PREPARATION OF HIDES AND SKINS FOR TANNING
Before beginning a description of manufacturing processes, a precise
definition of leather may be given. Laymen usually describe leather
as "hides tanned with bark," Since the introduction of modern
processes, however, this is only partly true. In any case, the
definition is very broad.
The primary objects of treating raw hides and skins to produce
leather is to make them imputrescible and impart various degrees
of pliability. These qualities are essential, but the simpler the
methods used to attain them, the greater the strength of the leather
produced.
The number of materials that will produce leather is legion. Whereas
oak bark was almost exclusively used for tanning until the last
century, there are now at least twenty useful vegetable tanning
materials. The active ingredient of all of these materials is tannin,
a colloidal or uncrystallisable substance. Correctly speaking, the
term "tanning," used to designate the process of converting hides
into leather, should be confined to the use of vegetable tannins;
unfortunately, the trade has largely adopted the word in many other
processes of making leather. For example, large quantities of leather
are produced by the use of minerals, and especially of chrome salts;
where the latter is used, the leather is said to be "chrome-tanned,"
although "chromed" would be a more accurate definition. One mineral
process of making leather, namely, that involving the use of alum,
or alumina sulphate, and salt, is technically described as "tawing,"
The conversion of skins into leather by the use of oxidised oil is
known as "chamoising," imitation chamois leather being made by that
process. Nearly every mineral has the property of converting skins
into leather, though most of them are of little practical value.
The most successful are the chrome salts, and alum and salt. The
use of iron salts would be by far the cheapest process, if means
could be found whereby they could be successfully used. Sixty years
ago, a chemist named Knapp experimented with iron salts, but failed
to produce satisfactory leather. Patents were taken out in Austria
in the early part of 1914 for the use of iron salts in tanning,
while a patent of more recent date covers the use of iron salts
in combination with chrome salts. Neither of these appears to be
of much practical value. The cheapest and most stable iron salts,
the sulphate and chloride, have strongly acidic properties, and,
therefore, have a somewhat destructive and hardening effect on
animal fibres. The iron salt that might convert skins into leather
is the carbonate, which is difficult to prepare cheaply enough for
commercial purposes, though it is used medicinally. A basic iron salt
might also be useful, but iron tannages would only be suitable for
black leather. Oils and fats also have leather-making properties,
fish oils being used for the manufacture of "chamois" and antelope
leather. There are other methods of producing leather which are
not used on a large scale, but sufficient examples have been given
to show that an exact definition of leather cannot be summed up in
one or two words. There are now many kinds of leather produced by
varying methods, and each class requires its own definition. Broadly
speaking, however, leather is an imputrescible material produced from
the raw skins of animals, chiefly of cattle, sheep, and goats, by
treating them with tannins extracted from the barks, wood, fruit, or
leaves of trees; or with chemicals (chiefly chrome salts, or alum and
common salt); or with oils and fats.
The preliminary processes are of great importance, as they determine
to a large extent the character of the finished leather. Mistakes
made in the early processes can never be effectually remedied.
The first operation is technically known as soaking, and its object
is to cleanse the hides or skins thoroughly. This is quite simple
in the case of raw hides received direct from the slaughterhouse,
as it is merely a question of soaking them in clean, soft water for
a few hours. If the only water available is hard, 1/2 lb. of borax
should be added for every 100 lb. of raw hides. Borax is useful in
any case, as it is a splendid cleanser and a very useful chemical in
the tannery. Most of the hides used in the United Kingdom, however,
are wet-salted (_i.e._, salted in the wet state), as it has become
customary for most butchers to send hides and skins to the auction
markets in all the principal towns, where they are offered for
sale every week. It is still a debatable point among tanners as to
whether it is better to buy hides direct from the butcher or through
these markets. It is certainly a great advantage to put hides into
work quickly (although not before they are quite cool), as time is
saved in the soaking process, and there is practically no loss of
gelatinous matter. On the other hand, the tanner is able to buy just
the selection of hides that he wants from the auction markets. Under
this arrangement, several days must elapse before the tanner receives
them, and it is, therefore, absolutely necessary to preserve them,
otherwise they begin to decompose quickly. The first sign of decay is
the slipping of the hair, which, in that condition, may be pulled out
of its roots quite easily. Micro-organisms multiply rapidly in the
gelatine of the hide, the grain comes away, and decomposition sets in
so quickly that, in a few days after the removal of the hide from the
carcase, it may lose nearly half its value for tanning purposes if it
has not been preserved by salting or by drying rapidly in the shade
in a current of air. Salted hides need a more prolonged soaking than
fresh hides, as it is essential that all trace of salt be removed
before the next process, otherwise the finished leather may be flat,
and poor in quality. Usually, two days' soaking in several changes of
water is necessary. For the soaking process, fresh or salted hides
and skins are either soaked in square, cement-lined, brick pits, or
in wooden vats filled with clean, cold water. A good system is to put
a pack of skins in one huge tank filled with clean water and leave
them in soak overnight before transferring them to the ordinary pits.
