CHAPTER XXVIII.

_WASTE PRODUCTS AND THEIR DISPOSAL._

Hair -- Fleshings and glue-stuff -- Fat -- Bate-shavings -- Horns --

Spent tan -- Tan-furnaces -- Sewage and other waste liquids --

Chemical purification of sewage -- Settling tanks -- Filter-presses --

Bacterial purification of sewage -- Tannery waste-liquors PAGE 460

APPENDIX A.

_METHOD OF THE INTERNATIONAL ASSOCIATION OF LEATHER-TRADES CHEMISTS

FOR THE ANALYSIS OF TANNING MATERIALS: Corrected to 1901._

Sampling from bulk -- Preparation for analysis -- Preparation of

infusion -- Determination of tanning matters and non-tannins --

Colour-measurement -- Analysis of used liquors PAGE 475

APPENDIX B.

_THE DECIMAL SYSTEM._

Metrical weights and measures -- Centigrade thermometer PAGE 481

APPENDIX C.

_METHOD OF ANALYSIS OF TANNING MATERIALS OF THE AMERICAN ASSOCIATION

OF OFFICIAL AGRICULTURAL CHEMISTS: Corrected to 1901._

Preparation of sample -- Quantity of material -- Moisture -- Total

solids -- Soluble solids -- Non-tannins -- Tannins -- Testing of

hide-powder -- Testing non-tannin filtrate PAGE 482

APPENDIX D.

_LISTS OF COAL-TAR DYES SUITABLE FOR DYEING AND STAINING LEATHER,

furnished by Mr. M. C. LAMB._

Colours for staining leather -- Colours for dyeing vegetable-tanned

leather -- Dyeing and finishing chrome-leather -- List of colours

suitable for chrome-leather PAGE 485

INDEX PAGE 499

PRINCIPLES

OF

LEATHER MANUFACTURE.

[Illustration]

CHAPTER I.

_INTRODUCTORY AND HISTORICAL._

The origin of leather manufacture dates far back in the prehistoric

ages, and was probably one of the earliest arts practised by mankind.

The relics which have come down to us from palæolithic times, and the

experience of the modern explorer, alike tell us that agriculture is a

later and a higher stage of development than the life of the hunter; and

since, in the colder regions, clothing of some kind must always have

been a necessity, we may conclude that it was first furnished by the

skins of animals.[1]

[1] See also Gen. iii. 21.

While wet skins putrefy and decay, dry ones are hard and horny; and

nothing could be more natural to the hunter than to try to remedy this

by rubbing the drying skin with the fat of the animal, of which he must

have noticed the softening effect on his own skin. By this means a soft

and durable leather may be produced, and this process of rubbing and

kneading with greasy and albuminous matters, such as fat, brains, milk,

butter and egg-yolks, is in use to this day, alike by the Tartars on

Asiatic steppes and the Indians on American prairies; and not only so,

but we ourselves still use the same principle in the dressing of our

finest furs, and in the manufacture of chamois, and many sorts of lace-

and belt-leathers.

Such a process is described in the _Iliad_ (xvii. 389-393) in the

account of the struggle over the body of Patroclus:

“As when a man

A huge ox-hide drunken with slippery lard

Gives to be stretched, his servants all around

Disposed, just intervals between, the task

Ply strenuous, and while many straining hard

Extend it equal on all sides, it sweats

The moisture out and drinks the unction in.”

It must also have been early noticed that wood smoke, which in those

days was inseparable from the use of fire, had an antiseptic and

preservative effect on skins which were dried in it, and smoked leathers

are still made in America, both by the Indians and by more civilised

leather manufacturers. To this method the Psalmist refers[2] when he

says, “I am become like a bottle in the smoke;” and such bottles, made

of the entire skin of the goat, are still familiar to travellers in the

East.

[2] Ps. cxix. 83.

The use of vegetable tanning materials, though prehistoric, is probably

less ancient than the methods I have described, and may possibly have

been discovered in early attempts at dyeing; an art which perhaps had

its origin even before the use of clothing! The tannins are very widely

distributed in the vegetable kingdom, and most barks, and many fruits,

are capable of making leather.

The employment of alum and salt in tanning was probably of still later

introduction, and must have originated in countries where alum is found

as a natural product. The art was lost or unknown in Europe till

introduced into Spain by the Moors.

Leather manufacture reached considerable perfection in ancient Egypt. A

granite carving, probably at least 4000 years old, is preserved in the

Berlin Museum, in which leather-dressers are represented. One is taking

a tiger-skin from a tub or pit, a second is employed at another tub,

while a third is working a skin upon a table. Embossed and gilt leather

straps have been found on a mummy of the ninth century B.C., and an

Egyptian boat-cover of embossed goat leather, as well as shoes of dyed

and painted morocco, are still in comparatively good preservation. The

art is of very early date in China, and was well understood by the

Greeks and Romans. In the Grosvenor Museum at Chester is the sole of a

Roman _caliga_, studded with bronze nails, which is yet pretty flexible.

After the fall of the Roman empire many arts were lost to Europe, and

it was not until the Moorish invasion of Spain that the art of dyeing

and finishing the finer kinds of leather was reintroduced.

England was very backward in this manufacture up to the end of the last

century, owing to the fossilising influence of much paternal

legislation, and of certain excise-duties, which were only repealed in

1830. Since this time the art has made rapid strides, especially in the

use of labour-saving machinery, and England may at the present moment be

considered fairly abreast of any other country as a whole; though in

some special manufactures we are surpassed by the Continent and by

America. In making comparisons of this kind, it must, however, be

remembered that, especially in sole-leather tannage, the most rapid

progress has been made during the last few years in those countries

which were more backward, and that therefore our superiority is much

less pronounced than formerly, and in a few years will probably cease to

exist unless marked improvements are introduced in the methods of

production.

In the sketch of the development of leather manufacture which has just

been given, it has been implied that its object is to convert the

putrescible animal skin into a material which is permanent, and not

readily subject to decay, while retaining sufficient softness or

flexibility for the purposes for which it is intended. As these range

from boot-soles to kid-gloves, there are wide divergences, not only in

the processes employed, but also in the materials used and in the

principles of their application.

The most important method of producing leather is by the use of

vegetable tanning materials, and this is perhaps the only one which is

really entitled to be called “tanning,” though the distinction is not

very strictly adhered to. It includes the whole range--from sole

leather, through strap, harness and dressing leather, to calf and goat

skins, and the various sumach tannages which yield morocco and its

imitations. All of these products but the first and the last undergo,

after tanning, the further processes of “currying,” of which the most

important operation consists in “stuffing” with oily and fatty matters,

both to increase the flexibility and to confer a certain amount of

resistance to water. Sumach-tanned skins are not strictly “curried” but

usually receive a certain amount of oil in the process of “finishing.”

Next in importance to the vegetable tannages are the “tawed” leathers

produced by the agency of alum and salt, including the “white leathers”

for belt laces and aprons, and calf- and glove-kid. A connecting link

between tanning and tawing is found in the “green leather,” “Dongola,”

and “combination” tannages, in which alum and salt are employed in

conjunction with vegetable tanning materials, and especially with

gambier.

Salts of several of the metals, and particularly those of aluminium,

iron, and chromium, have the power of converting skin into leather; and

processes in which salts of chromium are used have recently attained

very considerable commercial importance.

In the production of calf- and glove-kid, in addition to alum and salt,

albuminous and fatty matters, such as egg-yolk, olive oil and the gluten

of flour, play a considerable part, and are thus linked both to the

primitive methods in use by the Indians and Kalmucks, and to those by

which “crown” and “Helvetia” leather, and many other forms of belt- and

lace-leathers are now produced by treatment with fats and albumens.

From these again the step is a short one to the “chamois” and “buff”

leathers, and the German “_fettgar_” leathers, in which oils and fats

only are used; and these are probably again related chemically to

leather produced by the aid of formaldehyde and other aldehydes.

In an attempt to view all these complex processes from the scientific

standpoint, the reader should constantly realise that the present

methods of leather manufacture are the results of tens of centuries of

experience, and of innumerable forgotten failures, and must not

therefore expect that they can be easily superseded. Science must follow

before it can lead, and its first duty is to try to understand the

reasons and principles of our present practice, for we can only build

the new on the foundation of what has been already learned. Another

fact, which is scarcely understood by the practical man in his demands

on science, is that in leather manufacture every question which is

raised seems to rest on the most recondite problems of chemistry and

physics; the chemistry of some of the most complex of organic compounds,

and the physics of solution, of osmose, and of the structure of colloid

bodies--problems which are yet far from completely conquered by the

highest science of the day.

It may seem bold to attempt the scientific treatment of such a subject

at all; and, indeed, it must be admitted that our knowledge is still far

from adequate for its complete accomplishment, but enough has been done

to lay a foundation for future work, and this can at least be summarised

and arranged in an available form. The subject falls naturally into two

sections, in the first of which the processes of manufacture would only

be described in general terms, and with sufficient fulness to enable the

reader to understand the scientific considerations on which they are

based, and the methods of investigation which can be applied to them;

while in the second an effort should be made to give working details of

the various processes sufficient to enable those with a general

knowledge of the trade to experiment successfully in its various

branches. It was at first intended that these two sections should be

published in one book as a second edition to the Author’s ‘Text-book of

Tanning,’ but owing to the long delay in its publication, it was decided

to publish the first section under the present title ‘Principles of

Leather Manufacture,’ leaving the latter section ‘Processes of Leather

Manufacture’ to a later, and I fear, somewhat uncertain date; while the

strictly chemical portion has already appeared in the ‘Leather

Industries Laboratory Book,’ frequently referred to in the following

pages under the abbreviation “L.I.L.B.” Where quantities and details are

given, they must not be taken as recipes to be blindly followed; or

even, in every case, as the best known methods; but rather as mere

guides to experiment, which must be modified to suit varying conditions

and requirements. It is the special virtue of the scientific, as opposed

to the merely traditional way of looking at such questions, that knowing

the cause and effect of each part of the process, it can so adjust them

as to get over difficulties, and to suit novel conditions. It is

needless to add that many methods are jealously preserved as trade

secrets, and full details are frequently unattainable.

After what has just been said, it may be well to emphasise the great

importance of practical knowledge and experience to the leather

manufacturer. Even in trades which have reached the highest scientific

development, such, for instance, as the manufacture of the coal-tar

colours, the small experiments of the laboratory are not transformed

into manufacturing operations without experience and sometimes even

failure; and this must still more often be the case in a trade like

that of leather-making, where our knowledge of the actual changes

involved is still so incomplete. On the other hand, the cost of

experiments on a manufacturing scale is usually so heavy that the least

scientific must admit the advantage of learning all which the laboratory

can teach before venturing on anything more; while even our present

imperfect knowledge of the chemical changes involved will often warn us

off hopeless experiments, and give us hints of the directions in which

success may be attained. A knowledge of chemistry will probably prove at

least as important to the future of our trade as that of mechanics has

been in the past.

CHAPTER II.

_INTRODUCTORY SKETCH OF LEATHER MANUFACTURE._

The object of tanning has been stated to be the rendering of animal skin

imputrescible and pliable, but as we now rarely require leather with the

hair on, preliminary processes are needed to remove it, and to fit the

skin for tanning, and the nature of these processes has great influence

on the subsequent character of the leather produced.

The first step is usually a washing of the skin to remove blood and

dirt; while, where it has been salted or dried, a more thorough soaking

is needed to remove the salt, and to restore the skin to its original

soft and permeable condition.

The hair is then loosened by softening and partial solution of the

epidermis structures (see p. 47) in which it is rooted. This is most

generally accomplished by soaking for some days in milk of lime, which

is occasionally assisted by the addition of caustic alkalies or of

sulphides. When the latter are used in concentrated solution, the hair

itself, as well as the epidermis tissues, is softened and destroyed in

the course of a few hours. The lime not only serves to loosen the hair,

but swells and splits up the fibre-bundles of which the hide tissue is

composed, and so fits it to receive the tannage (cp. p. 125).

For some purposes a regulated putrefactive process is substituted for

the liming; the hides or skins being hung in a moist and warm chamber

(see p. 119), when the soft mucous layer which forms the inner part of

the epidermis is disintegrated, partly by direct putrefaction, partly by

the action of the ammonia evolved, so that the hair can be scraped off.

In this case the hide-fibre is not swollen, and the necessary swelling

has to be obtained by subsequent processes.

In whatever way the hair has been loosened, it is scraped off with a

blunt and somewhat curved two-handled knife on a sloping rounded “beam”

of wood or metal; this operation being termed “unhairing” (see p. 144).

This is generally followed by “fleshing,” which is performed on the same

beam with a somewhat similar knife, which, however, is two-edged and

sharp. In this operation, portions of flesh, and the fat and loose

tissue which underlie the true skin (see p. 147) are removed by scraping

and cutting. Machines for fleshing are also largely in use for certain

purposes (see p. 148).

For sole leather, the hide, after some washing in soft water to cleanse

from lime, is then ready for the actual tanning process; but for the

softer leathers more thorough treatment is needed to remove the lime,

and to still further soften the skin by solution and removal of a

portion of the cementing substance of the fibres.

