DRESSING LEATHER.
Hides which are intended for purposes where softness and flexibility
are required, as for instance, for the upper-leathers of boots, and
for saddlery purposes, are called "dressing" or "common" hides, or, if
they are shaved down to reduce their thickness before tanning, they are
denominated "shaved" hides. Hides for this purpose are limed much in
the same way as has been described for butts; but if they are required
very soft and flexible, a somewhat longer liming is permissible. After
unhairing, fleshing, and washing in water, they are usually transferred
to a "bate," composed of pigeon- or hen-dung, in the proportion of
about 1 peck to 25-30 hides.
In this they are retained for some days, being handled frequently.
They completely lose their plumpness, and become soft and slippery;
the caustic lime is entirely removed; and the remaining portions of
hair-sheaths and fat-glands are so loosened that they are easily worked
out by a blunt knife on the beam. This final cleansing process is
called "scudding." The theory of the action of the "bate," or "pure,"
as it is sometimes called, is somewhat imperfect. It is frequently
attributed to the action of ammonia salts, and phosphates, contained
in the fermenting dung. Ammonia salts certainly will remove caustic
lime, free ammonia being liberated in its place, and weak solutions
of ammonia sulphate or chloride will rapidly reduce hides, and remove
or neutralise the lime. The phosphates in dung are mostly, if not
entirely, in the form of lime phosphate, which is quite inert. In
point of fact, the process seems to be a fermentive one, the active
bate swarming with _bacteria_; to this, rather than to its chemical
constituents, its action must be attributed. The _bacteria_ act not
only on the organic constituents of the dung, but on those of the hide,
producing sulphuretted hydrogen, together with tyrosin and leucin,
and other weak organic acids, which neutralise and remove the lime,
and, at the same time, soften the hide by dissolving out the coriin,
and probably also portions of the gelatinous fibre. The truth of this
theory is supported by the fact that, in warm weather, the activity
of the bate is greatly increased, and that, if one pack of hides is
over-bated, the next following is much more severely affected, the
hides having in fact themselves furnished food for the multiplication
of the bacterian ferment from the destruction of their own tissues.
It also explains the effective use (as a substitute) of warm water
with a very small portion of glucose, which, in itself, would be
insufficient to dissolve the lime, but with a small quantity of
nitrogenous matter, forms an excellent _nidus_ for the multiplication
of these organisms. An American invention for bating is the use of old
lime-liquor neutralised with sulphuric acid, an idea which is much more
scientific than would at first sight appear. Old lime-liquors, as we
have seen (p. 143) contain much ammonia and weak organic acids, such as
caproic, amidocaproic (leucin), and tyrosin. On adding sulphuric acid,
the lime forms an inert sulphate, and the sulphate of ammonia and the
weak organic acids which remain dissolved are just what are required
in a chemical bate. The lime-liquor should of course be filtered or
settled clear before using, and enough acid added barely to neutralise
the lime, and the liquor again settled or filtered. By this means both
the dissolved gelatin and the iron of the acid will be got rid of. The
liquor might then be slightly acidified before use. The writer has no
experience of the method, but imagines that used as described it might
be worth trying, although it would have a very unpleasant smell. In
this connection may be mentioned the fact that, when bran drenches
are used, in which lactic acid is developed, the butyric fermentation
is liable, in hot weather, to take its place, and as butyric acid is
a powerful solvent of gelatinous tissue, and the dissolved tissue
itself feeds the fermentation, rapid destruction of the skins is the
result. Cleanliness, scalding out of the drench vats, and washing the
bran before using with cold water to remove adhering flour, are useful
precautions.
If the removal of the lime be the only object aimed at in bating, the
ordinary process is most wasteful, as well as disgusting, from the loss
of pelt it entails. It is easy to find chemical reagents which will
remove the lime; but the resultant leather has been found wanting in
softness, and it is probable that the solution of the inter-fibrillar
matter is in many cases advantageous. Probably one reason for the
non-use of such chemicals is their expense. Maynard has patented the
use of sulphurous acid for the purpose. If sugar, glucose, or ammonia
salts be used, and the alkalinity of the solution nearly neutralised
after each lot of hides by common vitriol, the same liquor may be used
again and again. In this case, if iron is contained in the acid it will
be precipitated by the ammonia and must be settled out. The writer is
convinced, from his own experience, that with suitable tannage such
bating would yield better weights and quite as satisfactory leather for
many purposes as the ordinary mode. French tanners, by the free use of
water, and careful working at the beam, and the employment of very weak
liquors at the commencement of tanning, make excellent dressing leather
without bating and this is also true of the celebrated French calf.
The bating required may be shortened, and probably with advantage, by
washing the hides with warm water in a "tumbler," or rotating drum,
Fig. 49, prior to putting them into the bate, or the whole bating may
be done in the tumbler. After a short bating, also, the hides may be
softened and cleansed by stocking for 15-20 minutes. Warm bates act
much more rapidly than cold ones.
[Illustration: Fig. 49.]
Various machines have been proposed to take the place of hand-labour
in the beam work, and, at least as regards the smaller skins, with
considerable success. As a type of these, may be mentioned Molinier's
hide-working machine, Fig. 29, which consists of a drum covered with
helical knives, rotating at a speed of about 500 rev. a minute, over a
cylinder coated with india-rubber. The skin is allowed to be drawn in
between these drums, and the two being pressed together by a treadle,
it is drawn out by a mechanical arrangement in a direction contrary to
the rotation of the knives, which scrape off the flesh, or work off the
hair.
