
through each pit. They remain in one pit about two days, and are shifted
forward. In the green or old limes the goods are handled up and down.
The old limes are, of course, mellower than the new and exert the
desired softening effect. The working is quite analogous to that of a
round of handlers. Unhairing is sometimes assisted by the use of arsenic
sulphide. E.I. kips need a thorough soaking before any liming; several
days are usually needed. The old methods involving putrid soaks and
stocks may be considered out of date, and it is usual to soften back in
caustic soda or sulphide soaks with some assistance by drumming. A
little sulphide is sometimes added to the older limes to continue the
treatment.
The goods are next thoroughly bated and delimed. The hen or pigeon dung
bate is still usual, and probably gives the best results, though closer
approximations have been made of recent years on artificial lines. Some
bating with solution of hide substance seems necessary for these goods.
The lighter goods are often drenched also to complete the deliming,
using 6 per cent. bran on the weight of pelt. The heavier goods are more
often treated with boric acid after bating, which not only delimes
completely and gives a soft relaxed felt, but also acts as antiseptic
and stops the action of the bate, a matter of some importance (see
Section II.). Lactic acid may substitute boric, in which case about 2
per cent. on the pelt weight of 50 per cent. acid may be required. It is
important to avoid a strong solution and local excess, hence lactic acid
must be added gradually so that the liquor is never stronger than 0.2
per cent. Drumming and paddling is an advantage in deliming.
The tannage is light in most cases, partly because some of the finished
goods are sold by area, but partly also because even if sold by weight,
the weight is obtained quicker and more easily by stuffing, which course
is also often preferable to obtain the desired mellow feel,
waterproofness and durability. Hence it is seldom that strong liquors
are employed. The tannage is also mellow, on account of the softness and
pliability required; no acids are consequently employed, and no material
which is liable to yield sour liquors. Gambier is easily the first
favourite amongst the tanning materials, whilst oak bark comes second.
It should be observed, however, that a hypothetical tannage of equal
weights of cube gambier and oak bark is in reality a tannage by
four-fifths gambier and one-fifth oak bark, on account of the relatively
greater strength of the former. This observation is so apposite with
respect to some tannages that it is nearly correct to say that the
tannage is gambier and the oak bark an excuse for having leaches through
which the gambier liquors may be run occasionally to clear and to
sharpen slightly. No serious theoretical objection to such a method is
possible if the liquors are weak and the system of working the liquors
is scientific and the process carefully regulated. Upper-leather
tannages, however, have scarcely merited scientific praise. It is often
a case, not of poor methods, but of no method at all. The same lack of
system, principle, and regularity observed with regard to the limeyard
has been equally obvious in the tanyard, when perhaps the need was even
greater. Even a mellow tannage has varying degrees of mellowness
possible to it; there still remains the question of the soluble
non-tans. However, method in the upper-leather tanyard has often been
conspicuously absent. There has been many a factory where any one tan
liquor was as good as any other in the yard. In the writer's experience
are two such cases: in one the liquors were all 25° Bkr., in the other
they were all 0° Bkr. In such cases, handling the goods from pit to pit
is somewhat futile, and handling forward from set to set still more so.
Hence it is possible to find dressing leather tanned by putting it
slowly through one round of handlers, adding a few buckets of gambier
where it apparently is necessary. It is, from one point of view,
surprising to see what serviceable and excellent-looking upper leather
can be manufactured by such happy-go-lucky processes. It is, however,
also possible to see how this may occur. Gambier is a stable tan, and no
souring and little decomposition take place in gambier liquors. It is
also extremely mild and non-astringent, and is always used in weak
liquors. The hides, moreover, are completely delimed, and there is
little danger of bad or uneven colour. Tanning under these conditions is
at its easiest; it is almost more difficult to spoil the goods than make
them right. Under such conditions tanning deteriorated rather than
improved in method. When neglecting it made little difference to the
finished leather, it was neglected.
This state of affairs, however, was embarrassing whenever a tanner
wished to try any other tanning material. The expense of gambier and oak
bark made valonia and mimosa bark into obviously desirable alternatives
and substitutes. Methods which would tan with gambier, however, would
not work with Natal bark or valonia, and many a tanner has had to revise
his method of tanning from end to end. The use of myrabs also raised the
problem of souring, and it has become evident that "working the liquors
down the yard" is as desirable a method for dressing leather as after
all other tannages. It will be clear from the above that types of
upper-leather tannages are less typical than for other leathers, but
nevertheless the more progressive manufacturers have for some years now
been working on sounder lines, economically and scientifically. In such
cases it is now usual to pass the goods through at least two sets of
handlers, and through liquors of gradually increasing strength.
Occasionally dusters or layers are given, especially for the heavier
goods. The tannage is nearly always commenced now by paddling the goods
in the oldest liquor. This paddling may be anything from half an hour up
to twenty-four hours. It is sometimes desired to work up a "grain," and
the old liquor is then often sharpened by the addition of fresh gambier
or leach liquor.
The same tendency to save labour in handling is to be observed in upper
leather tannages as in sole and other dressing leather factories. There
is also a tendency to obtain rather more weight in tanning by using
stronger liquors, and in the heavier goods to shorten somewhat the time
taken. The following methods may be taken to illustrate modern processes,
in order of evolution. They all last about seven weeks.
_Type 1_.--In this process the kips are first paddled in an old liquor
(3°), and passed to the first handlers (3°-30°) for three weeks. After
working through this set they pass through the second handlers
(20°-30°), in which they are not handled quite so frequently. They are
in this set also three weeks. Heavy goods may then receive a floater
(30°) for another week.
_Type 2_.--In this process the goods are paddled, and then enter a large
handler round (8°-30°), through which they pass in five weeks. The goods
are handled frequently in the early stages. The tannage is completed by
one layer of two weeks (30°). The layer is made by the ancient method of
putting the goods and dust alternately into an empty pit, and then
filling up with liquor from the best leach. Oak bark, valonia and myrabs
are used as dust, though sumach and gambler have been used.
_Type 3_.--In this process an attempt is made to save handling and
obtain more complete tannage. The goods are paddled for three to five
hours in a rather sharp liquor of 10°, and are then handled well for a
week in the first handlers (5°-20°). The goods then go through the
second handlers (20°-45°) in six weeks, and heavy goods may then receive
an extra floater (45°) for one week.
In type 1 the leaching material is two-thirds oak bark and one-third
valonia; in type 2 it is half oak bark and half mimosa bark; in type 3
it is one-third oak bark, one-third valonia or Natal bark, and one-third
myrabolans. In all cases the strongest handler is obtained from the
leaches, and made up to the required with strong infusion of gambier.
When the liquor has passed through the forward handlers, it is returned
to the leaches to clear and sharpen, and then run to the green handlers.
After passing through this round it is returned to the paddle, from
which it passes to the drain. The rest of the paddle liquor may be from
the forward handlers. It is often customary to obtain the best liquor
from the second leach, and allow the best leach to stand for a few days.
This allows the bloom to deposit in the leaches. The system secures the
result desired, but the deposition of bloom involves a loss of tannin,
which waste makes the system expensive.
Heavier dressing hides are tanned by methods similar to the above, but
with floaters, dusters and occasionally layers added after they have
passed through two sets of handlers. Thus they may have first handlers
(8°-18°) two weeks; second handlers (40°-45°) for six weeks, making
twelve weeks in all. Lighter goods may receive two rounds, being two
weeks in each.
After tanning, the kips are rounded usually into butts, shoulders and
bellies, to which different finishes are given. The currying may be
illustrated by selecting types, but it must be borne in mind that there
is much elasticity in this matter. Thus kips may be made into waxed
butts, satin shoulders and lining bellies, but also may be cut down the
back in "sides," both of which are finished limings.
Waxed kip butts are a type of many similar upper leathers (waxed shoe
butts, waxed calf, waxed splits, etc.). The finish is on the flesh side.
The kip butts are soaked carefully, and shaved by machine. They are then
drummed in sumach for an hour or two, slicked out and sammed for
stuffing. The sumaching is also the scouring unless the goods be too
heavily bloomed. The samming is often done by machine. Drum stuffing
follows, wool fat and stearin being staple greases, with varying amounts
of degras and cod oil, and of tallow and cod oil. A little paraffin wax
and resin are also used sometimes. The goods are well slicked out and
dried. They may be now dubbined and laid away to mellow for whitening,
which consists of a careful shaving of the flesh by a turned-edge
slicker or by machine. The grain is stoned, set out and "starched," and
the butts grained by boarding the flesh. In the waxing, one of two
courses may be adopted. The butts may be blacked with lampblack and oil,
"bottom sized" with glue, soap and logwood, and then "top sized" with
glue, dubbin, beeswax and turpentine; or they may be given a
"soap-blacking" of soap and logwood and lampblack, applied by machine,
and sized once only.
Dressing hide butts may also be given a grain finish, such as the "memel
butts" for heavy uppers. The butts are soaked, shaved or split, sumached
in drum, and preferably thoroughly scoured on flesh and grain. They are
then sammed and heavily stuffed in the drum. The grain is buffed and
stained black with logwood, ammonia and iron solution (curriers' ink).
The butts are then dried, set out, thinly sized and slowly dried.
When dry on the face they are printed or embossed by machine to give the
characteristic memel pattern and dried out completely. They are then
grained four ways. The grain is finished by a coating of linseed oil
containing resin, and the flesh is whitened, French chalked and glassed.
Shoulders for "satin" receive a currying which strongly resembles the
"waxed" finishes, but the smooth finish is on the grain side. The grain
is buffed, and blacked, dubbined, set and reset, with intermediate
drying, and is sized and finished by compositions similar to those used
for waxed leathers. The flesh is whitened. Satin hide and satin calf are
dressed similarly.
Shoulders may also be finished for "levant." After soaking, splitting,
and shaving to substance, they are drum-sumached, machine-sammed, and
oiled up to dry. They are stained with logwood on the grain, and at once
printed with the typical "levant grain," blacked and dried out. They are
then softened by machine, seasoned with logwood and albumen, glazed,
grained and oiled lightly with mineral oil. It will be observed that
stuffing is omitted.
Bellies may be dressed for linings. After soaking and splitting to the
required substance, they are bleached in a weak and warm solution of
oxalic acid, and drum-sumached at 110° F. After slicking well out they
are hand-stuffed on the grain with dubbin and water, or merely oiled,
and hung up to samm. They are then set-out flesh and grain. If the grain
be coarse, it is buffed and reset. After drying out the flesh is fluffed
and the grain dusted with French chalk.
In this section may be conveniently discussed the manufacture of legging
leather. Whilst in many respects a typical dressing leather there are
some rather important differences from the average upper leather.
Broadly speaking, the differences are that legging leather needs a
smooth grain, greater firmness and more thorough tannage on account of
the absence of stuffing.
The liming and bating are somewhat similar to dressing leather, though a
shorter liming with sulphides and a milder bating would be in order. The
tannage is mellow, but not so much as is usual for upper leather. Thus
gambier is used, but more valonia and myrabs are employed, and the
liquors may be strengthened with chestnut and quebracho extracts. The
hides are rounded before tanning into long butts or backs, and the
tannage is commenced in suspenders (18°-40°), which are kept acid by the
addition of lactic or acetic acid, in order to obtain the required
firmness; the goods are three weeks in these liquors. The backs next go
through rounds of dusters (40°-50°), in which they are put down with oak
bark and Natal bark. They are six weeks in this section, and then pass
to the layers. Three layers are given, first 50° for one week; second
55° for two weeks; and third 60° for two weeks. The tannage thus takes
fourteen weeks.
In finishing, the goods are soaked and split, and then scoured flesh and
grain. They are heavily sumached, slicked out thoroughly, oiled up with
linseed oil and dried out. They are then next damped back, stoned and
flatted. After further wetting and tempering they are dressed with Irish
moss and tallow on the flesh, and with gum tragacanth on the grain.
They are glassed whilst drying out, and then stained twice and glassed
again. They are again brushed, seasoned and glassed by machine.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 197-201 and 301-308.
SECTION IX.--BAG LEATHER
Hides to be tanned for bag leather receive a treatment which is little
different in fundamental principle from that of dressing hides for upper
leather, except that the tannage is usually shorter. Hides for bags and
portmanteaux represent a type of dressing leather in which the
outstanding features are that the goods are split but not rounded. The
splitting is done at all stages, according to the requirements of the
tanner. Some tanners split "green," _i.e._ split the pelt itself. The
advantage of this is that the fleshes may then be treated in quite a
different way, _e.g._ pickled or given a much cheaper tannage. Other
manufacturers split after tanning, the advantage being that there is
much less material to handle. The general opinion, however, favours a
middle course in which the hides are split after being in the tan
liquors for a short time. The advantage of this course is that the hides
are easiest to split under these conditions--a great
consideration--being coloured through with tan, just a little plumped,
but not hard. A smoother flesh is obtained together with more even
substance. Here again, however, are differences; some tanners prefer to
split after two days, others after two weeks in tan. Much depends upon
the nature of the tan and the strength of the liquors.
For this class of work, flat, spready and evenly grown cowhides are
obviously the most suitable material, and are invariably used. It is
important, however, that the grain be good, and free from scratches and
similar defects. The tannage must be sweet and mellow, _i.e._ contain no
acid and little astringent tan. Hence myrabolans and gambier have always
been the favourite tanning materials. A soft and mellow tannage is the
more important, inasmuch as the leather is not heavily stuffed with
grease in finishing. These types of method for tanning split hides will
now be outlined, and the nature of the currying then indicated.
_Type 1_.--In this a long mellow liming of 15-16 days is given, much
like that described for harness leather in Section III., Type 3.
Only lime is used, but the liquors are not allowed to get dirty. The
three-pit system is much the best. The hides are trimmed at the rounding
tables, and then bated in hen or pigeon dung for three days at 75°-85°
F. The deliming is commenced by washing in tepid water before bating,
and is completed by a bath of boric acid, using up to 30 lbs. acid per
100 hides as necessary. In this and other processes for split hides it
is essential to obtain all the lime out, but to do no plumping with
acid. Lactic acid may also be used, but it is not so convenient to hit
the neutral point with it.
The tannage consists of oak bark and myrabs together with gambier. These
may be partly replaced by Natal bark, valonia, and quebracho
respectively. It is sometimes desired to have a smooth finish, but
sometimes to work up a "grain." In the latter case the hides are first
put through colouring pits containing fresh leach liquor. In these they
are constantly handled for a few hours. A little experience indicates
which leach liquor will serve the purpose. The hides then go through the
"green handlers" (8°-20°) in two weeks. The liquor is the old forward
handler liquor made up with gambier. The hides may be sammed and split
up at this stage, but the heavier goods may be tanned further. These
heavies and the grains of the split hides now go through the "forward
handlers" (20°-40°) for four weeks, and the heaviest goods given two
layers (40°) of two weeks each, and making ten in all.
_Type 2_.--In this a shorter liming of 8-9 days is given with the help
of sulphide. No dung bate is used, but the goods are washed with water
and bated with ammonium chloride and boric acid. The tannage is chiefly
of myrabs, but some valonia or Natal bark may be used together with
chestnut extract and some quebracho. Gambier is used in the early
liquors. The goods are coloured off in drum or paddle and tanned in
several sets of handlers, viz. green handlers (15°-35°) three or four
days; second handlers (35°-60°) two weeks; forward handlers (60°-80°)
1-1/2 weeks; and floaters (80°-90°) for three weeks. The tannage is thus
6-1/2 weeks in all. The arrangement of pits is a matter of local
convenience, and the number of sets of equal strength is determined by
the number of hides being tanned. The hides are split green or after
passing through the green set. After tanning they are oiled with cod oil
and dried out.
_Type 3_ is illustrated by American methods. The goods are tacked on
laths or racks with copper nails in order to ensure smooth grain. They
are then suspended in tan liquors. The tannage is largely with gambier
and in weak liquors, which also help to give smooth grain. The tendency
is to employ handler rounds involving a rather large number of pits, and
to work these on the press system. Handling is also saved by plumping
the liquors instead of shifting the goods forward, and by rocking the
suspenders instead of handling up and down. The hides are split after
about a month, and the heavier grains laid away in hemlock liquors.
_Type 4_.--This is a rapid process throughout. The hides are limed in
6-7 days with the help of sulphide, and "bated" by washing in warm water
and then in cold to which hydrochloric acid is gradually added,
finishing off again in tepid water. The hides are now coloured off in
paddles, put through a small handler round (11°-20°) for half a week,
and then split. The grains are drum tanned in a mixture of chestnut and
quebracho extract, over a period of about three days in which the liquor
is strengthened gradually from 30° to 50°. The fleshes are drum tanned
with the old grain liquors after strengthening with quebracho.
The split hide grains for bag work, after tanning, are drummed in
sumach, rinsed, drained, and oiled up to dry out, with some setting out.
After wetting back they are shaved if necessary, hand scoured, and
heavily sumached again to get a light even colour. The goods are slicked
out, oiled up to samm, reset and dried out. They are next stained,
sammed, printed by machine, dubbined or tallowed, "grained" (see Part
II., Section I.), brushed and rubbed with flannel.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 202, 308.
SECTION X.--PICKING BAND BUTTS
It is the paradox of vegetable tannage that the less the pelt is tanned
the stronger is the leather produced. The manufacture of butts for
picking bands affords a good illustration. What is required is a leather
of maximum toughness, pliability and durability. Any factor reducing the
tensile strength of the leather is fatal. Hence, compared with most
other tannages, picking band butts are under-tanned. To ensure the
desired softness and pliability, moreover, it is necessary to have a
mellow liming, rather heavy bating, and a soft mellow tannage in sweet
and weak liquors. The required durability and the necessity for weak
liquors both point to oak bark as the most suitable tanning material,
assisted by some gambier in the early stages.
A good quality hide is chosen, and given a long and mellow liming of
about 15-16 days. The one-pit system may be used, and the hides are put
into an old lime for about five days with frequent handling and then
placed in a new lime which is made up in a pit containing about a foot
depth of the old liquor. After about twelve days another 1/3 cwt. of
lime may be added.
After unhairing and fleshing the goods are bated in pigeon dung for four
days at a temperature of about 78° F., handling twice on the first and
last days. The bating is stopped and the deliming completed by paddling
with boric acid (15 lbs. per 100 butts).
The tannage is commenced by paddling in a spent handler liquor (4°) to
which a little gambier has been added. The butts then go through the
first handlers (5°-15°), which are rounds of ten pits in which the goods
are handled every day in the first week, and alternate days in the
second week, and are shifted forward twice a week in the next pit. The
goods are therefore in this set for five weeks. Gambier is added to
these liquors as needed. The butts next pass to duster rounds of four
pits, in which they are dusted down in a liquor of 20° for four weeks
with 1-2 cwt. of oak bark. The liquor is obtained from the leaches, and
afterwards run alternately to the leaches and to the first handlers. As
many as six layers are now given of 20°-25° strength, in which the butts
are dusted down with 2-3 cwt. oak bark for three weeks. The layer
liquors are received from and returned to the leaches, which are made
from the "fishings" from the layers. The tannage lasts, therefore, 27
weeks, of which 18 weeks (two-thirds) are in layers.
Shorter tannages are now often given, using stronger liquors, much as in
ordinary dressing leather.
The tanned butts are rough dried, and then wet in for shaving. They are
thoroughly scoured, flesh and grain. They are next drummed for
three-quarters of an hour in sumach, struck out and hung up to samm.
Hand stuffing is best, to avoid any tendering owing to high temperature,
but drum stuffing is also used. After setting out and stoning on the
grain they are stuffed with warm cod oil and laid away in grease for
several weeks, re-oiling occasionally. They may be stained before
stuffing.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 203, 310.
PART II.--SKINS FOR LIGHT LEATHERS
SECTION I.--PRINCIPLES AND GENERAL METHODS OF LIGHT LEATHER MANUFACTURE
The term "skin," like the term "hide," in its widest sense applies to
the natural covering for the body of any animal, but is generally used
with a narrower meaning in which it applies only to the covering of the
smaller animals. Thus we speak of sheep skins, goat skins, seal skins,
pig skins, deer skins, and porpoise skins. It is in this sense that it
will be used in this volume. The treatment of such skins to fit them for
useful purposes comprises the light leather trade. Whilst this branch of
the leather industry is certainly utilitarian, the artistic element is a
great deal more prominent in it than in the heavy leather branch. Thus
the light leathers are often dyed and artistically finished, and their
final purposes (such as fancy goods, upholstery, bookbinding, slippers,
etc.) have rather more of the element of luxury than of essential
utility. The total weight and value of the skins prepared, and of the
materials used in their preparation, are naturally considerably smaller
than those of the heavy leather trade. In the latter, moreover, one has
to consider the purpose in view from the very commencement of
manufacture and vary the process accordingly, but in light leather
manufacture one aims rather, in the factory, at a type of leather such
as morocco leather, and only after manufacture is it fitted to such
purposes as may be particularly suited to the actual result. These
results depend very largely upon the "grain pattern" which is natural to
the skin of any one species of animals. Hence in Part II. of this volume
it has been found most convenient to deal with the different classes of
skins in different sections. Just as the hides of ox and heifer were
much the most numerous and important of hides, so also naturally are
sheepskins the most prominent section of the raw material of the light
leather trade. This is the more true because the skin is valued for its
wool as well as for its pelt; indeed, the wool is often considered of
primary importance, and receives first consideration in fellmongering.
Unfortunately for the light leather trade, sheepskins, though most
numerous, do not give the best class of light leather, the quality being
easily surpassed in strength, beauty and durability by the leather from
goat or seal skins.
In the wet work for the preparation of skins for tannage much the same
general principles and methods are embodied as in the case of hides, but
with appropriate modifications. As soft leathers are chiefly wanted, a
mellow liming is quite the usual requirement for all skins. It is also
usual to have a long liming, for some skins (like those of sheep and
seal) have much natural fat which needs the saponifying influence of
lime and lipolytic action of the enzymes of the lime liquors; whilst
other skins (like those of goat and calf) are very close textured and
need the plumping action of the lime and a certain solution of
interfibrillar substance. In consequence of the long mellow liming,
sulphides are not usually necessary, and indeed sodium sulphide is not
usually desirable, on account of its tendency to make the grain harsh.
It is used, however, for unwoolling sheepskins, in such a manner that
the grain is not touched. Similarly caustic soda is seldom required, and
the yield of pelt by weight is usually a small consideration. Systems of
liming show some variety. The one-pit system is very common, and is less
objectionable for a long mellow liming, but rounds of several pits are
also used, and in some cases even more than one round. This is obviously
conducive to regularity of treatment, and as the work involved in
shifting the goods is much less laborious than in the case of heavy ox
hides, it would seem a preferable alternative. The depilation of
sheepskins involves very special methods of treatment (sweating and
painting) on account of the importance and value of the wool, the
quality and value of which would be impaired by putting the skins
through ordinary lime liquors. The pelts, however, are limed after
unwoolling.
In deliming light leathers the process of puering is widely used. This
consists in immersing the skins after depilation in a warm fermenting
infusion of dog-dung. In principle this disgusting process presents a
close analogy with bating, and indeed the two terms are both used
somewhat loosely, but there are nevertheless several points in which the
two processes are radically different. The dog-dung puer is a process
carried out at a higher temperature than the fowl-dung bate; it is also
a much quicker process, and the infusion employed is generally more
concentrated. Whilst the fowl-dung bate is always slightly alkaline to
phenolphthalein the dog-dung puer is always acid to this indicator, and
the course of the puering may be conveniently followed by testing the
pelts with it. The mechanism of the two processes is also probably
somewhat different. The mechanism of the dog-dung puer has been largely
made clear by the researches of Wood and others, and been found due
partly to a deliming action by the amine salts of weak organic acids and
partly to the action of enzymes from a bacillus of the coli class, which
received the name of _B. erodiens_, and which effects a solvent action
on the interfibrillar substance. As we have noted (Part I., Section
II.), the fowl-dung bate involves two fermentations, in each of which
(ærobic and anærobic) several species of bacteria are probably active.
Wood found the bacteria of the bate to be chiefly cocci, and ascribed
part of the difference in mechanism by the nature of the media, which in
the bate includes also the urinary products. In the dog-dung puer, also,
a lipolytic action is probably an essential part of the total effect.
The puer gives a much more complete deliming and a much softer and more
relaxed pelt than the bate, it is therefore particularly suited to the
needs of light leather manufacture. The puering action has been imitated
fairly successfully by artificial methods. "Erodin" (Wood, Popp and
Becker) involves the use of _B. erodiens_ and a suitable culture medium
including organic deliming salts: "Oropon," "Pancreol" and others
involve the use of ammonium chloride and trypsin, together with some
inert matter.
Light-leather goods are usually drenched after puering. They are also
often split green after the wet work. Sheepskins thus yield "skivers"
(the grain split), whilst the flesh split is often given an oil tannage
(see Part IV., Section III.). The greasy nature of sheep and seal skins
necessitates the processes of "degreasing." In the case of sealskins
this is done largely before liming, but with sheepskins either after
being struck through with tan, or after tannage is complete. Sheepskins
are often preserved in the pelt by pickling with sulphuric acid and
salt, which process forms a temporary leather. The fibres of the pelt
are dried in a separate condition, but the adsorption is easily
reversible and the pelts may be "depickled" by weak alkalies and
afterwards given an ordinary vegetable tannage.
In the vegetable tannage of skins for light leathers, the same
theoretical considerations have force as in the heavy-leather section,
but the former has its own rather special requirements and aims.
Generally speaking, a softer and more flexible leather is required, but
these qualities must not be imparted by stuffing with grease as in the
currying of dressing leather, because a bright and grease-free result is
usually required. Hence it is important that a sweet mellow tannage be
given. The durability of the leather is also a primary consideration for
goods intended for bookbinding, upholstery, etc., and the tannage must
be arranged to impart this quality and avoid anything tending to cause
the perishing of the fibre. Thus oak bark is a popular tanning material,
and sulphuric acid very definitely avoided. The tannage must be fast,
and take the dyestuffs well, and for the production of light shades of
colour in dyeing must be a light-coloured tannage. All these qualities
are imparted by sumach, which also fits in excellently with the other
general requirements, such as softness, brightness and durability.
Hence sumach is the principal light-leather tanning material, but the
tendency is to employ other materials--oak bark, myrabs, and chestnut
extract--to do much of the intermediate tanning, so that the expensive
and useful sumach may be used for setting the colour and grain at the
commencement, and for brightening, bleaching and mordanting the leather
at the end of the tanning process. Weight is generally no consideration,
but area is often a definite aim, partly because some goods are sold by
area and partly because the striking out, setting out and similar
operations improve the quality of the leather by giving evenness of
finish. Leather well struck out, moreover, is less liable to go out of
shape. As the grain pattern is so important in the finished leather,
appropriate care must be taken during tannage. If a smooth or a fine
grain finish is wanted, for example, the goods must not be allowed to
get wrinkled, creased, doubled or unduly bent to and fro during the
tanning. For such goods, suspension, careful handling and even the "bag
tannage" may be desirable, whilst for coarser and larger grains paddles
or drums may be more extensively used.