This treatment is generally of a fermentive or putrefactive nature, and

the most common form is that known as “bating,” which consists in

steeping in a fermenting infusion of pigeon- or hen-dung. The theory of

its action is not yet thoroughly understood, but the effect is largely

due to the unorganised hydrolysing ferments produced by the _bacteria_

present; while at the same time the lime is neutralised and removed by

the weak organic acids and salts of ammonia which are produced; and the

fibre which had been plump and swollen with lime, becomes extremely

relaxed and flaccid.

In the lightest leathers, such as kid- and lamb-skins for gloves, and

goat and sheep for moroccos and the like, dog-dung is substituted for

that of fowls, and the process is then called “puering” (see p. 170).

These processes are often followed by “drenching,” which sometimes

indeed takes their place, the skins being soaked in a fermenting bran

infusion. In this, the small quantities of acetic and lactic acid formed

by fermentation are the active agents, neutralising and dissolving the

lime, and cleansing and slightly plumping the pelt (see p. 166).

The tanning process which follows consists in soaking the pelt in

infusions of various vegetable products containing bodies of the class

known as “tannins,” which have the power of combining with skin-fibre

and converting it into leather.

If at first strong infusions were used, they would act too violently on

the surface of the skin, hardening and contracting it so that the

subsequent tannage of the interior would be impeded, and the “grain” or

outer surface would be “drawn” and wrinkled. This is avoided by the use

at first of very weak infusions which have already been used on goods in

a more advanced stage. In the later part of the process much stronger

solutions are employed, and the hides are frequently “dusted” in them

with ground tanning material.

In the case of sole leather, these processes may require from two to

twelve months for completion; after which the leather is dried,

smoothed, and compressed by mechanical means, and is then ready for use.

Dressing-leathers, ranging from calf-skins to harness-hides, receive a

much shorter tannage, and the subsequent treatment with fats and oils,

which, together with mechanical manipulations, constitute “currying.”

The thin film of grease distributed over the surface of the fibres

renders them supple, and to some extent waterproof.

The lighter fancy leathers, such as morocco, are dyed, and undergo many

complex processes to fit them for their required purposes and improve

their appearance.

Many skins such as calf, glove, and glacé kid, are not tanned, but

“tawed” by a solution of alum and salt, which is often supplemented with

mixtures of flour and egg-yolk to fill and soften the leather.

Salts of chromium are also employed in place of alum and salt, and

produce an equally soft, but more permanent and enduring leather.

Lastly, wash-leather, or so-called “chamois,” and buff-leather are

produced by fulling the prepared pelt with fish or whale oil, which

converts the skin into leather by subsequent oxidation, during which

aldehydes are evolved.

CHAPTER III.

_THE LIVING CELL._

The larger part of the materials employed in leather manufacture are

organic in their origin, and the skin itself is an organised structure,

while the life-processes of putrefaction and fermentation play a large

part in the tannery. Some knowledge, therefore, of biological structures

and processes is necessary to a full understanding of much which

follows, and a few words are not out of place with regard to the

foundations of life itself.

The bricks of which all living structures are built are the living

“cells” and their products, and these first elements differ little, if

at all, whether the life is animal or vegetable, the distinction being

produced rather by the way in which they are put together, than by

differences in the cells themselves. This is so much the case that it is

often difficult to decide in which of the two classes to place the

simplest organisms, since most of these forms are capable of active

movement, and their modes of nutrition and reproduction are common to

both kingdoms.

In its simplest form, the cell, whether animal or vegetable, is strictly

speaking not a cell at all, but consists merely of a minute mass of

living jelly or protoplasm. Such is the amœba found in water and damp

soil, such are the lymph-cells and white blood-corpuscles of our bodies,

and such also some stages at least of the lowest forms of fungi, like

the _Æthalium septicum_ which is sometimes found on old tan-heaps as a

crawling mass of yellow slime. If a drop of saliva be examined with the

microscope under a cover-glass, with one-sixth objective and small

opening of diaphragm,[3] a few scattered semi-transparent objects will

be found, of the apparent size of a lentil or small pea, and of rounded

form. These are lymph-corpuscles (Fig. 1). Their contents are full of

small granules, and if they be observed quickly, or if the slide be kept

at about the warmth of the body, it will be noticed that these are in

constant streaming motion. If the warmth can be kept constant, which is

difficult without special apparatus, and the cells can be observed from

time to time, it may be seen that they lose their circular form, and put

out protuberances (pseudopodia, “false feet”) one of which will

gradually increase in bulk, till it absorbs the whole cell, which thus

crawls about. It will now readily be understood how these cells wander

through all the tissues of the body, passing through the smallest pores

like the fairy who put her finger through a keyhole, and grew on the

other side till she was all through! This independent vitality, in a

warm and suitable nutrient liquid, may continue for more than a week,

and, in the case of amœba, quite indefinitely.

[3] For details of microscopic manipulation in this and the following

chapter see L.I.L.B., p. 234 _et seq._

[Illustration: FIG. 1.--Lymph-corpuscle of frog, showing gradual change

of form.

(Ranvier.)]

It is possible that by close attention, a rounded or elongated body,

somewhat like an oil-globule, may be seen within the cell, though it is

generally more obvious when the latter has been killed and stained with

a weak solution of iodine. This is the nucleus, and within it is a still

smaller speck called the nucleolus, which bears an important, and as

yet little understood, part in the life-history of the cell. After a

period, it undergoes certain somewhat complicated changes, and divides

into two, the nucleus elongates, and also divides, each half carrying

with it a portion of the living protoplasmic jelly, and thus forming two

complete and independent cells. This is the life-history, not only of

the lymph-cell, but with more or less modification, of every living cell

or tissue.

[Illustration: FIG. 2.--Yeast-cells, much magnified.]

These cells, like all living things, feed on the nutriment which

surrounds them, and even enclose small particles of solid food, which

are gradually dissolved and disappear. In this way the white

blood-corpuscles are said to feed upon and destroy the still smaller

organisms which gain access to the blood, and which might otherwise

cause disease. The matter which cells consume is not, of course,

destroyed, but simply converted into other forms, some of which are

useless, or even poisonous to the cells, and which, like the secretions

of higher animals, are discharged into the surrounding fluids; while

others are retained, and contribute to the growth of the cell. Thus most

vegetable cells secrete cellulose, or plant-tissue, which forms a wall

enclosing the protoplasm, and so justifies the name of cell. If to warm

water and a little sugar we add enough yeast to render it slightly

milky, and examine it like the saliva, we shall have before us typical

vegetable cells of the simplest form (Fig. 2). There is the same

granular protoplasm, and there is the nucleus, though it cannot be seen

without special preparation, the rounded spaces which look like one,

being simply filled with transparent fluid, and called vacuoles. There

is, however, no motion, as in the case of amœba, for the cells are

enclosed in a tough skin of cellulose, which will be evident if they are

crushed by putting some folds of blotting paper on the cover-glass, and

pressing it with the handle of a needle or a rounded glass rod, when the

protoplasm will be forced out and the skin remain like a burst bladder.

This will be more obvious if the cells are previously stained with

iodine or magenta, which will stain the protoplasm, but not the

membrane. It is easy to observe the multiplication of the yeast-cells,

which is somewhat different to that of the corpuscles. Instead of

enlarging as a whole, and dividing into two equal cells, a small bud

appears on the side of the parent-cell, and enlarges till it becomes

itself a parent-cell with buds of its own. These do not break away at

once, and hence chains and groups of attached cells are formed which are

easily noticed in growing yeast if a microscope be employed. The

principal nutriment of yeast is grape-sugar or glucose; and much more of

this is consumed than is needed to produce the cellulose wall and the

substance of new cells; just as in the animal, sugar, starch and fat are

consumed to give heat and energy. In the yeast, this extra sugar is

split up into carbon dioxide, which escapes as gas, and to which yeast

owes its power of raising bread; and into alcohol, which in too large

proportion is poisonous to the yeast itself.

[Illustration: FIG. 3.--Epithelium-cells. Ranvier. _p_, pressure-marks;

_g_, granular protoplasm.]

In examining the saliva for lymph-cells, it is probable that some much

larger objects may have been noticed of irregular polygonal outline and

with a well-marked nucleus. These are cells from the lining _epithelium_

of the mouth, and only differ from those of the _epidermis_ of skin in

their form and size (Fig. 3). Note the markings caused by the pressure

of overlapping cells. In these cells the wall is formed of keratin or

horny tissue, which takes the place of the cellulose of the yeast.

[Illustration: FIG. 4.--_Penicillium glaucum_, a common green mould.]

Other simple forms of cell are those of _Saccharomyces mycoderma_ or

_torula_ which forms a skin on the surface of old liquors, and which

much resembles a small yeast; and of the various ferments which are

found in liquors, bates and drenches, which will be more fully described

in the chapter following.

Many of these, such as the acetic and lactic ferments, which, like all

other _bacteria_, multiply by division, do not separate, but remain

connected in chains or chaplets, like a string of beads. From these, the

step is not a long one to the _hyphæ_ or stems of the higher moulds,

which are too frequently found on leather which has been slowly dried,

and which consist simply of tubular cells which elongate and divide by

the formation of _septa_ or cross-partitions, and thus build up a

complicated plant-structure (Fig. 4). As we proceed higher in the scale

of plant and animal life, the forms and products of the cells become

more varied, and instead of one single cell, fulfilling all the

functions of the plant or animal, each class of cell has its own

peculiar duties and properties, while all work together for the

maintenance of the complex structure of which they form a part.

CHAPTER IV.

_PUTREFACTION AND FERMENTATION._

The chemical changes produced by the unicellular plants, such as yeasts

and bacteria, to which allusion has been made in the last chapter, are

known as fermentation and putrefaction, and are of such importance to

the tanner, both for good and evil, that the subject must be treated in

some detail. No scientific distinction exists between fermentation and

putrefaction, though it is customary to restrict the latter term to

those decompositions of nitrogenous animal matter which yield products

of disagreeable smell and taste.

The organisms which are the cause of both fermentation and putrefaction

are known by the general term of “ferments.” This term has also been

extended in recent years so as to include the so-called “unorganised

ferments” (enzymes, zymases) which are active products secreted by the

“organised ferments” or living organisms.

These latter are again divided into three classes:--

1. Moulds.

2. Yeasts (Saccharomycetes).

3. Bacteria.

The members of one class are distinguished from those of another by

their form, and, more especially, by the substances they produce during

their life-history. All three classes are now considered to be fungi.

All ferments possess the following three properties:--

1. They are nitrogenous bodies.

2. They are unstable, i.e. they are destroyed by heat, chemicals,

etc.

3. A relatively small quantity of the ferment is capable of producing

great changes in the substances upon which it acts, especially if the

products of the change can be removed as they are formed.

The general character of fermentation will be best understood by a

closer study of the yeast cell, which has already been described (p.

12), and its life-history briefly sketched. It has been shown that it is

a growing plant of a very simple type, belonging to the fungi. These are

devoid of the green colouring matter which enables the higher plants to

utilise the energy of sunlight to assimilate the carbonic acid of the

atmosphere, exhaling its oxygen, and employing its carbon for the

building up of tissue; and they must therefore, like animals, have their

nutriment ready formed, and capable of supplying energy by its

oxidation. For yeast, as has been stated, the appropriate nourishment is

glucose, or “grape-sugar.” This is broken down, in the main, into the

simpler compounds, alcohol and carbonic acid, while a small portion is

utilised for the building up of the cell and the formation of secondary

products. The main reaction is represented by the following equation:

C₆H₁₂O₆ = 2C₂H₆O + 2CO₂

_Glucose_ _Alcohol_ _Carbon dioxide_

Yeast cannot directly ferment ordinary cane-sugar (C₁₂H₂₂O₁₁), but

secretes a substance called invertase, which so acts on the sugar as to

break it up, with absorption of one molecule of water, into two

molecules of fermentable glucose (dextrose and levulose) which serve as

nourishment for the yeast.[4] This invertase is the type of the series

of bodies which are known as “unorganised ferments,” enzymes, or

zymases, differing from the organised ferments in being simply chemical

products without life or power of reproduction, but capable of breaking

up an unlimited quantity of the bodies on which they act, without

themselves suffering change. The way in which this is done is not

clearly understood, but some parallel may be found to it in the action

of sulphuric acid on alcohol, of which it will convert an unlimited

quantity into ether, without itself suffering any permanent change. The

action of enzymes is limited to breaking down complex bodies into

simpler forms, often with absorption of water, as in the case of sugar,

while some of the products of living ferments are often complex, a part

of their nutriment being broken down into simple products such as

carbonic acid, marsh gas and ammonia, to supply the necessary energy to

elaborate the remainder.

[4] Compare O’Sullivan and Thompson, Jour. Chem. Soc., 1890, p. 834;

1891, p. 46.

