Chemistry Reference
In-Depth Information
when such fibers are exposed to a humid or wet environment for some
length of time, they are hydrolyzed (react with the water) and begin to decay.
The water in such environments also becomes a source of decay processes:
it promotes bacterial activity and results in most oil-tanned leather having a
relatively short life.
Mineral Tanning.
Mineral tanning, a process often also called
tawing
,
entails immersing skins in a solution of specific minerals known as
alums
(composed of mixed sulfates of aluminum and other metals; alunite, also
known as
alum stone
, for example, an alum composed of mixed sulfates of
aluminum and potassium was widely used for mineral tanning in antiq-
uity). Skins to be mineral tanned are drenched in a solution of alum for a
rather long period of time (lasting from several days to several weeks,
depending on the thickness of the skins and the ambient temperature), until
the collagen fibers in the skin become thoroughly impregnated and react
with the alum. The alum-soaked skin is then taken out of the solution and
the excess alum is removed by repeated washing in water. The freshly pre-
pared leather is finally dried, either by exposure to the air or by
staking
(a
process that in addition to exposure to air, also includes mechanical stretch-
ing and smoothing).
Mineral tanning was probably first practiced in ancient Mesopotamia
and then spread to Egypt, the Middle East, and the Mediterranean Sea area
(Levey 1958). Mineral-tanned leather is soft to handle, has a velvety texture,
and is almost white, a color practically impossible to achieve by other
tanning processes. It is, however, very sensitive to humidity and water;
under wet conditions the alum in the leather is
hydrolyzed
(decomposed by
water), forming sulfuric acid, a very strong acid that attacks the leather and
causes its rapid decay. Mineral-tanned leather that has been humid or wet
for a more or less extended period of time loses some of its characteristic
properties, such as softness, pliability, and strength, and becomes hard,
horny, and brittle.
Smoke Tanning.
In the
smoke-tanning
process, clean skins are exposed to
the smoke of burning wood, which results in the reaction of the collagen
fibers in the skins with some of the compounds in the smoke, and the con-
version of the skins to tanned leather. Although the process can be practiced
exclusively of others, smoke tanning was often associated with initial oil
tanning: the skins were first rubbed with oil or animal fat and only then were
they smoked; the result was a combined oil-smoke tanned leather that was,
and still is, made by some people in central Asia, the Arctic region, and the
Americas.
