Animal skins and hides are treated to preserve them and make them
suitable for use.
The term hide is used to designate the skin of larger animals (e.g.,
cowhide or horsehide), whereas "skin" refers to that of smaller animals
(e.g., calfskin or kidskin). The preservation process employed is a
chemical treatment called tanning, which converts the otherwise perishable
skin to a stable and non-decaying material. Although the skins of such
diverse animals as ostrich, lizard, eel, and kangaroo have been used, the
more common leathers come from seven main groups: cattle, including calf
and ox; sheep and lamb; goat and kid; equine animals, including horse,
mule, and zebra; buffalo; pig and hog; and such aquatic animals as seal,
walrus, whale, and alligator.
The hides of mammals are composed of three layers: epidermis, a thin
outer layer; corium, or dermis, the thick central layer; and a
subcutaneous fatty layer. The corium is used to make leather after the two
sandwiching layers have been removed. Fresh hides contain between 60 and
70 percent water by weight and 30 to 35 percent protein. About 85 percent
of the protein is collagen, a fibrous protein held together by chemical
bonds. Basically, leather making is the science of using acids, bases,
salts, enzymes, and tannins to dissolve fats and nonfibrous proteins and
strengthen the bonds between the collagen fibers.
Leather making or leather work is an ancient art that has been
practiced for more than 7,000 years. Primitive man dried fresh skins in
the sun, softened them by pounding in animal fats and brains, and
preserved them by salting and smoking. Beginning with simple drying and
curing techniques, the process of vegetable tanning was developed by the
Egyptians and Hebrews about 400 BC. During the Middle Ages the Arabs
preserved the art of leather making and so improved it that morocco and
cordovan (from Córdoba, Spain) became highly prized leathers. By the 15th
century, leather tanning was once more widespread in Europe, and, by the
mid-19th century, power-driven machines that performed such operations as
splitting, fleshing, and dehairing were introduced. Toward the end of the
19th century, chemical tannage--in particular, the use of chrome
salts--was introduced.
The modern commercial leather-making process involves three basic
phases: preparation for tanning, tanning, and processing tanned leather.
As a preliminary step, a hide must be carefully skinned and protected both
in storage and transportation before reaching the tannery. A hide will
begin to decompose within hours of an animal's death; to prevent this from
happening, the hide is cured by a dehydrating process that involves either
air-drying, wet or dry salting, or pickling with acids and salts before
being shipped to a tannery.
At the tannery the hide is soaked to remove all water-soluble materials
and restore it to its original shape and softness. Hair is loosened
usually by a process called liming, accomplished by immersing the hides in
a mixture of lime and water; the hair and extraneous flesh and tissue are
removed by machine. The hide is then washed, delimed, bated (the enzymatic
removal of nonfibrous protein to enhance color and suppleness), and
pickled (to provide a final cleansing and softening).
The tanning process derives its name from tannin (tannic acid), the
agent that displaces water from the interstices of the hide's protein
fibers and cements these fibers together. Vegetable tanning, which is the
oldest of tanning methods, is still important. Extracts are taken from the
parts of plants (such as the roots, bark, leaves, and seed husks) that are
rich in tannin. The extracted material is processed into tanning liquors,
and the hides are soaked in vats or drums of increasingly strong liquor
until they are sufficiently tanned. The various vegetable-tanning
procedures can take weeks or months to complete. The end result is a firm,
water-resistant leather.
Mineral tanning, which uses mineral salts, produces a soft, pliable
leather and is the preferred method for producing most light leathers. Use
of this method can shorten the tanning period to days or even hours.
Chromium salt is the most widely used mineral agent, but salts from
aluminum and zirconium are also used. In mineral tanning the hides are
soaked in saline baths of increasing strength or in acidic baths in which
chemical reactions deposit salts in the skin fibers.
Oil tanning is an old method in which fish oil or other oil and fatty
substances are stocked, or pounded, into dried hide until they have
replaced the natural moisture of the original skin. Oil tanning is used
principally to make chamois leather, a soft, porous leather that can be
repeatedly wetted and dried without damage. A wide variety of synthetic
tanning agents (or syntans), derived from phenols and hydrocarbons, are
also used.
After the basic tanning process is completed, the pelts are ready for
processing, the final phase in leather production. The tanned pelt is
first thoroughly dried and then dyed to give it the appropriate color;
common methods include drum dyeing, spraying, brush dyeing, and staining.
Blended oils and greases are then incorporated into the leather to
lubricate it and to enhance its softness, strength, and ability to shed
water.
The leather is then dried to about 14 percent moisture, either in the
air or in a drying tunnel or by first stretching the leather and then air
or tunnel drying it. Other less frequently used methods include paste and
vacuum drying. The dried leather is finished by reconditioning with damp
sawdust to a uniform moisture content of 20 percent. It is then stretched
and softened, and the grain surface is coated to give it additional
resistance to abrasion, cracking, peeling, water, heat, and cold.
The leather is then ready to be fashioned into any of a multitude of
products. This leather work includes shoes and boots, outer apparel,
belts, upholstery materials, suede products, saddles, gloves, luggage and
purses, and recreational equipment as well as such industrial items as
buffing wheels and machine belts.
SOURCE: www.britannica.com