1. The art of tanning consists in converting hides and skins into
leather, by impregnating them with astringent matter.
2. It is impossible to determine the period at which the art of
tanning was discovered. It was doubtless known to the ancients, and
probably to the antediluvians, in some degree of perfection; since
skins were applied as means of clothing the human body, before the
arts of spinning and weaving were practised. It is likely, however,
that they were applied to this purpose, for a considerable time, in
their natural state; and that accident, at length, suggested the means
of rendering them more applicable, by saturating them with certain
mineral or vegetable substances.
3. Although the art of converting skins into leather was practised in
remote ages, yet it was not until near the end of the eighteenth
century, that the true principle of the process was understood. Before
this time, it was supposed, that the astringent principle of the
agents employed, was a resinous substance, which adhered mechanically
to the fibres, and thus rendered them firm and insoluble. The correct
explanation was first given by Deyeux, and afterwards more fully
developed by M. Seguin. These chemists clearly proved, that the
formation of leather was the result of a chemical union between a
substance called tannin, and the gelatinous part of the skin.
4. The subject, however, was not thoroughly understood, and reduced to
scientific principles, until the year 1803, when Sir Humphrey Davy
gave it a careful investigation, in a series of chemical experiments.
These inquiries resulted in the conviction, that the method of tanning
which had been in general use, may, with a few alterations, be
considered preferable to that by which the process is carried on with
more rapidity.
5. The skin which envelopes the bodies of animals, consists of three
layers. That on the outside is a thin, white, elastic membrane, called
the _cuticle_, or _scarf skin_; that on the inside is a strong
membrane, denominated the _cutis_, or _true skin_; between these two
is a very thin membrane, to which anatomists have given the name _rete
mucosum_, and in which is situated the substance which gives color to
the animal. The cutis is composed of fibres, which run in every
direction, and, being by far the thickest layer, is the one that is
converted into leather.
6. The skins of large animals, such as those of the ox and horse, are
denominated hides; and those of smaller animals, as of the calf, goat,
and sheep, are called skins. Of the former description, is made thick,
of the latter, thin leather. The process of tanning different skins
varies in many particulars, according to the nature of the leather,
and the uses to which it is to be applied.
7. The general process of changing thick hides into sole-leather, is
as follows: They are first soaked in water, to free them from dirt and
blood; and then, if rigid, they are beaten and rubbed, or rolled under
a large stone, to render them pliable. They are next soaked in
lime-water, or hung up in a warm room, and smoked, until a slight
putrescency takes place. The hair, cuticle, rete mucosum, on one side,
and the fleshy parts on the other, are then scraped off, on a _beam_,
with a circular knife.
8. Nothing now remains but the cutis, or true skin. Several hides, in
this state of preparation, are put together into a vat, for the
purpose of impregnating them with tannin. This substance is found in
astringent vegetables, and is obtained, in a proper state for
application, by infusion in water. In that condition, it is called
_ooze_, which is first applied in a weak state.
9. After the ooze, of different degrees of strength, has been renewed
several times, they are put between layers of bark, and suffered to
remain several months, fresh bark, from time to time, being supplied.
The whole process generally occupies from twelve to sixteen months.
When strong solutions of tannin are used, the leather is formed in a
much shorter time; but, in that case, it is much more rigid, and more
liable to crack. It is rendered smooth and compact, by beating it with
a wooden beetle, or by passing it between rollers.
10. Oak bark, on account of its cheapness, and the quantity of tannin
which it contains, is more extensively employed by tanners than any
other vegetable substance. In sections of country, where this kind
cannot be conveniently obtained, the bark of the hemlock, spruce, and
chestnut, the leaves of the sumach, and various other astringents, are
substituted.
11. The process of tanning calf-skins is somewhat different in many of
its details. They are first put into a solution of lime, where they
remain during ten or fifteen days, and are then scraped on both sides
on the beam, with a circular knife, as in the former case, and for the
same purpose. They are then washed in water, and afterwards immersed
in an infusion of hen or pigeon's dung. Here they are left for a week
or ten days, according to the state of the weather and other
circumstances; during which time, they are frequently _handled_, and
scraped on both sides. By these means, the lime, oil, and saponaceous
matter, are discharged, and the skin is rendered pliable.
12. They are next put into a vat containing weak ooze, and afterwards
removed to several others of regularly increasing strength. In the
mean time, they are taken up and handled every day, that they may be
equally acted upon by the tanning principle. The time occupied in the
whole process, is from two to six months. The light and thin sorts of
hides, designed for upper leather, harnesses, &c., are treated in a
similar manner.
13. The tanner procures his hides and skins from various sources, but
chiefly from the butcher, and from individuals who kill the animals
for their own consumption. Great quantities of dry hides are also
obtained from South America, where cattle are killed in great numbers,
principally for the sake of this valuable envelope of their bodies.