Each pit will take 50 hides, or 10 dozen calf skins, or 20 dozen goat
skins. Tanners designate as a "pack" each lot of hides or skins they
work through, irrespective of the quantity.
[Illustration: FIG. 13
DRUM TUMBLER]
[Illustration: FIG. 14
FALLER STOCKS]
Besides salting them, there are other ways of curing hides and
skins, and a large proportion is simply dried or dry-salted, the
salt in the latter method being applied while the hides are moist.
Dry hides, whether "flint" (i.e., simply dried) or salted, require
special treatment to make them soft and pliable, without which it
would be impossible to convert them into leather. Soaking in plain
water is insufficient, as it would need too much time, during which
there would be a great loss of gelatine. The use of chemicals and
mechanical motion are required; sometimes chemicals alone will
thoroughly soften the hides, but this is not often the case. Both
alkalies and acids may be used for softening dried hides, and it
is difficult to say which gives the better result, although the
former are frequently used, mainly because treatment with acids is
a comparatively recent innovation. Both kinds are effective, but
the use of acids retards the loosening of the hair, although it has
been proved experimentally that acid-softened hides give a slightly
improved yield of leather compared with the effect of the alkaline
process--a noteworthy advantage where the finished leather is to
be sold by weight. The acid generally used--formic--has antiseptic
properties, and can have no harmful effect on animal tissues, as is
generally the case where mineral acids, especially sulphuric, are
used. Sulphurous acid is also said to be a good softening agent for
hides, but it is rarely used. The most satisfactory alkalies are
sulphide of sodium (crystallised or concentrated) and caustic soda,
and of these two, the sulphide is preferred in nine cases out of ten.
The quantities used vary according to the condition of the hides, but
the average is about 1 lb. of caustic soda, or 1 lb. of concentrated
sulphide of sodium (65 per cent.), or 2 lb. of crystallised sulphide
of sodium (30 per cent.) for every 100 gal. of water. Of formic
acid, 1 lb. is sufficient for 100 gal. of water. It is possible to
soften hides by these means without mechanical acid, but the time
is considerably shortened by "drumming" the hides, _i.e._, placing
them in a round or, preferably, a square tumbler fitted inside with
shelves or staves (Fig. 13). The drum is rotated mechanically for a
few hours, during which time the hides are thoroughly kneaded and
softened by the alkali. Tanners often use a machine known as the
"faller stocks" (Fig. 14), which kneads the hides very thoroughly,
though somewhat drastically. The drum method is preferable, providing
the hides can be suitably softened. An old method, and one that is
used now by some of the small firms, is to "break over" (_i.e._,
vigorously scrape) the hides with a curved blunt knife fitted into
two wooden handles. For this manual operation, the hide is placed,
flesh side up, over the tanner's beam (Fig. 15) and is then scraped
with the knife.
[Illustration: FIG. 15
TANNER'S BEAM]
Another old method, which has not yet disappeared, is to make use of
stale soak liquors, which, although efficient for softening purposes,
cause a great loss of hide substance owing to the active growth of
bacteria, which are developed as a result of the water becoming foul
and putrid with dirt, blood, and dissolved gelatinous matter. This
method of soaking is always dangerous, apart from the great loss of
gelatine, for the thin grain (hyaline layer) is liable to be eaten
away in patches, a defect which greatly reduces the value of leather.
Dried hides are not only difficult to treat successfully in the
different processes of making them into leather, but they are also
of highly speculative value, although they are well preserved when
completely dry. It sometimes happens, however, that the drying
is imperfect; it may, for example, have been so rapid that the
exterior of both sides is thoroughly dried before the air is able to
penetrate into the centre. The result is that the interior of the
hide putrefies, but there may be no indication of this until the
hides are soaked and softened, when they may fall to pieces and are
only saleable to glue makers. Again, the hides may be dried in the
hot sun and be badly blistered, with the same result as regards their
value for leather. If the hides are dried in too high a temperature,
they become horny and rarely make satisfactory leather owing to the
difficulty of softening them. It is estimated that quite 10 per
cent. of the dried hides are improperly cured and, therefore, useless
for making into satisfactory leather.
Apart from the commercial risk in buying dry hides, their import,
especially from Russia and China, is a source of danger in conveying
the disease of anthrax to workmen. It has been conclusively proved
that dried hides are much more liable than wet-salted hides to cause
infection.