After bating, "shaved" hides are reduced in thickness in the stronger
parts by a shaving-knife, on an almost perpendicular beam. The workman
stands behind the beam, and works downwards. The knife is represented
at A, Fig. 26, and is a somewhat peculiar instrument. The blade is of
softish steel, and after sharpening, the edge is turned completely over
by pressure with a blunt tool, so as to cut at right angles to the
blade. There is an obvious economy in shaving before tanning, since the
raw shavings are valuable for glue-making, while, if taken off by the
currier, they are useless for this purpose. The hide also tans faster.
Instead of shaving, the untanned hide is frequently split, by drawing
it against a rapidly vibrating knife. The piece removed is tanned for
some inferior purpose, if sufficiently perfect. In sheep-skins, which
are split by a special machine, the grain-side is tanned for French
morocco or basil, while the flesh-side is dressed with oil, and forms
the ordinary chamois or wash-leather (see p. 210). Such a machine is
shown in Fig. 50.
[Illustration: Fig. 50.]
Tanned leather is frequently split by forcing it against a fixed knife,
as in the American "Union" machine, Fig. 51. This is however being
gradually superseded by the band-knife splitting machine, Fig. 52, in
which an endless steel blade travels over two pulleys like a belt, and
is kept constantly sharpened by a pair of emery-wheels seen below the
machine. I am indebted for the block to Messrs. Haley and Co., who have
made great numbers of these machines.
[Illustration: Fig. 51.]
After bating, scudding, and shaving, the hides are taken into the
tan-house, where they are grained, either by frequent handling, or by
working in a paddle-tumbler (a vat agitated with a paddle-wheel, and
known in America as an "England wheel"), with a liquor of suitable
strength. What this strength should be depends on whether a well-marked
grain is required or not. The stronger the liquor, the more it
contracts the hide, wrinkling the surface into a network of numberless
crossing furrows, which form the well-known marking of "grain-leather."
In bark tannage, the after management is much like that described with
sole-leather, except that weaker infusions are employed, and acid
liquors, which would swell the hide and produce a harsh leather, are
avoided. In old-fashioned country yards, which produce some of the best
bark-tanned shaved hides, the liquors rarely range above 10°-15° of
the barkometer, and the time employed is 3-6 months. The hides, after
passing through a set of handlers, of gradually increasing strength, in
which they are at first moved every day, are laid away with bark liquor
and a good dusting of bark, receiving perhaps 4-5 layers of 2-4 weeks
each. Unfortunately, these tannages are so unprofitable that they are
rapidly being supplanted by quicker and cheaper methods.
[Illustration: Fig. 52.]
These more rapid and cheap tannages mostly depend on the use of "terra"
(block or cube gambier) in combination with bark, valonia, mimosa,
and myrobalanes. Liquors warmed to 110° or even 140° F. (43°-60° C.)
are frequently employed, and a bright colour is finally imparted by
handling in a warm sumach or myrobalanes liquor, which dissolves out
much of the colour imparted by terra or extracts. The tannage is helped
forward by frequent handling, by working in tumblers, or sometimes by
suspension on rocking or travelling frames, after the American fashion.
To this class of tannage belongs that of East India kips, which is
largely carried on in the neighbourhood of Leeds. These kips are
the hides of the small cattle of India, and are imported in a dried
condition, and with their flesh-side protected (and loaded) with a coat
of salt and whitewash or plaster. They are usually softened in putrid
soaks, and unhaired with lime, and are used in England for many of the
purposes for which calf-skins were formerly employed. A variety of East
India kips, called "arsenic kips," are treated (instead of plastering)
with a small quantity of arsenic before drying, to prevent the ravages
of insects, which are often very destructive to these goods. Many kips
tanned in India have also been imported of late years, and have greatly
interfered with the profits of English tanners.
In yards where the leather is intended to be sold uncurried, it is
taken up into the drying-sheds, well oiled on the grain with cod-liver
oil, and either simply hung on the poles to dry, or stretched with a
"righter," a tool shaped somewhat like a spade-handle, and finally
set out with it to a smooth and rounded form. As in the case of
sole-leather, too much light or wind must be avoided, and it is very
difficult to use artificial heat successfully in the early stages
of the process. It is, however, now very common for the tanner who
produces such leather also to curry it, and, as this effects a
considerable economy, both in labour and material, it is likely to
become universal. When leather is to be sold rough, it is necessary
to tan it in such a way as to give it a white appearance, from the
deposit of "bloom" already mentioned; this being regarded by curriers
as an essential mark of a good tannage, although the first step in
the currying process is to completely scour it out. When the tanner
curries his own leather, he of course aims at putting in as little
bloom as possible, thus economising both tanning material and labour.
In addition, the leather goes direct from the tan-house to the
currying-shops, thus saving both drying and soaking again, and, it is
said, giving better weight and quality. The tanner, too, is enabled to
shave his hides or skins more completely, utilising the material for
glue-stuff, which, had the leather been for sale in the rough, must
have been left on to obtain a profitable weight.