Amongst the finishing processes dyeing holds an important position. The
nature of the process has many points of similarity with that of
tanning. The great specific surface of pelt is probably more enhanced
than otherwise during tannage, at any rate with light leathers, owing to
the isolation of fibres, and consequently leather is as liable as pelt
to exhibit adsorption. The dyestuffs, on the other hand, are substances
very easily adsorbed. Some (like eosin and methylene blue) are
crystalloids, some (like fuchsin and methyl violet) are semi-colloids,
whilst others (like Congo red and night blue) are undoubted colloids
forming sols (usually emulsoid) with water as dispersion medium. The
crystalloids and semi-colloids may also be obtained in colloidal
solution, sometimes being so changed on the mere addition of salts to
the solution. In addition, the pelt has been mordanted with tannin. If,
however, leather has been kept long in the rough-tanned or "crust"
state, this may not be so effective, owing probably to the secondary
changes in tanning (Part I., Section III.), but such leathers are
usually "retanned" or prepared for dyeing by sumaching (which process
also incidentally bleaches). The tannin mordant assists materially in
the fixation of the dyes. In the case of basic dyestuffs, lakes also are
formed, _i.e._ there is a mutual precipitation of oppositely charged
colloids (+dye, -tannin). The dyeing of leather is thus a case of
colloid reactions even more complicated than that of tanning.
Another finishing operation typical of the light leathers is "graining"
or "boarding." In this the skins after dyeing and drying are worked by a
board which is covered by cork, rubber, perforated tin or other
material, and so grips or "bites" the leather. The object of "graining"
is to work up the grain pattern by pushing or pulling a fold on the skin
with the board. The nature of the grain varies with the thickness and
the hardness of the skin, with the amount of pressure applied, with the
nature of the board, with the direction of the boarding and with the
total number of directions boarded. There is thus infinite scope for
variety of finish, and hence arise bold grain, fine grain, hard grain,
straight grain, cross grain, long grain, etc. The operation requires
considerable skill and experience. In the case of skins with little
natural grain (such as sheepskin) embossing and printing machines
impress the desired pattern.
In seasoning, a dressing is applied containing essentially albumins and
emulsified fats, _e.g._ egg albumin and milk. Colouring matters are also
often added to intensify or modify the shade. After seasoning the goods
are usually "glazed" by a machine which rubs the seasoned grain with
considerable pressure, by a glass or hardwood tool, and so produces a
high gloss, for which the seasoning is very largely a preparation. Light
leathers are very lightly oiled with linseed or mineral oil.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 220, 394.
Bennett, "Manufacture of Leather," pp. 36-41, 55, 85-90, 92-112,
312, 332.
Wood, "Puering, Bating and Drenching of Skins."
Lamb, "Leather Dyeing and Finishing."
SECTION II.--GOATSKINS
Goatskins are amongst the most valued raw material for the manufacture
of light leather. The leather obtained from them is of the very finest
quality in respect to durability and adaptability to the principal
purposes in view. The texture of the fibres in goatskin is exceedingly
compact and very strong, whilst the grain exhibits naturally a
characteristic pattern which renders it most suitable for a grained
finish. Hence for purposes like upholstery, bookbinding, slippers, it
forms almost an ideal material. The tanning and finishing of goatskins
into "morocco leather" may indeed be taken as a quite typical example of
light leather manufacture.
The skins are obtained from all quarters of the globe where goats exist,
and the excellent quality of the leather produced has created a demand
which is greater than the supply. This is due not only to the demand for
morocco leather, but also to the popularity of the goatskin chrome upper
leathers such as "glacé kid" (see Part III., Section IV.). The large
American trade in the latter has produced the saying that wherever there
is a goat there is an American waiting for it to die! The European
supply of skins is somewhat limited. They are obtained from the Balkans
and Bavaria, in which case they are small, fine-grained and plump skins.
The Swiss goatskins are larger, and have also a fine grain; they are
well grown and well flayed. Scandinavian skins have a poor reputation,
being very flat. The African supply is important; Abyssinian skins are
exceedingly compact and tough, and are very suitable for "bold grain"
finishes. The Cape skins are particularly large, strong and thick, but
their quality is often impaired by the cure, the skins being flint-dry,
and, like hides so cured, prone to unsoundness. Large quantities of
goatskins also come from the East. Many of these are imported in a
tanned state (E.I. Goat). These skins are tanned with turwar bark, which
contains a catechol tannin. They are also heavily oiled with sesame oil,
and need degreasing. The tannage is also stripped as far as practicable,
and the skins retanned with sumach before finishing. They make good
morocco leathers for many purposes, but the primary catechol tannage
renders them ineligible for finishing under the specifications of the
Committee of the Society of Arts. The skins have a Persian or Indian
origin. India also supplies a large number of raw dried goatskins which
are small and of variable quality. These, however, are more extensively
used for chrome uppers.
Goatskins are imported in either a salted or a dried condition. The
great aim of soaking is to obtain the skins in a thoroughly soft
condition. Hence the soaking is prolonged, and some mechanical treatment
is desirable in addition to various steepings in water. To be certain of
softness it is desirable to avoid the use of alkalies in the soak
waters, for although they cause hydration of the fibres by imbibition,
they also have a plumping effect which is not wanted at this stage.
Salted goatskins are first immersed in water and left until the
following day. This dissolves the salt. They are then stretched and
given a fresh soak liquor of water only to soften further, clean, and
remove the rest of the salt. This second water lasts only a few hours,
and the goods are then drummed well in running water. This not only
cleans quickly, but has an excellent softening effect. They are again
returned to a soak liquor, then softened mechanically by working them
over a beam. This treatment must be repeated, drumming again if
necessary, until the skins are perfectly relaxed and thoroughly
softened. If the treatment be very prolonged it becomes advisable to use
antiseptics in the soak waters after the first drumming. Solubilized (or
emulsified) cresols of the "Jeyes fluid" type are the most suitable
antiseptics, but too much must not be used or the sterilization affects
the liming, in which bacterial action is needed. Flint-dry skins are
left longer in the first soak, which should be of water only. They are
then given a fresh soak liquor containing 0.2 per cent. of sodium
sulphide. Sometimes a 1.0 per cent. solution of borax is used instead;
it softens excellently, is antiseptic, and avoids the plumping effect,
but is rather expensive. The goods are next drummed well, and resoaked
and worked as for salted skins. In either case the soaking takes about a
week.
The liming of goatskins presents some points of contrast with the
methods used for other skins. These differences are due to the
exceedingly tight and compact nature of the skin fibres. This
compactness of texture makes it quite necessary to dissolve the
interfibrillar substance to a greater extent than usual, and also to
plump the fibres and split them into the constituent fibrils. These
effects are essential to obtain a rapid and complete tannage and a soft
leather. Too much bacterial action should be avoided, however, or the
brightness and soundness of the grain may be impaired, which would be a
fatal defect in such a leather. Hence the liming is long rather than
mellow, and sharp limes rather similar to those required for sole
leather are often used. Another result of the tight texture of goatskin
is that depilation is not easily effected. This feature is rather
intensified by the deepness of the hair-root. Hence it is usual to
employ sulphides to assist the depilation. In one method two rounds of
five pits are used. The skins are given about two days in each pit, so
that the liming lasts approximately three weeks. In the first round,
which consists of rather mellow limes, arsenic sulphide is used to
assist depilation. Up to 6 per cent. on the weight of lime is added
during slaking. This is a comparatively large amount of arsenic
sulphide, and the depilation is considerably hastened; the skins indeed
are unhaired after passing through this round, _i.e._ after about 10
days' liming. In the next round the object is plumping, and caustic soda
(or carbonate) is added to the lime liquors in quantities comparable to
those suggested for sole leather (Part I., Section V.). In
this round the goods stay also for about 10 days. An alternative to the
above process is to hasten the earlier part of the liming by employing
sodium sulphide instead of realgar. More sulphydrate may be obtained in
solution in this way, and the unhairing may be in about half the time.
The sulphide of soda also commences the plumping action which follows in
the next round, but this alternative has the disadvantage that the skins
are unhaired whilst the pelt is swollen with sulphide, which renders the
grain both harsh and tender and consequently more liable to damage by
the unhairer's knife.
Deliming is by puering and drenching, and is often associated with a
further mechanical working of the goods. The skins are inserted into a
puer liquor at 85° F. and thoroughly pulled down. The caustic alkalies
should be completely neutralized. A slight cut into a thick part at the
butt end should develop no pink colour with phenolphthalein. The skins
should be thoroughly relaxed, and the swelling so much eliminated that
they are quite soft, weak and "fallen." The resilience and elasticity of
the plumped skins should have quite disappeared, and the impressions of
hand or thumb should be readily retained by the pelt. The grain should
appear white and possess a soft and silky feel. In this condition they
are again worked over the beam to soften further if possible. They are
then rinsed and again worked over the beam. Drenching follows with 10
per cent. of bran on the pelt weight, the operation commencing at 85° to
95° F., and lasting till next morning. The skins are next scudded
thoroughly to remove all dirt, but carefully so as not to damage the
grain.
In tanning, sumach and oak bark are the staple materials. Sumach gives a
much lighter colour, and hence it is used alone for goods that are to be
dyed the lighter shades, but oak bark is a "faster" tannage and more
preferable for dyeing in those cases where blacks and very dark shades
are wanted. For ordinary purposes a blend is usually employed. A feature
of oak bark, also, is that it tends to make a firmer leather, so that
the proportion used must be adjusted with this fact in mind as well as
the question of colour. For firmer moroccos the skins may pass through a
handler round of oak-bark liquors (10°-20°) in which a certain amount of
sumach is added to the liquors. The sumach is leached and assists both
in tanning and bleaching as the liquor works through the round. The old
liquor is run to a paddle, and the tannage is commenced by paddling the
drenched skins in this liquor. It is advantageous both for the tannage
and for the efficient "spending" of the sumach if this liquor be
slightly warmed. In the early pit liquors the goods are very frequently
handled. There is, however, the usual tendency of the times to save
labour in this direction, and hence it is common to have several paddles
with liquors of gradually increasing strength, followed by a shorter
round of handlers in which the handling is more infrequent. Instead of
paddles latticed drums may be inserted into pits containing liquors.
These, however, are not quite so convenient. In some tanneries,
especially where sumach only is employed, the tannage is in paddles
throughout. A new liquor is made up with fresh sumach and is used
repeatedly until exhausted. A three-paddle system sometimes obtains, in
which case the operation closely resembles the three-pit system of
liming (Part I., Section II.), and the skins pass through an "old"
liquor, a "medium" liquor and a "fresh" liquor. The goods need not be
paddled the whole day through, and indeed in the later stages this is
undesirable. The packs remain several days in each liquor and take up to
14 days to tan. Two to three bags of sumach are needed for about 20
dozen goatskins. This method of tanning is efficient and convenient for
bold-grain finishes, on account of the constant tumbling and bending of
the skins which tends to work up a grain. For very soft leathers and
fine-grain finishes, however, the "bag-tannage" or "bottle tannage" is
favoured. In this method the pelt is stitched up by machine to form a
bag, grain outwards, leaving a "neck" in the hind shank. The bag is
nearly filled with a fairly strong infusion of sumach, inflated with air
and tied up at the neck. The bags are then placed into a vat of warm
sumach liquor, in which they just float. The bags are pushed down and
the liquor stirred up, so that the goods are in constant motion. After a
few hours they are piled on a rack, and the tan liquor of the interior
is caused to diffuse through the skins by the pressure due to the weight
of the pile. The bags are refilled with fresh and stronger sumach liquor
and the process is repeated. The skins are thus lightly but effectively
tanned in about 24 hours, and the leather has very fine grain and soft
feel. However tanned the skins are dried out after tanning, and sorted
in the "crust" according to size and colour. The larger skins are
preferred for upholstery and the smaller for fancy goods and
bookbinding.
To illustrate the course of finishing operations, the case of hard-grain
morocco for bookbinding may be given as typical. The goods are wet back
with warm water and drummed for 1-2 hours in warm sumac to prepare for
dyeing. They are then struck out by machine, sammed and shaved. Dyeing
follows, with acid colours, in a drum. The goods are run first in a
little water and the dyestuff added very gradually through a hollow
axle. The acid required (preferably formic) is added later to develop
the full shade. Warm solutions are used, and the dye bath is practically
exhausted. The goods are next placed in cold water to wash off
superfluous liquor and free the skins from acid. They are then horsed to
drain, struck out and hung up to samm. They are seasoned with milk and
water and piled to temper. They are "tooth rolled" in the glazing
machine two ways: right-hand shank to left fore shank and _vice versâ_,
and piled again. After wetting back again they are "wet grained" by hand
with a cork board in four directions: belly to belly, shank to shank,
and across as before, and finally from neck to butt. They are
immediately hung up in a warm shed to dry, and to fix the grain. They
are then softened by "breaking down" with a rubber board, top seasoned,
piled to temper and dry, brushed lightly, piled again, brushed more
heavily, and dried out. They are finally softened by graining in three
directions: shank to shank and across, and neck to butt. They are then
brushed again. If these skins are wanted for upholstery they are shaved
after dyeing, and nailed on boards to samm. They are also dried out in a
cooler shed or "stove," to ensure softness.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 39, 55, 89, 111, 204, 344,
396.
SECTION III.--SEALSKINS
A special class of morocco leather is manufactured from the skins of
seals. This should not be confused with the "sealskin" of popular
parlance, which is manufactured from the skin of a different animal. All
the fin-footed mammals (_Pinnipedia_), except the walrus, are termed
seals, but they are divided into two families. The _Otariidæ_ are known
by their possession of small but distinct external ears: into this class
fall the fur-seals whose skin is dressed with the fur on, for women's
jackets, muffs and caps. The _Phocidæ_ are that family without external
ears: the skins of many species (_Phoce Greenlandica_, _Phoco barbata_,
etc.) of this family are unhaired and given a vegetable tannage, thus
forming the raw material of sealskin morocco leather. It is with the
latter that this section will deal.
As the seal is a marine animal and is partial to the colder seas, its
skin is very oily. The skins are imported in a salted condition from
both the Arctic and Antarctic regions. North Europe, North America and
Newfoundland supply many skins, and the southern material is supplied
chiefly through the Cape. Sealskin shares with goatskin the properties
of compact texture, strength of fibre, and great durability, all of
which fit it for the manufacture of moroccos for upholstery,
bookbinding, etc. It is, however, readily distinguishable from goatskin
by its characteristic grain pattern.
In soaking sealskins the object is not only to soften thoroughly, but
also to effect the recovery of as much seal oil as possible before the
liming commences. This is desired because the oil is in itself a
valuable bye-product, and because its removal is essential to a
satisfactory liming and tannage. The removal of the oil is materially
assisted by raising its temperature, so that the soaking of sealskins
is often done with warm water (85°-88° F.), after which treatment they
are laid over the beam and scraped with a blunt knife on both flesh and
grain. The oil flows away into a special receptacle. This treatment is
repeated until the bulk of loose oil is removed. The process is known as
"blubbering" or "brushing over." After some soaking the skins are
drummed to ensure softness. The skins are then fleshed. More oil may be
obtained from the fleshings.
By fleshing before liming a more regular action of the lime is obtained.
This is necessary to "kill" the grease still remaining in the skin. A
long and mellow liming is given for the same reason. Fully three weeks
are given, and old limes are much preferred, partly to obtain the
maximum lipolytic action and partly to avoid the intense ribbing of the
pelt which new limes so easily impart to the older animals. These ribs
are very difficult to eliminate in the subsequent work. Some factories
find it necessary to finish up in new limes, however, in order to plump
and split the compact fibre bundles into their component fibrils. The
plumped pelt is also easier to split green. No sulphides are usually
employed. Sweating (see Section IV.) is sometimes used for
depilation, and in this case the ribbing of the pelt does not take
place.
The puering is unusually thorough with sealskins. This is to obtain the
maximum softness and take full advantage of the lipolytic action. The
puer liquor is fully 95° F., and the skins are paddled for about three
hours, or until fully pulled down and completely delimed. Scudding
follows, now usually by machine. The skins are then well drenched. The
action is intensified by the use of peameal in addition to the bran.
About 10 per cent. of the mixture on the weight of pelt is used. It is
customary, however, to drench at a lower temperature (68°-70°) than in
the case of goatskins (Section II.), but the goods are left in
the drench overnight only, as is usual in drenching. It is quite
possible that drenches worked differently may have also a somewhat
different fermentation and be due to other organisms than the symbiotic
bacteria discovered by Wood. It is equally possible that the acids
produced are also different, in relative proportion, if not in nature,
and that consequently there is a real difference in the practical
effect. In the Author's opinion, the great probability is that in the
drench are several fermentations, and that if the action be reduced by
lowering the temperature, but intensified by adding peameal to the bran,
some of these fermentations are encouraged at the expense of others.
The tannage of sealskins depends upon the size of the skins, the purpose
for which they are intended, and whether they have been split or not in
the limed state. The largest and coarsest skins intended for boot
uppers, and those which have been heavily scratched on the grain and are
only suitable for enamels, are given a tannage which may last about 5
weeks. The liquors are made from oak bark and mimosa bark, and are made
up to 35° with gambier and possibly myrabolans extract. For fancy work
also heavy skins are used, but a softer tannage is needed. If for blacks
the tannage is with gambier and chestnut extract. Two sets of handlers
are given (10°-15° and 15°-20°), using only gambier in the green sets.
They are well sumached after tanning to bleach and to mordant. If for
colours, only sumach and oak bark are employed. The skins are first
paddled for 3-4 days in sumach liquors, in which they are coloured
through. The liquors may be warmed; this quickens the tannage and also
leaches the sumach. The skins are then split, and the grains pass
through a handler set with liquors made from oak bark (8°-24°). The
skins are in this set for 3 weeks, in the first half of which they are
very frequently handled. They are finished off by paddling for 1 or 2
days in a fresh liquor containing much sumach, which mordants the skins
and bleaches the bark tannage. The flesh splits are given a drum tannage
in chestnut and quebracho extracts. If small skins are being tanned for
bookbinding purposes, sumach only is employed, and usually the tannage
is entirely in paddles.
In finishing many types of grain may be obtained, in blacks and in
colours. The finishing of "black levant" may, however, be selected as a
typical case. The skins are soaked back, tempered, and either split or
shaved, according to their substance and the size of grain wanted. The
thin skins of course give the fine grains. Mixed tannages need scouring
and possibly sumaching. The skins are then oiled up with linseed oil,
sammed, set out and blacked. In this last operation the grain is brushed
over with a solution of logwood and ammonia, and afterwards with the
iron mordant which often contains glue. They are next hung up for a
while and then "wet grained" in four directions--belly to belly, shank
to shank, across, and neck to butt. After hanging up in a hot stove to
set the grain, they are cooled, fluffed on the flesh, and seasoned on
the grain with a solution of milk and blood. A little black dyestuff may
be added to the season. The season is well brushed in, the skins dried
somewhat, and then glazed. They are then grained four ways again as
above, dried out in the stove, and lightly oiled with warm linseed oil
on the grain.
REFERENCE.
Bennett, "Manufacture of Leather," 40, 56, 90, 112, 206, 251, 312,
346, 383.
SECTION IV.--SHEEPSKINS
The most numerous class of skins for light leathers is from the common
sheep. These skins have particular value inasmuch as they include the
wool as well as the pelt. This wool, which is actually the most valuable
part of the sheep's skin, is the raw material of our woollen industries,
and is one of the most important of animal proteids. We have, therefore,
in this section to consider this dual value of sheepskins, the proteid
of the epidermis (wool), and the proteid of the dermis (pelt); one the
raw material of the woollen industry, the other the principal raw
material of the light leather trade. The first problem is to separate
the two proteids. With other skins and hides the ordinary liming
processes were sufficient and appropriate, but in the case of sheepskins
the method is unsuitable, because the exposure of the wool to the action
of caustic lime and possibly other alkalies would seriously impair its
quality and reduce its commercial value. Hence this separation of wool
from pelt is usually quite a separate business, viz. that of the
"fellmonger," whose occupation it is to collect the sheepskins from
butchers and farmers, to separate the two important constituent
proteids, and to hand the wool in one direction to the "wool stapler,"
who sorts it according to quality, and to hand the pelt in another
direction to the light leather tanner, who tans and finishes the pelt to
fit it for light upper work, fancy goods, etc.
In the first instance, therefore, we have to consider the work of the
fellmonger, the separation of wool and pelt. In this work the wool
receives first consideration, and the raw material of the fellmonger is
usually classified accordingly into "long wools," "short wools," and
"mountain breeds." The skins vary very largely in quality of wool and in
quality of pelt, being influenced very strongly by the conditions under
which the sheep lived, and by the precise breed of animal from which the
skin has been taken. As in the case of hides (Part I., Section I.),
animals exposed to extremes of weather develop the best pelts, whereas
those sheep which have been carefully bred and reared for the sake of
their wool yield a thin and poor class of pelt. In Britain, and more
especially in England, are reared the finest and most valuable sheep.
This is evident from the prices paid for them by foreigners and colonial
breeders when seeking new blood for their flocks and fresh stock for
their lands. As much as 1000 guineas have been paid by an Argentine firm
for a single Lincoln ram.
Long wools are obtained from some of the best and most extensively bred
animals. The "Cotswolds" are the largest, and probably the original
breed of England are still found on the Cotswold Hills. They have long
wool, white fleeces, white faces, and white legs, and have no horns. The
wool is fine, but the pelts are particularly greasy, especially along
the back. A later breed originating in the Midlands was called the
"Leicester" long wool. This breed gives a great cut of wool and much
coarse mutton. It is very extensively distributed in the North of
England and has been much crossed, so that many sub-breeds are now well
known, _e.g._ the "Border Leicester"--the general utility sheep of
Scotland--and the "Yorkshire Leicester" or "Mashams," much bred in
Wensleydale. "Lincolns" are another long wool found only on the
Lincolnshire Wolds. They also have white faces and shanks and yield a
large pelt with fine grain. They give a big crop of wool. "Devons" are a
smaller breed common in Somerset, Devon and Cornwall. They yield a
fairly long wool of great strength, but not quite white. Romney Marsh
sheep ("Kents") are also long wools. They have white legs, white faces,
a tuft of wool on the head, and no horns. The pelt is large and good.
"Roscommons" are an Irish cross breed with much Leicester blood. They
yield a long wool and a spready pelt.
Short wools are typified by the "Down" sheep. These sheep are
extensively bred on the chalk lands which comprise a very large
percentage of the southern counties of England. The "South Downs" are
the best and most important, the breed being the general utility sheep
of England. They are small but well-shaped animals with grey faces, no
horns and fine close wool. The pelt is only fair, but the mutton is
excellent and provides the meat sold in our best shops. This breed has
largely stocked New Zealand. The "South Down" is a somewhat delicate
animal, and has therefore been largely crossed with Cotswolds and other
breeds. Many well-known cross-breeds are found in the eastern and
southern counties. The "Suffolks," for example, are found in the eastern
counties. They have black heads, faces and legs. "Oxfords" and
"Hampshires" are similar, but larger. "Shropshires" are another hardy
cross-breed, which yield a heavier fleece. All the cross-breeds are
larger than the South Down and yield bigger pelts.
Mountain breeds yield wool of varying quality but give the best pelts.
The "Cheviots"--much favoured by the Scotch farmers--have a wool of
medium length but with much hair in it. They have white faces and legs
and no horns, and yield excellent pelts. The "Black-faced Mountain
Sheep" have longer wool but coarse, and yield good pelts. They are kept
in the hilly parts of North England and in the Scottish Highlands.
"Lonks" yield a large and good pelt, but very coarse wool. The mutton is
good. They are a very large breed with much curved horns and black
faces. There are also some small breeds, "soft wools," "Shetlands," and
"Welsh Mountain Sheep." The wool of the last two is poor, but the Welsh
pelts are valued for their fine grain. There are large numbers of
sheepskins also imported, from South and Central America, and from
Australia, New Zealand and the Cape. The colonies, however, have often
done their own fellmongering, and we have imported pickled pelts. They
now tan the skins also, and many tanned sheepskins are now imported.
There are also many Indian skins imported after tannage with turwash
bark (cp. E.I. Goat, Section II.).
The depilation is brought about by "sweating" (or "staling") and by
"painting." The immediate object of both these types of method is to
avoid using any thing which will affect the wool. The sweating process
is the most ancient method of unhairing and is used in America for hides
as well as sheepskins. It consists of a more or less regulated
putrefaction. The loosening of hair or wool has long been accepted as
evidence that putrefaction had commenced in a hide or skin, and it is
the aim of the sweating process to stop the action at that stage, before
any damage has been done to the pelt. This aim is achieved rather
imperfectly by suspending the goods in closed chambers and regulating
the temperature and humidity by means of steam and water. Such chambers
are known as "sweat pits" or "tainting stoves". In the case of
sheepskins the "warm-sweat" system is generally used, and the operation
is carried out at 75°-80° F. A satisfactory yield of wool is obtained in
good condition, but the pelt is very liable to suffer bacterial damage
and show "weak grain." The skins are first cleaned by a few "soaks" in
clean fresh water, with intermediate help from a "burring machine" which
presents a rapidly revolving set of spiral blades to the wool, and in
the presence of a good stream of water quickly removes all dirt from the
wool. The skins then enter the tainting stove, and the operation is
commenced by a slight injection of live steam. In summer, about a week
is sufficient to loosen the hair, but in winter up to two weeks may be
necessary. Little control of the process is possible, and all that can
be done is to watch the goods carefully near the end of the operation.
In one variety of this method of unwoolling the skins are painted on the
flesh side with a creamy mixture of lime and water and piled for a day
or two until the pelt is distinctly plumped. They are then washed with
fresh water to remove the excess of lime, drained, and then enter the
tainting stove. By this method the pelts are obtained in better
condition and are less liable to damage by local excess of putrefaction.
In unwoolling the skins are placed over a beam and the true wool is
pulled out by hand. The wool is graded as it is pulled and different
qualities kept separate: ewe wool, lamb wool, hog wool, etc. The hair is
next removed from face and shanks by means of a blunt "rubbing knife,"
and the pelt then immersed in water.
In the other method of depilation, by painting, advantage is taken of
the loose texture of the sheepskin fibre and of the fact that the wool
root is nearly halfway through the skin. The flesh side of the clean
skin is painted with a creamy mixture of lime in a strong solution of
sodium sulphide (14°-24° Beaumé). Care is taken to keep the depilatant
off the wool. The skins are folded flesh to flesh and left for a few
hours or until next day before unwoolling, according to the strength of
the sulphide solution. The depilatory action is entirely chemical, being
due to the solvent action of the sulphide on the hair root. The lime is
sometimes omitted. After pulling, the skins are opened up and washed in
fresh water.
The various classes of wool are sold to the wool-stapler and so to the
woollen industry. As this is a mechanical rather than chemical industry,
its discussion is beyond the scope of this volume. However unwoolled,
the pelt still needs further treatment by the fellmonger. It needs
liming and unhairing. This is done in the ordinary way in pits of milk
of lime, through which the goods pass from old to new limes in the
course of about a week. This plumps the fibres, separates the fibrils
and kills the grease. Paddles are used also to save handling. Shearlings
are sometimes limed 9-14 days and unwoolled without sweating or
painting. After liming the skins are unhaired and fleshed, and placed in
clean strong limes until sold to the tanner.
Sheepskin pelts are sometimes preserved by pickling. This consists in
placing them first in a solution of sulphuric acid (about 3/4 per cent.)
together with some common salt. The pelts swell up and imbibe the acid
solution. They are then placed in saturated brine, which causes a very
complete repression of the swelling, the pelts being apparently
leathered. In this condition or partly dried out they may be kept for
years. The forces at work in this phenomenon are somewhat complex (see
Part V., Section I.). The skins may be depickled by paddling in a
10 per cent. salt solution to which weak alkalies such as borax,
whitening, carbonate and bicarbonate of soda, etc., have been added.