So far, only one country, the United States of America, has taken
steps to prevent the importation of the disease of anthrax, although
most countries have issued regulations in regard to the precaution
to be taken to avoid infection by anthrax in those factories where
imported hides, wool, hair, and bones are treated. It would seem
that the most rational method would be to disinfect the hides before
they are shipped, as it certainly appears to be unwise to import any
form of disease; unfortunately, the first regulations issued by the
American Government proved to be impracticable, as the suggested
method had the effect of lowering the quality of the hides, and
making the preliminary operations troublesome. Their second scheme,
consisting of baling a certain number of hides in canvas disinfected
with a 0.02 per cent. solution of mercuric chloride, is more
satisfactory, for, although it may not sterilise all the anthrax
spores inside the bale, it should prevent them reaching other goods.
This Government order only applies to hides, skins, hair, and wool
coming from countries where anthrax is known to be prevalent.
The disease of anthrax is generally contracted by workers through
sores or cuts in the hand; the bacilli multiply rapidly in contact
with the blood, and the first sign of disease is usually shown by
a red swelling or pimple in the neck. If treated at this stage by
anti-anthrax serum a cure is often effected, but if treatment is
delayed the disease quickly proves fatal, the patient dying in awful
agony. The germs of the disease may also be swallowed and the disease
develop internally, but cases of this kind are rare.
Besides the danger to workers, there is the risk of cattle being
infected. The effluent from tanneries where anthrax-infected skins
are treated contains millions of bacilli, and it is doubtful if the
latter are sterilised even when the effluents are precipitated and
aërated before they are run into streams or municipal sewers. In any
case, the sediment may be infected, and this ultimately finds its way
to the land.
Until the various European Governments insist on imported hides
from anthrax-infected areas being sterilised before shipment, the
use of a disinfectant such as lysol or a similar cresylic compound,
or bichloride of mercury, seems imperative in the first process of
soaking. The use of these disinfectants would make the waste liquors
fit for discharge into sewers or streams.
The English Public Health and River Pollution Acts have had a great
effect in improving the hygiene of the tannery, although leather
manufacturers have not welcomed them, as, in some cases, they have
meant considerable expense in providing settling tanks for the
treatment of waste liquors. The Public Health Act gives power to any
town corporation to declare as offensive trades such businesses as
tanning, hide and skin merchanting, fellmongering, tallow melting,
etc., and several boroughs have taken advantage of this law. In such
cases, anyone desiring to set up business in these trades must apply
to the Town Council, who may or may not give their consent; in fact,
a few applications to establish these businesses have lately been
refused.
While the curing of wet hides with salt or in brine is more
satisfactory than drying them, the use of ordinary salt is not an
ideal method, as 10 per cent. brine dissolves hide substance. The
recent introduction of a pure dry salt (99-98 per cent.) and of a
sterilised salt for commercial purposes has to a large extent removed
the objections to ordinary salt. Dry sulphate of soda is also a
satisfactory cure. It may be that, as hygienic conditions are further
advanced in the various industries, a suitable disinfectant will have
to be used for all hides, in addition to the salt, except where the
hides are sent direct to the tannery from the slaughterhouse.
The cure of hides in hot countries, especially where cheap salt is
unavailable, is often unsatisfactory. A method of obviating this
difficulty has been found in China, where, in one or two of the
principal towns, hides and skins are preserved by freezing them in
cold-storage. Although this process stiffens the hides, it is said to
be fairly satisfactory if they are allowed to soften naturally before
soaking them. If submitted to rough treatment before the stiffness
relaxes, there is a great danger of the hide fibres being ruptured.
Freezing removes the difficulty of softening which is experienced in
treating dried hides, while it preserves the hide substance.
After the operation of soaking, hides and skins are ready to be
treated for the removal of the hair. There are several ways of
loosening the hair sheaths, but most of them consist of treating
the hides in a solution of a caustic alkali. The use of a solution
of common lime was practically universal until a few years ago, but
nowadays sulphide of sodium, red arsenic sulphide (realgar), and
caustic soda are also used, generally in admixture with the lime.
Another process consists in sweating the hides in a heated room,
preferably a damp cellar, where rapid decomposition of the hides soon
loosens the hair.
This method is rarely used in England, but a few American tanners
seem to prefer it for certain classes of hides. In the American
process, the hides are first soaked, and then cut in half down the
back, forming what are known as sides. Dry hides are subjected to
the usual mechanical operations in the faller stocks, in which they
are kneaded by two large hammers (Fig. 14), or they are drummed in
the tumbler (Fig. 13). After the sides are thoroughly softened and
drained, they are transferred to the sweat pit, which is, preferably,
a dark underground chamber. The stock requires very careful attention
as the process is risky. The temperature should never exceed 75° F.,
otherwise the hides may be irretrievably ruined. The process may take
from one to four days, according to the varying conditions of the
hides and of the weather. The loss of nitrogenous matter gives rise
to the development of a strong odour of ammonia, which is sometimes
even too pungent for the workmen. When the hair is judged to be
sufficiently loose, the sides are washed in cold water and put in
the stocks again for about ten minutes, when all the hair will be
removed; or the hair may be scraped off in the unhairing machine.
