Illustration: TAN-YARD AND PITS.
Tanning is the name given to the process for converting the skins
of animals into leather, by combining them with a substance called
“Tannin.” This tannin exists in many vegetable substances, such
as oak-bark, gall-nuts, catechu, sumach, &c.: all of these, and
many more, are used for tanning, but, on account of its cheapness,
oak-bark is the usual substance employed. It is tannin which gives
the quality of astringency to many vegetables, and this very taste of
astringency is produced by a partial combination of the tannin with
the surface of the mouth.
Illustration: UNHAIRING THE SKIN.
The skins (called “hides” or “pelts”) are first freed from all loose
pieces of flesh, fat, or skin; the hair is then removed by soaking
them in lime and water. The skins are then laid in the “tan-pit”
between layers of crushed oak-bark until the pit is nearly full;
water is then pumped in, and the whole is allowed to remain for
several weeks or months (according to the thickness of the skin),
during which time, however, the skins are changed in position by
removing them from one pit to another with fresh bark in it, so that
those taken from the top of the first are placed at the bottom of
the next; and this is done from time to time, in order that all may
receive the same pressure and strength of tan-liquor. Very thick
hides take a year to tan perfectly in this way, and consequently many
processes have been tried to quicken the operation; but the leather
made most slowly seems to wear the best, and consequently fetches the
higher prices.
Illustration: DRYING ROOMS.
Skins which are thin, and to be used for fancy work, and for
book-binding and glove-making, are either tanned with “sumach,” or
with alum and salt made into a paste with flour and yolk of eggs;
this is put into a tub, and the mixture and skins worked together
with the hands till they are thoroughly united.
Illustration: SUMACH TAN TUBS.
Besides boots and shoes, leather is used for the harness of horses,
covers for seats, gloves, and innumerable other purposes.
Illustration: SPLITTING MACHINE.
For some purposes the leather is required to be very thin, and of
exactly one thickness. This is obtained by the process of splitting,
for which a machine is used whose exactness is such, that one slice
is taken from the inner part of the whole skin, reducing it in
thickness without cutting a hole in any part. The skin is stretched
tightly round a roller, which slowly revolves against a straight
knife-edge, fixed at a certain distance from it, according to the
thickness of the skin, and which is passed by the machine backwards
and forwards, cutting the skin a little further each time.