Very many different unorganised ferments are known to exist, as they are

not only produced by yeasts and bacteria, but are formed by the cells of

higher plants and animals; thus the digestive principles, pepsin,

trypsin, ptyalin, are of this character--ptyalin, like diastase,

converting starch into sugar; and such bodies fulfil many functions both

in animal and vegetable economy. In fermentation, as in disease, it is

often difficult to distinguish what is due to the direct action of

bacteria, and what to the unorganised ferments which they produce, and

the question is further complicated by the fact that in most natural

fermentations more than one ferment-organism is present. Sometimes the

action of the unorganised ferments may be distinguished by the fact that

the addition of chloroform has little effect on their activity while it

paralyses that of the living organism. By exposure to high temperature

both are destroyed, the bacteria, yeasts and moulds being killed and the

unorganised ferments coagulated like white of egg, and so rendered

inoperative. Many antiseptics also destroy the activity of both

organisms and enzymes; but others, like chloroform, have no action on

the latter. In some cases, as in that of invertase, the actual zymase

can be precipitated by alcohol from its aqueous solution, filtered off,

and restored to activity by transference into water. Since both

classes of ferments are destroyed by high temperatures, all

fermentation-processes are completely and permanently arrested by

exposure to sufficient heat, and subsequent preservation in vessels so

closed that no new ferment-germs can gain access. A familiar instance is

that of tinned meats. All fully developed bacteria are destroyed by a

very short exposure to a boiling temperature, and most by 60° to 70° C.,

but many species produce spores which are extremely difficult to

destroy. The thermophilic bacteria discovered by Globig and further

investigated by Rabinowitsch,[5] thrive at a temperature of 60° C. About

eight species are known, and they take part in the heating of hay and

similar fermentations where high temperatures are involved, and are

therefore presumably present in spent tan.

[5] Centr. Blatt für Bakt., II. Abth. vol. i. p. 585.

For absolute sterilisation it is therefore necessary either to boil

under pressure so as to raise the temperature to, say 110° C., or to

heat repeatedly for a short time to temperatures of 80°-100° C. at

successive intervals of 24 hours, in order to allow the spores to

develop. This process is frequently performed for bacteriological

observation in flasks or test-tubes merely stopped with a plug of

sterilised cotton-wool, which has been found to efficiently filter the

germs from the air which enters through it (see L.I.L.B., p. 270).

The ferment-organisms cannot thrive and multiply unless they have proper

nourishment and conditions of growth, the amount of moisture and the

temperature being two of the most important of the latter. Use is made

of this in the preservation of many articles of food, etc., since by

ensuring that at least one of the conditions necessary for growth shall

be absent, these substances are prevented from decomposing. For

instance, hides are preserved by drying them; the absence of sufficient

moisture hindering the growth of any organisms in them so long as they

are dry, but as soon as they become somewhat damp, putrefaction

commences at once.

The waste products of organisms are often poisonous to themselves, and

for this reason fermentations frequently come to an end before the whole

of the substance is fermented. Thus neither beer nor vinegar can be

obtained of more than a certain strength by direct fermentation, the

alcohol or acetic acid checking the growth of their respective ferments.

A solution of glucose “set” with the lactic ferment of sour milk will

only produce lactic acid to the extent of about half a per cent.; but if

chalk be added, the lactic acid will be neutralised as produced, and the

fermentation will go on till the whole of the glucose is converted into

insoluble calcium lactate.[6] When this is accomplished the lactic

ferment dies from want of nutriment, and its place is taken by another

organism, of which some germs are sure to be present, which ferments the

calcium lactate into calcium butyrate. If the nourishment fails, or the

conditions become less favourable for one ferment than for some other

which exists even in small quantity in a liquid, the former is quickly

overgrown and killed, and the latter takes its place. Thus the ordinary

ferment of the bran drench will die out rapidly unless constantly

transferred to fresh bran infusions.

[6] For the practical preparation of lactic acid, the solution may

contain 7¹⁄₂-11 per cent. of glucose, and some nitrogenous

nourishment. The solution should be slightly acid. See Journ. Soc. Ch.

Ind., 1897, p. 516.

Many of the products of bacteria (like those of some of the higher

plants) are intensely poisonous both to animals and man. Many of the

severe symptoms of disease are caused by these poisons produced in the

body. Thus the tetanus-bacteria produce a poison similar in its effects

to strychnine, and quite as virulent. Not only are such poisons produced

by disease-bacteria in the body, but frequently also in the earlier

stages of putrefactive fermentation. The latter are known as

_ptomaines_, and when present in cheese and preserved foods are liable

to cause poisoning. Such putrefactions are often unaccompanied by any

disagreeable odour or flavour.

The fermentations which are most important in the tannery are, firstly,

the ordinary putrefaction which attacks hides as well as other animal

matter, and which is usually a complicated process carried on by many

sorts of bacteria and other micro-organisms. This may be regarded as

generally injurious to the tanner; but it is utilised in the “sweating”

process for depilation and in the “staling” of sheepskins, in both of

which advantage is taken of the fact that the soft mucous layer of the

epidermis, which contains the hair-roots, putrefies more rapidly than

the fibrous structure of the hide itself. In soaking also, use is made

of the power of putrefactive ferments to dissolve the cementing

substance of the hide, though in this case with doubtful advantage to

the tanner. In the liming process putrefaction makes itself felt when

the limes are allowed to become stale and charged with animal matter,

softening the hide and finally rendering the leather loose, empty and

inclined to “pipe.” Here the effect is in many cases useful if not

carried too far.

In bating and puering, the action is almost entirely due to the enzymes

and other products of bacterial activity, the original chemical

constituents of the dung being apparently of minor importance. Naturally

the liquid is adapted to the growth of many other organisms beside those

acting most advantageously on the hide, and injury in the bates from

wrong forms of putrefaction is very common, if indeed it is not always

present in greater or less degree.

In drenching, the effect is, at first, entirely due to the weak acids

produced by bacterial fermentation of the bran, but becomes complicated

in its later stages by putrefactive and other fermentations which may be

desirable or otherwise.

In the tanning liquors, fermentation is not so marked, but is of great

importance owing to the production of acids by bacterial action from the

sugars present in the material. The acids themselves are apt to be

fermented and destroyed, principally by the oxidising action of

_Saccharomyces mycoderma_ and the higher moulds (see p. 14), which also

act destructively on the tannins.

The effect of these acids on the hides is to swell them and to

neutralise any lime they may contain. They also give to the liquors a

characteristic sour taste, as a consequence of which, liquors containing

acetic and lactic acids are usually known in the tannery as “sour

liquors.”

It is doubtful whether the action of fungi is completely stayed even by

the drying process. The heating of leather in the sheds is due to

bacteria and the higher moulds, and Eitner considers their growth one of

the causes of the “spueing” or “gumming” of curried leathers.

From what has been said, it is obvious that, with regard to

fermentations, a double problem is presented to the leather

manufacturer, since he desires to utilise those which make for his

advantage, while controlling or destroying those which are injurious.

The first step to a solution of these problems is a more complete

knowledge of the organisms which serve or injure us, that we may, as it

were, discriminate friends and enemies. We may then approach the

question in two ways. Taking the drenching process as an example, we may

on the one hand introduce a “pure cultivation” of the right ferment into

a sterilised bran infusion, and so induce only the one fermentation

which we require; or, on the other hand, as different ferments are

affected in varying degrees by antiseptics, we may perhaps choose such

as permit the growth of the organism we want, while killing or

discouraging the rest. We may also arrange the nutriment, temperature,

degree of acidity and other conditions, so as to favour one organism

rather than another. All three methods have been applied in brewing with

good results.

CHAPTER V.

_ANTISEPTICS AND DISINFECTANTS._

“Antiseptics” are often defined as substances which check putrefaction

without necessarily destroying bacteria and their spores, while

“disinfectants” are poisonous to ferment-organisms, and actually destroy

them; great differences exist in the extent of their sterilising power,

and the whole distinction is one rather of degree than of kind, and has

little practical value. Thus common salt is incapable of _killing_ most

bacteria, even in concentrated solution, though it holds putrefaction in

check both by withdrawing water from the hide and by directly preventing

the multiplication of bacteria. If the salt be washed out of the hide,

putrefaction is at once resumed by the organisms present. Hides, on the

other hand, which have once been sterilised by powerful disinfectants,

such as phenol (“carbolic acid”) or mercuric chloride, do not again

putrefy till the organisms which are killed are replaced by fresh ones

from outside. The action of sodium sulphate, and many other salts, is

similar to common salt in this respect, while a large proportion of the

aromatic compounds are permanently disinfectant, though their efficiency

varies with the species of bacteria involved.

Biernacki and others have shown that some disinfectants when extremely

diluted actually stimulate alcoholic fermentation, and probably the

growth of other ferments, e.g. mercuric chloride 1 in 300,000, salicylic

acid 1 in 6000, and boric acid 1 in 8000, and in many cases organisms

become habituated to antiseptics in doses which would at first have

proved fatal.

The number of antiseptics available is now so great that it is

impossible to give a detailed account of all, but the following are

among those which are best known and have been practically employed.

_Lime_ possesses some antiseptic properties, and is largely used in the

preservation of fleshings before they are sent off to the glue factory.

They are most conveniently stored in a large vat filled with a strong

milk of lime. Dilute solutions of caustic alkalies have an effect

similar to that of lime.

_Common salt_, sodium chloride, NaCl, acts to a certain extent by its

solubility and a dehydrating effect on animal tissues common to

chlorides, which removes water from hides and other materials which it

is used to preserve. Probably the latter characteristic has a good deal

to do with its effect in checking the development of bacteria, since

many species thrive quite well in weak salt solutions, and some even in

brine, and the dehydrating effect of the salt enables it to harden many

animal tissues if used in sufficient quantity, the water they contain

running away in the form of brine.

Ordinary rock salt frequently contains ferric chloride, and this, either

originally present in the salt, or in some cases derived from the action

of the latter upon the iron contained in the blood, is the cause of what

is known as “salt-stains.” These show but little during the liming of

the hides, unless sulphides are used, when stains appear of a greenish

black, from the formation of sulphide of iron; when, however, the hides

come into the tanning liquors, black or blue stains are produced by the

action of the tannin, which are partially removed by the acids of the

liquors during the tanning process, but generally show to some extent in

the finished hide. There is another species of salt-stain, not

apparently due to iron, but to the colouring matter produced by some

fungoid or bacterial growth, which it is practically impossible to

remove, and which is stated to be sometimes caused by the use of old

salt with which hides have been previously salted. Iron stains are most

readily recognised by the use of a solution of potassium ferrocyanide or

thiocyanate slightly acidified by hydrochloric acid. If this be applied

to the leather, the stains will be changed from a blackish to a blue, if

the former, or a red colour if the latter salt has been used. A more

absolutely conclusive proof is to lay a piece of filter paper soaked in

dilute hydrochloric acid upon the stain, and then to test for iron upon

the paper with ferrocyanide or thiocyanate. The freedom of the paper

itself from iron must be ascertained before use. Iron-stains produced in

the salted state are more difficult to discharge than those which are

caused later in the tanning process, since iron salts have distinct

tanning power, and attach themselves firmly to the untanned fibre. On

the Continent, where common salt is heavily taxed, alum, carbolic acid,

naphthalene and other materials are frequently added to it to

“denaturise,” or render it incapable of being used as food, and these

additions are often the cause of trouble to the tanner.

_Sodium sulphate_, Na₂SO₄, has little if any disinfectant power in

dilute solution, but if used in the calcined form (anhydrous sodium

sulphate) as proposed by Eitner[7] as a substitute for common salt in

preserving hides, it withdraws water from the hide and crystallises with

10 Aq (about 56 per cent.). This does not run away like brine, but

remains in the hide, which retains its weight, and remains plump and

swells well in the limes and liquors, which chlorides have a great

tendency to prevent; 10-15 per cent. on the weight of the hide is

sufficient, while salt must be used in nearly double this quantity. Care

must be taken that the sulphate used is free from bisulphate, NaHSO₄,

which has a powerful swelling effect upon the hide-fibre, like sulphuric

acid. The neutral sulphate does not redden methyl orange or litmus.

Pickled skivers may be in part preserved by the sodium sulphate formed

by the action of sulphuric acid upon the salt employed in the pickling

bath (see p. 90).

[7] Gerber, 1880, p. 185.

_The stronger mineral acids_ have considerable antiseptic power, and are

of course especially fatal to such ferments as thrive best in alkaline

solutions. The use of sulphuric acid in pickling skivers has already

been alluded to, and a very dilute solution applied without salt to raw

hides prevents putrefaction, though the principal object in using it is

to plump the hides and produce a fictitious weight and substance which

disappear on tanning. Such hides of course have a powerful acid reaction

to litmus. Sulphuric acid in small quantities has been used with

advantage in soaking E.I. kips. A very small excess of hydrochloric acid

will sterilise putrid effluents, and no doubt nitric or sulphuric acid

would have the same effect. The powerful effect of mineral acids on

animal fibre, and their solvent action on cements and iron, preclude

however, their general use as antiseptics.

More important is the use of sulphurous acid and sulphur dioxide, which,

from their mild acidity and great antiseptic powers, are capable of a

variety of useful applications. Considerable doubt has been raised as to

the germicide power of sulphur dioxide, and it is certain that the dry

gas is less effective on dry objects than when applied in solution, or

to moist materials, as is almost invariably the case in the tannery. It

may possibly be more efficient in its action on some moulds and

putrefaction-ferments than on the pathogenic bacteria which have been

most frequently used to test the power of disinfectants; but in practice

it is found extremely useful in the brewery and in gelatine manufacture,

and there is no reason that it should be less so in the tannery.