This method is not useful for sole leather, as it causes too great
a loss of gelatine, but it saves time in the production of sides
intended for boot upper leather, which is usually sold by measurement
(superficial area).
A dehairing process has lately been invented and patented, however,
which may supersede all of the methods just described. This process
consists in treating the hides with various enzymes which loosen the
hair so effectively that it can be removed more easily than the hair
of a limed hide. The fine, short hairs underneath are also removed,
whereas by the lime method a further process is needed to get rid of
these hairs. The only drawback to its use is that the inventor has
not yet been able to produce a material cheap enough to place on the
market, but as soon as this difficulty is overcome the enzyme method
may become fairly general. Neither the hair nor the gelatine of the
hide can be damaged by this method.
[Illustration: FIG. 16
LIME YARD
("Dri-ped" Tannery)]
The usual method of liming is carried out in brick pits of square
or rectangular shape with a sloping front on which the hides or
skins can be piled to drain (Fig. 16). There is a great variation in
the quality of lime, and in all cases it should be tested for the
available percentage of caustic lime. A good sample should contain
between 70 and 80 per cent. of pure lime. Buxton lime, which can be
obtained in powdered form, is particularly suitable for the tannery.
The ordinary lime is preferable to the chalk lime, as it is usually
stronger, though it sometimes contains more impurities. Gas lime is
the poorest of all. Lime should be stored in a dark place, otherwise
the outside of the heap carbonates quickly, forming chalk (CaO +
CO_{2} = CaCO_{3}) which is of no use for the liming process.
To prepare the lime for the pits it is slaked (_i.e._, formed into a
paste with water. All the lumps should be reduced to paste in order
to avoid lime burns, which are caused by direct contact of the hides
with pieces of unslaked lime. A certain quantity of the paste is then
added to the water or old lime liquor in the pit, and the liquor is
well plunged up to hasten solution and diffusion of the lime. A long
wooden pole, with a flat block of wood attached, is used for this
purpose and also for pressing down the hides under the surface of the
liquor.
Two methods of liming are in vogue: (1) the single-pit system, and
(2) the three-pit system; but the latter is the better method, as
it is more easily controlled, and causes less loss of gelatine than
the former. In this system the liquor is strengthened with fresh
lime for each pack of skins. Its great fault is that the pit is
only cleared out at long intervals in order to take advantage of
the mellowness of used lime liquors; hence, there is frequently
large accumulation of insoluble limestone and other sediment from
the lime, in addition to a quantity of dissolved gelatine, which
rapidly accumulates putrefactive bacteria. When the pit is cleared
out the process of liming is disturbed for a time, as fresh lime
liquors are not beneficial to hides and skins, and the loosening of
the hair is delayed. On the other hand, the three-pit system permits
liquors of uniform strength, and the process is continued without
interruption. Each new pack is first placed in the oldest of the
three liquors, which is then cleared out, and a new liquor prepared.
From the weakest pit the hides pass to a stronger liquor, and the
process is finished in the third pit, which should contain a new lime
liquor. The mellow liquors, being charged with bacteria, facilitate
the loosening of the hair, while the third liquor, consisting of
fresh lime, serves to swell the fibres of the hide, by means of which
the flesh is more effectually removed. The lime also forms a soap
with the natural grease of the hide; this grease can therefore be
removed. In some cases, however, especially in the cattle fed up for
Christmas, the hides contain a larger quantity of fat than the alkali
of the lime liquor can convert into a soap, and the surplus grease is
frequently seen in the finished leather, as it is difficult to remove
in later processes. A solution of hyposulphite of soda, or lactic
acid, given just before the process of tanning is said to remove the
grease, although a slight loss results in the case of those leathers
sold by weight. The objection to natural grease in leather can be
understood where the latter is intended for colours, but in the case
of sole leather it ought not to be a disadvantage, yet, owing merely
to the darker appearance of the leather where the grease reaches the
grain, its selling value is reduced by 2d. or 3d. per lb. The strange
part is that the grain of this sole leather, when made into boots, is
buffed on an emery wheel, then sometimes coloured with a paint, and
finally sold in boots at the same price as leather free from grease
and regular in colour.