The leather manufacturer classifies sheepskins according to the size of
the pelts. The large skins are tanned for light upper leathers and
similar work. These are called "basils." Many large skins are also split
green into "skivers" which after vegetable tannage are finished for
fancy goods, bookbinding, etc. The fleshes are often oil-tanned for
chamois leather (Part IV., Section III.). Medium-sized skins such as are
obtained from the Down sheep are tanned for "roans," and finished as a
kind of morocco leather. Small skins are mostly "tawed" (Part IV.,
Section I.) for glove leathers, but some are made into roller leather by
vegetable tannage.
Basils, which represent the heaviest sheepskin work, are tanned and
finished in the following manner. The limed pelts are first bated
lightly at about 80° F. for two days, scudded and drenched. They are
sometimes puered, but more often merely delimed with organic acids. In
this last case they are first paddled in warm water to remove excess of
lime, and a mixture of organic acids is very slowly added at definite
intervals. When nearly free from caustic alkali the skins are removed
and drenched overnight. There are two types of tannage. The West of
England tannage is similar to those noted for sealskins when oak bark
and sumach are employed (Section III.). There is also the tendency to
paddle more and handle less, and to use the stronger tanning materials
such as myrabs, gambier and other extracts. After about 12 hours'
tannage in paddles they are coloured through, and are then degreased by
hydraulic pressure. The skins are piled in the press with layers of
sawdust or bran between them, and the pressure applied very slowly. Much
grease runs out, for the natural sheepskin contains up to 15 per cent.
of oil and fat. Degreasing may be postponed till tannage is complete,
and the grease can then be extracted by solvents (benzene, acetone,
etc.). Degreasing after part tannage is usually considered preferable,
and the skins may be tanned out in pit or paddle in about a week. The
Scotch tannage is with larch bark from _Pinus larix_, which contains up
to 13 per cent. of a rather mellow catechol tan. This material has also
some sugars and yields sour and plumping liquors. The basils are paddled
in weak liquors (8°-11°) for about 2 days, and when struck through are
degreased by hydraulic pressure. They are then soaked back and tanned
out in stronger liquors (11°-20°), which takes up to one week. They are
then dried out and sorted in the crust. The finishing depends of course
upon the purpose in view. If for linings they are soaked, shaved,
sumached, struck out well, nailed on boards and dried right out. They
are next stained with a solution of starch, milk and red dyestuff. After
drying they are glazed by machine and softened with a hand board. For
fancy slippers the crust skins are starched and stained directly, then
"staked" (see Part III., Section II.), fluffed, seasoned and glazed. If
intended for leggings and gaiters a flesh finish is given. The skins are
soaked, stretched, shaved and sumached. They are then rinsed, drained,
sammed and stained. A brown stain mixed with linseed jelly is usual.
This is spread evenly over the flesh and glassed in. The skins are dried
out, restained if necessary, and staked to raise a nap. Basils for
gaiters are dyed in paddle and fluffed over the emery wheel.
Skivers are split in the limed state and sometimes immediately
degreased. They are next puered at 85° F. for about 3 hours in a paddle,
and scudded. They are drenched at a low temperature (68°-70° F.), but
often 2 or 3 days. They are again scudded and then rinsed and sent to
tan. The skivers are tanned in a few days by sumach liquors working the
goods up from mellow to fresh as usual. The liquors are warmed. Care
must be taken that the goods do not tear. A great variety of finish is
possible, but the "paste grain skiver" for fancy goods and the plain
finish for hat leathers are sufficiently typical. For paste grains they
are soaked and "cleared" for dyeing by immersion in very weak sulphuric
acid, excess of which is carefully washed out with water. Paddle-dyeing
follows, and is preferred to drum dyeing as the skins are so liable to
tear. After being struck out they are "pasted," by spreading on to the
flesh a glue jelly, using first the hand, then a stiff brush and finally
a cloth. The goods are then dried out. They are then seasoned, partly
dried and printed cross-grain. They are next grained two ways lightly;
shank to shank, and across, lightly tooth-rolled and glazed. They are
regrained two ways as before, dried out, and finally softened with a
graining board. They are sometimes sized on the grain to fix the pattern
and give a gloss. For hat leathers the skins are first soaked, sumached
and struck out. If for white or cream finishes they are now
lead-bleached. This consists of pigment dyeing with lead sulphate. They
are immersed alternately in lead acetate and in sulphuric acid solutions
until precipitation is sufficient. They are then dyed to shade. If for
browns it is common to mordant with titanium and use basic dyestuffs,
paddling afterwards in sumach to fix the dye. After dyeing the goods are
struck out again, starched, and dried out on boards. They are again
starched and rolled to give the plain finish.
Roans are not split. They are degreased, puered, scudded and drenched
overnight at 95° F. They are tanned with sumach usually in pits, and
take rather longer than usual to tan. They are finished in much the same
style as goatskins for morocco leather, but as the sheepskin has little
natural grain it needs embossing or printing according to the type
required. If for "hard grains," the skins are soaked, sumached,
seasoned, dried, glazed and damped back for printing. This is done by
the "hard grain" roller, and the goods are dried out to fix the pattern.
They are damped back, sammed, and grained in four directions (cp.
Section II.), dried out and boarded to soften. If for straight grains
they are printed with a straight-grain roller, or grained neck to butt.
After tooth rolling they are boarded, dried and glazed. They are
softened down and "aired off" in a cool store.
Roller leather is a special class of sheepskin leather which is used to
cover the rollers used in cotton spinning. The essential requirements
are that a smooth plain finish should be given, and the leather must not
stretch or be greasy. For this purpose small sheepskins with a fine
small grain are chosen, such as those obtained from the Welsh mountain
sheep. The pelts are machine fleshed, short haired and often puered, but
the deliming is also brought about by organic acids also. The pelts are
drenched in pits fitted with paddles, which are used to stir up the
infusion occasionally. A thorough scudding is given. For the
smooth-grain finish it is necessary to tan in weak liquors, and to give
plenty of time so as to ensure complete penetration. An oak-bark tannage
is preferred, but a little extract is usual to assist. The goods are
coloured through in paddle, like basils, and are then degreased by
hydraulic pressure. This should be as complete as possible, and a little
heat is used to assist the escape of grease. The pressed skins,
moreover, must be quite freed from creases, and this is attained first
by paddling in warm water to remove sawdust, and then by drumming in
fairly hot water, in which they are left overnight. The skins are tanned
out in suspenders, taking about 3 weeks. The crust skins need careful
sorting, and are soaked and hand shaved. They are sumached in drum,
rinsed, struck out, sammed and set. The striking and setting should be
thorough, in order to get rid of stretch. They are next "filled" by
coating with linseed jelly or similar material, and dried out on boards
in a thoroughly stretched condition. They are then trimmed, seasoned and
rolled with a steel roller. They are then staked or perched, fluffed,
reseasoned, dried and glazed. They are carefully short-haired, glazed
again and finally ironed.
E.I. sheepskins are imported in a tanned condition. These are soaked
back and the turwar bark tannage "stripped " as far as possible by
drumming with soda for 20-30 minutes at 95° F.; after washing they are
"soured" in weak (1/2 per cent.) sulphuric acid solution, and retanned
with sumach paste for an hour, drumming at 100° F. They may then be
finished for basils, moroccos or roller leather as described above, but
are often finished as imitation glacé kid. In this case they are drum
dyed, lightly fat-liquored (see Part III., Section IV.), struck out and
dried. They are staked by machine, fluffed, seasoned and glazed. They
may be re-staked and reglazed if desired.
REFERENCES.
A. Seymour Jones, "The Sheep and its Skin."
Bennett, "Manufacture of Leather," pp. 30, 85, 107, 208, 349-354,
385.
SECTION V.--CALFSKINS
Calfskins are the raw material for many classes of leather. The term
itself is rather broad. A calfskin may be obtained from a very young
animal and weigh only a very few pounds, or it may be anything just
short of a kip. Goat, seal, and sheep skins are obtained from adult
animals, but calfskins from the young of a large animal. Thus there are
many grades of quality, according to age, and the material must be
chosen with regard to the purpose in view. Some of these purposes have
already been discussed. Heavy calf is treated much like kip as a curried
leather for upper work. Even lighter skins are given the "waxed calf"
and "satin calf" finishes, and make upper leather of excellent quality.
To produce such leathers the treatment is much the same as described in
Part I., Section VIII. Calfskins were also used for very light upper
work, in which they were not so heavily greased in finishing, but rather
dyed and finished as a light leather. In this direction, however, the
vegetable tannage has been almost completely superseded by the mineral
tannages, first by "calf kid," an alumed leather (Part IV., Section I.),
and afterwards by the now popular chrome tannage of "box calf," "willow
calf," "glacé calf," "dull calf," etc. (Part III., Section III.). In
this section, therefore, we have only to consider calfskins as used to
make a vegetable-tanned light leather, such as may be employed in
bookbinding and in the manufacture of fancy goods. For these purposes
the skins receive a mellow liming of 2-1/2 - 3 weeks. No sulphide need
be employed, as the goods are soon fit to unhair. In such a mellow
liming it is important that the bacterial activity is not too prominent,
and hence it becomes advantageous to work the liming systematically in
the form of a round of pits. To avoid over-plumping in the newest limes
some old liquor is used in making up a new pit, and its bacterial
activity is reduced by adding it to the new caustic lime whilst slaking.
Thus for a pack of 200-250 skins, 14-16 stone of lime may be slaked with
about 30 gallons of old lime, and the pit filled up with water. If it be
necessary to shorten the process and to use sulphide, this should be
added only to the tail liquors of the round, and with it should be
added, if possible, some calcium chloride to reduce the harshness of the
soda. The skins should be puered thoroughly to obtain the necessary
softness, bate-shaved if desirable, and drenched with 8 per cent. of
bran overnight.
In tanning for fancy work and for dark colours, the goods are coloured
off and evenly struck through with sumach liquors, and then tanned
further with liquors made from oak bark, myrabolans or chestnut extract.
The methods are very closely similar to those used for goatskins and
sealskins (Part II., Sections II. and III.), and need not be described
in further detail. The tannage is finished off in sumach. For
bookbinding work, however, a pure sumach tannage is given, using liquor
slightly warm (70° F.). Paddle tannages are common, but for bookbinding
the bag or bottle tannage is often preferred. The skins are sewn
together in pairs, grain outwards, and nearly filled with warm sumach
infusion, just as described for goatskins. They are then handled in old
sumach liquors for about 3 days, and piled to drain and press. At this
stage the bag is cut open, the goods worked on the flesh, and the
tannage is completed with separated skins in newer sumach liquors,
handling at least once a day for 4-5 days, as necessary.
In finishing there is the usual variety, but a plain ungrained finish is
most typical, as the smooth and fine grain of the young animal lends
itself to this type of finish better than the skins of goat and seal,
and gives a better quality leather than those from the sheep. The crust
skins are wet back with water at about 110° F., and, if necessary,
sammed and shaved. Sumaching follows, the operation being carried out in
a drum for 1-2 hours. The skins are then well struck out. Striking and
setting should always be thorough for a plain finish, and this case
forms no exception. Dyeing follows next, the paddle being often
preferred to the drum, which is liable to work up a grain. The dyed
skins are placed in cold water for a while and again well struck out.
They are often nailed on boards to samm, and are then set out, lightly
oiled with linseed oil and dried out in a cool shed. Seasoning follows,
with milk and water only. The operation may be done with either brush or
sponge, after which the goods are piled grain to grain and flesh to
flesh to regulate. They may be next perched to soften and fluffed if
desired. After top seasoning with milk, water and albumin the skins are
hung up for a while, piled to regulate and brushed, first lightly and
then more vigorously. They may be then oiled very lightly and dried out
in a cool stove to ensure a soft leather.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 55, 84, 105, 201, 207, 303.
SECTION VI.--JAPANNED AND ENAMELLED LEATHERS
The leathers which receive a japanned or enamelled finish are usually
vegetable tannages, and so may be discussed at this stage. They are
popularly known as "patent" leather, but for no obvious reason. The
chief object is to obtain a leather with an exceedingly bright and
permanent gloss or polish, and this is attained by coating the leather
several times with suitable varnishes. The great difficulties are to
prevent the varnish cracking when the leather is bent or in use, and to
prevent it peeling off from the leather. Almost all classes of vegetable
tannage are japanned and enamelled. Hides are split and enamelled for
carriage, motor car and upholstery leathers, and enamelled calf, seal
and sheep skins are used for boot uppers, toe caps, dress shoes,
slippers, ladies' and children's belts, hat leathers, and so on. Broadly
speaking, a japanned leather is a smooth finish and is usually black,
whilst an enamelled leather is a grain finish with a grain pattern
worked up, and more often in colours. Hence japanned leathers are often
made from flesh splits or leathers with a damaged grain. It is in any
case advantageous to buff the grain lightly, for this permits the
varnishes to sink rather deeper and get a firmer grip, and avoids the
too sudden transition from phase to phase which is one cause of
stripping or peeling. Many flesh splits, however, are printed or
embossed to give an artificial grain and are then enamelled, which tends
to fix the embossed pattern.
Almost any method of preparing dressing hides for upper or bag work will
yield a suitable leather for enamelling and japanning (see Part I.,
Section VIII.; and Section IX.). If anything the liming should be
somewhat longer and mellower in order to eliminate grease, as the
natural grease of the hide causes the stripping of some varnishes. In
finishing it is important to obtain even substance, or the varnish is
liable to crack. Hides are soaked and sammed in, and often split.
Sometimes they are split twice, giving grain, middle and flesh, the two
former being enamelled and the last japanned. Other goods are shaved
very smooth. The goods should be next thoroughly scoured and stoned to
get as much "stretch" as possible removed. They are often sumached,
washed in warm water, slicked out again and sammed. They are then
lightly buffed on the grain, and after oiling lightly are thoroughly set
out and dried. Embossing or printing for enamels is done before the
goods are quite dry. Considerable difference of opinion obtains as to
the best oil to use in the above oiling. Linseed oil is widely preferred
as being most likely to agree with varnishes made from linseed oil. Some
manufacturers of japans do not dislike the use of mineral oil, but
strongly object to cod oil, tallow or other stuffing greases as tending
to cause the varnish to strip or peel. Other manufacturers, on the other
hand, will not have leather with mineral oil in it, and indicate that
nothing but cod oil should be used. In all probability these various
preferences are determined by the nature of the varnish, which differs
widely in various parts of the globe.
In this country the varnishes are made largely from linseed oil by
boiling it with "driers." This oil contains much triglyceride of an
unsaturated relative of stearic acid. The double bonds are very
susceptible to oxidation with the production of resinous bodies of
unknown constitution. This phenomenon is known as "drying the oil," and
has been extensively used in the manufacture of linoleums. The driers
are either oxidizing agents or oxygen carriers, such as litharge,
Prussian blue, raw umber, manganese dioxide, manganese borate, and
"resinate." Prussian blue is most preferred for British japans, as it
always materially assists the attainment of the desired black colour.
The exact details of the boiling, and the manufacture of the varnishes
is still largely the trade secret of the master japanners, and differs
indeed for the various stages of japanning. The varnish for the earlier
coats is boiled longer, and the drying carried further, than in the case
of the later coats. This is partly to obtain a product of such stiffness
that it will not penetrate the leather. The driers and the pigments
should be finely powdered and thoroughly mixed in. The boiling takes
several days when at a low temperature, but if done in 24 hours the
temperature may be up to 570° F. In the later coats driers are often not
used, and the product is often mixed with copal varnish, pyroxylin
varnish, etc., which greatly help in obtaining smoothness and gloss.
Turpentine, petroleum spirit and other solvents are also used to thin
the varnishes. Before boiling, the oil is often purified by a
preliminary heating with nitric acid, rose spirit and other oxidizing
agents, which precipitate impurities and thereby assist in obtaining a
bright gloss.
Before the application of the varnishes, the leather is first dried
thoroughly in a stretched condition. This is accomplished by nailing
down on boards which fit like movable shelves into a "stove," a closed
chamber heated by steam pipes. The temperature of the stove varies
widely in different factories, from 140°-200° F., according to the
nature of the varnishes. The first coat of warm and rather stiff japan
is laid over the hot leather in a warm room, being spread over first by
hand, then by a serrated slicker, and then again smoothed by hand. The
goods are then put into the stove for several hours to dry. When dry the
surface is pumiced and brushed and a second coat applied in a similar
manner, but with increased care. This is repeated with finer japans
until the desired result is obtained. Brushes are used to apply the
later coats. Up to seven coats may be applied for the production of a
smooth japan--three coats of ground japan, two coats of thinner japan,
and two coats of finishing varnish.
After the stoving is complete, the product is given a few days under
ordinary atmospheric conditions to permit the reabsorption of moisture
to the usual extent. Enamelled leathers are then grained to develop the
pattern.
REFERENCE.
Bennett, "Manufacture of Leather," p. 380.
PART III.--CHROME LEATHER
SECTION I.--THE NATURE OF CHROME LEATHERS
In these days the manufacture of chrome leather has attained a position
hardly less in importance than that occupied by the ancient method of
tanning by means of the vegetable tanning materials, and large
quantities of hides and skins are now "chrome tanned" after preparatory
processes analogous to those described in connection with vegetable
tannages (Part II., Section II.; and Part II., Section I.).
Chrome leathers are made by tanning pelts with the salts of chromium,
and are typical of what are known as "mineral tannages," in which
inorganic salts are the tanning agents. Tannage with alum and salt (see
Part IV., Section I.) is one of the earliest mineral tannages, but is
now of relatively minor importance. Chrome tanning was first
investigated by Knapp (1858), who experimented with chromic chloride
made "basic" by adding alkali, but his conclusions were unfavourable to
the process. A patent was taken out later by Cavallin in which skins
were to be tanned by treating with potassium dichromate and then with
ferrous sulphate which reduced the former to chromic salts, being itself
converted into ferric salt. The product, which was a combination of
iron-chrome tannage, did not yield a satisfactory commercial leather.
Another patent, taken out in 1879 by Heinzerling, specified the use of
potassium dichromate and alum. This in effect was a combination
chrome-alumina tannage. The alum had its own tanning action and the
dichromate was reduced to chromic salts by the organic matter of the
skin itself and by the greases employed in dressing. The process,
however, was not a commercial success. In 1881 patents were obtained by
Eitner, an Austrian, whose process was a combination chrome and fat
tannage. The chrome was employed as "basic chromium sulphate" made by
adding common soda to a solution of chrome alum until a salt
corresponding to the formula Cr(OH)SO{4} was obtained. Such a solution
is now known to be perfectly satisfactory, but at first it proved
difficult to devise satisfactory finishing processes, and to supplement
the chrome tannage with the fat tannage.
The first undoubted commercial success in chrome tanning was obtained by
the process of Augustas Schultz, whose patent was the now widely known
"two-bath process," in which the skins are treated successively with a
chromic acid solution and with an acidified solution of "hypo" (sodium
thiosulphate). The first bath was made up commercially of potassium
dichromate and hydrochloric acid, so that, strictly speaking, it
contained potassium chloride also. The second bath contained, in effect,
sulphurous acid, which reduced the chromic acid in the skin fibres to
the tanning chrome salts. Free sulphur is also formed in this bath and
in the skin, and contributes to the characteristic product obtained by
this process of tanning. Many minor deviations from the original process
of Schultz have been introduced, but the main features have been
unchanged, and this method of tanning is widely employed at the present
time for both light and heavy chrome leather. In 1893 tanning by basic
chromic salts was revived and the use of the basic chloride was patented
by Martin Dennis, who offered such a tanning solution for sale. The
validity of the patent has always been doubtful on account of the
previous work of Knapp and others, but the process itself was
commercially satisfactory, and the many variants of this and of the
basic sulphate tannages are now generally known as the "one-bath
process" in contradistinction to the variants of the Schultz process,
and are widely used for all classes of chrome leather. A one-bath
process which deserves special mention was published in 1897 by Prof. H.
R. Procter. In this the tanning liquor was made by reducing potassium
dichromate in the presence of a limited amount of hydrochloric or
sulphuric acid by adding glucose. Although a basic chrome salt is the
chief tanning agent thus produced, there is little doubt that the
organic oxidation products play an essential part in producing the
fullness and mellowness of the leather thus tanned, but their nature and
mode of action has not yet been fully made clear though lyotrope
influence is probable.
More recently Balderston has suggested the suitability of sulphurous
acid as reducing agent. A stream of sulphur dioxide gas is passed
through a solution of sodium dichromate until reduction is complete. The
resulting chrome liquor has been favourably reported upon by some chrome
tanners. Bisulphite of soda has also often been used as the reducing
agent. Other organic substances are also often used, instead of glucose,
to reduce the dichromate.
=Theory of Chrome Tannage.=--As to the theory of chrome tanning there is
still considerable difference of opinion and much room for experiment.
Some leather chemists regard the tannage as differing essentially from
the vegetable tannages. Mr. J. A. Wilson has even suggested that the
proteid molecule is in time partly hydrolyzed with the formation of a
chromic salt with the acid groups. The author, however, strongly favours
the view that in chrome tanning changes take place which are closely
analogous to those which occur in vegetable tannage, the differences
being mainly of degree. Thus the hide gel is immersed into a lyophile
sol--the chrome liquor--and there follows lyotrope influence,
adsorption, gelation of the tanning sol, as well as diffusion into the
gel, and finally also, probably, precipitation of the tanning sol at
this interface.
In chrome tannage the lyotrope influence is much more prominent than in
vegetable tannage, but the effect is in the same sense, viz., to reduce
the imbibition of the hide gel. Thus the potassium sulphate in a chrome
alum liquor has its own specific action of this kind and contributes to
the leather formation. Unhydrolyzed chromium sulphate and the sodium
sulphate formed in "making basic" act also in the same sense.
The tanning sol is probably chromium hydrate, formed by the hydrolysis
of chromium sulphate: it is a lyophile or emulsoid sol and is in
consequence very strongly adsorbed by the hide gel. This adsorption,
involving a concentration of lyophile sol, is the first stage in
gelation, which occupies a relatively more prominent place in chrome
than in vegetable tannage. Some diffusion into the gel also occurs, and
both the gelation and diffusion of the sol are affected by lyotrope
influence, but to a greater extent than in the vegetable tannage. Thus
far the analogy is almost complete.
There remains the question of the precipitation of the tanning colloid
at the interface. This is a point which has not yet been thoroughly
investigated, and which offers considerable difficulty to a clear
understanding, but the matter may be probably summarized thus: the
adsorbed chromium hydrate is precipitated at the interface of gel and
sol to some extent, chiefly through the neutralization of its charge by
the oppositely charged ions of the electrolytes present, but possibly
also--in the last stages of manufacture by the mutual precipitation of
oppositely charged gel and sol.
To illustrate the matter, the case of a basic chrome alum liquor will be
considered. The chromium hydrate sol is primarily a positive sol, just
like ferric and aluminium hydrate sols: _i.e._ in water they are
somewhat exceptional in that they adsorb H+ rather than OH-. To cause
precipitation therefore it is necessary to make the sol less positive
and more negative. The positive charge of the sol, however, is greater
than in water, because of the free acid formed in the hydrolysis, which
results in the adsorption of more hydrions by the sol. Hence to ensure
precipitation steps must be taken to reduce the adsorption of hydrions
by the chromium hydrate sol. In practice such steps are taken, and to
such an extent that there can be little doubt that the chrome sol is not
far from its isoelectric point. Amongst these "steps" are (1) making the
liquor "basic," _i.e._ adding alkali to neutralize much of the free
acid, which involves a considerable reduction in the stabilizing effect
of the hydrions; (2) the adsorption of hydrions by the hide gel when
first immersed in approximately neutral condition; (3) the operation of
the "valency rule" that the predominant ionic effect in discharging is
due to the multivalent anions. In this case the divalent SO{4}-- ions
assist materially in discharging the positive charge on the chrome sol;
(4) the final process of neutralization in which still more alkali is
added. The operation of the valency rule is the most complex of these
factors, for there is also to be considered the stabilizing effect of
the kations, especially of the trivalent kation Cr+++ from the
unhydrolyzed chromium sulphate. It is quite possible also that in the
last stages of chrome tanning there are "zones of non-precipitation" due
to the total effect of multivalent ions, and it is quite conceivable
that the chrome sol may change its sign, _i.e._ become a negative sol
and thus give also a mutual precipitation with the hide-gel. This is
particularly probable where a local excess of alkali occurs in
neutralization. However that may be, it is probable that most of the
tannage is accomplished by chromium hydrate in acid solution, and it is
therefore legitimate to conclude that adsorption and gelation have a
relatively greater part in chrome tannage. The operation of the valency
rule makes it easy to understand why basic chlorides do not tan so well
as sulphates; the precipitating anion is only monovalent (Cl-) and
chromic chloride contains no substance analogous to the potassium
sulphate of chrome alum and hence contains a less concentration of the
precipitating anion. Hence also the stabilizing influence of common salt
added to a basic alum liquor, the effect being to replace partially the
divalent SO{4}-- by the monovalent Cl-. Lyotrope influence, however,
may be here at work.
It is possible to make out a rather weak case that the tanning sol is
not chromium hydrate at all, but a basic salt of chrome also in
colloidal solution, and to contend that this salt, like most substances,
forms a negative sol, but in practice not negative enough, hence the
desirability of alkali, divalent anions, etc. From this point of view
the analogy with vegetable tannage becomes more complete and the
stabilizing effect of the soda salts of organic acids becomes easy to
understand.
It is highly probable that the electrical properties of the chrome sol
need closer investigation on account of the complexity due to the
prominent effect of multivalent ions. It is desirable to bear in mind
the remarkable phenomenon observed by Burton (_Phil. Mag._, 1905, vi,
=12=, 472), who added various concentrations of aluminium sulphate to a
silver sol (negative). He observed (1) a zone of non-precipitation due
to protection; (2) a zone of precipitation due to the trivalent kation;
(3) a second zone of non-precipitation due to protection after the sol
has passed through the isoelectric point and become a positive sol; (4)
a second zone of precipitation due to the precipitating effect of the
anion on the now positive sol. It seems to the writer that similar
phenomena may possibly occur in chrome tanning, for whatever the sol
actually is, it is not far from the isoelectric point.
A few observations on the vegetable-chrome combination tannages will not
be out of place at this stage. Wilson refers to the well-known practical
fact that chrome leather can take up about as much vegetable tan as if
it were unchromed pelt, and considers this evidence that the two
tannages are of fundamentally different nature. "In mineral-tanned
leathers the metal is combined with carboxyl groups, while in
vegetable-tanned leather the tannin is combined with the amino groups.
This strongly suggests the possibility that the two methods of tanning
are to some extent independent of one another, and that a piece of
leather tanned by one method may remain as capable of being tanned by
the other method as though it were still raw pelt" (_Collegium_
(London), 1917, 110-111). To the writer, however, it seems that the
facts are evidence for the contrary proposition, that the tannages are
fundamentally of the same nature. On the adsorption theory, one would
expect chrome leather to adsorb as much tan as pelt; the readily
adsorbable tan is the same, and the chrome leather is an adsorbent of
very much the same order of specific surface as pelt. The adsorption
theory would find it difficult to account for chrome leather not
adsorbing as much tan as pelt. It is quite conceivable that a chrome
leather could adsorb more tan than pelt, owing to the more complete
isolation of the fibrils by the chrome tannage and to their being coated
over by a more adsorbent gel. Adsorption is often deliberately increased
by a preparatory adsorption. Thus sumach-tanned goatskins are wet back
from the crust and "retanned" in sumach before dyeing, to coat the
fibres with a fresh and more adsorbent gel and so ensure the even and
thorough adsorption of the dyestuff. Mordanting fabrics has a similar
object,--the adsorption of colloidogenic substances which give rise to
an adsorbent gel on the fibre. Unless vegetable-tanned leather is so
much loaded with tan that its specific surface is effectively reduced,
one would similarly expect that vegetable-tanned leather would adsorb
the chrome sol. This, of course, is exactly the case of semi-chrome
leather. If, on the "chemical combination" theory, the vegetable tan
combines with the amino groups and the chrome with carboxyl groups, it
is natural to inquire which groups the dyestuffs combine with. As either
tannage does not interfere with the adsorption of dye, are we to
conclude similarly that tanning and dyeing are fundamentally different
processes?
