[Illustration: THE PITS.]
Leather is a substance universally used amongst civilized and very
generally amongst barbarous nations; it is made from the skins of
animals, which are tanned, or prepared with some substance, having the
power of converting the perishable skin, that decays readily when wet or
moist, into a lasting and comparatively imperishable leather.
The preparation of skins by tanning or other similar processes has
been practised from the earliest times; and although it has engaged
the attention of several scientific men, and has been the subject
of many curious experiments, it has received less alteration from
recent improvements in chemical science than many other manufacturing
processes. Several plans, which have been suggested with a view to
expediting the process, which on the old system is a very tedious one,
have been found to injure the quality of the leather, and have therefore
been wholly or partially abandoned; and others, which appear to be more
successful, are as yet adopted by a few manufacturers only.
The larger and heavier skins operated upon by the Tanner, as those of
bulls, buffaloes, oxen, and cows, are technically distinguished as
_hides_, while the name _skins_ is applied to those of smaller animals,
as calves, sheep, and goats. The process necessary to convert hides into
the thick hard leather used for the soles of boots and shoes, and for
similar purposes will first be noticed. The hides are brought to the
Tanner either in a fresh state, when from animals recently slaughtered,
or, when imported from other countries, dried or salted, and sometimes
both, for the sake of preserving them from decomposition. In the former
case the horns are removed, and the hide is scraped to cleanse it from
any small portions of flesh or fatty matter that may adhere to the inner
skin; but in the latter it is necessary to soften the hides, and bring
them as nearly as possible to the fresh state, by steeping them in
water, and repeated rubbing or beating. After this the hair is removed,
sometimes by steeping the hides for several days in a solution of lime
and water, which has the effect of loosening the hair and epidermis, or
outer skin; and sometimes by suspending them in a close chamber called
a smoke-house, heated a little above the ordinary temperature of the
atmosphere by means of a smouldering fire, in which case the epidermis is
loosened by a very slight putrefaction. In either case, when the hair and
epidermis, or cuticle, are sufficiently loosened, they are removed by
scraping with a curved knife, the hide being laid upon a convex bench or
_beam_.
[Illustration: Unhairing the Hides. Striking the Hides.]
The hides are prepared for the actual tanning, or immersion in a solution
of bark, by steeping them for a few days in a pit containing a sour
solution of rye or barley flour, or in a very weak menstruum, consisting
of one part of sulphuric acid mixed with from five hundred to a thousand
parts of water. By this process, which is called “raising,” the pores of
the hides are distended and rendered more susceptible of the action of
the tan.
Oak-bark is the substance most commonly used to supply the astringent
principle, and it is crushed or ground to powder in a _bark-mill_. In
the old method of tanning, which is not yet entirely abandoned, the hides
and powdered bark were laid in alternate layers in the _tan pit_, which
was then filled with water to the brim. After some months the pit was
emptied, and refilled with fresh bark and water; and this process was
repeated whenever the strength of the bark was exhausted. In this way,
the time required for impregnating the hides varied, according to their
thickness and other circumstances, from one to four years. The process
has been greatly expedited by the improvement, introduced in consequence
of the experiments of M. Seguin, a French chemist, of tanning with
concentrated solutions of bark, formed by passing water through a mass
of powdered bark, until, by successive filtrations, it is completely
deprived of its soluble tanning principle.
[Illustration: Bark Box. Barrow.]
The variations of practice among different Tanners extend to the
substance used as an astringent, as well as to the manner of applying
it. Ground oak-bark, which was formerly the only material in common use,
and is still the most general, produces good leather of a light fawn
colour. Valonia, of which considerable quantities are imported for the
use of Tanners, produces leather of great solidity and weight, the colour
of which is inclined to grey, and which is more impervious to water than
that made with oak-bark. Valonia consists of the acorns of the _Quercus
Ægilops_, and is brought from the Levant and the Morea. Catechu, or terra
japonica, the extract of the _Acacia Catechu_, produces leather of a dark
reddish fawn colour, which is light, spongy, and very pervious to water.
[Illustration: Fleshing Knife, and Section. Shaving Knife, and Section.
Striking Pin, and Section. Unhairing Knife, and Section.]
When the process is complete, the hides are hung up in a shed and allowed
to dry slowly; and while they are drying they are compressed by beating,
or rubbing, or by passing them between rollers, to give them firmness and
density. A yellow deposit is now found upon the surface of the leather,
to which the name of “bloom” or “pitching” is technically given.
We have hitherto alluded chiefly to the preparation of the thick hides
used for sole leather, among which several varieties may be found, each
distinguished by a different technical name, by which its thickness,
quality, or mode of preparation is known; but the thinnest and weakest
hides, as well as the skins of calves and other animals, are also
prepared for use as upper leathers, in which case it is necessary to
reduce their thickness by _shaving_ or _paring_ them down upon the flesh
or inner side, before they are subjected to the action of the tanning
infusions. Such hides or skins also require, after leaving the hands of
the Tanner, to be rubbed, softened, and dressed by the currier, in order
to bring them to the necessary degree of flexibility and smoothness.
The currier also has recourse to shaving or paring with a peculiarly
formed _knife_, to bring the skin to the requisite tenuity; and it is his
office to blacken the surface, which, for common shoe leather, is done
on the flesh side, although for some purposes leather is blackened on
the outer or grain side. Horse-hides, which are comparatively weak and
thin, are sometimes dressed in the latter way, under the name of Cordovan
hides, from the circumstance of such leather having been formerly made
at Cordova, in Spain. Calf-skins supply the quality of leather most
generally preferred for the upper part of boots and shoes.
Of the thin skins prepared for ornamental purposes, many are tanned with
a substance called _sumach_, prepared from a plant of the same name. The
tanning is performed by sewing up each skin into the form of a bag, with
the grain or hair side outwards, and nearly filling it with a strong
solution of _sumach_ in water. The bag is then fully distended by blowing
into it, and the aperture is tied up; after which it is thrown into a
large shallow vessel filled with hot water containing a little sumach.
The distended bags float in this vessel, and are occasionally moved
about with a wooden instrument until the solution which they contain
has thoroughly penetrated their substance. Owing to the thinness of the
skins and the heat to which they are exposed, this operation is performed
in a few hours. The process is expedited by taking the bags out of the
solution, and piling them upon a perforated bench or rack at the side of
the tub, so that their own weight may force the confined liquid through
the pores.
[Illustration: Apron. Gloves. Whetter. Roller. Leoline.]
When the tanning is completed, the bags are opened to remove the sediment
of the sumach; the skins are washed, rubbed on a board, and dried; after
which they are ready for dyeing and finishing with a ridged instrument,
which imparts to the surface that peculiar grain by which morocco leather
is distinguished. An inferior kind of leather, known as “imitation
morocco,” is prepared in a similar manner from sheepskins.
[Illustration: Plungers. Tongs. Jet. Hook.]
“Tawing” is the name applied to the process by which the skins of lambs
and kids are converted into soft leather by the action of alum. Of this
kind of leather gloves are usually made.
The _jet_ and _plungers_ used for immersing the hides in the tan, the
_tongs_ and _hook_ for removing them, and the other implements, are
easily understood as applied to the various processes here mentioned.
[Illustration]