The gas is most conveniently produced by burning sulphur, which produces

double its weight of sulphur dioxide. If used for “stoving” drying rooms

and other places infested with moulds, care must be taken to avoid risk

of fire. A shallow cast-iron pot set on bricks or sand is generally the

most suitable vessel, and the sulphur may be ignited by a piece of

red-hot iron or a rag which has been previously dipped in melted

sulphur. It is corrosive to metalwork, and bleaches many colours, but

does not produce any marked injurious effect on leather, though the

sulphuric acid formed by oxidation may, if not removed, ultimately make

it tender.

For many purposes a solution of the gas is required, and this is most

easily made by burning the sulphur in a small metal or firebrick stove

from which the fumes are sucked through a “scrubber,” which, on a small

scale, is conveniently made of large glazed sanitary pipes, packed with

coke or broken earthenware, over which water is allowed to trickle. The

lowest pipe has an opening for a branch pipe, which is connected with

the stove and rests on three bricks in a tub, which collects the acid

solution and forms a water-seal to prevent the escape of gas. Above the

inlet for the gases is fixed a wooden grating on which the coke rests.

The scrubber may be 10-15 feet in height and connected at the top with a

chimney or steam ejector to produce the draught. The arrangement is

illustrated in Fig. 5. Another method is to burn the sulphur in a closed

cylinder and to force the products through water with an air-compressor

or steam-jet injector.

In place of using a scrubber, the fumes may be blown by a steam ejector

direct into a tank. This is a very good arrangement for washing and

bleaching hair, etc., but where large quantities of solution are

required is inferior to the scrubber. Ejectors of hard lead or regulus

metal should be used, and are less acted on by the dry gases than by the

very dilute moist exhaust from the scrubber (see p. 335).

[Illustration: FIG. 5--Sulphurous acid apparatus.]

_Bisulphites_ have also strong antiseptic properties. “Bisulphite of

soda” (hydric sodic sulphite) solution may be made by supplying the

scrubber with solution of soda-ash or washing soda; bisulphite of lime,

by using milk of lime or packing the scrubber with chalk or limestone

(free from much iron) in place of the coke. In either case a much

stronger solution is obtained than with water alone.

Boakes’ “metabisulphite of soda”[8] is a very convenient source of

sulphurous acid when the latter is wanted in small quantities. It is an

anhydrosulphite, Na₂O.2(SO₂), and contains 67·4 per cent. of its weight

of SO₂. One molecule of the salt (= 190) requires one molecule of H₂SO₄

(= 98) to set free the whole of the sulphurous acid. For many purposes

the sulphate of soda formed may be neglected and the acidified solution

used direct.

[8] Patented by Boakes, Ltd., Stratford, London, E.

For analysis of sulphites and sulphurous acid solution, see L.I.L.B.,

pp. 16 and 37.

_Boric acid_, _borax_ and other _borates_ are not very powerful

disinfectants. They have no injurious action upon the skin, but to be

effective require to be employed in pretty strong solutions, say 1 per

cent., and their comparatively high cost unfits them for general use as

antiseptics in the tannery, though boric (boracic) acid is very useful

as a drenching and deliming agent (see pp. 156, 229, and L.I.L.B., p.

37).

_Mercuric chloride_, corrosive sublimate, HgCl₂, is an extremely

powerful antiseptic, preventing the growth of some species of bacteria

in solutions so dilute as 1 in 300,000 (Koch). 1 in 14,000 is

disinfectant (Miquel), but its power varies very much upon different

organisms (Jörgensen states that 1 in 400 is required to kill

_Penicillium glaucum_), and it is unsuited for most purposes in leather

manufacture, both from its extremely poisonous character, and because it

is rendered inactive by various substances present in the materials

used.

_Mercuric iodide_ dissolved in iodide of potassium solution was patented

by Messrs. Collin and Benoist as an antiseptic in tanning, but it is

ineffective for the same reasons as mercuric chloride; although under

favourable circumstances it is even more powerful than the latter.

_Copper sulphate_, _zinc chloride_ and _sulphate_, and many other

metallic salts are powerful antiseptics, but have only a limited

application in leather industries, and do not usually actually

sterilise. _Arsenic_ (arsenious acid), which has been used in curing

hides, is an excellent insecticide, but not particularly effective as an

antiseptic; and sulphide of arsenic (realgar) when used in limes (see p.

139) seems to have but little antiseptic effect. Arsenious acid is

easily soluble in alkaline solutions.

_Fluorides_ have been suggested as antiseptics in the tannery, but do

not seem of much practical value.

The most important antiseptics at present are those derived from coal

tar, and belonging to the aromatic series. Of these, the phenols

(carbolic acid, cresol, etc.) are the most used.

_Pure phenol_, “pure crystallised carbolic acid,” is hydroxybenzene

C₆H₅(OH), but the crude forms which are generally employed contain

cresols and higher members of the series in which one or more of the

atoms of hydrogen are substituted by CH₃ groups. These are oily bodies

scarcely soluble in water, and even pure phenol is only soluble in cold

water to the extent of some 7 per cent. Crude carbolic acid should not

be employed in the tannery, since the insoluble oily particles stain the

hide, and render it unsusceptible of tanning. Suitable carbolic acid

should be of a pale yellow colour when fresh (though it will darken on

exposure to air and light), and it should be wholly soluble in a

sufficient quantity of water. Its specific gravity should be 1·050 to

1·065. For methods of chemical examination, see L.I.L.B., p. 40. A

saturated solution of carbolic acid sterilises hide completely against

most putrefactive organisms, but has a sort of tanning effect, adhering

obstinately to the fibre so that it cannot be removed by washing; and

hides which have been cured with it cannot be unhaired by sweating,

though they may be limed in the usual manner, if somewhat more slowly.

Care should be taken in mixing with water or liquor, as undissolved

drops will produce the same effects as those of the crude acid. Hides

are occasionally stained, as has just been described, by salt which has

been denaturised with common sorts of carbolic acid. Eitner recommends

the use of a solution of carbolic acid in an equal weight of crude

glycerine, which readily dissolves in water, and seems to prevent any

injurious effect on the hide.

An aqueous solution containing 1 per cent. of carbolic acid is

sufficient for mere sterilising of hides, but if it be desired to

preserve them for a long period, stronger solutions (up to 4 per cent.)

may be employed.[9]

[9] Gerber, 1889, p. 98.

Quantities so small as 1 part per 1000 control the fermentation of

liquors, and prevent the formation of moulds on the surface, economising

tannin, and preserving vegetable acids already present, but at the same

time lessening their production by fermentation, and therefore sometimes

leading to difficulties in the early stages of tanning. Carbolic acid is

not, strictly speaking, an acid, but rather of the nature of an alcohol,

although it forms weak combinations with bases. It is a powerful

narcotic poison, and if dropped on the skin in a concentrated form it

produces severe burns; these are best treated with oil, while in cases

of poisoning, oil and chalk must be administered internally, but if the

quantity of carbolic acid taken has been large, are not likely to be

effective. From its cheapness and efficiency, carbolic acid is likely to

be increasingly used, although for special uses some of the newer

antiseptics have great advantages.

_Eudermin_ is a tar-oil manufactured by Speyer and Grund, of

Frankfort-on-Main, which is intended as an antiseptic addition to

stuffing greases to prevent mould and spueing. It is recommended for the

purpose by Eitner[10] and can be used in proportions such as 10 per

cent. of the grease. Creasotes and cresols can be dissolved in oils and

stuffing greases, and act as antiseptics, though less powerfully than

in aqueous solution. Rosin oils and turpentine have also antiseptic

properties.

[10] Ibid., 1893, p. 41.

_Creasote_, “heavy coal oil,” or “dead oil,” is a complex mixture of

hydrocarbons, phenols and cresols, obtained by distillation of coal tar,

heavier than water, and almost insoluble in it. It is largely used as a

preservative for timber. Carbolineum is an oil of this class, boiling at

over 300° C., and intended for application to wood. One or more coats

are applied to the dry wood at a temperature of 80° C. The workman’s

hands must be protected by gloves, as the hot creasote raises painful

blisters. Eitner[11] recommends its use for preserving pits, posts and

other woodwork in tanneries. Wood-creasote is a somewhat similar product

obtained from wood-tar.

[11] Gerber, 1889, p. 183.

The heavier cresols are so little soluble in water as to be valueless in

their ordinary form as antiseptics, but several preparations are made

under the names of “Creolin,” “Jeye’s fluid,” “Lysol,” “Izal,” “Soluble

phenyl,” etc., in which they are treated with additions of soap or

alkalies, which cause them to emulsify or dissolve in water, generally

as milky liquids. These are powerful germicides and have the advantage

over phenol of being non-poisonous. 0·1 to 0·5 per cent. solution of

creolin will sterilise hides after bating so that no putrefaction takes

place in the liquors. Mr. J. T. Wood specially recommends creolin for

the general purposes of the tannery, disinfecting pits and tubs, and for

checking the action of puers and drenches on goods which have gone a

little too far, by throwing them into a 0·2 per cent. solution.

_Salicylic acid_, orthohydroxybenzoic acid, C₆H₄OH(COOH), is now

artificially prepared from phenol. It is much less poisonous than the

latter and has no smell, which makes it valuable for certain purposes,

but is too dear for most technical applications. Many bacteria appear to

become gradually habituated to its action, and the same is true of

phenol to a less degree.

Salicylic acid is closely related to protocatechuic and gallic acids,

and, like these, gives a blackish colour with iron salts. It is freely

soluble in hot water, but very sparingly in cold. The addition of 1-2¹⁄₂

parts of sodium phosphate, sulphate, or potassium nitrate to each part

of salicylic acid greatly increases its solubility. It seems much more

powerful in preventing the development of bacteria than carbolic acid;

a solution of 1 part of salicylic acid in 666 of water is said to be

equal in this respect to 1 part of carbolic in 200.

_Benzoic acid_, C₆H₅COOH, though not much employed, except in medicine,

is a still more powerful disinfectant, and has the advantage of being

non-poisonous to human beings.

“_Cresotinic acid_,” which is derived from the cresols as salicylic acid

is derived from phenol, is more soluble than salicylic acid. It is not

very poisonous, and a powerful disinfectant. In a crude form it has been

introduced by Hauff, of Feuerbach, for bating or removing lime from

hides. This it does very well, though without the softening action of a

true bate. It has a tendency to produce a pinkish stain, and in some

degree a sort of tanning of the fibre. Its price, moreover, is rather

high for extensive technical use. (See also p. 162.)

“_Anticalcium_” is a more recent preparation introduced as a bate by the

same firm.[12] It is a solution of mixed sulphonic acids derived from

cresols, and has considerable disinfectant powers. It removes lime very

effectively, but from its acid character somewhat swells the skin. It is

used very successfully as a drench for thin skins (p. 163).

[12] Gerber, 1895, p. 133.

“C.T.” (coal-tar) bate is a grey crystalline pasty mass, with a tarry

smell, and is chemically very similar to anticalcium if not identical

with it.

_Naphthalene sulphonic acid_ has strong antiseptic properties. Its use

in bating has been patented by Burns and Cross. (See p. 163.)

_Naphthols_, C₁₀H₇(OH).--These bodies, which have the same relation to

naphthalene as the phenols to benzene, are powerful antiseptics; and

naphthalene itself appears to have antiseptic power, and is occasionally

used for denaturising salt. There are two naphthols, varying in the

position of the OH group in the molecule, and denominated α and β, of

which α naphthol is the more powerful antiseptic and the less poisonous,

though β, being cheaper, is the common commercial article. It is said

that quantities so small as 0·1-0·4 grams of α naphthol per liter are

sufficient to prevent the development of microbes, while of β naphthol

about ten times that quantity is required.

Naphthols are not very expensive, but their value is diminished by the

fact that they are insoluble in water. They are soluble in alkaline

solutions, but their compounds with bases are of much lower antiseptic

value, and the same is true of their alcoholic solutions; when an

alcoholic solution is added to water the naphthol is precipitated, but

if an addition of soap or camphor be made to the alcoholic solution, the

naphthol remains in a very finely divided condition, if not dissolved.

Adopting Eitner’s suggestion with regard to oxynaphthoic acid (see

below), hides may no doubt be sterilised by treatment first with an

alkaline naphthol solution, and then with a very dilute acid to set the

naphthol free.

“_Hydronaphthol_,” β tetra-hydro-naphthol, C₁₀H₁₂O, is obtained by the

reduction of β naphthol by sodium (Rideal). It seems to be an excellent

disinfectant.

_Oxynaphthoic acid_, α hydroxynaphthoic acid, C₁₀H₆(OH)COOH, which bears

the same relation to naphthol as salicylic acid does to phenol, is

cheaper than salicylic acid, and said to be a more powerful antiseptic.