Those who favour this chemical combination theory, and who offer
equations for the formation of vegetable and of chrome leather, should
likewise suggest an equation for the formation of leather from pelt by
the action of dyestuffs--a practical though hardly an economic process.
The remarks made earlier in this volume (Part I., Section III.) as to
the occurrence of what have been called "irreversible changes"
subsequent to the mutual precipitation of oppositely charged gel and
sol, are equally applicable to the chrome tannages. Chrome tannage was
once thought to embrace such irreversible changes, but the process can
now be "reversed" with ease. The reversibility of the chrome tannage is
an easier proposition than that of vegetable tannage, partly because the
leather is comparatively much less tanned, and partly because the
acidity or alkalinity of the stripping agent may be adjusted, as
desired, without the oxidation trouble. In approaching this question
from the theoretical side one must consider mainly whether to solate the
tanning agent to a positive or to a negative sol. Our imperfect
knowledge of the electrical forces in operation in the chrome tannage is
thus a serious drawback, but the evidence on the whole points to the
precipitation being effected by a negative sol near its isoelectric
point but in faintly acid solution. Hence, we should theoretically
expect that reversion should take place into a negative sol in nearly
neutral or even faintly alkaline solution. Thus, suitable stripping
agents for chrome leather would be the alkali salts of organic acids
(especially if multivalent). Now, Procter and Wilson have recently
accomplished this stripping of chrome leather by the use of such salts.
They approached the question from an empirical and practical point of
view and found that Rochelle salt, sodium citrate, and sodium lactate
would strip the chrome tannage with ease. This important and very
creditable achievement will have great practical and commercial
importance. Procter and Wilson have deliberately and carefully refrained
from offering an exact explanation of this reversible action, but point
out that all their stripping agents are salts of _hydroxy-acids_, and
strongly insist that these form soluble complexes with the chrome.
Whilst not denying this in the least, the present author would point out
that according to the views advanced in this book, the salts of organic
acids which do _not_ contain hydroxyl groups should, when combined with
a monacid base, also strip the chrome tannage. This he has found to be
the case. Thus the chrome tannage is reversible in solutions of ammonium
or potassium oxalate and of ammonium acetate. With these salts the full
effect of multivalent anions is not attained, so that somewhat strong
solutions are necessary. A 10 per cent. solution of ammonium acetate
shows some stripping effect after a few days, but a 40 per cent.
solution after a few hours. Saturated ammonium oxalate is only a 4.2 per
cent. solution, but shows a stripping effect in 2-3 days. Potassium
oxalate (33 per cent.) shows distinct stripping in 24 hours. Potassium
acetate and sodium acetate show only slight action, because the solution
is too alkaline, but strip if acetic acid be added until litmus is just
reddened. It is noteworthy from a theoretical point of view that a 40
per cent. solution of ammonium acetate is distinctly acid, and indeed
smells of acetic acid. There can be little doubt that such stripping
actions are also connected with the solubility of the stripping agent in
the gel, for the liquid must pass through the walls of the gel to dilute
the liquid in the interior. This view fits in with the facts that
hydroxy acids and ammonium salts are particularly efficient, for the
tendency of chrome to form ammonia-complexes as well as hydroxy
complexes is well known. From this point of view we should not expect a
stripping action from a salt such as disodium phosphate, which would
form an insoluble substance. Actually sodium phosphate does not strip,
and indeed reduces the stripping power of ammonium acetate. Similarly,
we might expect some stripping action by ammonia and ammonium chloride,
with the formation of chrome ammonia complexes. This actually occurs, a
pink solution being obtained. Sodium sulphite does not strip, possibly
partly on account of its too great alkalinity, but is interesting
theoretically to observe that sodium sulphite as well as Rochelle salt
will strip salt stains (see Yocum's patent, _Collegium_ (London), 1917,
6; also Procter and Wilson, _loc. cit._). This points to the formation
of a negative sol, and suggests many other substances for removing salt
stains.
=Special Qualities of Chrome Leather.=--A few words on the special
peculiarities of the leather formed by chroming will not be out of place
at this stage. One of the greatest disadvantages of the chrome tannage
has been the absence of what is known as the "crust" or "rough leather"
stage. In chrome tanning, the finishing operations have had to follow on
immediately after the tannage. Chrome leather, after tanning, may be
dried out like other leathers, but if thoroughly dried, or if kept in a
dried condition for any time, it will not "wet back" again with water.
Various suggestions have been made to overcome this difficulty but none
yet have found much favour in practice. The discovery of the
reversibility of the tannage, however, ought to solve this difficulty,
and the author would suggest that any of the substances used for
"dechroming" might also be suitable for "wetting in" chrome leather
which has been well dried out. A piece of chrome leather, dried out well
after neutralizing, and kept in a warm place for four years, wetted back
easily in ammonium acetate, in the author's laboratory.
Another peculiarity of the chrome tannage is that any defects in the raw
material always seem more obvious in chrome than in vegetable leather.
This often necessitates the use of a better quality hide or skin. Weak
grain or loose grain becomes very obvious. The presence of short hair
which both unhairing and scudding have failed to remove also is usually
more evident.
A more serious disadvantage of chrome leather is its tendency to
stretch. In the case of belting leather this feature is an obvious
nuisance, and has inevitably led manufacturers to use powerful
stretching machines upon the goods before they are marketed. In chrome
sole leather also there is a tendency to spread and throw the boot out
of shape.
Further disadvantages arise from the fact that the chrome tannage is an
"empty" tannage. Compared with the vegetable tannage, very little of the
tanning agent is adsorbed. Hence there is little matter of any kind
between the hide fibres isolated during tannage. The inevitable effect
of this is that the leather has not the same solidity and firmness, and
needs filling out with other materials. A commercial consequence is also
that it is impossible to obtain the same yield of leather from any given
quantity of raw material. In trade parlance chrome tannage does not give
good "weight." Another consequence is that (even when well filled with
greases in finishing) chrome leather tends to be "woolly" on the flesh
side or at cut edges.
On the other hand, chrome tanning has very many advantages over the
older process. The most obvious of these is the great saving in time.
Many chrome tannages involve only a day or two, and none more than a
week or two. A chrome leather factory therefore needs less capital on
account of the quicker turnover. If, moreover, the market be
unfavourable, a chrome tanner can stop or reduce his output in a very
short time, whereas a vegetable tanner is committed to many weeks'
supply of the goods he is manufacturing. Another notable advantage of
chrome leather is its durability. In the finishing processes more grease
is usually employed than in vegetable tannage, and this has a
preservative effect upon leathers which often get wet. Chrome sole
leather and hydraulic leathers are cases in point. Chrome leather will
also stand changes of temperature and friction much better than
vegetable tannages. The light chrome tannage results, further, in
yielding a leather which has great tensile strength, and it is not
surprising to find that chrome harness and chrome picking bands are
highly thought of. The empty nature of the tannage necessitates the use
of stuffing greases, but such large proportions of these may be used
that chrome tannage becomes obviously suitable if one wishes to produce
a waterproof leather. Hence the popularity of chrome tannage for
waterproof soling and hydraulic leathers.
The advantages of the chrome process are very real, and very obviously
such as will appeal to manufacturers. Chrome leathers have now been for
some time in the forefront as far as boot-uppers are concerned,
especially for the best quality goods, in which the popular "box-calf"
and "glacé kid" are so largely employed. There seems little doubt that
this will continue to be the case. It is an unfortunate fact that in
this important branch of tanning, British manufacturers have not quite
risen to the occasion. Their products have in the past been faced with
very serious competition from Continental and American manufacturers of
chrome uppers, and there can be no doubt that these competitors produced
a better article, and produced it more economically. The disorganization
of the Continental factories owing to the war should give British
manufacturers a valuable opportunity of putting such businesses on a
better basis. For sole leather also the chrome tannage makes constant
headway, and the relative proportion of it becomes gradually greater. A
great impetus to chrome sole leather has been given by the war
conditions of Britain. Owing to the submarine campaigns of Germany the
tonnage question became all-important, and the bulky imports of
vegetable tanning materials became a serious item. British tanners were
therefore encouraged to make more chrome sole and less vegetable sole.
The urgent need of leather for our armies also assisted in the same
sense. The production of chrome sole progressed therefore enormously
during 1917 and 1918, and although some reaction will doubtless occur,
there seems little doubt that chrome sole leather has taken a definite
and permanent leap forward. Once the general public fully appreciate its
qualities of waterproofness and durability its future will be assured.
On the whole the position and prospects of chrome tanning are good. The
chrome tannages are making headway in all directions, and undoubtedly
threaten the existence of many of the older processes of vegetable
tanning.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 198-220.
Bennett, "Manufacture of Leather," pp. 210, 355.
Bennett, _J.S.L.T.C._, 1917, 176.
Stiasny, _Collegium_, 1908, 117.
SECTION II.--GENERAL METHODS OF CHROME LEATHER MANUFACTURE
It has been previously pointed out that the chrome tannage is an "empty"
one; the primary principle in the wet work of goods for chrome leather
is to avoid anything which will make this feature more obvious. In the
vegetable tannages relatively larger amounts of the tanning agents are
used, and these fill the interfibrillar spaces; indeed, as we have seen
(Part I., Sections III., V. and VI.), effort is made to increase these
spaces and to fill them to their maximum capacity, thus yielding a
leather of which 50 per cent. is the tanning agent. In chrome tanning,
however, the tanning agent may only be approximately 5 per cent. of the
finished leather, so that any trouble taken to split the hide fibres or
to dissolve hide substance is usually not only superfluous, but also
calculated to enhance the "emptiness." The governing principle of all
the preparatory processes for chrome tannage is therefore the
conservation of hide substance, and this principle determines the
modifications of the processes of soaking, liming, and deliming, which
are in vogue. Now, in most of these processes there is usually some loss
of hide substance, and it is the particular problem of chrome tanning to
reduce this loss to a minimum in each stage. Whether the loss of hide
substance be due to alkaline or fermentive hydrolysis, or to solation of
the hide gel, the effect is increased by swelling, and in the wet-work
for chrome, therefore, any variations in the degree of swelling are
objectionable. The preparatory processes should be carried out with as
little change as possible in the volume and elasticity of the pelt.
Whether also the loss of hide be due to hydrolysis or solation, it is
increased by time, hence short processes are (other things being equal)
much to be preferred. Fermentive hydrolysis is minimized by cleanliness,
alkaline hydrolysis by avoiding strongly alkaline liquors, and solation
of collagen is reduced by both, and also by avoiding, as far as
possible, the presence of calcium and ammonium salts.
Soaking should be quick and clean. The use of the paddle or drum gives
the greatest efficiency and also assists in procuring the softness so
essential to the bulk of chrome leathers.
Liming chrome leather satisfactorily is almost an impossible ideal.
Every conceivable arrangement has some objection to it. The time of the
process may be shortened either by the use of sulphide or by the use of
mellow or old limes. To shorten time by the use of sodium sulphide
unfortunately involves the employment of more alkali than is desirable,
with a consequent plumping effect and tendency to harshness. If
sufficient sulphide be used to make the liming very short, then the
grease is not "killed" (saponified or emulsified). If the harshness and
alkalinity be removed by using also an excess of calcium chloride, then
the lyotrope influence of this substance enhances the solation of the
hide gel. On the other hand the use of old lime liquors avoids the
plumping effect, but increases considerably the bacterial activity, and
the bacterial enzymes produce both hydrolysis and solation of the pelt.
In practice what is generally done is to shorten time by both methods
and so to admit both disadvantages to a limited extent. This is
theoretically more sound than would appear, for in mellow limes sulphide
has less plumping power but is just as strong a depilatant; whilst, on
the other hand, a mellow liming shortened by sulphide is less
objectionable as there is some evidence that bacterial activity is
relatively less in the first few days. Hence a mellow sulphide liming of
7-10 days is very common in practice, but sometimes a 3-4 days' process
with more sulphide is also found satisfactory.
It would seem probable that the real solution of the problem would be
found by a different process altogether. In this connection it is
interesting to note that a Continental proposal to unhair by enzyme
action only has been found most practicable with goods for chrome, and,
in the author's opinion, some development on these lines, in which a
lipolytic enzyme is used in addition to a proteolytic, might solve the
difficulty, and give a rapid depilation which dispenses with liming,
plumping and deliming with the consequent loss of valuable hide
substance.
In the usual short, mellow, sulphide liming it is clear that there is
not much advantage in a "round" or "set" of pits. Hence the one-pit
system is popular on account of the less labour involved.
The above remarks are less applicable in the case of chrome sole
leather. In this case weight is a great consideration and plumping is
necessary. In such leather the chrome tannage is supplemented by the use
of waxes, which fill up the spaces between the fibres and give solidity
and waterproofness to the finished article. With this leather an
ordinary sole leather liming in sharp liquors is not unsuitable,
handling the goods from "mellow to fresh," but there is, on the whole, a
tendency to shorten the process to about a week by using more sulphide.
Processes for deliming pelt for chrome leather should also be chosen by
our guiding principle of hide substance conservation. Here again short
processes involving little change in swelling should be preferred. Now,
the ordinary bating and puering processes give (1) neutralization of
lime by organic acids combined with weak bases; (2) the solation of some
hide substance; and (3) a "pulling down" effect on the swollen pelt.
Now, neutralization is quite superfluous, as the acids of the chrome
liquor (one-bath or two-bath) can quite well accomplish this; the
solvent effect is undesirable altogether; and the "pulling down" effect
is also unnecessary if the goods are not plumped up. With any method of
liming, however, some plumping is obtained, and this creates a problem
of practical importance. In the huge quantities of pelt which go for
chrome upper leathers, a primary consideration is the soft, "kind," or
mellow feel of the grain in the finished leather. This is obtained only
by tanning the pelt when the grain at least is in a thoroughly deplumped
and inelastic condition. It is essential to delime not only so that the
alkaline plumping effect is completely removed, but also so that no acid
plumping effect succeeds it. The practical problem is to decide whether,
in any particular instance, dung puers and bates are necessary to obtain
this result. Bating is clearly not very desirable, on account of the
length of the process, during which hide substance would be lost
unnecessarily, and also because there will usually be a slight alkaline
swelling. Puering with dog-dung infusions is preferable; it is not such
a long process, the liquor is just acid to phenolphthalein, and the
action is more intense, and by puering for a short time only the loss of
hide may be confined to the grain and flesh only, whilst the desired
inelasticity of grain-pelt is soon obtained. Many large firms have
admittedly found themselves unable to dispense with puering, but others
have succeeded in substituting for it the use of non-swelling deliming
and lyotrope agents like ammonium chloride and boric acid. In all cases
it is futile to delime or puer the grain and then allow the goods to
stand until the centre lime has diffused outwards. The goods must pass
into the chrome liquors when in the correct condition. For heavy chrome
leather a surface deliming with boric acid is all that is necessary.
Even that is superfluous when the goods are to be pickled before
tanning.
=Types of Two-bath Chrome Tannage.=--Although the original process of the
Schultz patent is quite a practicable one, many modifications have been
introduced. These modifications have been made to suit the class of
goods under treatment, to suit the particular mode of application which
is available or suitable, and to effect economies of chrome and other
material, and of time, and also to combine with the tannage a pickling
effect or a partial alum tannage. Other modifications arise from the
precise acid, neutral, or alkaline condition of the pelt, being for
example obviously necessary when pickled stock replace neutral pelts.
The many two-bath processes which have been found useful have been
classified previously by the author[6] into three types: (1) The
"Schultz type," in which such quantities of dichromate and acid are used
that there is no excess of free acid (other than chromic), but an excess
of unaltered dichromate; (2) the "Acid type," in which the chromic acid
is completely free and the liquor contains also some excess of mineral
acid also; and (3) the "Neutral type," in which neither of these main
constituents is in excess, just sufficient mineral acid having been used
to liberate all the chromic acid from the dichromate.
[Footnote 6: "Types of Two-bath Chrome Tannage," _Leather_, 1909,
227-259.]
Now:--
K{2}Cr{2}O{7} + 2HCl = 2KCl + 2CrO{3} + H{2}O
204 73
Taking the commercial hydrochloric acid as a 30 per cent. solution, 73
parts will be obtained in about 250 parts of commercial acid. Hence 294
parts dichromate need 250 parts commercial hydrochloric acid for the
above reaction;[7] in other words, 5 per cent. dichromate needs 4-1/4
per cent. commercial acid. Similarly 6 per cent. and 4 per cent. of
dichromate need 5.1 per cent. and 3.4 per cent. respectively of
commercial acid. If therefore such quantities be used we have the
so-called "Neutral type" of chroming bath. If less quantities of acid be
used we have the "Schultz type," and if greater quantities of acid be
used we have the "Acid type." The original Schultz patent used 5 per
cent. dichromate and 2-1/2 per cent. hydrochloric acid, and well
exemplifies its type, for there is much undecomposed dichromate. The
composition of some chroming baths in common use on a practical scale
are given below under the heading of their type:--
----------+--------------+--------------+-------+------------
Type. | Dichromate. | Hydrochloric | Salt. | Aluminium
| | Acid. | | Sulphate.
----------+--------------+--------------+-------+------------
| 5 | 2-1/2 | -- | --
| 5 | 2-1/2 | -- | 3
Schultz | 5 | 2-1/2 | 5 | --
| 5 | 2-1/2 | 10 | --
| 6 | 3 | -- | --
----------+--------------+--------------+-------+------------
| 4 | 4 | -- | --
| 4 | 4 | 5 | --
| 5 | 5 | 5 | 3
Acid | 5 | 5 | 10 | --
| 6 | 6 | 15 | --
| 3 | 3 | 15 | 4
| 2 | 4 | 10 | --
| 4 | 15 | 24 | --
----------+--------------+--------------+-------+------------
| 5 | 4-1/4 | 5 | --
| 5 | 4 | -- | 2-1/2
Neutral | Chromic acid | | |
| 5 | -- | 5 | --
| 6 | -- | 8 | --
| 4 | -- | 10 | --
----------+--------------+--------------+-------+------------
[Footnote 7: Commercial acids of course vary in strength, and the amount
needed varies accordingly.]
All the figures are percentages of the weight of pelt. As K{2}Cr{2}O7
has a molecular weight of 294, and Na{2}Cr{2}O{7}·2H{2}O a molecular
weight of 298, in practice they may be considered as interchangeable,
weight for weight. The sodium salt is cheaper and more often used. The
corresponding amount of chromic acid, 2CrO{3}, has an equivalent weight
of 200, hence any weight of dichromate may in practice be substituted by
two-thirds the weight of commercial chromic acid. Equivalent weights of
commercial sulphuric acid are sometimes used in place of hydrochloric.
The quantity depends upon the strength of the sulphuric acid used.
Aluminium sulphate, Al{2}(SO{4}){3}·18H{2}O (mol. wt. 666), may be
replaced by ordinary potash alum, K{2}SO{4}·Al{2}(SO{4}){3}·24H{2}O
(mol. wt. 948). In practice 7 parts of the former and 10 parts of the
latter may be considered equivalent. It should be remembered that both
these salts are hydrolyzed in solution, and therefore increase slightly
the amount of free acid present. Their presence decreases the amount of
chrome taken up, and as little or no alumina is found in the leather,
there is usually small advantage in their employment. The use of salt is
common but often unnecessary. It is considered desirable in baths of the
acid type to prevent swelling by the excess of acid, and in baths made
up from commercial chromic acid to replace correspondingly that normally
formed from the reaction of dichromate and acid. It is used also in all
baths which are intended to treat pickled goods. Like all electrolytes
its presence decreases the adsorption of chromic acid.
All these conceivable modifications will make good leather, and the
choice of a process often depends largely upon market prices. On the
whole the tendency is to prefer the neutral or acid type, on account of
the greater ease and completeness with which the bath may be exhausted.
Pickled stock may be depickled before tanning, by a bath of salt, mixed
with borax, whitening, or basic alum solutions. It may also be placed
direct in the chroming bath, but the amount of acid thus added with the
goods must be determined and allowed for when making up the bath. No
allowance is usually necessary, however, if the "pickle" consist only of
alum and salt.
The chroming operation is carried out usually in drums or paddles. Drums
are preferable because more concentrated baths may be used; these
solutions penetrate quicker and are easier to exhaust economically. They
are also preferable for hides and heavy skins. Paddles are preferable
where grain is important, and for light skins in which little time is
needed. Small variations in the ratio of chrome to pelt, or in
concentration of liquor, have little influence upon the resulting
leather.
The analytical investigation and control of chroming baths is usually
simple. A suitable volume of liquor is titrated with N/10 thiosulphate
after acidifying with hydrochloric acid and adding potassium iodide. The
operation should be conducted in a stoppered bottle, and the liquor
allowed to stand for 10-15 minutes after adding the iodide and before
titrating. A little fresh starch infusion should be added towards the
end of the reaction. Each c.c. N/10 thiosulphate corresponds to 0.0033
gram CrO{3} or 0.0049 gram K{2}Cr{2}O{7}. The same volume of liquor
should also be titrated with N/10 caustic soda and phenolphthalein.
Potassium chromate is neutral to this indicator, _i.e._ chromic acid
acts as a dibasic acid. Any excess of hydrochloric acid is also
titrated. More indicator should be added towards the end of the
titration, as it is often oxidized. Each c.c. N/10 soda corresponds to
0.005 gram CrO{3}, 0.01 gram "half-bound" CrO{3} (_i.e._ present as
dichromate), 0.0147 gram K{2}Cr{2}O{7}, or 0.00365 gram HCl. If _a_
c.c. N/10 thiosulphate and _b_ c.c. N/10 soda be needed the type of
chroming bath may be seen at a glance--
---------------------------+-------------+-----------------------
If | The type is | The bath contains
---------------------------+-------------+-----------------------
_b_ is greater than 1/3_a_ | Schultz | potassium dichromate
but is less than 2/3_a_ | | and chromic acid
_b_ is greater than 2/3_a_ | Acid | chromic acid and free
| | hydrochloric acid
_b_ equals 1/3_a_ | Neutral | chromic acid only
---------------------------+-------------+-----------------------
If 10 c.c. chrome liquor require _a_ and _b_ c.c. of thiosulphate and
soda respectively--
I. 10 c.c. of a Schultz bath contain (b - 1/3×a) × 0.01 gram CrO{3}
and [(a×0.0033) - [(b - 1/3×a) × 0.01]] × 1.47 grams K{2}Cr{2}O{7}
II. 10 c.c. of an acid bath contain (a×0.0033) grams CrO{3} and
[(b - 2/3×a) × 0.00365] grams HCl
III. 10 c.c. of a neutral bath (a×0.0033) grams \
> CrO{3}
_or_ (b×0.005) grams /
The second bath of the two-bath chrome tannage consists of a solution of
sodium thiosulphate acidified with hydrochloric acid. The reactions in
this bath are somewhat complicated, several occurring simultaneously.
Broadly speaking, the final result is due to (1) the reduction of the
chromic acid to a chromic salt by the sulphurous acid; (2) the
formation of a basic chromic salt owing to the excess of thiosulphate;
(3) the reaction of the added acid and thiosulphate to give free
sulphur, which is deposited in and on the leather. The relative
intensity of these effects is variable, according to the conditions of
operation, _e.g._ the amounts of chemicals used, their concentration,
the nature and condition of the goods, the time of application, the
manner of application, etc. In practice the most favourable conditions
are usually discovered empirically, but, broadly speaking, the goods are
usually added soon after the thiosulphate and acid are well mixed. There
is some evidence that the reduction is in steps, intermediate products
such as sodium tetrathionate and chromium dioxide are known to be
formed. The goods change from yellow to dark brown, then to green, and
finally to the familiar blue. The sulphur makes the final colour a
lighter blue than in the case of a one-bath tannage, hence the two-bath
process is often preferred for "colours."
On account of the empirical character of this "hypo bath," it is
impossible to fix any exact relation between the quantities of material
used in the chroming bath, and the quantities of "hypo" and acid used in
the reducing bath. The following rules, therefore, must be understood as
rough approximations for practical use, and though they have been
empirically discovered their theoretical significance is often fairly
obvious.
1. The amount of hypo necessary is almost directly proportional to the
amount of dichromate used. In chroming with baths of the acid or
neutral type, the percentage of hypo should be about three times the
percentage of dichromate used. Thus 4 per cent. dichromate needs 12
per cent. hypo; and 6 per cent. dichromate needs 18 per cent. hypo on
the pelt weight. In baths of the Schultz type a less proportion of
hypo may suffice, but the 10 per cent. hypo for 5 per cent.
dichromate, recommended by the Schultz patent, is generally considered
rather insufficient.
2. The proportion of hypo is increased somewhat for the heavier
classes of goods, and may even reach 20 per cent. of the pelt weight.
3. An increase in the proportion of hypo is usual with an increase in
the amount of free acid in an acid chroming bath.
4. The percentage of hydrochloric acid in the reducing bath is roughly
half that of the hypo, but is the most variable factor. The quantity
varies with the rate and mode of addition, the class of goods under
treatment, and the composition of the chroming bath.
5. In baths of the Schultz and neutral type it is better to add some
acid to the hypo bath before adding the goods, but this is less
essential for goods from an acid chroming bath.
6. In the case of goods from acid chroming baths, the amount of acid
used in the reducing bath is an inverse function of the excess of acid
in the first bath, _e.g._ take the following two processes:--
------------------------------------+-------------------------------
Chroming bath. | Hypo bath.
---------------+--------------------+----------+--------------------
Dichromate. | Hydrochloric acid. | Hypo. | Hydrochloric acid.
---------------+--------------------+----------+--------------------
4 | 4 | 12 | 5
4 | 15 | 15 | 1
---------------+--------------------+----------+--------------------
7. There should be some excess of hypo at the end of the process. This
acts as a feeble alkali, and commences the neutralization.
The process can be carried out in paddles or in drums as preferred, for
reasons similar to those applicable in the case of the first bath. On
the whole, however, drums are less popular for the second bath, for the
dilute solutions of the paddle effect some economy of sulphurous acid,
which is apt to escape into the air. A preliminary "hypo dip" is
sometimes used to prevent the "bleeding" of the chromic acid. The use of
many other reducing agents has been suggested as substitutes for hypo.
Sulphides, sulphuretted hydrogen, polysulphides, sulphites, bisulphites,
hydrogen peroxide, nitrous acid, lactic acid, etc., have been used, but
none are so easy to manipulate as thiosulphate.
=Types of One-bath Chrome Tannage.=--The one-bath process is simpler
than the two-bath process inasmuch as only one kind of liquor is
involved, viz. one in which the chromium is in the chromic state. Hence
the variants of the one-bath process consist mainly of variations in the
composition of this liquor. The chief point of variation is in the
readiness with which chromium hydrate is adsorbed. This is determined by
the extent to which the chromic salt is hydrolyzed to form the tanning
sol and free acid, and by the concentration and nature of this free acid
as well as of other substances. It is difficult unfortunately to express
these factors in terms which are comparable under general conditions.