The liming process in pits takes from six days to a month, according
to the character of the leather required. Light calf skins may
be ready for unhairing within a week, while hides intended to be
finished for "raw hide leather" may be left in the pits quite a
month, the object being to distend and harden the fibres. Lamb
skins intended for parchment, and small calf skins for vellum, are
also subjected to prolonged liming. Between these extremes, there
are several stages in the process which have varying effects on the
character of the finished leather. In fact, it is a tanner's dictum
that leather is made or marred in the lime liquor, though this, of
course, only applies to a certain extent. Generally, however, the
heavier the hides, the longer the liming required. Fortunately,
the limited solubility of the lime in water affords a wide margin
of safety in working, and the only danger to guard against is the
too prolonged use of old liquors, which are readily detected by the
strong odour of ammonia. One important property of lime is its lower
solubility in hot than in cold water.
Lime by itself does not readily attack the hair bulbs, and the
slowness of the process has led to the introduction of other
chemicals, generally for use in conjunction with lime. The principal
of these are sulphide of sodium and red arsenic. Mixtures of sulphide
of sodium and lime, or red arsenic and lime are now largely used,
the former for hides, calf, and sheep skins, and the latter for
goat and kid skins. Both sulphide of sodium and arsenic dissolve
keratinous matters (horns, hair, etc.) and workmen should, therefore,
be provided with rubber gloves to prevent the loss of their finger
nails. Sulphides naturally lower the commercial value of the hair
removed and, if used alone, destroy it. In admixture, however, the
hair has some value, although it is not so good as that removed by
the use of pure lime. Against this, however, there is a great saving
of time and less loss of hide substance and, therefore, increased
weight of leather. Sulphide of sodium is prepared in crystallised
or concentrated form; the former, about 30 per cent. strength,
is preferred in Great Britain, while the latter--65 per cent.
strength--is prepared for export, the main object being to save the
cost of transit of 35 per cent. of water. About thirty different
sulphide salts may be used, but the sodium and arsenic disulphides
are the best, as it has been shown that the most rapid loosening of
the hair occurs where the quantities of sulphur and alkali are nearly
the same. There are two arsenic salts used in the trade, namely,
realgar, or red sulphide of arsenic (As_{2}S_{2}) and orpiment, or
the yellow sulphide (As_{2}S_{3}), but the former is often preferred
as it is said to give better results than the latter. In practice,
the proportion of arsenic used is 1 part in 20 parts of lime,
although it naturally varies a little according to the class of skin
under treatment. Sulphide of sodium is used in the proportion of 1
part to 10 of lime, or, if concentrated sulphide be used, 1 in 20.
The quantities of lime, or lime and sulphide, used are estimated on
the weight of the raw hides. For hides intended for sole leather, 5
per cent. of lime on the weight of hides is ample, while a little
more may be used for hides intended for dressing leather (_i.e._,
leather which has to be dressed or finished with a certain degree of
flexibility for bags, boot uppers, etc.). When a mixture of lime and
sulphide is used, 3 per cent. and 0.3 per cent. respectively is a
satisfactory quantity. The action of this mixture on hides is complex
and has not yet been definitely ascertained, but it is thought that
the calcium sulphydrate formed by the chemical reaction between
sulphide of sodium and calcium hydrate (slaked lime) is the active
principle.
In the pit method of liming, it is essential that the liquors be
frequently plunged, while the hides should be taken out ("hauled"),
piled to drain for a few hours, and put back again ("set"), or
transferred to another pit. Although lime is more soluble in cold
than in warm water, it is found in practice that the process may
be stopped or considerably retarded in very cold weather, and the
activity of the liquors is increased by the application of waste
steam (conveyed through iron pipes from the boiler).
Sulphide of sodium and lime are sometimes made into a thin paste,
which is applied to the hair side of hides and skins with a mop or
fibre brush. The hides are then folded down the back with the flesh
sides out; other hides are similarly treated and placed in a pile.
This saves a great amount of labour in pitting, and, if the paste is
fairly strong, consisting of 2 to 2-1/2 per cent. sulphide, the hair
can be removed after a few hours.
With so many depilatories available, it is not surprising that
several patents have been granted and numerous suggestions made with
the object of trying to improve on the old process of liming. While
there may be some objections to lime, it has a few advantages which
are lacking in other depilatories. These advantages are not perfectly
understood theoretically, but the tanner recognises them in practice.
Hence, there are very few tanneries where lime is not used at all,
and the only progress that seems to have been made in the process of
liming consists in the admixture of sulphide of sodium or arsenic to
hasten the process, reduce the loss of gelatine, and, in the case of
arsenic, to improve the fineness of the grain of skins for boot upper
and glove leathers.
One patented method consisted in forming the calcium hydrate within
the hide by treating it with a 1 per cent. solution of caustic soda
and then with a 1-1/2 per cent. solution of calcium chloride, the
reaction of these two chemicals forming calcium hydrate (lime) and
sodium chloride (salt). This method, however, does not loosen the
hair at all and has to be supplemented by soaking the hides in putrid
water. It is a good method of liming hides or skins dressed in the
hair, as it opens up the fibres without weakening the hair roots and
prepares the hides in a suitable condition for tanning.