Its salts have no antiseptic power. In its commercial form it is a

reddish crystalline powder, inodorous, but with a burning taste, and its

dust causes violent sneezing. It is scarcely soluble in water, and is

said to undergo some change on keeping which lessens its germicide

power; it is readily soluble in alcohol, and the solution produces a

milky fluid on mixture with water. Such a solution containing 15 grams

of the acid in 4 liters of water, will sterilise a hide. Eitner

recommends[13] that it should be dissolved in dilute soda solution, and

the hides, after soaking in it, passed through water slightly acidified

with hydrochloric acid, as has been suggested in the case of naphthol;

the method is also applicable to creosotinic acid, the hides being

permanently sterilised so that they cannot be unhaired by sweating,

though they will lime in the usual manner.

[13] Gerber, 1888, p. 101; 1889, pp. 99 _et seq._ See also p. 163.

_Carbon disulphide._--Moret has suggested an aqueous solution of this

compound as an antiseptic, and it seems to have considerable sterilising

powers, but from its inflammability, poisonous character, and unpleasant

smell, it is not likely to come largely into use.

_Formaldehyde_, COH₂, has recently been introduced as an antiseptic in

aqueous solution containing 40 per cent. of formaldehyde together with

a little formic acid, under the names of “formalin,” “formol,” etc. It

seems to have great disinfectant powers, and may possibly be valuable in

various processes of leather manufacture as it becomes cheaper, but has

a curious hardening tanning effect on hide fibre and gelatinous matters,

so that in very dilute solution it will produce leather.[14] The vapour

of formaldehyde, or of its condensation-product paraform, may be

employed to harden microscopic preparations. 1 part of formaldehyde, and

consequently 2¹⁄₂ parts of “formalin” in 12,000 parts of water, is said

to sterilise, and this proportion would form a good disinfectant

solution. Even in considerably larger proportion than the above, it does

not appear to be poisonous, and thus possesses the bactericidal power of

sublimate without the latter’s poisonous properties. Formaldehyde has

another advantage over most, if not all other antiseptics, in that it

may be used as well in the gaseous as in the liquid state, and on that

account it is largely employed in the disinfection of rooms or of

articles which would be spoiled if they were to be wetted, as the

gaseous formaldehyde, though thoroughly disinfecting them, will not

injure the colours of materials of the most delicate fabrics.

[14] Gerber, 1897, p. 67; ibid., 1899, pp. 101, 205, 218.

On account of its capability of rendering gelatinous matters hard and

insoluble in water, formaldehyde requires to be employed with great

care, but 0·2-0·3 per cent. may be successfully used in admixture with

egg-albumen in the preparation of “seasoning” in the finishing of

morocco leather. It is also used commercially to produce different

varieties of white leather for soldiers’ accoutrements and similar

purposes (p. 380).

_Triformol_ (tri-oxymethylene, “paraform”) is a product of the

polymerisation of formaldehyde, and is prepared by evaporating a

solution of the latter to dryness on the water-bath. It is said to be

more powerful than formalin in its antiseptic properties, but has not

entered very largely into use as a disinfectant, though considerable use

is made of it to “fix” bacteria in gelatin for bacteriological purposes.

_Camphor and essential oils_, as well as oil of turpentine, have

considerable antiseptic powers, and the cheaper essential oils such as

those of winter-green, black birch, sassafras and aniseed are frequently

employed, especially in America, in preserving pastes, finishes and

seasonings, and at the same time covering offensive odours. The odour of

essential oils becomes much more powerful as they are diluted, and very

small quantities suffice for the purposes mentioned. Birch-tar oil, such

as is used to give the scent to Russian leather (p. 372), has

considerable antiseptic effect.

CHAPTER VI.

_THE ORIGIN AND CURING OF HIDES AND SKINS._

A considerable proportion of the hides and skins used in leather

manufacture are those of animals killed by the butcher for food, and

these are frequently employed by the tanner without any preliminary

curing. Domestic hides and skins are now generally sold by auction in

weekly markets in the principal towns, after sorting and classification

in weight and quality.[15] This is in many respects an improvement on

the old method of purchase direct from the butcher, but it often leads

to delay in delivery, and in hot weather hides suffer from putrefaction.

In most cases, the damage is not sufficient seriously to affect the

durability of the leather, but the delicate membrane of the “grain” is

injured, and the hide or skin unfitted for coloured leather, or any

purpose where small damages to appearance are important. Butchers are

adverse to the use of salt, because it withdraws water from the hide in

the form of brine, and so causes it to lose weight; but much injury

would be saved by a light salting, and all hides or skins on which the

hair is “slipping” should be regarded as damaged for fine leather

manufacture.

[15] The weight of English market-hides as credited to the butcher is

usually marked on the edge of the butt near the tail, by cuts with a

knife, the mode of numeration being sufficiently explained by Fig. 6,

in which cuts crossing the horizontal line each represent 20 lb., that

above it 10 lb., while less amounts are expressed in Roman figures.

On the Continent weights are usually given in pounds of half a

kilogramme (50 kilos = 110 lb. English). In Paris the marking is on

the tail, and is also shown on Fig. 6.

[Illustration: FIG. 6.--Method of marking weight on hides; 97 lb.]

Sheep-skins are not usually bought direct by the tanner, but by the

fellmonger, who removes the wool; and as this is usually of much greater

value than the skin, the latter is frequently handled very carelessly,

and its quality sacrificed for the sake of real or fancied improvement

to the wool. In very many cases the skin is “sweated” or “staled” by

hanging in a warm and moist chamber, heavily charged with ammonia

derived from the putrefaction of the skin, until the wool is

sufficiently loosened to be “pulled.” If this treatment is conducted

with extreme care the skin may escape serious injury, but in most cases

the grain is weakened, and the foundation is laid of damage, which makes

itself felt throughout the tanning process. For the purposes of the

tanner, a much better way is to lime the skins by painting with thick

limewash on the flesh-side, and after folding the skins down the back,

flesh-side in, to prevent as much as possible the access of the lime to

the wool, to place them in a pit, and cover them with water, till the

wool is loosened by the penetration of the lime through the skin. A

still more satisfactory method, and one which is in general use in the

American stockyards, and to some extent also in Europe, is to wash the

skins in water to free them from blood and dirt, and then, laying them

in a wet condition, flesh side up, to paint them with a solution

containing about 25 per cent. of sulphide of sodium, thickened with

lime. The skins, as they are painted, are doubled down the back,

flesh-side in, and laid on a floor, overlapping each other like tiles on

a roof, for some hours, or overnight, till the wool is sufficiently

loosened to pull, after which the pelts are limed and treated in the

ordinary way. As a general rule the English fellmonger keeps his skins

in lime till they are sold to the tanner, and as in small yards some

time is taken to accumulate a parcel, the earlier skins may suffer great

injury from overliming. Even sweet fresh limes dissolve the cementing

substance of the fibre, and increase the naturally loose texture of the

sheep-skin, but the injury is much more considerable when old and stale

limes, charged with ammonia and bacterial products, are employed, as is

frequently the case. In the American stockyards the skins are generally

limed only for the necessary time to act upon the grease, and to swell

and differentiate the fibres, and are then at once puered, drenched and

preserved by “pickling.” For details of “pickling” see p. 89. It is very

probable that the Pullman process of liming (p. 137) would answer well

for fellmongered skins, as goods will keep for a considerable length of

time uninjured after treatment with calcium chloride.

Where hides or skins cannot be used at once in the fresh state, there is

probably no better method of preserving them than the use of salt.

Although salt is not fatal to bacteria, it so slows bacterial growth,

partly by its direct antiseptic effect on many organisms, and partly by

withdrawing water from the skin, that well-salted skins can be kept in

good condition for almost an unlimited time. Where it is only required

to preserve goods for a week or two, a moderate sprinkling on the flesh

side is efficient, but if they are to be preserved for any length of

time, more thorough treatment is necessary. It is said that however

carefully hides are salted they deteriorate if kept in this condition

above twelve months.

The method of salting employed in the Chicago stockyards for “packer”

hides may be taken as a good type of a thorough salting. The hides are

first trimmed from useless “switches,” and any large portions of

adhering fat are removed. The curing takes place in large and cool

cellars, with concrete floors. The detail is well given in the following

extract from the ‘Shoe and Leather Reporter’:--

“Great care is taken to make the sides of a pack higher than the middle,

so that the brine which is made by the juices of the hide coming in

contact with the salt will be retained. The brine can only escape by

percolation and hence the fibre of the hides is thoroughly cured. The

floor of a hide cellar is usually of concrete, and a pack is from 15 to

20 feet long and as wide as the space between the posts which support

the floor above. The sides of a pack are built first to a height of from

4 to 6 inches; the cross layers are then put on, generally three on each

side, two being inside and one having the butts drawn out to the edge.

In a pack 20 feet long, the side layers will contain about 25

medium-sized hides each, and a cross-layer 12 or 14. To begin a pack a

truck-load of hides is run along to the front of the place selected,

one spreader grasps the butt and his partner the head of a hide, and

together they carry it to what is to be the rear of the bed. The hide is

then dropped, so that the folded back is parallel to and from 15 to 20

inches from the inside line of the posts, the head a trifle closer than

the butt. The front man takes the dewlap and front shank in his left

hand, and extends his right along the belly of the hide as far as is

necessary to raise the edge, the rear man holding the flank with one

hand and the hind shank with the other. They keep their legs well out of

the way of the salt thrower, who with a single throw covers the whole

hide, being particular that enough salt strikes against the edges held

by the men to make a pronounced ridge when they are lapped down. A

little salt is thrown on the hair surface and the butt folded over about

a foot. The folded edge is then drawn out even with the outer line of

the pack. More hides are placed the same way until the corner is high

enough. After this, each hide is put further forward to make a level

surface from rear to front, the heads at the front corner being folded

back as the butts were at the starting place. The other side is built

the same way, and then the cross layers are put on alternately until the

pack is level, when sides are again built as before. In putting on the

first hides of the cross layers, they are thrown over the edge, to lap

back again when the salt is thrown on; the layer is then continued on to

the front. The spreader who holds the butt does the guiding in every

case. He drops the butt down at exactly the proper place, takes the

upper flank and shank in each hand, sets one foot on the lower shank to

keep it firm, and throws the one in his hands from him with considerable

force. The man at the head watches his partner, keeps the folded hide

taut, and drops it at the same time as the latter. He takes the

fore-shank at the knee in the one hand and the upper head-piece in the

other, and setting his foot on the lower side, throws the upper side

forward simultaneously with the rear man. Two expert spreaders,

accustomed to working together, spread a hide at a single throw, but

some little straightening has to be done by hand before the hide is

ready for the salt. A gang composed of two spreaders, one salt thrower

and a salt trucker put down forty hides an hour. When gangs are doubled,

two men do all the spreading; the other two place the hides where they

can be got at conveniently. A double gang put down eighty hides an

hour. The salt trucker brings the salt to the pack in box-trucks open at

one end to permit the entrance of a shovel. The salt thrower keeps the

edges and corners of the pack full of salt. He must see that every part

of the flesh-side is well covered. Each hide takes two scoop shovelfuls

of ground rock or coarse white salt, mixed with an equal quantity of old

or second salt. The salt thrower throws the shovel forward and to one

side and back again with a peculiar swinging jerk, causing the salt to

fall regularly over the entire surface of the hide. The ease and

rapidity with which a gang operates depends greatly upon the efficiency

of the salt thrower. When the pack gets too high to be comfortable for

the men, it is brought to a dead level and covered over with clean salt.

It then presents a very neat and workmanlike appearance. Spreaders and

salt throwers receive 20 cents an hour, and truckers get 17¹⁄₂ cents.

When the temperature is kept at an even average, two weeks is ample time

to cure the hides.

“In ‘taking up,’ two men strip the hides from the pack. As they were put

down from the rear to the front, they are taken up in the reverse

direction. No matter how much loose salt is lying on the top, the man

knows exactly where to place his hand on a shank; as the hides are moved

forward, the loose salt is thrown off toward the front. One man takes

away the salt as it accumulates and trucks it to the salt bins, where it

is mixed with new, to be used again. A ‘horse’ made of a network of

scantling about 3¹⁄₂ feet wide by 6 feet long, and standing 2¹⁄₂ feet

from the floor, is placed in front of the pack, on this the hides, flesh

side down, are shaken to remove the salt that is clinging to them. This

process requires four men, one at each corner. The hide is brought down

heavily on the horse twice, and then spread on the floor flesh side up

for examination by the inspectors, of which there are two, one

representing the house and the other the buyer of the hides. They sweep

off any salt that may be left, and examine for cuts, sores, brands,

manure and grubs. They also see that the hide is properly weighed and

classified. If the contract calls for a special trim it is now done. Two

men then roll the hide, beginning by lapping over the shanks, head and

neck. Then the sides are folded over and lapped again, leaving the roll

15 to 18 inches wide. The ends are thrown inward, slightly overlapping

each other; a final fold is then given, and the hide is ready to be

tied. Rope the size of clothesline is used for tying, and is cut into

lengths of about seven feet. It takes three men to tie for a gang such

as we have described. After tying, the neat bundles are weighed and

loaded on the cars for shipment. A small tare is allowed the buyer.