Chromic salts are usually hydrolyzed to some extent, but this extent is
very different even in water, according to the nature of the acid
radical. The degree of hydrolysis is also largely affected by the extent
to which the solution has been "made basic" by the addition of alkalies.
By the neutralization of the free acid in this way there is further
hydrolysis, the extent of which is again influenced by the nature of the
acid radical involved and other dissolved substances, especially of
organic matters. Again, the hydrolysis is largely affected by the
concentration of the solution even when the proportions of the
ingredients are constant, and this is practically important on account
of the necessity for exhausting the chrome liquors economically. Nor is
the matter entirely one of degree of hydrolysis, for (as we have noted
in the preceding section) the electrical condition of the chroming sol
is of great importance owing to the operation of the valency rule and
the possibility of zones of non-precipitation. The alkaline, neutral or
acid condition of the goods when first introduced has also its influence
on all these points.
It will be readily understood, therefore, that there is some difficulty
in expressing the tanning power of a chrome liquor. As near as can be
yet said this is determined by (1) the concentration of the actual
tanning sol, and (2) its nearness to the isoelectric point. Now, these
points are not readily determined by analytical methods, and the best
that can yet be done is to determine the conditions which have large
influence upon these points. Thus the degree to which the liquor is
"made basic" by adding alkali is known, and can be expressed in formulæ
by assuming that the acid neutralized by this alkali is replaced in the
chrome salt by hydroxy groups. Chromic chloride, Cr{2}Cl{6}, with the
addition of soda to correspond to half the acid formed upon complete
hydrolysis, would be considered then to be a solution of the salt,
Cr{2}(OH){3}Cl{3}. This has given rise to the conception of the
"basicity" of a chrome liquor, which may be expressed in many ways, the
most common of which in practice is the number of grams SO{4} still
combined with 52 grams Cr. Thus the salt corresponding to the
composition Cr(OH)SO{4} is said to have a basicity of 96. The practical
importance of such determinations of basicity has been much exaggerated,
for they are but a rough guide to the degree of hydrolysis of the chrome
and to the extent to which the sol is positive. Thus if the chrome salt
be actually a sulphate, a liquor of basicity 96 has about the same
_practical_ value as a chloride liquor of basicity 72, and in each case
the figures are of little significance if many organic substances be
present. If, however, as is usual in practice, there be approximately
the same acid radicals throughout the tannage and about the same
relative proportion of organic matters or of inorganic salts, then these
determinations have some practical value for comparative purposes. The
determination is itself simple: a portion of liquor is titrated direct
with caustic soda. The titration is at boiling-point, and is continued
until a permanent pink is obtained with phenolphthalein. The amount of
SO{4} corresponding to the soda required is then relative to the amount
of Cr in the same volume of liquor. A chromium estimation is therefore
also necessary and is most readily done by evaporating a portion of
liquor to dryness, igniting the residue and oxidizing the chrome to
chromate by heating in a muffle furnace with magnesia and sodium
carbonate in equal parts, or fusing in a blowpipe with sodium and
potassium carbonates in equal parts. The oxidized residue is dissolved
in hydrochloric acid and titrated with thiosulphate as described for the
two-bath process.
Another attempt to determine the practical value of a chrome liquor is
the empirical test suggested by McCandlish, in which 10 c.c. of the
liquor is titrated with standard alkali until the precipitation point is
reached and a turbidity appears. The figure thus indicates approximately
the degree of nearness to the precipitation point and the amount of free
acid in the liquor. The author has found this a useful test taken in
conjunction with the basicity determination. It is best expressed in the
same units, _e.g._ grams SO{4} per 52 grams Cr.
Another method is the determination of the hydrion concentration of the
liquor. This has useful possibilities for research work, but is usually
too laborious for rapid commercial control. The results, moreover, are
not less empirical, for the hydrion concentration of the liquor
indicates but imperfectly the electrical condition of the particles of
the tanning sol.
In classifying one-bath liquors into types, it is best to take together
those in which the usual "basicity" and "acidity" determinations have at
any rate approximate comparative value, and this is determined in the
main by the method by which the liquor is manufactured. Broadly
speaking, there are three types of chrome liquor: (1) those made from
chromic salts by adding suitable amounts of alkali; (2) those made from
sodium dichromate by reduction with organic matter; and (3) those made
from sodium dichromate by reduction with sulphurous acid or its salts.
Of the first type the most common is that in which chrome alum (a
bye-product of the dyeing industry) is the starting-point. To a solution
of this a solution of washing soda is gradually added, with constant
stirring, until the salt corresponding with the formula Cr(OH)SO{4} is
obtained.
Now:--
K{2}SO{4}Cr{2}(SO{4}){3}·24H{2}O + Na{2}SO{4}
\___________________________________________/
998
+ Na{2}CO{3}·10H{2}O
\________________/
286
= 2Cr(OH)SO{4} + K{2}SO{4} + CO{2} + 33H{2}O
Hence, in practice, for every ten parts of chrome alum 2.86 parts of
soda crystals (or 1.06 parts anhydrous soda) are used. A convenient
"stock solution" is of 10 per cent. strength. Thus 10 lbs. of chrome
alum is dissolved, made basic, and made up to 10 gallons. To dissolve
the alum a mechanical stirrer is necessary, for the water must not be
more than warm. The disadvantage of this liquor is the limited
solubility of chrome alum and the need for its solution in the cold.
Liquors may be also made by dissolving chromium hydrate in hydrochloric
acid, and making basic to correspond to the formula
Cr{2}(OH){3}Cl{3}. Many preparations are on the market containing
both chlorides and sulphates with appropriate basicity. Chrome alum
liquors have been less often used in Britain of recent years owing to
the high price of chrome alum, caused in part by the presence in the
salt of potassium, all the salts of which have been scarce and dear
under war conditions.
Of the second type Procter's "glucose liquor" is a good example. Use 5
lbs. sulphuric acid, 6 lbs. sodium dichromate, and 7 lbs. of glucose, or
quantities in similar proportion. The dichromate is first dissolved, and
the acid added gradually. The glucose is then added cautiously on
account of the brisk effervescence of carbon dioxide. A glucose of good
quality is necessary, and the proportion to be used is not quite
definite, for sufficient only is needed to effect the reduction, and
this amount is influenced by the rate of addition and temperature of the
mixture. The reduction should be careful and regular, or the oxidation
products will be irregular and have a varying effect upon the tanning.
Molasses can be substituted for glucose, in amounts varying with its
strength.
Of the third type the most common is that in which the dichromate is
reduced by sulphuric acid and sodium bisulphite. Solid bisulphite may be
used, but it is usually dear, and solutions are more commonly employed.
Into this type fall also the liquors formed by passing sulphur dioxide
gas into dichromate solution. Stock liquors of this type have the
advantage that strong solutions may be made (up to 18 per cent.
Cr{2}O{3}); they have the disadvantage that they are liable to contain
excess of free sulphurous acid.
The method of application of chrome liquors is usually by paddling or
drumming the goods in solutions of appropriate strength--broadly
speaking, paddles used for lighter goods and plain finishes, and slowly
revolving drums for heavier hides and grained finishes. Heavy chrome
leather is often tanned in pits by suspension just as in vegetable
tanning. In such instances rockers may be usefully employed.
In any case, the goods are successively brought into contact with
liquors of increasing strength, as in vegetable tannage, and the liquors
are thus most conveniently exhausted economically. The green goods thus
receive first nearly spent liquor and finish out of fresh strong liquor.
The goods may be, of course, handled from drum to drum, or from pit to
pit, but the modern tendency is to save labour by moving the liquors
instead. Thus in drum tanning the liquor is run out and pumped into the
next drum. In pits air ejectors have proved suitable, not only as lift
pumps, but also as agitators of the liquor in which goods are suspended.
The press system is also used.
=Finishing Operations.=--In nearly all cases the chrome leather has
to be "neutralized" after tanning. This consists in removing the acid
"reversibly adsorbed". This removal is necessary to the finishing
processes, as well as to bring the tanning sol into condition for more
permanent tannage. Neutralization gets rid of soluble chrome salts as
well as free mineral acid, and is the final stage in rendering the
tanning sol less positive, and perhaps even negative. It is brought
about by the use of weak alkalies, of which borax is the easiest and
safest, but not the cheapest. Sodium silicate, phosphate, carbonate, and
bicarbonate have been also used, and a mixture of soda and an ammonium
salt has been suggested by Stiasny. Whitening has also been tried, but
is very slow-acting. Considerable economy in alkali may be effected by a
thorough washing of the leather before using the alkali. If the water be
hard, so much the better, and if warm water be available the process is
hastened. For most leathers it is necessary to remove excess of alkali
just as much as excess of acid, so that a thorough washing in water
generally follows the treatment with alkali. Anything from 1/4 to 3 per
cent. borax (or its equivalent) on the pelt weight may be used, and,
generally speaking, it is better to use solutions as dilute as
practicable in order to avoid local over-neutralization and tender
leather.
Fat liquoring is a process which is very largely typical of chrome
leather manufacture; it consists in drumming the goods with an oil
emulsion, the grease of which is entirely taken up by the leather. It
thus strongly resembles drum stuffing (Part I., Section IV.) in method,
but the "fat liquor" is such that it mixes easily with water, and
usually contains soap in order to assist in this sense, and may
sometimes indeed consist of soap only. Mineral oil is also used
frequently in fat liquors. The object of fat liquoring is to give
softness, pliability, or waterproofness, and to feed the "empty" chrome
tannage. It is also used as a preparation for more complete impregnation
of grease, _e.g._ as in "stuffing" chrome harness, and in "dipping"
chrome sole leather. Fat liquors are usually made by dissolving the soap
in boiling water and gradually adding the oil with constant agitation.
Perfect emulsification is essential, and this is assisted by the use of
casein, albumen, gelatine, starch, egg yolk in addition to soap and
oil. Soda and borax also assist, and degras and sod oil are also useful
and are admissible where the leather is to receive a dull finish. The
operation of fat liquoring is greatly assisted by heat, and temperatures
of about 110° to 130° F. are usual. Chrome leather may be dyed before or
after fat liquoring: if before, the fat liquor sometimes tends to alter
the shade; if after, the dyeing tends to be uneven. Logwood extract and
iron salts are largely used for blacks. It is common to mordant chrome
leather with vegetable tanning before dyeing. Sumach and gambier are
often used for this purpose, and the usual "fixing agents" (tartar
emetic, titanium salts, etc.) may also be used.
Of the mechanical finishing operations staking is the most
characteristic. It is now done entirely by machines, and the primary
purpose is to soften the leather, which otherwise dries out in a
non-pliant and stiff condition. In the staking machine, the "blade" is
fixed between two rollers, which are however on the other side of the
leather. The leather is held by the operator, and the machine "head"
pulls a fold of the leather over the blade. Seasoning and glazing are
also common for many chrome leathers.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 198-220.
Bennett, "Manufacture of Leather," pp. 210, 312, 355, 375.
Bennett, "Types of Two-bath Chrome Tannage," _Leather_, 1909, Aug.
and Sept.
SECTION III.--CHROME CALF
The tannage of calfskins by the chrome processes for the manufacture of
upper leathers is one of the most extensive branches of leather
manufacture. The deservedly popular =box calf= is typical of these
leathers, and the observations of this section are primarily applicable
to it. A chrome-tanned calf skin, fat liquored and blacked, provides as
suitable an upper leather as could be desired for ordinary boots. It is
at once supple and durable. It is also sufficiently waterproof, but can
be given a bright glazed finish.
In regard to the wet work for chrome calf, the general principles and
methods discussed in the previous section are much to the point. It is
essential to avoid undue plumping and the loss of hide substance. The
skins should be washed clean as soon as possible. Three fresh waters are
desirable, the goods remaining only a short time in each. Salted skins
need more time, but the liquors must be kept sweet. Drumming the skins
in running water is very suitable for the first and last stages of
soaking.
The liming should be short but not "sharp," _i.e._ mellow sulphide limes
are suitable, depilation being carried out after about 7 days. The
one-pit system is usual, but two liquors may be given, the green goods
being first inserted into a used liquor, and after handling reinserted
into the same pit with a new lime liquor made up with lime, sulphide and
a proportion of the old liquor. Scudding should be carefully done, as
hair on the finished leather is very objectionable.
In deliming it is essential to have the grain of the skins thoroughly
relaxed and pulled down. The finished box calf should have a
characteristic soft and silky feel, and this is only attained by
procuring the inelastic pelt. It is not surprising that a light puering
is a popular method for attaining this, but there is also a tendency to
use artificial bates such as are made from ammonium chloride and
pancreatin, together with organic acids, or non-swelling acids like
boric acid. Drenching is also common after a preliminary deliming with
acid. The skins may be half or two-thirds delimed with lactic acid,
rinsed and drenched over night at 85° F. with 6 per cent. bran on the
pelt weight. Less acid may be also used, in tepid water, and the drench
made up with 10 per cent. bran and a little pea meal. It is very common
to pickle the skins in 5 per cent. alum and 5 to 10 per cent. salt
before tanning. This is often of doubtful advantage, but sometimes
prevents drawn grain when the goods are moved rapidly into strong chrome
liquors. This pickling is said to give fullness to the leather.
The tannage of box calf is usually by the one-bath process, though the
two-bath process gives quite as good a result and is sometimes used.
Again, drum tannages are the most popular on account of their speed and
the economy of chrome. The practical problem is to use up all the
chrome, and to tan quickly without "drawing" the goods. It is, in any
case, usual to commence the tannage in a used and nearly spent liquor
and finish in a fresh liquor. The most appropriate way depends largely
upon local convenience, the number of drums available, supply of labour,
etc. In a one-drum system the goods may be started in an old liquor,
which is run off when exhausted by the green goods. Fresh stock solution
is then added at intervals of an hour or two and the drumming continued
till tannage is complete, which is usually in less than 24 hours. The
remaining liquor is used to commence the tannage of the next pack.
In another system the operation is similar except that the liquors are
weaker, and the goods are then removed and finished in another drum. A
three-liquor system, however, is often combined with a one-drum method;
the goods are thus not handled. The liquors are run off and pumped to
other drums, the once-used liquor to a drum containing goods already
treated with a twice-used liquor; the twice-used liquor to a drum
containing green goods, and the thrice-used liquor pumped to the drain.
In any of these methods the chrome alum liquor is suitable, using 10 per
cent. alum and 3 per cent. soda on the pelt weight. The glucose liquor
has also proved very suitable for chrome calf, and the liquors made with
sulphurous acid or its salts have increasing popularity on account of
lower costs. Many tanners use bought liquors--"chrome extracts" which
are supposed to be specially devised to suit the tannage of chrome calf.
When thoroughly tanned through, as can be readily judged from a
sectional cut of the leather, and also by the strength of the liquor
remaining, the goods are horsed in pelt overnight, and are then ready
for finishing.
In finishing box calf the neutralization should be thorough, or the acid
may cause trouble in dyeing and fat liquoring. Imperfect removal of
excess chrome salts may cause the formation of "chrome soaps" which are
very difficult to remove; the goods should therefore be well washed.
There are two general types of treatment before blacking. In one, the
skins are first well washed with water at 110° F., neutralized with
about 3 per cent. borax, and well washed again. Striking follows and is
usually very thorough, partly because it assists in producing evenly the
characteristic box grain, and partly because the finished leather is
sold by the square foot. Machine striking is now almost universal, and
may be done several times at different stages in the drying. When half
dry ("sammed") the skins are shaved by machine and, at this stage
usually, weighed. Dyeing and fat liquoring then follows. In the other
type, the goods are merely washed, and then struck out, sammed, shaved
and weighed. The skins are then neutralized, washed and immediately dyed
and fat liquored. The advantages of this latter course are that the
goods remain in the drum for the last four processes, which is
economical of labour, and also that by neutralizing immediately before
dyeing and fat liquoring there is less danger of a further diffusion of
acid.
In dyeing logwood extract is largely used, occasionally a little fustic
is used also, and by using a "striker" of iron and copper sulphates a
good black is obtained. Logwood is often used also in conjunction with
coal-tar dyestuffs. The goods are first warmed in the drum up to 140°
F., and the dyestuff solution gradually run into the drum whilst it is
revolving. Up to 3/4 hour may be necessary to exhaust the bath, the
goods being constantly drummed. The fat liquor is then run in similarly,
and the drumming continued until the grease is all absorbed by the
leather, which may take another hour. The skins are horsed till next
day, during which time the grease penetrates more completely.
The skins are now dried out, sometimes by suspending from the hind
shanks and sometimes by nailing on boards or wooden frames. They are
damped back for staking by leaving for 1-1/2 to 2 days in moist sawdust.
After staking they are dried strained in a "stove" at about 105° F.
In finishing off, the grain is "cleared" by sponging with 10 per cent.
lactic acid, and seasoned with a mixture of milk, blood and black
dyestuff. When dry on the surface the skins are glazed by machine, and
grained two ways--neck to butt and belly to belly. They are usually
reseasoned, dried out, reglazed, regrained, lightly oiled with mineral
oil, and finally trimmed. These various operations are fairly typical,
but there is obviously ample scope for divergence. Thus one may fat
liquor before dyeing, and the skins may be staked before drying out, and
may be re-staked after glazing.
Much so-called "box calf" is not made from calf skins. A very close
approximation, however, is obtained from rather older animals, and
"box-kip" is largely manufactured by similar methods. Light hides are
also widely used, being similarly treated except that they are split
and also cut into two along the spine. The finished article is sold as
"box-sides." To yield the characteristic grain pattern, the goods are
frequently printed and embossed. Even the flesh splits are sometimes
made into box calf imitations, some filling material being used and an
artificial grain pattern embossed.
=Willow calf= typifies the chrome calf which is finished in colours. The
soaking, liming and deliming processes are the same as for box calf. The
tannage, however, is generally by the two-bath process on account of the
lighter colour thereby obtained. This colour is largely due to the
deposition of sulphur in and on the leather in the second bath.
In one tanning process the skins are first pickled in 2 per cent.
hydrochloric acid and 10 per cent. salt. They are then drummed in
solution containing 2 per cent. dichromate (strength 1 in 60) for about
half an hour. A solution containing 4 per cent. dichromate, 3-1/2 per
cent. hydrochloric acid, and 5 per cent. salt is gradually added, and
the skins drummed until well struck through. They are then horsed
overnight and struck out and passed through a "hypo dip,"--a 2 per cent.
solution of thiosulphate,--and then into the reducing bath, which
contains 10 per cent. of thiosulphate, to which 5 per cent. hydrochloric
acid is added.
Another process employs paddles instead of drums. The chroming liquor is
made up with 4-1/2 per cent. chromic acid and 10 per cent. salt. The
bath is exhausted by commencing the tannage of a succeeding pack. The
skins are reduced as in the last process.
In another process the "acid" type of chroming bath is used. The skins
are paddled with a solution containing 5 per cent. dichromate, 5 per
cent. hydrochloric acid, 2 per cent. aluminium sulphate, and 10 per
cent. salt. In the reducing bath 14 per cent. hypo and 4 per cent.
hydrochloric acid are used.
In yet another process the skins are pickled first in 5 per cent.
aluminium sulphate, 7-1/2 per cent. salt, and 3 per cent. sulphuric
acid, and are then dried out and sorted. The tannage proper is in the
drum, using 6 per cent. dichromate, 5 per cent. hydrochloric acid, and 5
per cent. salt. In the reducing drum 15 per cent. hypo is used and 4-1/2
per cent. hydrochloric acid.
Whichever process of tanning has been used, the skins are neutralized
and washed thoroughly, as for box calf, sammed and shaved. In dyeing,
the skins are first mordanted with a filtered infusion of leaf sumach,
used at 110° F. for half an hour. As fixing agent, 4 oz. tartar emetic
per dozen skins is then added and the drumming continued for half an
hour. The goods are washed, struck out and drum dyed at 140° F. with
basic colours, and immediately fat liquored. In the fat liquors olive
oil and castor oil, with the corresponding soaps, have been popular, but
substitutes are now used on economical grounds. The skins are next
horsed a while, well struck out again and dried strained. They are now
finished off as for box calf, except that it is usual to grain only one
way--neck to butt--and the season should consist of milk, water and
albumin only, though sometimes other mucilagenous matters are added. As
with box calf, the finishing may be varied in many ways. The skins may
be dyed with acid colours after fat liquoring. For pale shades direct
dyes are used without a mordant. For darker shades of brown and red, the
dyewoods are used both as mordants and ground colours, and titanium
salts are useful as fixing agents.
Both the "box" and "willow" finish are largely a matter of public taste,
and the fashion varies from time to time on such points as to whether
the grain should be one way or two ways, and whether it should be faint
or bold. There are also other common finishes besides the typical box
grain. =Glacé calf= is made much in the same way as box calf, but there
is no graining at all. The goods are usually seasoned and glazed three
times. Small skins are preferred for this finish. =Dull calf= is also a
plain finish. The leather contains more grease, and the fat liquor is
made up with greater proportions of degras. The goods are not seasoned
or glazed, but ironed, "sized" with gum, oil, soap and logwood, and
after brushing are dried and rolled. In both these plain finishes a
one-bath paddle or pit tannage is common, in order to ensure the smooth
finish.
REFERENCES.
Procter, "Principles of Leather Manufacture," p. 198.
Bennett, "Manufacture of Leather," pp. 55, 84, 105, 227, 360-363,
375.
Bennett, "Theory and Practice in Wetwork of Chrome Calf," _Shoe and
Leather Reporter_, Sept., 1909.
SECTION IV.--CHROME GOAT AND SHEEP
Immense quantities of goat and sheep skins are chrome tanned for upper
leathers. Most of them are manufactured into the well-known and popular
=Glacé kid=, to the manufacture of which this section is chiefly
devoted. To be quite strict, glacé kid should be made from kid skins,
but actually comparatively few of such skins are used, they being
reserved rather for glove leathers. The popular upper leather is made
from goatskins.
Chrome goat is deservedly popular; it is an ideal upper leather for
shoes and light boots. As compared with chrome calf (thickness and other
factors being equal), it is not only softer and more pliant, but also
more durable. It is usually, however, not quite so thick, and perhaps
therefore not quite so warm and waterproof. The popularity of glacé is
probably enhanced by the brighter and more glassy finish than is usual
with box.
As the supply of goatskins is unfortunately too limited, an even more
widely used glacé upper leather is made from sheepskins, and often sold
as glacé kid. From what has been previously said as to the quality of
goat and sheepskin leathers (Part II., Sections II. and IV.), it will be
readily understood that glacé sheep is by no means so good a leather as
glacé goat. It is perhaps as soft, but is more spongy and loose
textured, and is neither so waterproof nor so durable as chrome goat.
The ubiquitous sheep, however, provides an immense supply of raw
material, and the resulting leather, which should strictly be regarded
as a glacé kid imitation, finds a ready sale. When well finished it is
indeed a good imitation in respect of appearance, and this fact,
together with its comparatively low cost, causes it to meet an undoubted
public need.
The production of glacé goat will first be considered. The soaking
process is quite similar to that before described for the production of
goatskin moroccos (_q.v._) and need not be here repeated. The liming is
similar in many respects also, but from what was said in Section II.
about the undesirability of excessive plumping of pelt for chrome
leather, it will be clear that caustic soda should be omitted from the
limes. The liming should also be shorter for glacé than for moroccos,
and this is attained both by using a greater proportion of sulphide and
by using mellower lime liquors, preferably the latter, as soft pelts are
better ensured. Calcium chloride has sometimes been added to the limes:
this reacts with the soda from the sulphide, yielding salt and probably
precipitating lime, and has its own lyotrope influence, thus reducing
the plumping effect possibly in two ways. To obtain either effect it is
necessary to use considerable amounts of calcium chloride. As goatskins
are so tight fibred, a longer liming and a greater loss of collagen is
permissible than with most pelts for chrome. The deliming operations
should be exceedingly thorough in order to obtain the desired softness
and the smooth grain. Puering is largely used to the full extent, _i.e._
the goods are thoroughly pulled down at 85°-90° F., and are carefully
delimed in the puer liquor. After puering it is common to give a low
temperature drench (60°-65° F.), which of course acts slowly over a day
or two. The skins must be well scudded after puering or after drenching;
sometimes after both. The drenching is often substituted for purely
deliming processes, of which may be mentioned the use of boric acid and
also the use of warm solutions of the commercial organic acids (lactic,
formic, acetic, butyric, etc.), together with calcium chloride. In place
of the chloride, a salt of the acid may be employed, and the deliming
bath may be regenerated by oxalic acid and used repeatedly. Sometimes
puering is omitted and the desired result obtained by washing in warm
water, nearly deliming with warm solutions of organic acid, washing
again and drenching. Skins are also washed often after drenching.
In tanning chrome goat for glacé the two-bath process is mostly
preferred. This is partly because the sulphur deposited in the reducing
bath assists materially in producing the mellowness and fullness which
are so essential, and partly because a large proportion of skins are
finished in colours. The two-bath process also lends itself to a paddle
tannage, which is necessary for the smooth grain finish. One or two
illustrative processes may be given.
One process presents many points of resemblance to the first process
suggested for willow calf in Section III. (_q.v._). The skins are first
pickled in a paddle with 2 per cent. hydrochloric acid and 10 per cent.
salt, and then pass into the chroming paddle, which contains at first
only 2 per cent. dichromate. Subsequently 4 per cent. dichromate, 3-1/2
per cent. hydrochloric acid, and 5 per cent. salt are added to the
paddle liquor, and the skins paddled until well struck through. After
being horsed overnight the skins are struck out by machine, passed
through a hypo dip if desired, and reduced with 12 per cent. of
thiosulphate and about 5 per cent. of acid. The skins may be left
overnight in the hypo paddle, and the excess of thiosulphate, which is a
feeble alkali, commences the neutralization.
In another process the chroming bath is made up of 5-1/2 per cent.
chromic acid and 6-1/2 per cent. of salt, and to this paddle liquor 2 or
3 per cent. of aluminium sulphate may be added if desired. The reduction
is with 14 per cent. hypo and 7 per cent. hydrochloric acid. A little of
the acid is added to the reducing bath; when the liquor turns milky, the
skins are rapidly inserted, and the rest of the acid gradually added.
In the finishing processes the mechanical operation of "striking" is
very prominent, on account of the necessity of obtaining area and smooth
grain. The skin of goats has rather a tendency to bold grain, and this
enhances the need of striking. Most manufacturers lay great stress upon
thorough neutralization and washing. An important point also is that the
staking should be carried out at the proper condition of dryness. If
either too damp or too dry, the requisite mellow feel is not obtained.
There is, of course, ample scope for variation and ingenuity, and the
following processes for blacks and colours must be taken as broadly
typical.
The skins from the reducing bath are first machine-struck, and then
immediately neutralized with one per cent. borax until this is
thoroughly used up, and the skins are then paddled for many hours in
running water. They are again struck out and lightly shaved, possibly
after a little drying. There is a tendency to save time by using a
stronger borax solution, and by using warm or tepid water, and some
factories save borax by washing well first in warm water. If for blacks
a common plan is to dye grain and flesh a violet-blue and then black the
grain only with logwood and iron. The skins are drum dyed blue with a
coal-tar dyestuff, drumming half an hour in the solution at 110° F., and
again struck out. They are then paired or pleated, and rapidly passed
successively through three vats containing respectively cold weak
ammonia, a logwood and fustic infusion at 120° F., and a solution of
ferrous sulphate containing a little copper sulphate. The skins must be
immediately washed well to remove excess of iron. Instead of this
process the skins may be passed through vats containing coal-tar blacks.
