1. ORIGIN.-Though its proper care depends largely
upon a proper knowledge and understanding of its origin and the processes of its
manufacture, it is perhaps unnecessary to give a definition of leather. Suffice it to say
that leather originally was the skin of an animal, and that through a chemical process
called "tanning" this skin is made into leather. It is not so generally known,
however, that while the antiquity of the processes of tanning are unknown, the industry
was practiced in the remote ages by the Chinese. It was practiced as early as 900 B.C. by
the Egyptians (as evidenced by pieces of eather now in the British Museum taken from a
mummy case of that age). It was practiced at the beginning of the Christian Era by the
Romans, who accidentally discovered the tanning processes and grew to be much skilled in
the preparation and finishing of leather. Strange does it seem, therefore, that despite
the almost universal use of skins by savages, so many aboriginal peoples remained, almost
up to the present time, ignorant of the art of tanning. The answer probably lies partly in
the fact that the chemical processes are more or less complicated, depending upon the
method of tanning used, and partly in the fact that proper chemicals were either unknown
or unavailable.
2. TANNING.-In its original raw state an
animal's skin is made up of three layers, the outer, or hairy, which has no blood vessels;
the middle, which is the true skin, made up of gelatinous fibers; and the inner, a fatty
layer, in which are imbedded the perspiratory and sebaceous glands. All processes of
tanning have generally for their first object the reduction of the skin to the middle
layer, or true skin. The inner, fatty layer and the outer, hairy layer are removed by
steps called "cleaning" and "depilation."
The cleaning of the hides is accomplished by
soaking them in frequently changed water until they are soft, which may take two to three
weeks, after which all adherent fatty tissue is scraped from the inside. Depilation is
accomplished in two ways, - by sweating and by liming. In the sweating process,
for the production of what is called "raw" leather, the hides are suspended in
pits at a uniform temperature (64.50 F.) and in a moist atmosphere until they undergo a
partial decomposition, the ammonia produced saponifying the fats around the roots of the
hairs, after which the hairs may be scraped off. In the liming process, for the
production of what is called "finished" leather, the softened hides are soaked
in lime water until the hair may be removed, the lime acting as a saponifier, after which
the excess lime must be removed from the hides by an acid (sulphuric) which, in turn,
"plumps" the fibers, materially increasing the hide's thickness. The hides must
then be treated with a putrefactive or fermenting element called "bate" (such as
sour bran), which softens the hides and abates the "plumpness". After these
processes have been accomplished, the hides are ready for tanning.
In this country there are two common methods of
tanning called, respectively, the vegetable (or tannic acid) and mineral methods. The
vegetable method, in which the tannin generally comes from oak or hemlock bark, produces
what is commonly conceded to be the best leather for many purposes, especially heavy
leather, but takes a long time and is therefore relatively expensive. Generally, in this
process, the bark is thoroughly ground, then leached to extract the tanning principle from
it. The hides are first placed in a weak solution of this tannic acid and then transferred
successively to vats where the liquor is stronger until the processes are complete.
Depending on many conditions, the complete process may require from two to fifteen months.
The chrome tanning process, which is the principal
process of the mineral tanning method, was discovered by an American, and first used about
1890. It is a quick, relatively cheap method, requiring much less time to accomplish tha
ii the vegetable method. Used mainly for tanning lighter leather, it produces leather with
great water repellant and heat resisting properties, generally of great tensile strength,
but much more difficult to dye than that tanned by the tannic acid method. Briefly, it
consists of treating the hides with a weak solution of dichromate of potash to which
sufficient hydrochloric acid has been added to liberate chromic acid, which is the tanning
principle. After the skins have taken up a bright yellow color throughout their texture,
they are drained and transferred to a bath 6f hyposulphite of soda to which hydrochloric
acid again is added. This liberates sulphurous acid, which reduces the excess of chromic
acid to green chromic oxide, which, in effect, coats each fiber with a casing that
preserves it, and gives the resulting leather a characteristic pale bluish-green color.
Upon removal from the bath, the leather must, because of its water repellant properties,
be dyed while still moist by applying the color dye desired. If allowed to dry before
dyeing, it can never again be sufficiently wetted to dye properly. Even then the dye does
not penetrate the surface greatly, which is shown by the finish of boot uppers wearing
where the stirrup strap crosses the shin, and prevents its being ever redyed properly.
There are also for different animal skins, such as
chamois, sheep, pig, calf, etc., different tanning processes, such as oil tanning, smoke
tanning, Indian tanning, etc., followed in this and different countries, all of which,
however, are directed generally to the same end, i.e., the conversion of the gelatinous
tissues of the skin into the non-putrescible substance we know as leather. (See section II
and V, TM 10-226.)
3. CLASSIFICATION.-During the tanning processes
if the hides have not been "bated" (steeped in a mixture to soften and reduce
the swelling of the fibers), the resulting product is called "raw" leather; if bated
during the tanning process, the resulting product is called "finished"
leather. In order to finish raw leather it is necessary to scour it and compress it. by a
strike pin and roller. If the leather is stuffed with oils to increase its resistance to
water and to make it more flexible, it is then called "dressed". It may then be
dyed or stained, after which it may be grained
by rolling, if desired. Nearly all items of manufactured leather equipment (harness,
saddles, bridles, etc.) are made from dressed
leather. (See Section I, TM 10-226.)