Ordinary workmen in hide cellars get 17¹⁄₂ cents an hour, and inspectors

25 cents an hour.”

About 25 per cent. of salt on the green weight of the hide is required

for thorough curing. Rock salt merely crushed is frequently employed,

but this is very liable to contain iron in the form of oxide and

chloride, which causes the peculiar marbled markings known as

“salt-stains.” It is therefore much better to use a white crystallised

salt, though it is possible even in this case that stains may arise from

the iron present in the blood. Some salt-stains appear also to be due to

the action of pigment-bacteria, and not to contain iron. A reddening of

the flesh side is often noticed in hides which have been kept in salt

long or under unsatisfactory conditions, and is very frequent in

wet-salted South American hides. Such hides are said never to produce so

firm a leather as those which are sound.

Hides are not unfrequently cured by steeping in salt brine, instead of

strewing with dry salt. This method is principally resorted to in order

to give fictitious weight. Brined hides do not plump well in tanning,

the leather is not so good in quality as from those salted with dry

salt, and the cure is much less efficient.

Many hides are not only salted but also dried in order to preserve them.

Not much detail has been published with regard to the methods used,

which no doubt vary much in different places, but probably in some cases

the hides are salted in pile and in others by brining, and then hung up

to dry. The principal object of this drying is to economise weight and

cost of transport, but it makes the hides much more difficult to wash

and soften for tanning, and probably the crystallisation of the salt has

a weakening effect on the fibre. Hides cured in this way are styled “dry

salted.”

A large number of the hides of the small native cattle of India are

imported into this country in a dry-salted condition. The following

particulars of their cure are taken from a paper by the Author and Mr.

W. Towse.[16]

[16] Journ. Soc. Ch. Ind., 1895, p. 1025.

Dry-salted, or, as they are commonly called “plaster cures,” such as

those of Dacca and Mehapore, are thickly coated with a white material,

which in the first instance is merely the insoluble portion of a saline

earth used in the cure; though in many cases it is applied in larger

quantities than necessary, with the simple object of giving weight. The

salting is thus described by Mr. W. G. Evans, who some years since had

considerable experience as a tanner at Cawnpore:--

“The salt used by the natives is a salt-earth; and is so called by them.

It is found extensively in the districts of Cawnpore, Agra, Delhi,

Lucknow, Patna, etc., and has no doubt something to do with the

localisation of the hide-curing and kindred industries in these places.

The mode of procedure used is pretty much as follows:--the salt-earth is

mixed into a very thin paste, and this is lightly brushed on to the

flesh side one day, and the hide allowed to remain over night under

cover. Next day, for best hides, the same solution is again spread on

the flesh side of the outstretched hide and rubbed into it with a porous

brick, and then for legitimate salting, the hide is allowed to dry under

cover. If for export, the saltings may be three or four, and the hides

are treated out in the open, subject to the intense heat of the sun;

which accounts for the number of hides which go back in the soaks in

England and elsewhere.”

“We had a clause in our agreement with hide-factors, that any hides

which did not come down to natural suppleness in two days in clean water

were to be returned. Of arsenic curing I know nothing, and it is not so

much in vogue as formerly. There is quite a trade in Cawnpore, Lucknow,

Allahabad, etc., in treating old and inferior hides with new for export,

and great efforts are made by native holders to get their stocks down

before the rains commence, as they say, and rightly I think, that hides

are not worth so much after the rains by 30 per cent. The peculiar

latent moisture of the rains affects them very detrimentally.”

Under certain circumstances this mode of cure gives rise to extensive

iron-staining of the skins, and analyses of the material scraped off

Dacca and Mehapore kips were undertaken with a view to elucidating the

causes of this injury. The following are the results of the analyses

referred to, which were made upon the residue after the rather

considerable quantity of fibrous organic matter, which had been scraped

off with the cure, had been destroyed by ignition, together no doubt

with traces of ammoniacal salts:--

------+---------------+------------+------------+------

| ---- | Dacca. | Mehapore. |

| |Entire Cure.|Entire Cure.|

|---------------+------------+------------+------

| | | |

|Sand and silica| 20·55 | 27·38 |

|Fe₂O₃ | 2·77 | 1·86 |

|Al₂O₃ | 2·48 | 2·74 |

|Mn₃O₄ | 0·60 | 0·40 |

|CaO | 2·60 | 3·70 |

|MgO | 3·38 | 3·69 |

|Na₂O | 28·97 | 26·80 |

|SO₃ | 38·90 | 33·75 |

|Cl | 0·22 | 0·18 |

|H₃PO₄ and CO₂ | Traces | Traces |

| +------------+------------+

| | 100·47 | 100·50 |

------+---------------+------------+------------+------

The soluble salts of the Dacca cure were also analysed separately with

the following result:--

CaO 0·70

MgO 0·60

Na₂O 29·00

SO₃ 37·90

Cl ·22

Insoluble 32·12

------

100·54

It thus consisted exclusively of sulphates, with the exception of a

trace of chloride. The cures, after ignition, were both neutral to

phenolphthalein, but before ignition the Dacca was distinctly alkaline,

in consequence probably of the presence of ammonium salts, and both

showed considerably larger traces of carbonates before than after.

The most striking feature of these analyses is the absence of more than

the smallest traces of chlorides. The cures are thus practically free

from common salt, and owe their antiseptic power to the sodium sulphate

which they contain, and which indeed forms their principal constituent.

Nitrates appear to be entirely absent. Sodium sulphate sometimes forms

large crystals in pits used for soaking these kips.

The iron-staining of hides which has been mentioned appears to result

only when the hides after cure are exposed for a lengthened period to a

moist atmosphere, in which the carbonic acid present probably also plays

its part, the iron passing into solution as hydric carbonate.

The analyses show a striking resemblance to those of the soda deposits

of Wyoming, given by Dr. Attfield,[17] except that their percentage of

sodium carbonate is smaller, which is quite intelligible in the light of

Mr. Brunner’s abstract on the ‘Probable origin of natural deposits of

sodium carbonate,’[18] which supports the view that the sodium carbonate

is derived from sodium sulphate by the reducing and carbonating action

of low organisms.

[17] Journ. Soc. Ch. Ind., 1895, p. 4.

[18] Ibid., 1893, p. 116.

It may be noted here that the preservative properties of sodium sulphate

are well known, and the anhydrous sulphate has been recommended as a

substitute for common salt (see p. 23).

Drying is a very common method of preserving hides as well as other

putrescible matters. It has no effect in killing bacteria, but

putrefaction can only go on in presence of a considerable amount of

moisture. As applied to hides, it is, to the tanner, one of the least

satisfactory modes of cure, involving very considerable difficulties in

bringing hides back to the moist and swollen condition which is

necessary at the outset of his operations, but it is the only practical

method in districts far from the coast and with primitive modes of

transit, both on account of the cost of salt, and the lessened weight of

the dried hide. Great differences are found in the ease with which dried

hides soften, according to the way in which the drying has been

accomplished, the difficulty being greater the higher the temperature

which has been used (see p. 111). The best mode of drying is to hang in

the shade in a good draught of cool air, with the flesh side out. Hides

or skins dried in a tropical sun are not only difficult to soften, but

are liable to damaged portions, which either refuse to soften, or

blister and go to pieces in liming, owing to the structure of the hide

being destroyed by heat, the outer surface drying first and forming an

impervious layer which hinders evaporation from the inside, so that the

moist interior becomes melted, while the outside appears quite sound.

Such injuries are often only to be discovered by soaking and liming.

Very similar damage may occur from putrefaction of the interior after

the outside has become dry, and to get good results, the drying must be

gradual, but rapid, especially in hot climates. South American hides are

mostly dried in the sun, suspended by head and tail from stakes, with

the hair side out.

The risk of injury by putrefaction during drying is diminished by the

use of antiseptics. Solutions of arsenic have been frequently used for

this purpose, and many of the dried Indian kips are of what are known as

“arsenic cures,” although the writer has never been able to detect

arsenic in any which he has examined, and its use seems by no means

general. The arsenious acid is usually dissolved in soda solutions.

Unless used pretty freely it has little antiseptic effect, but is useful

in preventing the attacks of insects, which are often very destructive.

The larva of a small beetle, _Dermestes vulpinus_, frequently devours

the whole tissue of patches of the hide, leaving only the epidermis.

It may be well here to say a few words about the injuries and defects to

which hides and skins are liable, although some of them are not strictly

due to the cure. The most serious, and yet preventable injury is that

due to butchers’ cuts. As the value of the hide bears only a small

proportion to that of the meat, many butchers do their work extremely

carelessly, and this is encouraged by the loose classification of

“damaged hides” in some markets. There is also an idea that the

appearance of the meat is improved by a thin layer of the white

skin-tissue being left on it, and for this reason as well as mere

carelessness, butchers frequently score the flanks of the hide with

shallow cuts which greatly diminish its value. The “packer hides” of the

United States, and the products of the large saladeros or slaughtering

(“salting”) establishments of South America, such as Liebig’s, show what

can be done by skilled work in this respect. In the United States, much

of the flaying is done by means of a wooden cleaver, instead of a sharp

knife. Another method to some extent in use, and which may be

recommended for calf and sheep skins, is to inflate the carcase before

skinning, with air from a compressing syringe, which tears the

connecting tissue between the skin and the body, and renders flaying

much easier.

Brands are a great source of damage to hides, but where cattle roam at

large on unfenced plains, as on the prairies of Texas and the Pampas of

South America, it seems indispensable for the recognition of ownership;

no other mode of marking being sufficiently permanent and conspicuous.

It is unfortunate, that as the animals crowd together, and cannot be

closely approached, it is necessary that the brands should not only be

large, but placed on the most valuable part of the hide. Generally on

the Pampas an effort is made to keep them on one side only, so that in

South American hides it is possible to select clear and branded sides.

In the United States much land is now fenced with barbed wire, which

while it obviates the necessity of branding, introduces another evil in

the form of “barbed wire scratches,” which are frequently troublesome in

“packer hides.”

[Illustration: FIG. 7.--_Hypoderma bovis._ 1, egg; 2, maggot; 4,

chrysalis case; 6, fly, magnified (Brauer); 3, 5, chrysalis and fly,

natural size (B. Clark).]

[Illustration: FIG. 8.--Sac of warble, showing growth of epidermis round

aperture.]

In countries where cattle are used for draught purposes, goadmarks are a

frequent source of injury, and some of the large cattle-ticks do

considerable damage to the hides of Spain and South America. From the

tanners’ point of view, however, the most injurious insects are the

“bot-flies” or “warble-flies” (_Hypoderma bovis_ and allied species,

Fig. 7). There is still some controversy as to how the eggs of these

insects are deposited. In the horse-bot fly it is known that the eggs,

first deposited on the skin, are licked off and swallowed by the animal,

and develop in the stomach, where they pass their larval and pupal life

hanging on to its interior coats, and only drop off and are passed out

with the dung before their final change to the complete fly. Fortified

by this, and by some direct observation, some American naturalists are

of opinion that the American species at least, hatches in the stomach,

and as a minute larva wanders through all the intervening tissues till

it reaches the skin, where it undergoes its further development. The

late Miss Ormerod, who has made a careful study of the English

species,[19] states that the egg hatches on the hair, and that the

_larva_ simply eats its way below the skin, leaving a minute red

puncture which it subsequently enlarges to obtain air for its spiracles,

which are in the tail. As it grows it continues to irritate the lower

part of the cavity with hooked mandibles, and lives on the pus and

matter so produced. It grows to a length of fully ³⁄₄ inch, and the

cavity, Fig. 8, situated between the skin and the subcutaneous tissue is

often as large as half a walnut. It remains in the sac not only during

its larval, but its pupal stage, which do not differ much in appearance,

and falls out on the ground before complete development. In small

numbers, the warble seems to do little injury to the general health of

the animal, but cases have been known where animals have actually died

of the inflammation produced. Some idea of the extent of the plague may

be realised from the statement that an Indian kip in the possession of

the writer has not less than 680 warble holes, and that almost equal

numbers have been counted in English hides. Preventive measures are the

sheltering of the cattle during the summer months when the fly is most

prevalent; the application of mixtures of oil or grease with tar-oil and

sulphur to the hair, to prevent egg-laying; and the destruction of the

_larva_ in its early stages, in autumn and winter by smearing the

breathing aperture with grease, or better, with mercurial ointment.

When this is done sufficiently early, the hole heals up without

permanent injury, but when it is allowed to remain open during the

period of growth, its sides become partially coated by the growth of

epidermis, and this permanently prevents their proper union by

skin-tissue. It is believed that if the _larvæ_ were systematically

destroyed in a district, they would soon become extinct, as they are not

supposed to travel far.

[19] ‘Some Observations on the Œstridæ,’ E. A. Ormerod, Simpkin and

Marshall, London, 1884, price 4_d._

A very troublesome injury to the skins of lambs and sheep is the disease

known as “cockle,” in which the skin becomes thickly dotted with spots

of thickened tissue, which bear some fanciful resemblance in form to a

cockleshell. The affection is prevalent during the spring while the wool

is thick, and disappears almost immediately on shearing, but little is

known of its causes or mode of prevention.