Instead of blue backing the skins may be drum-dyed black on flesh and
grain with either coal-tar blacks or with logwood and iron. In the
latter case the skins must be drummed in water for an hour to remove
excess of iron. However dyed, the skins are often struck out again after
dyeing, and sammed slightly for fat liquoring. Neatsfoot oil is a
popular ingredient of the fat liquor. The skins are drummed dry for a
few minutes in a hot drum, and the fat liquor added at 130° F., and the
drumming continued after the grease has been taken up in order that it
may be thoroughly distributed. The skins are struck out again, rapidly
dried out, and wet back for staking in damp sawdust. The staking should
be thorough, and, if necessary, repeated when the goods are rather
drier.
In finishing off the skins may be fluffed if desired, and are then
"cleared" by sponging with 10 per cent. lactic or acetic acid. They are
then seasoned and glazed after some drying. This is repeated until the
required gloss has been obtained. They are finally oiled lightly with a
mixture of linseed and mineral oils. On finishing =dull kid= a heavier
fat liquor is given, in which degras is used, and the skins are not
seasoned and glazed, but are ironed and oiled. In finishing for
=coloured glacé=, the skins are mordanted before dyeing by the use of
dyewood extracts, antimony and titanium salts being used as fixing
agents. The fat liquor should contain less soap and more egg yolk, and
for fancy shades even egg yolk only is sometimes used.
The production of chrome glacé sheep follows the same general lines as
glacé goat. There is less difficulty in obtaining smooth grain, so that
"striking" is perhaps less prominent, and drum tannages are preferred,
whether one bath or two bath. The skins are received after
fellmongering (see Part II., Section IV.) and need thorough puering to
remove scud, and may be then rinsed through boric acid. Pickling is very
common with these goods. In the pickled state they are often sorted out
before tanning. The pickling is usually a one-bath process in which
vitriol and salt or else alum and salt are used, but sometimes all three
substances. The skins may indeed be received in a pickled state. They
may be depickled by paddling with salt and borax, bicarbonate, or basic
alum solution. They may also be tanned without depickling if the
composition of the pickle be allowed for in the first chroming liquor. A
commonly used pickle consists of 3 per cent. aluminium sulphate and 9
per cent. salt. If these goods are to be dried out, flour also may be
used with the pickle, which thus becomes practically a light preliminary
alum tannage (see Part IV., Section I.). A commonly used acid pickle is
of 5 per cent. commercial sulphuric acid and 25 per cent. salt.
The delimed or depickled stock may be tanned as now described. The
two-bath process may be used with drums. The chroming bath contains 5
per cent. dichromate, 5 per cent. hydrochloric acid, and 10 per cent.
salt. After the skins are thoroughly penetrated they are horsed
overnight and reduced with 20 per cent. thiosulphate, up to 7 per cent.
of hydrochloric acid being added after half an hour in thiosulphate
only.
Alum pickled or tawed skins are wet back by drumming for about an hour
in water, and are then tanned by the one-bath process in drums. Only a
few hours are needed. Towards the end of the operation about 1/2 per
cent. of bicarbonate of soda may be added to the chrome liquor. Acid
pickled skins may be wet back with 10 per cent. salt, and depickled by
adding a basic alum solution and the chrome tannage superimposed after
about half an hour without handling the goods. The basic chrome alum
liquor is suitable for this purpose.
In finishing glacé sheep much the same methods are used as in the case
of glacé goat. Sheepskins are perhaps more lightly fat liquored, being
naturally soft and porous. Degreasing is often necessary to obtain an
even finish. As sheep gives an empty pelt and chrome an empty tannage, a
slight retannage is often given in gambier, especially for blacks, in
which case the skins are well mordanted. This retannage makes the
leather less stretchy. Logwood and iron blacks are usual. For colours,
fustic or sumach are the usual mordants, with tartar emetic to fix. If
for glove leathers, skins pickled in alum and salt or tawed should be
preferred, and flour may be used in the fat liquor.
Sheepskin splits are sometimes given a chrome tannage and finished as
=chrome chamois=. This leather may be used for linings, but not for
polishing silver on account of the sulphur originating from the
reduction bath. The splits are puered heavily, and pickled in 6 per
cent. vitriol and 24 per cent. salt. They are paddled in this pickle
liquor, and 4 per cent. dichromate added in successive portions. The
fleshes are horsed overnight and reduced in 15 per cent. thiosulphate,
to which a little hydrochloric acid is added if needed.
In finishing the splits are washed in warm water, neutralized in weak
soda, and washed again. They are sammed by machine striking, and fat
liquored, using much soap. They are then horsed, struck and dried out.
They are staked several times after damping back, drying out again
between stakings. They are finally fluffed.
REFERENCES.
Procter, "Principles of Leather Manufacture," p. 198.
Bennett, "Manufacture of Leather," pp. 55, 84, 105, 230, 364.
Bennett, "Theory and Practice in Wetwork of Chrome Goat," _Shoe and
Leather Reporter_, Sept., 1910
SECTION V.--HEAVY CHROME LEATHERS
The term "heavy chrome leather" is taken to include chrome sole leather,
chrome strap and harness butts, waterproof chrome upper leathers, motor
butts and picking band butts. These will be discussed in turn.
=Chrome sole leather=, as stated in Section I., has made headway in
Britain during the European War, the Army authorities having recognized
its great advantages in durability and waterproofness. At the time of
writing, however, its manufacture has received a set back, and many
factories are reducing their output. The primary cause of this is that
the Army purchases have largely ceased, whilst the general public have
not yet been educated to its value. Men who take chrome uppers for
granted talk of chrome sole as a "leather substitute" with an
implication that it is of inferior value. It must be recognized, too,
that there is some interested opposition to its development. Cobblers
and bootmakers complain that it ruins their tools, being so hard to cut.
Now, it is manifestly impossible for it to be soft to cut and hard to
wear out; the complaint is therefore an excellent testimonial. There is
also a stupid fear that an article which lasts twice as long will reduce
repairs and retail sales by 50 per cent. Even the manufacturer has
sometimes a suspicion that a demand reduced in proportion to durability
will not be balanced by an extended export trade. These points of view
will become minor considerations when the public realize its relative
economy, and when the community as a whole grasp that a durable article
is a natural asset. Meanwhile credit is due to those firms who persevere
in their pioneering work of educating the public.
The manufacture of chrome sole leather presents many analogies with the
vegetable tannages. The soaking and liming should be about identical,
but the hides for chrome are generally given more sulphide and the
depilation is reduced to about a week. The methods used for deliming
differ widely in different factories. Some delime completely with
mineral acids, some even pickle in acid and salt, whilst others merely
delime the grain with boric acid. The last is really quite sufficient.
Again, in tanning one finds similar divergences of method. Drum tanning
is practised, but tannage in pits by suspension is more usual, though,
as this last involves more dilute liquors, it involves also greater time
to tan. In drum tannages a few days only are sufficient. In pit tanning
at least a week is given, but sometimes up to a month, according to the
strength of the final liquor and the rate of progress of the goods into
stronger liquors. Liquors containing over 1 per cent. of chromium may
easily be spent out so as to contain only 0.01 per cent. Labour and time
are saved in pit tanning by the use of rockers. The press system of
avoiding handling, however, so complicates the analytical control that
its advantage is doubtful, a better way being to shift the liquors by an
air ejector, which may also be used as an agitator of the liquor and
thus abolish the need for rockers ("Forsare" patent). Chrome butts are
tanned out in suspension. No floats or layers are used. The
neutralization need not be so thorough as for light chrome uppers, as
dyeing is not practised and trouble does not arise with emulsions made
from sulphonated oils. Thorough washing is advisable, and the butts are
usually then cut into bends and may be oiled before drying if desired.
The bends are dried strained, to obtain flatness and smooth grain, for
no machines, such as strikers and rollers, are usually employed. It is
necessary to dry very thoroughly, for the bends are waterproofed by
dipping the dry leather into molten waxes. The most commonly used wax
and the cheapest is paraffin wax with a m.p. of about 127° F. It is
rather a brittle wax, however, and as the finished leather consists of
up to one-third of the wax, it is better to use at least some proportion
of hard fat, Japan wax or ceresin wax, to obtain a stuffing material
with less crystalline texture. The use of 10-30 per cent. rosin in the
stuffing grease is also usual. This prevents the leather from being so
slippery when in wear. The stuffing should take place at temperatures
from 150°-195° F., according to the melting-point of the grease
employed. The bends are taken out and laid in pile to cool and set in a
flat condition, and are then finished.
The chrome tannage of butts for strapping and harness backs, and for
motor butts and picking bands may be similar to that for chrome sole,
but drum tannages are more common and the two-bath process is often
used. In the latter case the acid chroming bath is preferred, using 6
per cent. of dichromate and of acid, with up to 15 per cent. of salt,
and reducing with 15 per cent. thiosulphate and acid as needed. This
process assists in the production of the light colour which is preferred
in the case of some of these leathers.
Strap butts after tanning are very thoroughly washed with cold water in
pits, and repeatedly struck out by machine between the washings. They
are then oiled with heavy mineral oil, and stretched by powerful
machines. They are dried and curried during the stretching. Degras, wool
fat and vaseline are greases used, and the drying and stretching
finished off at 120° F. They are then fluffed on the flesh,
French-chalked and heavily rolled.
Harness backs are neutralized, machine sammed, and lightly fat liquored
with 4-1/2 per cent. soap. They are then struck and oiled with heavy
mineral oil and dried for stuffing. Hand stuffing, drum stuffing, and
"burning in" are all used (see Part I., Section IV.). Stearin, paraffin
wax, ceresin wax, wool fat, sod oil and mineral oil are the greases
employed. The butts are blacked after stuffing with lamp black and oil,
glassed well and buck-tallowed on the grain.
Motor butts are fat liquored lightly, using soap only. They have to be
softened, therefore, during the drying by being mechanically worked. A
boarding machine is repeatedly used during the drying. They are
finished off with French chalk on flesh and grain.
Picking band butts are neutralized by using warm water and then borax
solution, and are then sammed by machine and very heavily fat liquored
with cod oil and tallow and hard soap, to which degras may also be
added. Up to 20 per cent. of greases (on the pelt weight) may be used.
They are well drummed in this, struck out, French chalked, and dried
out. They are softened finally by machine.
Waterproof chrome upper leathers are manufactured usually from hides
tanned by the two-bath process, which is said to give a mellower
leather. The neutral type of chroming bath is common. The butts are
neutralized, machine sammed and struck, and then fat liquored with 2 per
cent. each of neatsfoot oil and soft soap. They are then sammed, shaved
and blacked on the grain with logwood and iron, and dried further. They
are stuffed then by brushing with an abundant amount of concentrated fat
liquor. This gives the waterproofness. They are staked after drying
further, and often grained three ways. A further waterproof finish is
given consisting of a fat liquor containing beeswax. They are finally
brushed and re-oiled with linseed oil, to which some mineral oil may be
added. This leather is much the most durable type for a shooting boot,
or where waterproof uppers are desirable.
REFERENCES.
Procter, "Principles of Leather Manufacture," p. 198.
Bennett, "Manufacture of Leather," pp. 234, 368.
PART IV.--MISCELLANEOUS TANNAGES
SECTION I.--ALUM TANNAGES
The use of alum for making pelt into leather is several centuries old.
It was the first case of what are called "mineral tannages." The tannage
is closely analogous in theory to the chrome tannages discussed in Part
III., and if soda be added to ordinary potash alum in solution, a basic
alum liquor is obtained which is quite capable of yielding a
satisfactory leather, and which is thus a strict analogy of the basic
chrome alum liquor described in Part III., Section II. The range of
basicity which is practicable is very limited, however, and it is much
more usual to use common salt with the alum instead of soda. The alum
is, of course, hydrolyzed and free sulphuric acid is quickly adsorbed,
whilst the colloidal solution of alumina is adsorbed also but more
slowly. The adsorbed acid tends to swell the pelt and to cause it to
take up the alumina less readily. The function of the salt is to repress
the swelling by a pickling action. The actual result is thus partly due
to the alum tannage and partly due to the temporary tannage given by the
pickle. Hence such tannages are not firmly "fixed," nor is the result
water-resisting, for much of the tanning material may be washed out. If,
however, such leathers be stored for a time in a dry condition, the
alumina becomes much more firmly fixed, owing probably to a further
dehydration of the alumina gel deposited upon the fibres. The tannage is
thus relatively more "irreversible," and such storage is practised in
commerce for this purpose, being known as the "ageing" of the leather.
It will be understood that it is possible to use too great a proportion
of salt, the hygroscopic nature of which would keep the leather moist
and thus interfere with a glossy finish. About one-third the weight of
the alum used is usually sufficient.
All that has been said in Part III. as to the empty nature of the chrome
tannage is equally applicable to the alum tannages. It is as necessary
therefore to employ filling agents. A fat liquor is quite satisfactory
for many purposes, but is too dark coloured and greasy for glove
leather. Egg yolk is the favourite emulsion in these cases. It contains
about 30 per cent. of an oil very similar to olein and in very perfect
emulsion. Olive oil is also largely olein and is also used, being
emulsified by the egg yolk and effectively reducing the proportion
required of this expensive material. Flour is also used as a filling
agent. It acts also as a whitening agent and as an emulsifier. Its use
enables the tanner to obtain the required fullness without so much
greasiness. Thus softness and fullness may be obtained, and yet a glossy
finish be possible. It will be clear that the more flour is used, the
more oil may also be used.
The materials mentioned, viz. alum, salt, flour, egg yolk and olive oil,
are all mixed together into a paste with some amount of water. The goods
are drummed in this paste and then dried out. This operation is known as
"tawing." The goods are then "aged" for several weeks and finished as
required.
The manufacture of "glove kid" from lambskins and kid skins is the most
typical example of alum tannage. Lambskins are unwoolled very usually by
painting the flesh with a mixture of lime and sodium sulphide. There
must not be too much of the latter on account of its tendency to give
harshness, a fatal defect in glove kid. The addition of calcium chloride
is desirable, and the skins, which should be pulled as soon as possible,
should be quickly placed in soft water or weak lime. For kidskins a set
of lime liquors may be used, and in preference to sodium sulphide red
arsenic is employed. About one per cent. realgar on the weight of the
lime is used, but more often larger quantities are preferred, even up to
6 per cent. The liming is thus shortened to 4 or 5 days. Fresh lime
liquors are sometimes used without any sulphides. Another method is to
place the skins in a paste of lime to which realgar has been added in
slaking. In any method it is necessary to saponify or emulsify the
grease on the grain, or difficulties occur in dyeing and finishing.
Skins which are to be tawed for glove kid are both puered and drenched.
They are heavily puered at 70° F. for 3 hours, or even longer for the
heavier skins. After scudding they are drenched with 10 per cent. bran
and some pea meal at 95° F. for a few hours only.
In preparing the tawing paste, the flour should be mixed with tepid
water; the egg yolk should also be diluted with tepid water slightly,
and strained if necessary, and then added to the flour. The oil is then
carefully mixed in. The alum and salt are dissolved separately at 110°
F. and added to the flour and oil. The tawing paste should be used at
about 105° F. For every hundred medium-sized lambskins there will be
required: 10 lbs. flour in 2-1/2 gallons water, 1 quart preserved egg
yolk, 3-3/4 lbs. alum and 1-1/4 lbs. salt. The skins are drummed in this
for an hour or so and dried out on poles rapidly, but not with great
heat. This is essential to get "stretch." They are next wet back,
staked, dried and staked again. They are then "aged."
To wet back for dyeing and finishing the skins are drawn through warm
water and then drummed in water at 95° F. for 15 minutes to wet evenly
and thoroughly. This liquor, which contains much of the tawing material,
is run off and replaced by the dye solution, _e.g._ fustic or turmeric,
with which the goods are drummed for half an hour. Iron, chrome or
copper salts may be used for saddening. After this "bottom" colour is
obtained, a coal tar colour is added for "topping" and the drumming
continued until the required shade is obtained. The excess liquor is now
run off, and the materials lost in soaking are replaced by drumming
further with egg yolk and salt for 15 minutes. This is known as
"re-egging." Blacks are obtained with logwood and iron. After
re-egging, the skins are dried out and staked. They are "seasoned" with
a weak emulsion of soap and oil, dried, oiled lightly with linseed oil,
ironed, re-oiled and finally brushed. Whites are undyed, and 10 lbs.
French chalk per 100 skins is used in re-egging.
"Calf kid" is a once popular but now obsolete upper leather made by
tawing calfskins. The skins were well plumped in limes, delimed by
washing and drenching, tawed much as for glove kid, split, dried out
rapidly, staked and aged. They were finished dull and black with soap
and wax.
The various white leathers used for belts, laces, whip lashes, aprons,
covers for stoppered bottles, etc., are very usually made with an alum
tannage. Alum, salt and flour only are used. Whitening is also mixed in
and acts as neutralizing agent as well as pigment dye.
Wool rugs are manufactured from suitable sheepskins by an alum tannage.
They are first well cleaned, using soap on wool and flesh. They are next
degreased by painting with fuller's earth paste and drying. They are
tawed by painting the flesh with a strong solution of alum and salt, or
even by rubbing on the solid salts. They are dried out, aged and sorted
for suitable colours. The dyeing is rather difficult, as many artificial
dyestuffs are of no use. It is usual to bleach the skins first in a weak
solution of bleaching powder, and afterwards to dye with infusions of
the dyewoods, _e.g._ logwood, fustic, sandalwood, terra japonica,
quercitron bark, turmeric, indigo, etc. Vat dyeing is usual. After
dyeing, retanning with alum and salt is necessary, on account of the
loss of these in bleaching and dyeing. Rugs are usually finished black,
white, grey, brown, walnut, crimson, blue or green.
REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 184, 236.
Bennett, "Manufacture of Leather," pp. 239, 371.
SECTION II.--FAT TANNAGES
For the manufacture of a permanent leather the essential requirements
are that the fibres of the hide or skins gel should be dried in a
separate condition, and that they should be coated by some waterproof or
insoluble material. Many substances fulfil the first but not the second
of these conditions. For example, the dehydration only may be
accomplished more or less by salt (as in curing hides), still better by
salt if a little mineral acid be used (as in pickling), and by other
salts such as potassium carbonate and ammonium sulphate, and dehydrating
agents such as alcohol. Such "temporary leathers," however, are not
water-resisting, as the second requirement has not been fulfilled, viz.
the coating of the fibres with some more or less waterproof material.
Thus if pelts dehydrated with alcohol be treated with an alcoholic
solution of stearic acid, the second condition is fulfilled and a
permanent leather is obtained.
Now, many tanning agents accomplish these two requirements only
imperfectly. As we have noted in the preceding section, the alum-tanned
leathers are not very water resisting, and much of the tannage will wash
out. Leathers made by the vegetable tannages usually contain some excess
of vegetable tanning matters which are soluble in and removed by water,
though much tannin can no longer be thus removed, owing to the mutual
precipitation of the oppositely charged tannin sol and hide gel. The
necessity for fulfilling the second requirement mentioned is one reason
for the practice of following these tannages by applications of oil, fat
or of both. In this way the isolated fibres are not only dried
separately, but are coated with a typical water-resisting material.
In the fat tannages an attempt is made to fulfil this second requirement
without the use of any specific "tanning agent" for producing the first
requirements; _i.e._ an attempt is made to dry the fibres separately in
an "untanned" condition, and to coat them simultaneously with fat so
that a permanent leather is obtained. It is only possible to do this, if
the pelt is constantly during drying subjected to mechanical working,
_e.g._ by twisting, folding, bending, drumming, staking, etc. The
resulting leather is often called "rawhide leather," and presents a real
advantage over other leathers in its great tensile strength. Where
toughness is an essential quality, there is much to be said for the fat
tannages. It is also possible, of course, to effect compromises between
ordinary tannages and the straight fat tannages; thus picking band
butts, which must be tough, are often very lightly tanned with oak bark
or chrome, and then given what is practically a heavy fat tannage. In
the most typical of fat tannages, moreover, it is often common to
"colour" the goods by a brief immersion in a weak vegetable tan liquor.
Further, the employment of fats in the currying of dressing leather is
in effect a fat tannage superimposed upon the vegetable tannage. (See
Combination Tannages, Section VI.)
The fat tannage is undoubtedly one of the earliest methods for making
leather. Prehistoric man discovered that the skins of animals killed in
hunting could, by alternately rubbing with fats and then drying
slightly, be eventually converted into a useful leather, whereas without
the fat it was stiff and horny. Even yet similar methods are in use,
thongs of raw hide being continually twisted during drying, with
intermittent application of fats.
In the modern fat tannages drums are used to give the necessary
mechanical working to the goods. The raw hide leather produced in the
U.S.A. is made by drumming the nearly delimed goods with tallow and
neatsfoot oil. In this country the fat tannages have been typified by
the "Crown" and "Helvetia" leathers. The hides are thoroughly limed in
mellow limes, and after the beam work are delimed by drenching, scudded,
and sometimes fleshed again, and then coloured off in tan liquor. After
partial drying, they are drummed warm for some hours to ensure isolation
of the fibres. After further drying they are coated with the tanning
paste, which consists essentially of soft fats and flour to produce
partial emulsification. Equal parts of soft fats and of flour may be
used, to which may be added smaller proportions of degras, cod oil,
mutton tallow, salt, together with about 25 per cent. water. The goods
are coated with this mixture, drummed, and dried further, and this
routine repeated as often as necessary to fill the interstices
thoroughly with fat. The temperature in the drum may reach 95° F. In
finishing an attempt is made to stuff further with grease. The goods are
thoroughly set out, dried a little, and coated again, flesh and grain,
with a mixture of tallow, cod oil, glycerine and degras, and dried
further. The excess grease is slicked off and the goods again set out
and grained. They are then dried out.
REFERENCES.
Bennett, "Manufacture of Leather," pp. 245, 246 and 376.
Procter, "Principles of Leather Manufacture," p. 378.
SECTION III.--OIL TANNAGES
There are very obvious analogies between the fat tannages discussed in
Section II. and the oil tannages now to be dealt with, but there is
nevertheless a distinct departure in principle involved. In the oil
tannages the mechanical treatment is generally more vigorous, and the
"drying" process is conducted at a much higher temperature, with the
result that there is a vigorous oxidation of the oil. This results in
the formation of insoluble oxidation products which coat the fibre and
play an essential part in the production of a permanent leather. Pungent
vapours are evolved in the drying operations, amongst which is acrolein
and probably also other aldehydes, and it is thought by Procter that
these aldehydes also are essential tanning agents and typical of the
process (cf. Section IV.). Fahrion considers that the tanning action is
due solely to unsaturated fatty acids with more than one double linkage.
Garelli and Apostolo, however, believe that the tannage is due to a
coating of fatty acid whether saturated or not. These observers made
leather with stearic and palmatic acids in colloidal aqueous solution.
The manufacture of chamois leather from the flesh splits of sheepskins
comprises the largest and most typical branch of the oil tannages. The
sheep pelts are split in the limed state, and the fleshes are given
another sharp liming which may last up to a fortnight. They are next
"frized," _i.e._ scraped over the beam with a sharp two-handled knife,
to remove roughness and loose fat. The goods are next thoroughly washed
in running water and drenched. A paddle drench is often preferred, and
if not used the handling should be frequent. Paddling drenching reduces
the time required from about 16 hours to about 6 hours. An hour or more
in a hydraulic press removes superfluous liquor and some more grease.
The fleshes are separated, cooled and then stocked for 30 minutes to
equalize the moisture in them. After removing from the stocks they are
sprinkled on both sides with cod oil and thrown back into the stocks for
a few hours. They are then dried cold for a day or two. The stocks used
are similar to those once popular for softening dried hides during
soaking, and consist of two heavy hammers which fall alternately upon
the goods which are contained in a curved box below. The result is a
mechanical kneading action. The fleshes are again sprinkled with cod
oil, restocked for a few hours and dried again, this time at 100° F.
They are then repeatedly sprinkled, stocked and dried, the last
operation being conducted always at an increasing temperature until
finally the final "heater" is even up to 160° F. As the operation
proceeds it is advantageous to hang the splits also nearer one another,
and in the final "heater" they are quite close. The next stage is to
pack the goods quickly into suitable boxes and allow them to "heat,"
_i.e._ to oxidize further. This is a rather critical stage in the
process, and to prevent overheating ("burns") it is often necessary to
open out and repack into another box, with possibly some little
intermediate cooling. They are turned over thus repeatedly until the
oxidation is complete, and then spread out to cool.
The fleshes are now a dark brown colour, and are next treated to remove
excess of oxidized oil products. The goods are dipped through water at
110° F. and then subjected to hydraulic pressure. The grease and water
which exude are allowed to separate by settling, and the thick yellow
oil so obtained, known as "degras," forms a valuable material for
leather dressing, as it more readily emulsifies with water than many
oils, and impart this quality to other greases mixed with it. A further
quantity of a similar oil is obtained by paddling the goods with a weak
soda solution. The liquor obtained is treated with sulphuric acid to
neutralize the alkali, and the grease recovered is known as "sod oil."
The fleshes are now well washed with hot water (140° F.), fat liquored
with cod oil and soft soap, machine sammed, either by a wringer or a
centrifuge, and then dried out.
Much chamois leather is also made in France by closely similar methods.
The skins are usually oiled on tables and folded up before stocking.
Other marine oils (seal, whale, etc.) replace cod oil. Generally
speaking the oxidation is more moderate, and the grease from the
hydraulic press (_moellon_) is mixed with other fish oils to form
commercial degras. An inferior quality of degras is obtained by
subsequent treatment with soda.
The crust chamois obtained as above has only to be thoroughly staked to
soften, "grounded" and "fluffed" to raise the nap, and then trimmed, and
the ordinary wash-leather is obtained.
If intended for glove leathers superior skins are selected. These are
fluffed carefully upon emery wheels, using first a coarse surface and
eventually a fine surface so that a fine velvet effect is attained. The
skins are next bleached.
In the "sun bleach" or "grass bleach" the goods are soaked in a 1-1/2
per cent. soft soap solution and exposed to sunlight after being wrung.
They are bleached in about 3 days in summer, but nearly a fortnight may
be necessary in winter.
In the permanganate bleach, which is less tedious, the skins are first
degreased by soaking in a warm 3/4 per cent. solution of soda crystals
and then drumming for 30 minutes in water at 95° F. They are then
paddled in a 1/8 per cent. solution of commercial permanganate for an
hour at the same temperature, rinsed through water, and the brown
manganese dioxide is then removed by paddling or drumming the goods in a
3 per cent. solution of sodium bisulphite to which hydrochloric acid is
added as required. The goods are well washed in warm water, and are then
"tucked," _i.e._ placed in a vat of boiling water containing a little
soft soap, just for a few seconds. The goods shrink and curl up, and
they are then dried out at 120°-140° F. to fix the tuck. They are then
staked, fluffed, and dyed.
In dyeing with coal tar colours the alizarin colours may be used after
mordanting with chrome alum. Direct dyes, natural dyestuffs and pigment
dyes are also used. The goods are struck out after dyeing, lightly fat
liquored with commercial egg yolk, dried out at 110° to 120° F., staked
and fluffed on the face side.
Buff leather is a similar leather made from hides. They are limed mellow
for a fortnight, unhaired, fleshed, and then limed again for another
week in sharp limes. The grain is then split off, and the goods rinsed
and scudded, slightly delimed and hung up to dry. They are then treated
in much the same way as fleshes for chamois, but lime is often added to
the cod oil used in stocking.