Another method consists in mixing a small quantity of soda ash with
the lime, thereby hastening the process by increasing the alkalinity
of the liquor.
A somewhat complicated method was introduced a few years ago, but,
although it seemed advantageous from a chemical point of view, it
has not proved successful in practice so far as is known, probably
because of its expense. In consisted of four distinct processes. The
hides were first mopped on the hair side with a thin paste of lime
and arsenic, and dehaired after twenty-four hours. In the second
process, they were treated in a drum for twenty-four hours with a
solution of sodium sulphide, they were then drummed for twenty-four
hours in a solution of hyposulphite of soda, and finally placed in
a vat or pit containing a solution of lime and a little arsenic for
two to five days. After washing, etc., they were ready for tanning.
The sulphide of sodium swelled the hides or skins by distending
the fibres, and the natural fat is converted into a soluble soap.
The hyposulphite arrests the action of the sulphide and acts as an
antiseptic.
In modern yards it is a growing practice to use mechanical power to
keep the hides in motion, instead of handling them. The hides are
attached to poles joined to a strong cross beam, which, in its turn,
is connected by stout iron rods with the main pullies and shafting.
The installation is expensive, but it saves an enormous amount of
manual labour and time, while ensuring uniform treatment. This
method is not so much used for liming as for the tanning process.
After loosening the hair and opening up the fibres, the next
operation is dehairing or depilation, or, as it is generally
described in the trade, "unhairing." The hair must slip quite easily
before beginning this operation, otherwise a number of them will be
left in the hide and will be difficult, or almost impossible, to
remove later on if the hides are being made into sole leather. These
hairs present an unsightly appearance and lower the value of the
leather. Depilation may be done by hand or machine; but the latter
method is rapidly superseding the former, as it is in nearly every
other process of leather manufacture.
In the manual process, the hair is removed by scraping it off in a
downward direction with a blunt, convex-shaped knife, fitted into two
wooden handles (Fig. 17), the hides being placed on a sloping convex
beam (Fig. 15) supported by a trestle. A series of grooves under the
beam permits it to be placed at any angle. The beam (different from
that used by curriers) may be of wood, iron, steel, or zinc-covered
wood.
[Illustration: FIG. 17
DEHAIRING KNIFE]
There are several types of unhairing machines, of which the Leidgen
more nearly approaches hand work than any other. The skins are placed
on a soft bed of felt, and the working roller, fitted with spiral
knives, is brought into contact with the hide and scrapes off the
hair.
[Illustration: FIG. 18
DEHAIRING AND FLESHING MACHINE]
The type of machine often used, however, is shown in Figure 18;
the working part is a long spindle fitted with helical knives. The
advantages of this machine are its large output and its use for other
operations by simply changing the working roller. The blades must be
blunt for dehairing, but sharpened blades are needed for fleshing.
If hides intended for sole leather are being dehaired, the short
hairs which are not removed by the dehairing knife are carefully
scraped off with a sharp knife. Other kinds of hides and skins are
freed from short hairs in a later operation.
When the hides are dehaired, they are sometimes submitted at once to
the next operation of fleshing, which, as its name implies, consists
in removing loose flesh and fat from the "flesh" side, that is, the
side near the carcase. The extent to which this operation is carried
out depends on the quality of the finished leather. Naturally, the
more flesh left on the greater the weight, particularly as loose
flesh will absorb a large quantity of tannin, and, unfortunately, of
adulterants which are frequently used for weighting common leathers.
The flesh ought to be removed in all cases, for the loose flesh forms
very poor and spongy leather. Where hides and skins are tanned and
then dried for sale to leather dressers and finishers, there is often
the tendency to leave far too much loose flesh on them, with the
object of producing as much weight of leather as possible. From the
point of view of economy, this is a mistake, for the loose flesh must
be removed during the dressing process, whereas, if it had been taken
off at first, both material and time would have been saved in the
process of tanning.
If the fleshing is not done directly after dehairing, the hides or
skins should be placed in a weak lime liquor, and this method is
to be recommended. The great point to be observed is to keep the
limed hides, both before and after dehairing, away from contact
with the air, as the chemical action of the carbonic acid on lime
results in the formation of chalk, which tends to harden the hides
and to roughen the grain, so that it is likely to be scratched in
later operations. Where the hides are intended for sole leather, and
not treated with an acid before entering the tanning liquors, the
presence of chalk would lead to an uneven colouring of the leather.