Climate and breed have a considerable effect on the quality of hides and

skins. As a rule the less highly bred races, and those which are most

exposed to the extremes of weather, have the thickest hides, and in most

cases highly bred animals have had their meat-producing, or in the case

of sheep, their wool-bearing qualities developed at the expense of the

characteristics most valued by the tanner.

CHAPTER VII.

_STRUCTURE AND GROWTH OF SKIN._

Although, at first sight, the skins of different animals appear to have

little in common, a closer examination shows that all the Mammalia

possess skins which have the same general structure, and thus an

anatomical description of the skin of an ox applies almost equally to

that of a sheep, goat, or calf, though on account of the difference in

texture and thickness the practical uses of these various materials may

differ widely. The skins of lizards, alligators, fishes and serpents

differ from those of the higher animals, chiefly in having considerable

modifications in the epidermis, so that it becomes harder and forms

“scales,” and the arrangement of the fibres presents considerable

difference. In many fish-skins for instance, the fibres are in

successive layers, at right angles to each other and diagonal to the

skin, but not interlaced.

In its natural condition, the skin is not merely a covering for the

animal, but at the same time an organ of sense and of secretion, and

hence its structure is somewhat complicated. It consists of two

principal layers, the _epidermis_ (_epithelium_, cuticle) and the

_corium_ (_derma_, _cutis_ or true skin). These are totally distinct,

not only in structure and functions, but in their origin. In the egg of

a bird and the _ovum_ of a higher animal, the living germ consists of a

single cell, which, as soon as fertilised, begins to multiply by

repeated division. The mass of cells thus formed early differentiates

into three distinct layers, from the upper of which the epithelium

arises, while the true skin, together with the bones and cartilages, is

derived from the middle one.

This distinction of origin corresponds with a wide difference of both

anatomical and chemical characteristics. A diagrammatic section of

calf-skin is shown in Fig. 9, and a more correct representation of its

actual appearance is given in Plate I. (Frontispiece). The _epidermis_

is very thin as compared with the true skin which it covers, and is

entirely removed preparatory to tanning; it nevertheless possesses

important functions. It is shown in Fig. 10 at _a_ and _b_, more highly

magnified. Its inner mucous layer _b_, the _rete malpighi_, which rests

upon the true skin _c_, is soft, and composed of living nucleated cells,

which multiply by division and form cell-walls of keratin. These are

elongated in the deeper layers, and gradually become flattened as they

approach the surface, where they dry up, and form the horny layer _a_.

This last is being constantly worn away, thrown off as dead scales of

skin, and as constantly renewed from below, by the multiplication of the

cells. It is from the epithelial layer that the hair, as well as the

sweat and fat-glands, are developed.

[Illustration: FIG. 9.--Vertical section of calf-skin, magnified about

50 diameters. _a_, epidermis; _b_, grain or papillary layer; _c_,

fibrous layer of skin; _d_, hairs; _e_, fat-glands; _f_, sweat-glands;

_g_, opening of ducts of sweat-glands; _h_, hair-muscles.]

Each hair is surrounded by a sheath which is continuous with the

epidermis, and into which the young hair usually grows as the old one

falls out. The hair itself is covered with a layer of overlapping

scales, like the slates on a roof, but of irregular form. These give it

a serrated outline at the sides, and when strongly developed as in wool

and some furs, confer the property of felting. Within these scales,

which are called the “hair cuticle,” is a fibrous substance which forms

the body of the hair; and sometimes but not always, there is also a

central and cellular pith, which under the microscope frequently appears

black and opaque, from the optical effect of imprisoned air. On boiling

or long soaking in water, alcohol, or turpentine, the air-spaces become

saturated with the liquid, and then appear transparent.

[Illustration: FIG. 10.--Epidermis layer.]

The fibrous part of the hair is made up of long spindle-shaped cells,

and contains the pigment which gives the hair its colour. The hair of

the deer differs from that of most other animals in being almost wholly

formed of polygonal cells, which, in white hairs, are usually filled

with air. In dark hairs, both the hair and sheath are strongly

pigmented, but the hair is much the most so, and hence the bulb has

usually a distinct dark form. The dark-haired portions of a hide from

which the hair has been removed by liming still remain coloured by the

pigmented cells of the hair-sheaths, which can only be completely

removed by “bating and scudding.”

[Illustration: FIG. 11.--_a_, sebaceous gland; _b_, hair; _c_, erector

muscle. Mag. 200.]

[Illustration: FIG. 12.--_a_, hair; _b_, hair cuticle; _c_, inner

root-sheath; _d_, outer root-sheath; _e_, dermic coat of hair-sheath;

_f_, origin of inner sheath; _g_, bulb; _h_, hair-papilla.]

Near the opening of the hair-sheath to the surface of the skin the ducts

of the sebaceous or fat-glands pass into the sheath and secrete a sort

of oil to lubricate the hair. The glands themselves are formed of large

nucleated cells arranged somewhat like a bunch of grapes; the upper and

more central ones being highly charged with fatty matter. Their

appearance is shown in Fig. 11. The base of the hair is a bulb,

enclosing the hair papilla _h_ (Fig. 12), which is a projecting knob of

the true skin and which by means of the blood-vessels contained in it

supplies nourishment to the hair. The hair-bulb is composed of round

soft cells, which multiply rapidly, and pressing upwards through the

hair-sheath, become hardened, thus increasing the length of the hair.

The cells outside the bulb, shown at _f_ in Fig. 12, pass upwards as

they grow, and form a coating around the hair, known as the “inner

root-sheath.”

In embryonic development, a small knob of cells forms on the under side

of the _epidermis_, over a knot of capillary blood-vessels in the

_corium_, and enlarges and sinks deeper into the latter, while the

root-bulb of the young hair is formed within it, surrounding the

capillaries from which it derives nourishment, and which form the

hair-papilla, Fig. 13. In the renewal of hair in the adult animal the

process is very similar. The bulb of the old hair withers, and the hair

falls out, and in the meantime a thickening takes place in the epidermal

coating of the bottom of the sheath, and the young hair is formed below,

and usually to one side of the old one, growing into the sheath, and

taking the place of the old hair. This is one cause of the difficulty of

removing ground-hairs in the process of unhairing, since they are not

only short, but deeper seated than the old ones.

The process of development of the sudoriferous or sweat-glands is very

similar to that of the hairs. They consist of more or less convoluted

tubes with walls formed of longitudinal fibres of connective tissue of

the _corium_, lined with a single layer of large nucleated cells, which

secrete the perspiration. The ducts, which are exceedingly narrow, and

with walls of nucleated cells like those of the outer hair-sheaths,

sometimes open directly through the epidermis, but more frequently into

the orifice of a hair-sheath, just at the surface of the skin. Each hair

is provided with a slanting muscle called the _arrector_ or _erector

pili_ (see Fig. 11), which is contracted by cold or fear, and causes the

hair to “bristle,” or stand on end; by forcing up the attached skin, it

produces the effect known as “goose-skin.” The muscle, which is of the

unstriped or involuntary kind, passes from near the hair-bulb to the

epidermis, and just under the sebaceous glands, which it compresses when

it contracts.

Beside the hair, and hair-sheaths, and the sebaceous and sudoriferous

glands, the _epidermis_ layer produces other structures of a horny

character, including horns, hoofs, claws and finger-nails; which both

chemically and anatomically are analogous to exaggerated hairs, such as

the quills of the porcupine.

The whole of the epidermis, together with the hairs, is separated from

the _corium_ by an exceedingly fine membrane, called the hyaline or

glassy layer. This forms the very thin buff-coloured “grain-” surface of

tanned leather, which is evidently of different structure from the rest

of the _corium_, since, if it gets scraped off before tanning, the

exposed portion of the underlying skin remains nearly white, instead of

colouring. The whole of the hair-sheath is enclosed in a coating of

elastic and connective-tissue fibres, which are supplied with nerves and

blood-vessels, and form part of the _corium_.

[Illustration: FIG. 13.--Development of young hair.]

[Illustration: FIG. 14.--Connective-tissue fibres. (Ranvier.)]

The structure of the corium or true skin is quite different from that of

the epidermis which has just been described, as it is principally

composed of interlacing bundles of white fibres, of the kind known as

“connective tissue” (see Fig. 14); these are composed of fibrils of

extreme fineness, cemented together by a substance somewhat more soluble

than the fibres themselves. The fibres are not themselves living cells,

but are apparently produced by narrow spindle-shaped cells lying against

them. The felted fibre-bundles are more loosely interwoven in the middle

portion of the skin, but become compacter again near the flesh. In the

case of sheep-skins this is especially marked, the middle part being

full of fat-cells and very loose. Any ill treatment of the pelt during

the wet-work is liable to still further loosen this middle layer so that

grain and flesh may sometimes be torn apart. The flesh-splits of

sheep-skins must have this loose fatty layer frized off before

chamoising, and American “waxed fleshes” from ox-hides are levelled by

splitting away this portion, and finished on the flesh. The outermost

layer, just beneath the epidermis, is exceedingly close and compact, the

fibre-bundles that run into it being separated into their elementary

fibrils, which are so interlaced that they can scarcely be recognised.

This is the _pars papillaris_, and forms the lighter-coloured layer,

called (together with its very fine outer coating) the “grain” of

leather. It is in this part that the fat-glands are embedded, while the

hair-roots and sweat-glands pass through it into the looser tissue

beneath. It receives its name from the small projections or _papillæ_,

with which its outer surface is studded, and which form the

characteristic grain of the various kinds of skin.[20] (See Fig. 9 and

Plate I.)

[20] It will be noted that the word “grain” is used by the tanner in

at least three different senses, which are productive of much

confusion. The extremely thin hyaline layer forms a natural glaze to

the skin, and might well be spoken of as such; the form and

arrangement of the _papillæ_ and hair-pores might be called the

“pattern” of the grain, leaving the use of the word “grain” itself

restricted to the _pars papillaris_.

[Illustration: FIG. 15.--Fat-cells in connective tissue. _a_,

fat-globule; _p_, protoplasm; _n_, nucleus; _m_, cell-wall. (Ranvier.)]

The study of the structure of the grain, and especially of the

arrangement of the hair-pores is very important, as it is usually the

readiest means of identifying the kind of skin of which a leather is

made, which in finished skins with artificially printed grain is often

very difficult. (Plate II.) The examination is facilitated by wetting

and stretching the skin, and by the use of a good lens, or a low power

of the microscope.[21]

[21] Under the microscope, the skin is of course lighted from above by

direct light from a window, or by that of a lamp concentrated by a

“bullseye” condenser. The reversal of the image in the microscope

often causes a pseudoscopic effect very puzzling to the beginner,

prominences appearing as hollows, and _vice versa_ till the real

direction of the lighting is considered.

[Illustration: PLATE II.

PHOTO-MICROGRAPHS OF GRAIN OF VARIOUS SKINS (_A. Seymour-Jones_).

1. Cow-hide; 2. Calf-skin; 3. East India Goat; 4. Pig-skin; 5. East

India Sheep; 6. Welsh Sheep.

[_Face p. 52._]

[Illustration: FIG. 16.--Striped, or voluntary muscular fibre.

(Ranvier.)]

As stated above, the surface of skin which is next to the flesh is

firmer than that in the centre, and as the fibres run nearly parallel

with the surface it has a more or less membranous character. The skin is

united to the body of the animal by a network of connective tissue

(_panniculus adiposus_), which is frequently full of fat-cells and is

then called adipose tissue. This constitutes the whitish layer which is

removed, together with portions of actual flesh, in the operation of

“fleshing.” If a minute portion of adipose tissue be examined

microscopically, it will appear to consist of a mere mass of

fat-globules entangled in connective tissue. If, however, it be stained

with carmine or logwood it may be at once observed that each globule is

contained in a cell, of which the nucleated protoplasm, by which the fat

was secreted, is pressed closely against the wall (Fig. 15). Similar

cells are contained in considerable quantities throughout the hide, and

especially in the loose tissue of the central part; hence in leather

manufacture it is impossible to expel or wash out the fat until the

cells have been broken down by “liming” or in some other way.

Many animals (ox, horse, etc.) possess a thin layer of voluntary muscle

(red flesh) spread over the inner side of the skin, and used for

twitching to drive off flies. In rough fleshing this is sometimes left

on and may be a cause of dark flesh in sole leather. Even in the

finished leather its striped structure may be detected microscopically

(Fig. 16).

Besides the connective-tissue fibres, the skin contains a small

proportion of fine yellow “elastic” fibres. If a thin section of hide be

soaked for a few minutes in a mixture of equal parts of water,

glycerine, and strong acetic acid, and then examined under the

microscope, the white connective-tissue fibres become swollen and

transparent, and the yellow “elastic” fibres may be seen, as they are

scarcely affected by the acid. The hair-bulbs and sweat- and fat-glands

are also rendered distinctly visible by this treatment. On the other

hand, the white gelatinous fibres are most easily seen by examining the

section in a strong solution of common salt, or in one of ammonium

sulphate; or by staining with some aniline dyes such as safranine.

Sections are most readily cut for these purposes by the use of the

freezing microtome, or after previous hardening in alcohol. For further

details see L.I.L.B., p. 254.