Buck leather is a similar product obtained from deerskins, but much mock
buck is made from cheaper raw material.
REFERENCES.
Bennett, "Manufacture of Leather," pp. 247-250 and 376-379.
Procter, "Principles of Leather Manufacture," p. 378.
SECTION IV.--FORMALDEHYDE TANNAGE
The use of formalin for hardening gelatin has long been known, but it
was left for Payne and Pullman to devise a commercial process for
tanning pelt into leather by means of formaldehyde (H·CHO) solutions.
Their process, which was patented, specified the use of alkalies in
conjunction with formaldehyde or other aldehydes. The function of the
alkalies is not very obvious, for it has been shown that formaldehyde
will tan also in neutral and in acid solution. The precise action of the
aldehydes is also as yet somewhat obscure, but it is noteworthy that
very small proportions of formalin will give a complete tannage. It is
probable that the action of formaldehyde is not perfectly analogous with
that of its homologues, for it is a most reactive substance, and will
certainly with proteids undergo reactions which are not analogous to
those with other aldehydes. The leather obtained by tanning with
formalin is quite white and resembles buff leather, but has advantages
over the latter in that no bleaching is necessary.
According to the patent specifications the pelt should be drummed in
water and the tanning liquor--a solution of formalin and sodium
carbonate--added gradually at 15-minute intervals. Up to 6 hours for
light skins, and up to 48 hours for heavy hides, are required for
complete tannage. The temperature is raised during the process from 100°
to 118° F. The tanning liquor may be made from 16 lbs. of commercial
formalin (36 per cent. formaldehyde) and 32 lbs. soda (80 per cent.
Na{2}CO{3}) in 10-15 gallons of water. This should be added, one
gallon at a time, to 4 cwt. pelt in 100-120 gallons of water. After
tannage is complete the goods should be paddled with a 1-1/2 per cent.
solution of ammonium sulphate to remove the soda, and "nourished" in a
solution of soft soap and salt, about 2-1/4 per cent. of each on the
weight of pelt. The goods are then dried out, and may be finished like
chamois, buff, and buck leathers (Section III.).
REFERENCES.
Payne and Pullman, English Patent 1898, 2872.
Bennett, "Manufacture of Leather," pp. 250 and 379.
SECTION V.--SYNTHETIC TANNING MATERIALS
In spite of much valuable work on the constitution of the vegetable
tannins and the compounds usually associated with them, such as that of
E. Fischer, K. Freudenberg and their collaborators on gallo-tannic acid,
and that of A. G. Perkin on ellagic acid and catechin, we are still in
the dark with respect to the constitution of the tannins which are of
commercial importance, and any synthetic production of these materials
is thus out of the question as yet. Attempts, however, have been made to
produce artificially substances which possess similar properties to the
tannins and which may be used for converting pelt into leather. Into
this category fall some of the earlier attempts to synthesize
gallo-tannic acid by heating gallic acid with condensing reagents.
The first commercial success in this direction was attained by Stiasny,
who produced condensation products of the phenolsulphonic acids, to
which products he gave the general name of "syntans" (synthetic
tannins). The Badische Co. placed one of these products on the market as
"Neradol D," and later took out subsidiary patents for the manufacture
of similar products by slightly differing methods of productions. Since
the outbreak of the European War such patent rights have been suspended,
and several British firms have been manufacturing synthetic tanning
materials by similar methods, but doubtless with developments and
improvements of their own discovery. These products (_e.g._ Cresyntan,
Maxyntan, Paradol, Syntan, etc.) are now in use in many factories, and
assist rather than substitute the vegetable tannins in producing leather
of the desired colour and quality.
These synthetic tanning materials resemble the vegetable tannins in the
following respects. They are organic acids containing phenolic groups.
They are semi-colloidal, passing slowly through semipermeable membranes.
They precipitate gelatin, basic dyestuffs and lead acetate, give a
violet-blue colour with ferric salts, and convert hide into an undoubted
leather. They differ from the vegetable tannins in that they contain
sulphur and sulphonic acid groups, but they agree in that both are
aromatic derivatives. In each case the tanning effect is diminished by
alkalies, but the synthetic materials are the more sensitive.
=Methods of Manufacture.=--There are, broadly speaking, three types of
method by which these condensation products are produced, viz.,
condensation by formaldehyde, condensation by phosphorus trichloride or
similar reagents, and condensation by heat alone. Illustrative methods
will now be given.
Condensation by formaldehyde was the first method used. The procedure is
given by the Austrian patent 58,405. A phenol, _e.g._ crude cresylic
acid, is heated with the equivalent amount of sulphuric acid for a few
hours to 100°-210° C., cooled, and formaldehyde added slowly whilst
cooling and stirring, in the proportion of one molecule of formaldehyde
to 2 molecules of phenol. The free mineral acid is neutralized, and the
resulting product is the syntan "Neradol." By this procedure only
water-soluble products are obtained, but an alternative process is to
heat the phenols in slightly acid solution, and then to render soluble
the resinous products obtained by treating with sulphuric acid. The
proportion of formaldehyde to phenol used led Steasny to conclude that
the resulting products were diphenyl-methane derivatives which
polymerize to form molecules of considerable size. The formaldehyde
supplies the "carbon bridge." This view was criticized by A. G. Green as
too simple, and he suggested the alternative theory that polymerization
does not take place at all, but that more advanced or higher
condensation products are formed; he thought that o-hydroxy-benzyl
alcohols were first produced, that these condensed with another
molecule, and afterwards the process was repeated. The result was a
"colourless dyestuff." This view receives some support from the other
types of method of manufacture.
With the use of other condensing reagents the procedure may be as in the
process of the B.A.S.F. (Fr. pat. 451,875-6), thus: 225 parts of
o-cresol-sulphonic acid are heated to 60° C. for 4 hours with 262.5 parts
of phosphorus oxychloride. The excess of oxychloride is removed by
distillation under reduced pressure and the residue washed with dilute
hydrochloric acid.
Condensation by heat alone is illustrated by the method given in the
same patents, thus: phenol-p-sulphonic acid is heated to 130° C. for 24
hours under a pressure of 20 mm. or in a current of dry air at
atmospheric pressure. The product may be used direct or may be purified
by dissolving in water, neutralizing with caustic soda, filtering and
evaporating to dryness. A white powder is obtained which tans when its
solution is acidified. An alternative is to mix phenol with sulphuric
acid and heat the mixture to 140° C. for 72 hours under 20 mm. pressure
and purify as before.
=Methods of Use.=--The synthetic tanning materials may be put to many
uses. When well manufactured they make practically a white leather, and
this fact makes a valuable opening for their use in connection with
light leather tannages and the dressing of rugs. It is also claimed that
they improve the colour usually obtained in the ordinary vegetable
tannages. If used in the suspenders to the extent of 5-10 per cent. they
are said to brighten the colour throughout the tannage. If used in
bleaching and finishing they are said to lighten the colour of the
finished leather. About 5 per cent. on the weight of the goods may be
added to the bleach or vat liquors; they may be also mixed with sumac
during finishing, and in effect act as a sumac substitute; solutions are
also brushed over the grain before oiling, with a view to obtaining good
colour. It is also claimed that their use prevents vegetable-tanned
leather from becoming red under the action of sunlight. The syntans are
also used to lighten the colour of chrome leather, even of chrome sole
leather after it has been dipped.
It is claimed also that syntans produce a tough leather, and if used for
heavy leather in the early stages they give a tough grain and assist in
avoiding a cracky grain. On this account they are also recommended for
retanning E.I. tanned kips. When used in heavy leather suspenders they
are said to get rid of lime blast (CaCO{3}) and to quicken the tannage,
_i.e._ to enable the same weight to be obtained in less time. Procter
suggests that a tannage of commercial value might be obtained by
blending them with wood pulp extract.
If used alone for tanning a series of pits containing liquors of 4° to
37° Bkr. may be used, but drum tannages may be given using liquors of
14°-29° Bkr., the goods being tanned in 6-8 hours. About 30 per cent. of
syntans are said to be necessary for complete tannage.
REFERENCES.
E. Stiasny, "A New Synthetic Tannin," _Collegium_, 1913, 142-145.
(See also _J.S.C.I._, Abs. 1913, 500.)
E. Stiasny, "Syntans--New Artificial Tanning Materials,"
_J.S.C.I._, 1913, 775.
Patents:--Austrian 58,405.
German 262,558, Sept. 12, 1911.
French 451,875, Dec. 13, 1912; 451,876, Dec. 13, 1912;
451,877, Dec. 13, 1912.
SECTION VI.--COMBINATION TANNAGES
The formation of leather being due to the adsorption of colloidogenic
substances at the interface of the tanning liquor and the hide gel,
there is the obvious possibility that several such substances may be
used simultaneously, and that the resulting leather may be due to the
combined effect of these substances. Indeed, the average vegetable
tannage consists of such a combination tannage, each tanning material
contributing its own individual tannin and characteristic astringent
non-tannins. There is evidently also the possibility that the different
_types_ of tannage discussed above might be used either simultaneously
or successively, and that a leather might be obtained which combines to
some extent the qualities of each of the types in combination. It is
such a case that is generally called a "combination tannage." There are
many conceivable combinations, and in this section will be chiefly
discussed a few which have demonstrated some commercial possibilities.
Some of these have already received notice in the preceding sections.
The manufacture of curried dressing leathers is a combination of
vegetable and fat tannages. The manufacture of waterproof chrome uppers
illustrates a combination of chrome and fat tannages. The use of
"syntans" in conjunction with vegetable tanning materials is also a
combination tannage. The case of chamois leather is possibly a
combination of aldehyde tannage with fatty acid tannage. Two-bath chrome
leather is a combination of chrome, sulphur and fat tannage.
Formaldehyde and vegetable tannage is also a known possibility. It is
clear that there are possibilities of endless complexity, and that what
normally may appear as a simple tannage is in reality a very complex
combination tannage. From this standpoint one might instructively
consider the successive adsorptions involved in a goatskin tanned first
with syntans, then with oak bark, "retanned" in sumac, mordanted with
chrome, dyed with coal-tar dyestuffs and finally oiled with linseed oil.
It will be easily seen that in a very strict sense nearly all tannages
are combinations.
Usually, however, the term "combination tannage" is confined to those
cases where the main tanning agents not only differ in type, but where
none are in predominant quantity. A typical case is that of "semichrome
leather," in which a vegetable tannage is succeeded by a chrome tannage.
E.I. tanned sheep and goat skins are rather heavily "stripped" of their
vegetable tannage and heavy oiling, by drumming with warm soda
solutions, and after washing with water are chromed with the one-bath
process; they are neutralized, dyed, fat liquored and finished for glacé
upper leather.
In a precisely similar way kips and split hides which have received
vegetable tannage are stripped and retanned in chrome and finished as
for box calf, of which they are a good imitation. Such vegetable-chrome
combination tannages possess many of the properties of chrome leather.
To chrome the pelt first and afterwards to subject it to vegetable
tannage is also an obvious possibility, but has not yet been made a
commercial success in this country, but has been increasingly used in
the U.S.A. during the War.
Another typical case of combination tannage is the dongola leather
produced by the use of gambier and of alum and salt. This is a
vegetable-alum combination, and yields a good quality leather for light
uppers, gloves, etc. Goatskins for "glazed dongola" are paddled tanned
in gambier liquors, and alum and salt are subsequently added. They are
tanned in 24 hours, well washed, and are fat liquored without ageing.
The E.I. tanned skins may also be stripped with soda, and retanned in
alum and salt, using flour also if desired. Dull dongola are first tawed
and then retanned in gambier liquor. "Suède" and "velvet calf" are also
tawed and retanned with gambier.
Yet another case of combination tannage is that of sheepskins for glacé
uppers, which are first tawed thoroughly with alum, salt and flour and
dried out for sorting, and are then retanned in chrome by the one-bath
process, and finished as usual. Closely related to this is the method of
"pickling" in alum and salt and then chrome tanning.
Another case is the combined one-bath, two-bath method of chrome
tanning. The goods are chromed by a one-bath liquor containing
dichromate (say 2 per cent.), and then pass into a reducing bath. There
is not much advantage in such procedure, however.
From a strictly commercial point of view the "dongola" and "semichrome"
leathers have proved the most successful combination tannages, but there
seem to be possibilities in combinations of the vegetable tannins with
synthetic tanning materials.
Many other substances are known to tan, _e.g._ iron salts, cerium salts,
sulphur, quinones, fatty acids, the halogens, etc., etc.; hence there is
always the possibility that new useful combination tannages may be
discovered.
REFERENCES.
Bennett, "Manufacture of Leather," pp. 243, 374-5.
Procter, "Principles of Leather Manufacture," p. 236.
SECTION VII.--THE EVOLUTION OF THE LEATHER INDUSTRY
The leather trades are amongst the oldest of all industries, but their
evolution has been much more rapid during the last two or three decades
than at any other period of their history. The European War, moreover,
has caused the commencement of another period of rapid development, and
it is the aim of this section to point out some of the principal lines
of change which have already become apparent.
Many of these lines of evolution in the methods of manufacture have been
previously discussed in their appropriate sections. They may all be
summarized as attempts at more economical production. Prominent amongst
them is the persistent effort to attain quicker processes. During the
last twenty-five years the time necessary to produce the heavy leathers
has been reduced from 12 months to as many weeks. The tendency is to
reduce the time further still, but this is of course increasingly
difficult to accomplish. On the other hand, it is more urgent to strive
in this direction than ever, because a needless week involves more
capital lying idle than ever before. Moreover, as most leather factories
are now large works, a saving even of 24 hours has become a serious item
in economic production. Hence in liming, bating, tanning, drying and in
warehousing there are increased efforts to make a quicker turnover.
A good illustration of this "speeding up" in modern tanneries is the
adoption by all large factories of much more rapid methods of extracting
tannin. On the old press-leach system liquors may be percolating through
the material for possibly a fortnight. The extract manufacturer reduces
this operation to about two days. Steam generated from the spent bark is
used to heat the extracting vats, and to work a vacuum pan or evaporator
whereby more water can be used and a more complete as well as a more
rapid extraction obtained. The evaporator also makes easy the
preparation of the strong liquors used in modern tanning.
Hand-in-hand with quicker production and manipulation are the attempts
to obtain a larger turnover. It is realized that the big business
attains cheap production. Even before the war the smaller factories were
disappearing. A small tannery must now either extend or close down. This
has been better realized in the heavy than in the light leather trades.
In the sole leather tanneries very often many thousand hides per week
are put into work, but in the glacé kid factories there is nothing yet
to correspond to the output of American glacé factories, which sometimes
reaches three or four thousand dozen a day.
Another very prominent feature of factory evolution is the increased use
of labour-saving machinery. This practice has been in operation for a
considerable time, but with marked acceleration during the last few
years owing to the labour shortage occasioned by military service. This
development of machine work has largely dispensed with that labour which
involved any skill or training. The journeyman currier is now
practically extinct. In the beam house, too, fleshing, unhairing and
scudding are rapidly becoming machine instead of hand operations. Many
devices are now being adopted also which reduce the quantity of
unskilled labour needed. Instead of "handling" the goods from pit to
pit, modern tanneries aim at moving the liquors. Thus in the "Forsare"
and "Tilston" systems of liming, hides are placed in a pit and lie
undisturbed until ready for depilation, the soak liquors and lime
liquors being supplied and run off just as required, whilst these
liquors are agitated as often as desired by means of a current of
compressed air. This agitation replaces the "handling" up and down once
practised. In the tanyard proper the same tendency is at work, "rockers"
are increasingly preferred to "handlers," and an inversion of the press
leach system permits the exhaustion of tan liquors by a gravity flow,
and so avoids the handling forward from pit to pit. There is also a
tendency to install lifts, overhead runways, trucks on lines, motor
lorries, etc., to replace carrying, barrowing, carting, etc., and so to
arrange the tannery that the minimum transport is needed.
All these lines of evolution involve more intensive production, and
necessitate much more careful supervision. It is not surprising,
therefore, that the industry now feels that scientific oversight and
administration are essential. A dozen years ago the trade chemists were
largely unqualified men, whose work lay solely in the laboratory, and
consisted mainly in the analysis of materials bought. To-day all large
tanneries have qualified chemists, and it is realized that they are the
practical tanners. Their function is so to control the manufacturing
processes that all waste is avoided, and so to correlate and co-ordinate
the manufacturing results with the analytical and experimental records
of the laboratory, that constant improvements are made in the methods of
production. The extended use of machinery, and the necessity for economy
in coal and power, give the engineer also very large scope for useful
work. Modern business conditions, moreover, have made necessary more
skilful clerical work and accountancy in the large offices of a modern
tannery.
In the creation of cordial relationships between capital and labour in
the leather trades, there has been unfortunately little progress. The
leather trade is not a sweated industry. Its workers have always enjoyed
reasonable hours of work. In most factories an approximate 48-hour
working week (involving no night work) has long been in operation. The
industry, however, is not one in which high wages obtain. The average
tannery worker receives a wage which is never much above the level of
subsistence. This is mostly due to the fact that he is usually a quite
unskilled labourer, and is therefore on the bottom rung of the labour
ladder. In addition to this the work itself is often distressingly
monotonous, and makes little demand upon the intelligence of the worker.
The trade consequently offers little attraction to the intelligent
labourer. The old system of apprenticeship is now quite obsolete, partly
owing to the rapidity of the changes in the methods of manufacture,
partly to the specialization of labour which results from the
development of large factories, and partly also, because to understand
modern tanning involves a better general education than most workmen
receive. It is indeed frequently difficult to find competent
under-foremen for the different departments of the modern leather
factory. Until recently leather workers have been either unorganized or
badly organized, and their views and complaints have been confused and
sporadic, but during the war period there has been a very rapid
extension of trade union movements, and consequently a more articulate
expression of the demands for "democratization" as well as "a greater
share in the fruits" of the industry. In the leather trades, however,
the gulf between the unskilled labourers and the wealthy employers is
perhaps unusually wide, and there is little disposition on the part of
capital to recognize the equity of either of the above demands of
labour. Generally speaking, the leather trade firms are not public but
private companies. There is absolutely no trace of "co-partnership" or
"profit-sharing" schemes, or of co-operative production. There is little
recognition that the trades' prosperity should be shared in any way by
the workpeople, and still less recognition of any right to a voice in
industrial conditions. This condition of affairs has an ominous reaction
upon the attitude of labour, which believes that it is producing great
wealth but not obtaining much more than subsistence. It is not the
function of this volume to pronounce a verdict upon the wages question
or upon the democratization of the leather trades, but one may be
permitted earnestly to hope that if such be the future lines of
development, there will be also, as an absolutely essential part of any
such schemes, a much higher standard of education amongst the workers,
for this is the only satisfactory guarantee that the voice of labour in
council will have any practical value, or that higher wages will be at
all wisely used by the recipients.
In his instructive and valuable volume on "The Evolution of Industry,"
Prof. MacGregor points out that modern industry has evolved three
outstanding types, viz. the Co-operative Movement, the Trusts, and the
methods of Public Trading. He also suggests that these types tend to
blend. In the leather industry co-operative and municipal production
are unheard of, but the industry has certainly developed along the lines
of the large trusts. Large businesses have replaced small, and later
still have formed local federations, which in turn have combined to form
the "United Tanners' Federation." War conditions have certainly
stimulated evolution towards the trust type. The United Tanners'
Federation has become possessed of powers which were not originally
contemplated, such as the purchase and distribution to its members of
hides, bark, extract, sulphide and other materials. How far some of
these arrangements will be permanent is problematical, but one
beneficial result is that the allied trades have certainly realized more
thoroughly their unity of interests. This is shown by the much freer
collaboration of the tanners, and by the encouragement now given to
similar collaboration between their chemists. More evidence is found in
the proposals for combined research.
There is also considerable reason to believe that there is some movement
in the direction of partial State control. There is little doubt that
evolution along trust lines will make this less difficult and possibly
more desirable. The country cannot afford the spectacle of a Leather
Trust permanently at war with a Labourers' Union. The public has
realized that the well-being of the leather industry is vital to the
national safety. It has realized that the leather trades are great
producers of national wealth, and that increased production with the
development of the export trade will materially assist to restore the
country's financial position. It has realized also its own right to
protection from bad leather and from exorbitant prices. On all these
grounds it is probable, though there may be some reaction from the
present position, that the State, which has already got its fingers in
the pie, will refuse to draw them out altogether. The Imperial aspect of
the question affords some further justification for this attitude. The
leather trades operate very largely upon imported material, and it is
clearly desirable that there should be close co-operation between the
home industry and the colonial supplies of material. Here too the war
has also given a great stimulus in this direction. Indian myrabolans has
long been a staple tanning material. South African wattle bark has
during the last few years replaced almost completely, and probably to a
large extent permanently, Turkish valonia. There has also been great
increase in the imports of Indian kips and of South African hides, and
it is not at all an impossible proposition to maintain a self-contained
Imperial Leather Trade, should this be necessary. French chestnut
extract, and quebracho extract, however, are much too valuable tanning
materials to exclude for merely sentimental reasons. These instances
indicate possible advantages in Imperial co-operation, but also show the
need for caution in the elaboration of such schemes.
Although a partial, and indeed increasing, measure of State Control is
probable, there has been as yet no serious proposal to nationalize the
leather industry. Such a proposition, indeed, is hardly ripe even for
discussion. Until the nationalization of transport and of mines is a
proved success, and until the merely distributive undertakings of the
municipalities (_e.g._ of coal and of milk and other foods) are past the
experimental stage, any proposition to nationalize the leather trades
seems premature. It is noteworthy, however, that in Queensland,
Australia, the Government have the right to commence and to administer
State Tanneries.
Any progress in the direction either of democratization or of
nationalization, has been certainly postponed by the sudden and
unprecedented trade slump which commenced in the earlier part of 1920.
This depression, in spite of heavy falls in the prices of raw materials,
has made economic production a much more difficult problem. It has
undoubtedly given a further stimulus to evolution towards the trust
type, and created a further tendency towards the closing of the smaller
factories, and the employment of labour-saving devices. When the general
fall in prices has made an appreciable fall in the cost of living, some
reduction in the leather workers' wages, together with more efficient
work, will also contribute to the solution of the difficulty. It is
chiefly to be desired, however, that the export trade should be
restored. The realization of this hope depends largely upon the
establishment of peace and prosperity abroad, and the consequent
stabilization of the various foreign exchanges.
PART V.--GELATINE AND GLUE.
SECTION I.--PROPERTIES OF GELATINE AND GLUE
Many of the chemical properties of gelatine, especially those which
distinguish it from other proteins, have been described in the
Introduction to this volume, and need no further comment. In this
section its colloid nature and behaviour will chiefly be considered, for
these points have greatest importance from the standpoint of industrial
chemistry.
It is hoped, moreover, that this section will be of interest not only to
the chemist concerned in the manufacture of gelatine and glue, but that
it will be of value also to those concerned in leather manufacture. The
difference between the "collagen" which composes the hide fibre and the
high-grade gelatines is so small that for many practical purposes it may
be considered negligible. Thus the description of the behaviour of a
gelatine gel is very largely applicable to a hide gel also.
Gelatine has been crystallized by von Weimarn by evaporating a dilute
solution in aqueous alcohol whilst in a desiccator containing potassium
carbonate, the temperature being maintained at 60°-70° C. The carbonate
takes up water only, and the concentration of the alcohol therefore
slowly increases until the gelatine is no longer soluble. Gelatine is
usually found and known in the colloid state, however, and its behaviour
in this state only is of practical importance.
The fundamental idea of modern colloid chemistry is that colloids are
heterogeneous systems, usually two-phased, in which one phase is liquid
and the other phase either liquid or solid. The latter phase, which is
divided into small separate volumes, is known as the "disperse phase,"
whilst the other is the "continuous phase" or "dispersion medium." The
"dispersity" is the degree to which the reduction of the dimensions of
the disperse phase has been carried, and is best expressed numerically
in terms of "specific surface," _i.e._ surface area divided by volume,
but it is also often expressed as the thickness or diameter of a film or
particle. When the dispersity is not high, we have ordinary
"suspensions" and "emulsions," which with increasing dispersity merge
into the typical colloids. By analogy, colloids have been divided into
"suspensoids" and "emulsoids," when the disperse phase is solid and
liquid respectively. The classification, however, has not been found
satisfactory, for some systems in which the disperse phase is
undoubtedly liquid, exhibit characteristic properties of suspensoids,
and _vice versâ_. A more satisfactory division, therefore, is found in
the presence or absence of affinity between the two phases, the systems
being termed "lyophile" and "lyophobe" respectively. If water be the
continuous phase the terms "hydrophile" and "hydrophobe" are often used.
Broadly speaking, the lyophile colloids correspond to the emulsoids, and
the lyophobe colloids to the suspensoids. Gelatine is a typical
hydrophile colloid.
Another fundamental idea of colloid chemistry is that the great
extension of surface involved in a high dispersity causes the surface
energy to be no longer a negligible fraction of the total energy of the
system, and that the recent advances in knowledge respecting surface
phenomena may be called in to assist in the explanation of the special
properties of the colloid state. Particles which exhibit the Brownian
movement, about 10^(-5) cm. diameter, down to the limit of microscopic
visibility (10^(-3) cm.) are termed _microns_. Particles less than this,
but just visible in the ultra-microscope (5×10^(-7) cm.) are termed
_submicrons_. Particles still less, approximately 10^(-7) cm., have been
shown to exist, and are termed _amicrons_. The dimensions of molecules
such as may exist in true solutions are of the order of 10^(-8) cm. A
colloid sol may contain particles of various sizes. Thus a gelatine sol
(like other lyophile systems) contains chiefly amicrons, but submicrons
are also observable.
1. THE CONTINUOUS PHASE
Owing to the contractile force of surface tension, it is concluded that
the surface layer of a liquid is under very great pressure, much greater
than the bulk of the liquid. Any extension of the surface of the liquid
naturally causes a corresponding extension of the proportion of liquid
which is thus compressed. If in a beaker of water there be placed a
porous substance, such as animal charcoal, there is a great extension of
the surface of the water, and a corresponding increase in the amount of
compressed water. If instead there be substituted a large number of very
small particles of a substance, a still further increase in the amount
of compressed water is involved. As the specific surface of the
substance inserted is increased, and its amount, the proportion of
compressed and denser water increases also, until it is a practically
appreciable percentage of the total volume. It is clear also that the
extent of the zone of compression will be determined also by the nature
of the substance with which the water is in contact at its surface,
_i.e._ by the extent to which it is hydrophile, and this indeed may be
the more important factor.
Now in a gelatine sol we have the necessary conditions for a system in
which the compressed water bears an unusually large ratio to the total,
owing to the enormous surface developed by the minute particles of the
disperse phase (amicrons) and to the unusually wide zone of compression
surrounding each particle caused by the strongly hydrophile nature of
gelatine. It should be pointed out that these zones of compression do
not involve any abrupt transition from the zone of non-compression, the
layer nearest the particle is under the greatest pressure, and the
concentric layers under less and less pressures, the actual compression
being thus an inverse function of the distance from the particle. Now if
there be a gradual increase in the concentration of the sol, the time
will come when these zones of compression begin to come in contact, and
the system will then show a considerably increased viscosity. With
further increase in concentration the zones of compression will overlap
throughout the system, and when the layers under considerable pressure
are thus continuous, the whole system will acquire a rigidity much
greater than water and approaching that of a solid body. This is a
gelatine gel, or "jelly." With increasing concentration the jelly
becomes increasingly rigid, and if it be eventually dried out under
suitable conditions it forms what is practically solid
body--gelatine--which, however, still contains from 12 to 18 per cent.
of water.