[Illustration: FIG. 19
FLESHING KNIFE]
Fleshing by hand demands great skill. The knife used (Fig. 19) is
similar to the unhairing knife, except that it has two edges. The
cutting is done with the convex edge, which has to be kept very
sharp. The concave edge need not be sharp, its use being limited to
scraping off loose particles of flesh, while the parts not removed
by this means are cut off with the sharp edge. The knife is held
slantingly, with the blade almost parallel with the beam. The
strokes should be short and in a semicircular direction, otherwise
it is difficult to avoid cutting the skins. This manual operation
is now largely superseded by machinery, and will, no doubt, soon be
obsolete. The early types of fleshing machines were not a success,
but the modern machine is very effective. There are several makes
on the market, but in most cases the working tool is of similar
construction and consists of a long cylinder to which spiral knives
are fixed. (Fig. 18.) Half of these blades converge to the left and
half to the right, the object being not only to cut away the flesh
but also to stretch the hides outward, thus ensuring an evenly cut
surface. The fleshings and useless pieces of skin are kept in a weak
lime liquor until there is a sufficient quantity to send to the
glue maker; although in some of the larger tanneries this offal,
technically termed "spetches," or glue pieces, is converted into glue
on the premises. In hot weather, a large accumulation of fleshings
is liable to putrefaction, despite the use of plenty of lime water.
While lime certainly arrests putrefaction of gelatinous matter for
a time, decomposition afterwards sets in and serious damage may be
done. To avoid this, it has been a common practice in Germany to use
formaldehyde, but, while this acts as a preservative, it hardens
animal tissues, and has a tanning effect, with the result that the
pieces are rendered insoluble and cannot, of course, be reduced to
a liquid gelatine by boiling. Glue makers have condemned the use of
formaldehyde, but other preservatives, such as "lysol" (a cresylic
compound) and "arasol," have no tanning effect and may be used with
safety.
After the operation of fleshing, it is necessary to get rid of the
lime in the hides, for, if they were put directly into tan liquors,
the lime, being alkaline, would neutralise the acidity of the tan
liquors and retard the beginning of the tanning process for a long
period. The leather would ultimately be poor, thin, stained, and
brittle.
Up to the process of deliming, there is not a great deal of
difference in preparing the hides and skins for the large variety
of leathers, but between the fleshing and the tanning processes
the work varies considerably, and largely determines the character
of the finished leather. When the hides or skins are in the limed
state, they are gristly and firm in texture. A certain amount of this
firmness is desirable in some leathers, such as sole and belting,
and, therefore, it is deemed advisable to leave a small quantity of
lime in the hides, although, to get them evenly coloured in tanning,
it is essential that the lime should be completely removed from the
surface of the hide. An old method, which is even in use to-day
in some tanneries, is to wash the fleshed hides in a cubical or
hexagonal drum for several hours in running water, which is conveyed
through an iron pipe in the journal of the drum, and escapes through
small holes in the drum. The effect of using a hard water for washing
out the lime is shown in the interior of these washing drums, the
sides of which become incrustrated with a thick, hard deposit. The
surface of this incrustration is irregular, and small projections are
sometimes formed which mark the hides and reduce their value, as the
impressions are not removed in later processes.
The modern method of deliming hides intended for sole leather is
to use a weak solution of acid, or an acid salt. For this purpose,
sulphuric and hydrochloric acids are by far the cheapest, but require
handling with great care, as any excess used has a corrosive and
destructive effect on the fibres, which results in weak leather.
Mild organic acids are much safer, and those generally preferred
are lactic, formic, and butyric, although boracic and acetic acids
are favoured by some tanners. Butyrate of ammonia is used for the
same purpose by a number of French tanners. The same acid bath can
be used for a second lot of hides, but sufficient acid should be
added to raise the acidity of the liquor to its former standard. It
is inadvisable to use one bath more than four or five times. The
salt formed during the process by the combination of the lime with
the acid--calcium lactate, formate, butyrate, acetate, or borate,
according to the acid used--must be washed from the hides, either in
a pit or drum, before they are ready for the tan liquors.
[Illustration: FIG. 20
SCUDDING KNIFE]
Other kinds of hides and skins require additional treatment,
according to the class of leather it is intended to make. Hides to
be dressed for such purposes as bags, portmanteaus, cases, harness,
belting, and stout uppers are usually steeped in an infusion of hen
or pigeon dung. A vat is filled with tepid water, a quantity of the
dung, usually about half a pailful, is added and well stirred in the
water before putting the hides in. The acid fermentation evolved
neutralises the lime, while bacteria multiply and rapidly reduce
the rubbery limed hide to a soft, flaccid condition. The hides are
then well washed in clear water and scudded, after which they are
transferred to the tan pits. "Scudding" is the technical name of an
operation performed on hides and skins with a special tool, known
as the scudding knife, which consists of a convex piece of slate or
vulcanite fitted into a wooden or steel handle (Fig. 20). The hides
are placed on the beam, grain side up, and vigorously worked with
the knife to scrape off scud (_i.e._, short hairs, dirt, and soluble
lime salts). Both sides of the hides should be scudded for best work.