Ordinarily in the production of leather only the corium, or true skin is

used, and in order to obtain it in a suitable condition for the various

tanning processes, the hair or wool, together with the epithelium, must

be completely removed without damaging the skin itself; and especial

care must be taken that the grain, or portion next to the epidermis,

does not suffer any injury during the treatment. All the methods

employed depend upon the fact that the epidermis cells, especially the

soft growing ones next to the corium, and those of the epidermis layer

which surround the hair-roots, are more easily destroyed than the corium

itself owing to their different chemical character. The “unhairing”

process consists essentially in breaking down these cells by chemical or

putrefactive agents, and removing the hair together with the rest of the

epidermis by mechanical means. Of the various substances which may be

used for this purpose, lime is one of the most convenient, as its

solubility in water is so slight, that a solution of such a strength as

to injure the hide cannot be easily made. Caustic alkalies, on the other

hand, are much more soluble, and unless care be taken to use only the

proper quantity, a dangerously strong solution may be made with

consequent damage to the skin. The addition of small amounts of

sulphides to the lime-solution accelerates the unhairing owing to their

special solvent action on the epidermis-structures, and also in the case

of alkaline sulphides, by the caustic alkali which is produced by their

reaction with the lime. Even if used alone, strong solutions of alkaline

sulphides rapidly destroy both hair and epidermis, converting them into

a mass which may be swept off the skin like wet pulp, and yet they have

practically no injurious action on the true skin.

In the “sweating” process the epidermic cells are broken down by

putrefactive organisms and their products, so that the hair becomes

loose and may then be either rubbed or scraped off. Ammonia, which is

produced during the putrefaction, has also an important solvent action,

and its presence doubtless tends to quicken the processes both of

unhairing and of destruction.

To obtain useful knowledge of the structure of any particular skin, it

is not necessary to have a very elaborate or expensive microscope, and

it is quite possible to obtain useful information merely by the use of a

good pocket lens, as for instance, in the examination of various forms

of “grain,” and the embossing of one skin to imitate another.

For further details of the manipulation and selection of the microscope,

the reader must consult L.I.L.B., pp. 234 _et seq._

CHAPTER VIII.

_THE CHEMICAL CONSTITUENTS OF SKIN._

The chemistry of the various constituents of skin is still very

imperfectly understood, but Beilstein, in his great handbook of organic

chemistry, places gelatin, albumens and keratins in the “aromatic”

series, and implies therefore that they contain the “benzene” ring. It

is at least certain that all are very complex.

The epidermis structures belong to the class of keratins, which are

closely related to coagulated albumin; while the white fibres of the

corium (or true skin) are either identical with gelatin, or only differ

from it in their molecular condition or degree of hydration. This

gelatinous tissue constitutes the bulk of the corium, but it also

contains albumen as a constituent of the lymph and blood which supply

its nourishment, keratins in the epithelial structures of the blood and

lymph vessels, and “yellow fibres,” which are perhaps allied to the

keratins, but which cannot well be isolated for analysis.

The white connective tissue of the corium is converted into gelatin

(glutin) by boiling with water. Owing to the impossibility of obtaining

unaltered hide-fibre free from the other constituents, and still more to

that of deciding to what point it should be dried to remove uncombined

water, it is impossible to prove by analysis whether its composition is

identical with that of glutin; but as the white fibre constitutes by far

the largest part of the corium, and the other constituents do not differ

largely from it in their percentage composition, an analysis of

carefully purified corium is practically identical with that of the

actual fibre. The following analyses of hide and gelatin are therefore

of interest.

The analyses of Von Schroeder and Paessler[22] are of special importance

as being the average of a large number of separate determinations.

Their nitrogen determinations are by Kjeldahl’s method. Small amounts of

ash and traces of sulphur are neglected, and probably included in the O,

which is obtained by difference.

[22] Ding. Polyt. Journ., 1893, cclxxxvii. pp. 258, 283, 300.

ANALYSES OF PURIFIED CORIUM.

--------------------------+------------------+----+----+----+----+----

Analyst. | Material. | C | H | N | O | S

--------------------------+------------------+----+----+----+----+----

Stohmann and Langbein | .. |49·9| 5·8|18·0|26·0| 0·3

| | | | | |

Müntz |Ox-hide |51·8| 6·7|18·3|23·2| ..

| | | | | |

|{Ox, calf, horse,}| | | | |

Von Schroeder and Paessler|{camel, pig, }|50·2| 6·4|17·8|25·4| ..

|{rhinoceros }| | | | |

„ |Goat and deer |50·3| 6·4|17·4|25·9| ..

„ |Sheep and dog |50·2| 6·5|17·0|26·3| ..

„ |Cat .. .. |51·1| 6·5|17·1|25·3| ..

--------------------------+------------------+----+----+----+----+----

ANALYSES OF GELATIN (free from Ash).

--------------------------+----+----+----+----

Analyst. | C | H | N | O

--------------------------+----+----+----+----

Von Schroeder and Paessler|51·2| 6·5|18·1|24·2

Mulder |50·1| 6·6|18·3|25·0

Fremy |50·0| 6·5|17·5|26·0

Schützenberger |50·0| 6·7|18·3|25·0

Chittenden and Solly[23] |49·4| 6·8|18·0|25·1

--------------------------+----+----+----+----

[23] Contained also 0·7 sulphur. Journ. Physiol., xii. p. 23.

It will be noted that the above analyses of skin differ more widely

among themselves than their average does from that of the gelatin

analyses, though on the whole the nitrogen is somewhat higher in the

latter. The molecular weight of gelatin must be very high,[24] and any

empirical formula founded on ultimate analysis therefore quite

hypothetical. Bleunard,[25] Schützenberger and Bourgois,[26] and

Hofmeister agree on the formula C₇₆H₁₂₄N₂₄O₂₉, which leads to the

following percentage composition:--

per cent.

C₇₆ = 912 = 49·7

H₁₂₄ = 124 = 6·8

N₂₄ = 336 = 18·3

O₂₉ = 464 = 25·2

---- -----

1836 100·0

[24] Paal (Berichte D. Ch. Ges., xxv. (1892) pp. 1202-36, and Ch. Soc.

Abst., 1892, pp. 895-7) calculates a molecular weight of about 900

from physical (freezing, boiling point) methods.

[25] Annales de Chimie [5] xxvi. p. 18.

[26] Compt. Rend., lxxxii. pp. 262-4.

The addition of a molecule of water would make a difference in the

percentage composition indicated by these formulæ which would be less

than their probable experimental error, and the change may therefore be

one of hydration.

Gelatin certainly contains both carboxyl and amido-groups, and is

capable of combining with both acids and alkalies (see p. 84).

Reimer[27] obtained what he supposed to be pure unaltered

fibre-substance by digestion of purified hide with ¹⁄₂ per cent. acetic

acid for many days and subsequent neutralisation. His analysis showed C

= 48·45 per cent., H = 6·66 per cent., N = 18·45 per cent., O = 26·44

per cent., thus deviating considerably from direct analysis of unaltered

skin. It is obvious that little weight can be placed on this result,

Reimer’s precipitate being probably a mere decomposition product.

[27] Ding. Polyt., ccv. p. 164.

Hofmeister[28] notes that on heating gelatin it loses water and forms an

anhydride which he considers identical with collagen or hide-fibre. When

gelatin is dried at a temperature of 130° C. it becomes incapable of

solution in water, even at boiling temperature, and can only be

dissolved by heating under pressure. It is certain that collagen

(hide-fibre, ossein) is less easily soluble in hot water than ordinary

gelatin.

[28] Bied. Centr., 1880, p. 772.

So far as our present knowledge goes, we may regard hide-fibre as merely

an organised and perhaps dehydrated gelatin.

_Gelatin_ or _glutin_ (not to be confounded with the gluten of cereals),

when pure and dry is a colourless, transparent solid of horny toughness

and of sp. gr. 1·3. It begins to melt about 140° C., at the same time

undergoing decomposition. It is insoluble in hydrocarbons, in ether, or

in strong alcohol. In cold water it swells to a transparent jelly,

absorbing several times its weight of water, but does not dissolve. In

hot water it is soluble, but a solution containing even 1 per cent. of

good gelatin sets to a weak jelly on cooling. Gelatin jellies melt at

temperatures which vary considerably with the quality or freedom from

degradation products, but which within pretty wide limits (5-10 per

cent.) are little affected by the concentration. A 10 per cent. solution

of best hard gelatin melts about 38° C., while low glue may fail to set

at 15° C. A useful technical test for the setting power of gelatin,

based on this fact, consists in placing an angular fragment of the jelly

in a small tube attached to a thermometer, and stirring in a beaker of

water, which is slowly heated till the jelly melts, when the temperature

is noted. The exact point is perhaps more easily seen if the tube is

drawn to a conical point. The jelly may also be allowed to set in

capillary tubes open at the bottom, and the moment noted when water

rises into the tube. The temperature of fusion is raised by the addition

of formaldehyde, salts of chromium, alumina and ferric salts, which

produce a tanning effect, and in a less degree by sulphates, tartrates,

acetates, some other salts, and diminished by iodides, bromides,

chlorides and nitrates.[29] Solutions of gelatin too weak or too warm to

gelatinise possess considerable viscosity. Gelatin may therefore be

estimated, in the absence of other viscous matters, by the viscosimeter,

an instrument which measures the time taken by a liquid in flowing

through a capillary tube.[30] The firmness of a jelly, which is often

important for commercial purposes, is frequently measured by Lipowitz’s

method, in which a slightly convex disc, conveniently of exactly 1 cm.

diameter, and cemented to the bottom of a thistle-head funnel tube, is

loaded gradually with mercury till it sinks in the jelly. The jelly (5

or 10 per cent.) should be allowed to set some hours before the test is

made.

[29] See Pascheles, ‘Versuche über Quellung,’ Archiv für ges. Path.,

Bd. 71.

[30] See Prollius, Ding. Polyt. Journ., ccxlix. p. 425, who employs a

1 per cent. solution; also Stützer, Zeit. Ann. Ch., xxxi. pp. 501-15.

Solutions of gelatin from skin and bone are powerfully lævorotatory to

polarised light. At 30° C. (A)_{D} = -130°, but temperature and the

reaction of the solution have much influence on the value found.

Gelatin is precipitated from aqueous solution by the addition of strong

alcohol and concentrated solutions of ammonium sulphate and some other

salts. Many other colloid bodies such as dextrin and gums behave

similarly. In the absence of these substances, precipitation by alcohol

may be utilised for the technical analysis of gelatins and glues,

printers’ roller compositions and gelatin confectionery. 25 c.c. of the

gelatinous solution, which is preferably of about 10 per cent., is

placed in a small beaker tared together with a glass stirring rod, and

thrice its volume of absolute alcohol added. On stirring, the gelatin

sets firmly on the rod and sides of the beaker, and may be washed with

dilute alcohol or even with cold water, dried and weighed. A very pure

French gelatin gave 98·6 per cent., while a common bone-glue only

yielded about 60 per cent. precipitate. Absolute alcohol withdraws water

from gelatin-jelly, leaving a horny mass. Gelatin may also be

precipitated completely by saturating its solution with sodium chloride,

and then acidifying slightly with sulphuric or hydrochloric acid; and

masses of jelly become hardened in acidified salt solution as in

alcohol, though a neutral solution has little effect. The cause of this

is difficult of explanation, but its bearing on the pickling of

sheep-skins (p. 89) and the production of white leather (p. 186) is

obvious.

_Decompositions._--When aqueous solutions of gelatin are heated under

pressure, or in presence of glycerin and other bodies which raise the

boiling-point, or more slowly at lower temperatures, they gradually lose

the power of gelatinising on cooling, the gelatin being converted into

modifications soluble in cold water, but still capable of being

precipitated by tannin. Hofmeister[31] states that the gelatin takes up

3 molecules water and is split up into _hemicollin_, soluble in alcohol

and not precipitated by platinic chloride solution; and _semiglutin_,

insoluble in alcohol and precipitated by platinic chloride solution.

Both are precipitated by mercuric chloride. Dry gelatin is soluble in

glycerin at high temperatures, but probably suffers a similar change.

Hence high temperatures and long-continued heating must be avoided in

gelatin manufacture; and in making printers’ roller compositions, which

are mixtures of gelatin and glycerin, the gelatin must be swollen with

water and melted at a low temperature with the glycerin.

[31] Bied. Centr., 1880, p. 772, and Ch. Soc. Abs., 1881, p. 294.

Gelatin is also converted into soluble forms (peptones), perhaps

identical with the above, by the action of heat in presence of dilute

acids and alkalies. These, like gelatin, are precipitated by tannin and

by metaphosphoric acid.[32] Heated for longer periods or to higher

temperatures with aqueous solutions of the caustic alkalies, baryta, or

lime, gelatin is gradually broken down into simpler and simpler

products, ending in nitrogen or ammonia, water and carbonic acid. Among

the intermediate products may be mentioned various acids of the

amido-acetic series, as amido-acetic (glycocine, glycocoll),

amido-propionic (alanine), and amido-caproic (leucine); and of the

amido-succinic series (amido-succinic = aspartic acid).[33]