It will be clear that, in the case of gelatine jellies (_e.g._ of 3-10
per cent. strength), an increase in temperature will cause an increase
in the kinetic energy of the particles and effectively reduce the zones
of compression. Indeed, they may be reduced to such an extent that they
are no longer in contact, and the rigidity due to the continuous contact
of the layers of great compression will then disappear; as we say
usually, the jelly melts. On cooling, the decreased kinetic energy of
the water molecules results in the return of the state of compression,
with rapidly increasing viscosity and eventual gelation; as we say
usually, the jelly sets. Neither of these changes takes place at a
definite temperature (like a melting-point), and in "melting" (solation)
or in "setting" (gelation) the temperature-viscosity curve is quite
continuous. By various arbitrary devices, however, approximate melting
and setting points may approximately be determined. The results also
vary somewhat with the concentration of the gel or sol. Gels between 5
and 15 per cent. strong melt about 26°-30° C. and set at 18°-26° C.
On this view, we must regard a gelatine gel as a continuous network of
water under great compression, and in this network are zones of still
greater compression, which surround the particles of the disperse
phase--the gelatine itself, and zones of less compression which in a
weak gel, at any rate, have a compression equal to or much the same as
the normal state of compression in water.
One consequence of this system is, that when a piece of gelatine
swells, there is a considerable enlargement in the zones of compression;
in other words, some, at least, of the imbibed water is compressed. Now
the compression of water means that work is done, and when gelatine
swells, therefore, we expect--and actually find--that heat is liberated
(5.7 cal per g. gel). Hence also by the Le Chatelier theorem, we
expect--and find--that gelatine swells best in _cold_ water. Further,
the compression of water involves a decrease in volume, and we therefore
expect--and actually find--that the volume of the swollen jelly is
appreciably less than the volume of gelatine plus the volume of water
imbibed.
Another consequence of such a compressed system is that a gelatine
jelly, even in water, will have a surface tension towards water just as
the water itself has such a tension to the water vapour above the
liquid. This interfacial tension of the jelly will of course have a
contractile effect, and will tend to resist swelling and to limit it as
far as it possibly can. This force, tending to contract the jelly and
resist imbibition is therefore one of the main influences at work in the
swelling of gelatine, and is one of the two principal factors which
determine the extent of the maximum swelling when equilibrium is
established. The force tending to resist swelling is, in the ultimate,
just surface tension. Its actual magnitude depends, of course, mainly
upon the extent of compression in the dispersion medium of the gel, and
will be a resultant which is a function of this compression. The
magnitude will thus vary with the average compression in the continuous
network of compressed water. It will be obvious that as the jelly swells
the power of resisting the swelling will decrease, and the interfacial
tension with the external water will tend to disappear. If the force
tending to swell were great enough the swelling would continue until the
zones of compression were no longer in contact and the gel would become
sol.
As suggested above, it is probable that the extent of the zones of
compression is determined by another factor in addition to the great
development of surface. That factor is connected if not identical with
that power which makes the system lyophile, and is evidently connected
also with the solubility of the disperse phase, and may indeed be
electrochemical forces tending to form a series of hydrates, or at least
to cause an orientation or definite arrangements of the water molecules
in the zone of compression. This idea receives some support from the
hydrate theory of solution, and the zones of compression and orientation
are the colloid analogue of the hydrates supposed to exist in solutions
of electrolytes. The extension of such zones on cooling are then
analogous with the series of hydrates formed, for instance, by manganese
chloride with 2, 4, 6, 11, or 12 molecules of water when crystallized at
temperatures of 20°, 15°, -21°, -30°, and -48° C. respectively, the idea
being that the salts most hydrated in solution crystallize with most
water.
As the compression is the result of two factors, one of which depends
upon the nature of the disperse phase, we expect--and find--in other
lyophile systems a considerable variation in their power of gelation.
Some indeed, though very viscous, _e.g._ egg albumin, never quite set
like gelatine, and others (_e.g._ agar-agar) set to a stiff gel from a
much weaker sol than gelatine. When the zones of compression are large,
as in gelatine, the magnitude of the compressing force on the outermost
part of the zone is relatively small, and it is not surprising that time
is necessary for the victory of this force over the kinetic energy of
the water molecules. Hence we find a 5 per cent. jelly sets readily on
cooling, but its elasticity increases steadily for many hours after it
has set. This phenomenon, known as hysteresis, we should expect--and
find--to be much more marked in a case where the zone of compression is
unusually large (_e.g._ an agar gel). We should also expect--and
find--that hysteresis is more marked in a high-grade gelatine than in a
low-grade gelatine where both eventually form gels of equal elasticity.
We should expect too--and we find--that hysteresis is more prominent in
weak gels than in strong. These points are of obvious importance in
testing gelatine by its elasticity, _e.g._ the well-known "finger test."
There are also other facts and considerations which have an important
bearing upon the point under discussion. It is necessary ultimately to
regard true solutions of electrolytes and other bodies as heterogeneous,
though perhaps of a rather different order. From this point of view
molecules and ions existing in an aqueous solution will present a
surface and have associated zones of compression analogous with those
suggested for the minute particles of gelatine.
Now recent investigations have shown that the essential physical
properties of water are affected by dissolved substances in a definite
manner and to a fixed extent, and that these substances exhibit a
sequence in order of their effect. This sequence is also exhibited in
the essential properties of water as solvent and as dispersion medium
for colloid sols. The sequence is known as the "lyotrope series." Thus
the numerical value of the compressibility of aqueous solutions is
reduced below that of water by salts which, with the same kation,
exhibit an effect in the following order:--
CO{3} > SO{4} > Cl > Br > NO{3} > I
This same order is observed, in the effect on the increased values for
the surface tension, density and viscosity of these solutions. On the
other hand, the kations have a similar sequence of effects,
Mg < NH{4} < Li < K < Na < Rb < Cs
which appears when salts of the same anion are chosen. It is not
surprising to find that this lyotrope series exhibit an analogous
influence on the chemical reactions of water, _e.g._ the hydrolysis of
esters. In the hydrolysis by acids SO{4} retards the action, the other
anions and the kations accelerate it, in the lyotrope order. In the
hydrolysis by bases the series is reversed. Similarly the lyotrope
series exert the same order of effect upon the inversion of cane sugar
and other reactions.
This lyotrope influence has also been shown to exert considerable effect
in the behaviour of lyophile sols. With the lyophobe sols the addition of
foreign substances apparently affects the disperse phase only, but with
the lyophile sols the effect on the continuous phase is also important,
and may overshadow the other. Now, in gelatine and in hide gels and
tanning sols we are dealing with lyophile systems, and there are many
points of behaviour in which lyotrope influences become prominent.
Similar effects are observed upon other lyophile sols (_e.g._ albumin,
agar-agar, etc.) which differ widely in chemical nature. Thus the
salting out of albumin (reversible precipitation) is influenced by
sodium salts in lyotropic sequence as follows. The anions hinder
precipitation; in order of precipitating power they are:
citrate > tartrate > SO{4} > acetate > Cl > NO{3} > ClO{3} > I > CNS
The sulphates illustrate the kation effect, which is independent and
which favours precipitation:
Li > K > Na > NH{4} > Mg
If the experiments be carried out in faintly acid solution this order of
effect is exactly reversed, iodide and thiocyanate having the greatest
effect and citrates the least. The coagulation temperature of albumin
and the coagulation by other organic substances are similarly influenced
by the lyotrope series.
Lyotrope influence also exerts a powerful effect on the behaviour of
gelatine sols and gels. The gelation temperature is influenced thus:--
raised by SO{4} > citrate > tartrate > acetate
lowered by Cl < ClO{3} < NO{3} < Br < I
The kation effect (small) is Na > K > NH{4} > Mg
Other lyotrope substances raise or lower the temperature thus:--
glucose > glycerol--(H{2}O)--alcohol < urea
The effect on gelation is also illustrated by the change of viscosity of
the sol with time. The same lyotrope order is found.
In the salting out or precipitating of gelatine with salts, the order of
anions is lyotrope:
SO{4} > citrate > tartrate > acetate > Cl
Also the osmotic pressure of gelatine sols is markedly lowered by
neutral electrolytes in lyotrope sequence:
Cl > SO{4} > NO{3} > Br > I > CNS
Similarly lyotrope influences are shown in the modulus of elasticity:
substances which favour gelation increase elasticity, whilst substances
which favour solation decrease elasticity. The order is again lyotrope.
The permeability of the gel is affected by lyotrope influences; alcohol
and glycerol reduce diffusion through gelatine (or agar); and urea,
chloride and iodide increase it. (Similarly the diffusion of sols
through "semipermeable" membranes is affected by lyotrope influence.)
The lyotrope series also influence the optical activity of gelatine sols
and the double refraction of strained gels.
The swelling of gelatine (and other gels) is very strongly influenced by
the lyotrope substances and merits more attention than it has received.
Hence this lyotrope influence exerts a profound effect in the
manufacture of gelatin, and perhaps even greater in the manufacture of
leather. This is only to be expected. If a gel comprise a continuous
network of compressed water, as suggested above, the presence of other
substances in the gel which cause increases or decreases in the
compression must modify accordingly the properties which depend upon
this state of compression, such as the viscosity of the melted gel, the
rate of gelation, the elasticity of the gel, and the rate and extent of
its imbibition. This indeed we find to be the case. Now the substances
which affect the compressibility, surface tension, etc., of water
_least_, _i.e._ the substances producing little or no compression of
water, are just those which reduce the compression of water in a
gelatine jelly, and cause a decreased viscosity, elasticity, surface
tension, etc., and which therefore naturally allow the gel to swell more
than in pure water. Conversely, the substances which cause the greatest
compression of water, the greatest increase in its surface tension and
viscosity, are also the substances which increase the compression,
viscosity, elasticity, and surface tension of gels, and which therefore
hinder imbibition. The effect on swelling is as follows:--
Sodium sulphate > tartrate > citrate > acetate; > alcohol > glucose >
cane sugar; (water) chlorides-potassium < sodium < ammonium;
< sodium chlorate < nitrate < bromide < iodide < thiocyanate < urea.
As the amount of compression will depend upon the amount of substance,
we expect--and find--that the effect is usually additive, and that
suitable mixtures of substances having an effect in the opposite sense
will produce no change.
The interpretation of lyotrope influence is of course somewhat
speculative, but considered as a surface phenomenon, the surface
specific of the molecules and ions of the lyotrope substance must be one
of the factors involved. One naturally also connects the effect with
solubility and the tendency to form hydrates in solution, the zones of
compression being zones of orientation and of electrochemical
attraction. The hydrate theory of solution again affords an instructive
commentary. The fact that, broadly speaking, the polyvalent anions and
the monovalent anions also group themselves together, suggests that
electrical forces are at work, and the order of effect of monovalent
anions almost suggests that what are called "residual valencies" are in
operation. It is difficult to resist the conclusion that in the lyotrope
influence, in the crystallizing of salts, and in the formation of a gel,
we have zones of compression and orientation which are manifestations of
the same forces--surface and electrical; the chief differences in the
case of gelatine being that the zones are larger and that the electrical
effect is perhaps of less definite magnitude.
However these things may be, the fact of water compression determines
the rigidity of the gel, and the changes in this compression of the
continuous phase determine the surface tension resultant which hinders
swelling, and which is one of the two main factors fixing both the rate
at which gelatine swells in water, and the final volume attained by the
gel.
Before leaving this point, it is desirable to note the effect on the
swelling of gelatine of the extremes of this lyotrope influence.
Substances like iodides, thiocyanates and urea prevent a gelatine sol
from setting to a gel at all, and a piece of gelatine in such solutions
swells rapidly until it solates. On the other hand, sulphates,
tartrates, etc., make a stiffer gel on account of the enhanced
compression. Gelatine in such solutions may swell, but at a much slower
rate than in water and with a decreased maximum extent. A gelatine gel
may in such solutions not only fail to swell at all, but actually
contract and in some cases, indeed, be practically dehydrated. If a gel
be in a very concentrated solution of such a substance, it may be that
the lyotrope compression in the external solution is greater than the
compression in the dispersion medium of the gel; in which case the
surface tension effect is reversed, and the external solution tends to
increase in volume and the gel to contract. Hence we find that the
saturated solutions of such substances as ammonium sulphate and
potassium carbonate will dehydrate a gel almost completely, and will
also, by a similar action on pelt, make a kind of white leather. It is
important to remember this contractile effect of strong solutions of
salts, because it is very easy to confuse this effect with a similar
result produced in another manner, viz., by a reduction of the force
tending to swell.
2. THE DISPERSE PHASE
A very important feature of the colloid state is that the particles of
the disperse phase appear to possess an electric charge, and if this
charge be removed a colloid sol no longer remains such, but
precipitates, flocculates, coagulates, etc. As to the origin of this
charge several theories have been advanced, but the most generally
accepted is that it is a result of the adsorption of electrically
charged ions by the particles of the disperse phase. The enormous
specific surface possessed by this phase renders it particularly liable
to such adsorption. This view harmonizes well also with the general
behaviour, of colloid sols and gels, in endosmosis, kataphoresis,
precipitation, etc. According to this point of view the particles of the
disperse phase are surrounded by a surface layer in which these ions are
in much greater concentration than in the volume concentration of the
dispersion medium. The hydrion and hydroxyl ion are particularly liable
to such adsorption. In the case of a lyophile colloid, like gelatine,
the charge may be either positive or negative, according to the nature
of the predominant ions in the dispersion medium, and the amount of
adsorption is determined by the concentration of these ions in
accordance with the adsorption law.
In effect, therefore, the particles of the disperse phase each carry an
electric charge of the same nature, and as similarly charged bodies
repel one another, the particles of the disperse phase will tend to
separate and to occupy a bigger volume. It is the author's opinion that
this repulsion of similarly charged particles is the cause of the
swelling of gelatine. The amount of charge and force--tending to
swell--is due possibly to several ionic adsorptions, which may be
considered to operate independently, and the power of repulsion is
determined by the nett charge, which in the case of a "positive colloid"
is positive, and in the case of a "negative colloid" is negative. As
ions possess different electric charges, the charge on the disperse
phase is subject to the valency rule.
Now the repulsive force between two similar and similarly charged bodies
is proportional to the amount of charge and is inversely proportional to
the square of the distance between them. The amount of charge on a
colloid particle will be determined by the dispersity--best signified by
the specific surface (s)--and by the operation of the adsorption law
y = mac^(1/n)
The distance between the particles varies with the degree
of swelling, and is determined by the cube root of the volume of the gel
(_v_). Hence if F be the force tending to make the gelatine swell, we
may write
F = Q/(d^2) = (sy)/v^(2/3)
Now with all electrolytes, even with water, we have both positively and
negatively charged ions, and y is consequently determined by the
difference in the amounts adsorbed. Hence in the case of an electrolyte
with an equal number of oppositely charged ions
y = ma{1}c^(1/n{1}) - ma{2}c^(1/n{2}), where a{1}, a{2}, and
n{1}, n{2}, are the appropriate constants for the particular ions
concerned. Hence at constant temperature, pressure, etc., we may write
F = [ sm( a{1}c^(1/n{1}) - a{2}c^(1/n{2}) ) ] / v^(2/3)
The force tending to make a piece of gelatine swell is proportional to
its mass, which is perhaps fairly obvious. The swelling force is also an
inverse function of the volume of the gel, and as swelling proceeds
therefore the force tending to swell further decreases. The force
tending to swell is proportional to the specific surface of the disperse
phase, other factors being constant. To illustrate this one has only to
imagine that one particle of the disperse phase be split into two
particles each carrying half the original charge. It is clear that a new
repulsive force becomes operative, which did not before influence the
swelling, and that the distance between the particles is halved. In the
swelling of gelatine, however, we may consider the dispersity constant
for constant temperature, and if we consider unit mass we see that the
force causing swelling depends upon the operation of the adsorption law
and upon the degree to which the gel is already swollen.
In the swelling of (say) one gram of gelatine to its maximum, both the
contractile force of surface tension and the expanding force of
electrical repulsion are in operation. At the commencement the latter is
much the greater force--hence the rapid imbibition. Both these forces
decrease in magnitude as the swelling proceeds, but the force tending to
swell decreases at a more rapid rate, and the time comes when it has
decreased to the precise value of the force tending to resist swelling.
At this point equilibrium is established and the maximum swelling
attained. Obviously this maximum will in many cases be determined
largely by the value of a{1}c^(1/n{1}) - a{2}c^(1/n{2}).
This factor, therefore, demands particular consideration.
Now, unfortunately, the adsorption law constants for the different ions
have not yet been numerically determined, so that we are still somewhat
in the dark as to the operation of ionic adsorptions. It is
possible, however, to form conclusions of a qualitative or relative
order, and these are such as to throw much light upon the question at
issue. In the first place, we know that in general the various ions are
not usually very widely different in the extent to which they are liable
to be adsorbed. If this were otherwise, the valency rule would hardly
operate so well in endosmosis, kataphoresis, and precipitation. In
consequence we must expect the differences between the ions to appear in
small rather than in large concentrations, the amounts adsorbed being
under those conditions more affected by changes in the volume
concentration. At the larger concentrations, therefore, the value of
a{1}c^(1/n{1}) - a{2}c^(1/n{2}) is small, and the force causing
swelling often tends to zero.
There are, however, noticeable differences at lower concentrations. Thus
we know that if a substance be primarily a positive colloid, it will
absorb kations more readily than anions. As gelatine falls into this
class, we may therefore conclude that usually a{1} > a{2}. Further,
it often happens that very adsorbable substances are less affected by
concentration changes, and in the case under consideration, therefore,
we should expect that n{1} > n{2}. Moreover, we know that the hydrion
and hydroxyl ion are much more readily adsorbed than other ions, _i.e._
have a large value for _a_. Hence in the case of gelatine we expect that
a{1}c^(1/n{1}) - a{2}c^(1/n{2}) will have a comparatively large
value when one of the ions is H+ or OH-. Also we know that organic
anions are usually much more strongly adsorbed than inorganic anions,
and hence that in such cases a{1} is more nearly approached by the
value of a{2}. It should be emphasized perhaps, at this point, that
these various considerations are not based upon any facts relating to
the phenomena of imbibition in gels, or in gelatine in particular, but
are based upon the behaviour of colloids in endosmosis, kataphoresis,
electrolytic precipitation, adsorption, etc.
[Illustration: FIG. 1.]
Now if we select a few simple figures which are in accord with the above
considerations, we can examine the value of the factor
a{1}c^(1/n{1}) - a{2}c^(1/n{2}) in a purely illustrative and
typical way, and at any rate form some idea as to the manner in which it
is likely to vary. The figures might be:--
Ion. | _n_. | _a_.
-------------------------+---------+---------
Hydrion _or_ hydroxylion | 20 | 10
Kation of a metal | 15 | 7
Organic anion | 10 | 8
Inorganic anion | 6 | 6
For the sake of simplicity we can assume that these ions are all
monovalent. The ions adsorbed by unit mass will then be 10c^(1/20),
etc. If these hypothetical adsorption isotherms be plotted as usual we
get the fairly typical curves shown in Fig. 1.
Now in practice there are always two of these ions, each giving its own
specific effect in opposite senses, and the difference
( a{1}c^(1/n{1}) - a{2}c^(1/n{2}) ) represents the nett charge
adsorbed. Hence we have the following combinations:--
Inorganic acid 10c^(1/20) - 6c^(1/6)
Organic acid 10c^(1/20) - 8c^(1/10)
Alkali 10c^(1/20) - 7c^(1/15)
Inorganic salt 7c^(1/15) - 6c^(1/6)
If we plot these values of nett adsorption against the concentration we
obtain the curves shown in Fig. 2.
[Illustration: FIG. 2.]
On the assumption that the nett charge adsorbed is the dominant factor
in determining the maximum swelling at equilibrium, one must therefore
regard the curves of Fig. 2 as representing the changes in volume of the
swollen gel as the concentration is increased. Now in _type_ these
curves correspond to those obtained by experiment from hydrochloric
acid, acetic acid, caustic soda, and common salt. The maximum swelling
with hydrochloric acid increases rapidly with the concentration at first
and then rapidly decreases, though not at such a great rate. The
swelling with acetic acid increases less rapidly and to a less maximum,
but decreases more slowly. With common salt there is a slight swelling
followed by contraction. Caustic soda gives a rapid increase in volume
at first, afterwards much less so, and finally yields an exceedingly
slow decrease. The correspondence of these facts with the type-curves
inevitably suggests that the phenomenon of swelling might be accounted
for, in part at least, along these lines.
Of course it is not likely that the simple figures selected for the
illustration of the argument are either relatively or absolutely
correct. Thus we know that the adsorption curve for hydrions and
hydroxylions are not likely to be quite identical, as assumed above. As
gelatin is primarily slightly positive, it is probable that the values
of _a_ and of _n_ for hydrion adsorption will be relatively slightly
greater. The relative values supposed, however, are near enough to
illustrate the contention that the type of the maximum volume curve can
be explained on this assumption of different adsorption isotherms for
each of the ions.
If the remarks on the compression of the continuous phase be recalled,
it will be obvious that in the present paragraphs we have been giving
the question of equilibrium-volume a rather one-sided consideration. The
volume of the gel when equilibrium is established may be determined in
type by the nett charge adsorbed by the disperse phase, but it will be
modified also by the lyotrope influence of the particular substance on
the continuous phase. When gelatine swells in solutions the influences
on both phases are always in operation, and either upon occasion may
become predominant. In the case of neutral organic substances, such as
cane-sugar, the lyotrope influence is the determining factor. In the
case of neutral salts the predominant influence is decided by the place
occupied by the salts in the lyotrope series. If at either end of the
series the lyotrope influence is uppermost and the effect of ionic
adsorptions is practically swamped. Thus sodium sulphate and sodium
iodide hinder and promote imbibition respectively as could be expected
from their strong lyotrope power. On the other hand, in the case of
sodium chloride, which has comparatively feeble lyotrope influence, the
relatively different adsorptions of its ions comes to the fore. With
acids and alkalies the relatively large adsorption of the hydrion and
hydroxylion causes this to be the predominant influence, but we must
concede the possibility that purely lyotrope influences may be at work
in some cases, and especially at the greater concentrations. Indeed, it
is sometimes a difficult problem to decide whether an increase or
decrease in swelling is due to lyotrope or adsorptive influence, but,
broadly speaking, we can expect strong lyotrope effects at either end of
the series and also at large concentrations, and we can expect strong
adsorptive effects in dilute solutions, in the middle of the lyotrope
series and in the case of alkalies and acids.
For much of the above explanation of the nature and behaviour of
gelatine, the author must himself take responsibility, and in this
section he has freely quoted from his own papers upon the subject (see
References). He claims that his view of a gelatine gel as involving a
network of compressed water, liable to modification by lyotrope
influence upon the continuous phase and by ionic adsorptions of the
disperse phase, is most in harmony with the recent advances in our
knowledge of colloids; that much of the theory is a necessary corollary
of those discoveries; and also that he has found this view to be a sound
guide in practice, both in tanning and in gelatine manufacture.
Many other theories have been advanced, but most are generalizations
over too limited a field, and from experiments with only a few
substances, and show little or no correlation with the wider facts of
colloid behaviour. That of Procter, for example, discards altogether the
idea of a two-phased structure of the gel as an "unproved and rather
gratuitous assumption," dismisses surface tension considerations as
"more complicated and less verified," and adsorption as "wholly
empirical," whilst it ignores lyotrope influence and the analogy with
agar gels completely. Procter's theory applies mainly to the swelling of
gelatine by acids, which swelling he considers to be due to the osmotic
pressure of the anion of a highly ionizable salt formed by the chemical
combination of the acid with gelatine. On this assumption, mathematical
considerations show that the electric charge on the gelatine is given by
the expression z = sqrt(4ex + e^2), where z = the amount of ion taken
up, x the concentration of the surrounding solution, and e the excess
concentration of diffusible ions in the jelly.
The property of gelatine and glue which is chiefly used in classifying
them into grades of different commercial value, is the strength of the
jelly obtained as compared with any arbitrary standard gelatine. An
enormous number of other physical tests have been devised, but none are
nearly so simple or so reliable. Gelatine is unfortunately very liable
to hydrolysis even by water, and long before any amido-acids, etc., have
appeared there is a change to a not greatly hydrolyzed product
(sometimes called [beta] gelatine) which has lost the power of setting
to an elastic gel. It is thus the lyophile nature which has been
altered, and the fall in elasticity corresponds to the fall in power of
compressing water, which is proportional to the concentration of [alpha]
gelatine. Now the elasticity of a gelatine gel varies as the square of
the concentration. Hence if one so arranges the concentrations of
standard and unknown samples that gels of equal elasticity are obtained,
the concentration of [alpha] gelatine is the same in both gels, and the
_relative_ amounts of [alpha] gelatine in the original samples are
inversely proportional to the weights used to give gels of equal
elasticity. The "strength" of a gelatine or glue is therefore usually
stated as the number of grams of a standard gelatine which will yield a
gel with elasticity equal to that from 100 grams of the gelatine or glue
being tested. Elasticity is matched by lightly pressing with the
finger-tips.
It is also possible to grade samples of gelatine and glue by the
estimation of "peptones," whose amount indicates the degree of
hydrolysis. Nitrogen is estimated by Kjeldahl's method in the sample and
in the precipitate obtained by saturating a solution with zinc sulphate.
The difference is calculated as peptones by multiplying by 5.33. Trotman
and Hackford say that the results are in the same sequence as those of
the finger test. The method, however, is much more laborious than the
"finger test."
Gelatine is also graded according to the results of bleaching and
clarifying, but with quite arbitrary standards, largely determined by
the fancy of the customer.
Chemical analyses, involving estimations of ash, lime, fat, acid, water,
insoluble matter, and poisonous metals, _e.g._ arsenic, copper, zinc and
lead, are of value for special cases according to the destiny of the
goods. Special physical tests, such as "breaking strain" and "foam
test," are also of some little value in special cases.
REFERENCES.
"The Chemistry of Colloids," W. W. Taylor. 1915.
"Handbook of Colloid Chemistry," W. Ostwald. 1919.
"Chemistry of Colloids," Zsigmondy and Spear. 1918.
"Introduction to the Chemistry and Physics of Colloids," E.
Hatschek.
"Surface Tension and Surface Energy," Willows and Hatschek.
"Chemistry of Colloids," V. Pöschl.
"Grundzüge d. Dispersoid Chemie," von Weimarn.
"The Lyotrope Series and the Theory of Tanning," Bennett,
J.S.L.T.C., 1917, p. 130.
"The Swelling of Gelatine," Bennett, J.S.L.T.C, 1918, p. 40.
"The Swelling of Gelatine," Procter, _J.C.S. Trans._, 1914, =105=,
313; and _Koll. Chem. Beihefts_, 1911, =2=, 234.
"The Swelling of Gelatinous Tissues," Procter, J.S.C.I., April 16,
1916.
"Summary of Procter's Views, and Bibliography," Collegium (London),
p. 3, 1917.
"Lyotrope Influence and Adsorption in the theory of wet work,"
Bennett, J.S.T.C., 1920, p. 75.
For the "finger test," see--
"Glue and Glue Testing," Rideal, 2nd ed., p. 158.
"Leather Trades' Chemistry," Trotman, p. 241.