The process of treating hides with excrement is known technically as
"bating,"
Calf skins, which are curried or dressed after tanning, should be
reduced to a more supple condition than hides intended for harness,
belting, and military leathers. A more active dung is, therefore,
used for skins of all kinds which have to be rendered supple. Dog
manure is generally used, that from the hunting kennels being
preferred. Imported dry dung has to be used by some firms, as the
supply of fresh dung is insufficient. The demand for the latter is
very keen, as it is more effective than the dry product. Goat skins
dressed for the famous shoe leather known as glazed or _glacé_ kid,
and kid skins for glove leather, need a larger proportion of "puer"
than nearly every other kind of leather, for the grain of goat skins
is naturally hard and requires a large quantity of dung to reduce it
to the necessary softness and suppleness of "kid" leather.
Although these processes can only be described as disagreeable, they
apparently have no injurious effect on the workmen. Further, the
hides and skins are thoroughly cleaned before putting them in the tan
liquors, in which the bacterial activity caused by the infusion of
dung is quickly arrested.
Fortunately, from the hygienic point of view, the use of natural
"bates" and "puers," although still extensive, is likely to be
superseded everywhere by artificial products. So far there are nearly
2,000 tanneries throughout the world where the artificial bating
materials are preferred, German and American tanners being the
principal users.
The best known artificial bate is "Oropon," which consists of a
mixture of pancreatin or trypsin, ammonium salts, and a large
quantity of sawdust, the last-named merely acting as a mechanical
agent. The enzyme, pancreatin, is the active ingredient, and may
be prepared from the intestines of the pig. It has the effect of
breaking up the albuminous matters of hides and skins, which are
rapidly reduced to a soft and supple condition, while the ammonium
salts cleanse them. The quantities used vary according to the degree
of suppleness required in the finished leather.
The patentee and vendor of this proprietary article claims that
it is suitable for all classes of leather. Hides for sole leather
are sometimes treated with a weak "Oropon" liquor in Continental
tanneries and, as a result of the cleansing properties of this
bating material and its effect in opening up the fibres, the tannage
proceeds rapidly. The use of enzymes for bating was discovered by
an English leather manufacturer and chemist, who did not take out a
patent for his invention, probably because he had previously patented
and worked on a commercial scale a bacterial bate which gives very
good results but requires much skill in application.
The artificial product, "Oropon," has many advantages over
excrements. It is simpler, cleaner, and more rapid in working, and
never damages the grain of the skin. On the contrary, great care
and experience are needed in using excrements, and the skins may be
so badly damaged through negligence as to be almost worthless. Bate
burns are fairly frequent when dung is used, and are generally due to
hard pieces being insufficiently broken up and diffused in the liquor.
Other useful artificial products are: "Erodin," and "Puerine," a
patented American product which consists of a weak organic acid and
a small proportion of molasses. Possibly, malt enzymes or diastase
could be utilised for the manufacture of an artificial bate or puer,
although they would not be so effective as animal products.
Where hides and skins have been treated with Oropon, they may be
transferred to the tan liquors after being rinsed in water, although
it is better to submit them to the operation of scudding.
There are two tests to determine the end of the process of bating
and puering, although the extent to which the reduction is carried
depends on the kind of leather wanted. For this reason, it is not
advisable to remove every trace of lime from hides which must possess
a certain degree of firmness when finished into leather. Of the two
tests, one is chemical and the other mechanical. In the former case,
a cross section is made in the thickest part of the hide and a few
drops of phenolphthalein are added to the cross section; if the whole
of the lime has been removed, no coloration is given, but, if lime
be present, a purplish colour is given, which varies in intensity
according to the contents of lime in the hide. The other test is made
by pressure with the thumb nail, and, if the impression be permanent,
the hides or skins are in a sufficiently reduced, or, as it is
technically known, "fallen" condition for all practical purposes.
The latter test is really only useful when the process has to be
carried to its fullest practical extent, as it affords no idea of the
intermediate stage. The experienced workman can judge the progress
made by appearance and touch.
In cases where skins have been puered with excrement it is often
necessary, after scudding them, to submit them to a further process
before tanning. This is known as "drenching," and consists in
treating the skins in an infusion of wheaten bran or pea flour. The
acid fermentation produced by these ingredients effectually cleanses
the skins by neutralising the last traces of lime and scud, and
prepares them in an ideal condition for the process of tanning. The
combined processes of bating with Oropon and drenching in bran are
extremely useful for skins to be dressed into glove leather.
Following the processes of bating, puering, or drenching, the skins
are washed in water to remove all mechanical impurities, and are then
in a perfectly clean condition for tanning.