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Animal skin that has been chemically modified to produce a strong, flexible material that resists decay. Almost all the world output of leather is produced from cattle hides and calfskins, goatskins and kidskins, and sheepskins and lambskins. Other hides and skins used include those of the horse, pig, kangaroo, deer, reptile, seal, and walrus.
Leather is used for a wide range of products. The variety of skins and the way they are processed can produce leather as soft as cloth or as hard as a shoe sole. Cattle hides, the major raw material for leather production, range from being lightweight and supple to tough and strong. Tough hides are used in the production of the durable leather required for soles of shoes, machine belting, engine gaskets, and harnesses. Calfskin is lighter and finer grained, and is used for making fine leather suitable for such articles as shoe uppers. Sheepskin is soft and supple; it yields the type of leather suitable for gloves, jackets, and other apparel. Since ancient times, human beings have used animal skins and learned to make leather. The process of using chemicals to turn skins into leather is called tanning.

The raw materials used by the leather industry originate largely as by-products of the meat-packing industry. Before entering the tanning process, the raw skins are "cured" by salting or drying them promptly after being removed from the slaughtered animal. The more common methods used in curing require the use of salt (sodium chloride) in one of two ways: wet-salting or brine-curing. In wet-salting, the skins are liberally salted and piled on top of one another until they form a pack. They are left in the pack for about 30 days to allow the salt to thoroughly penetrate the skin. Brine-curing is a much quicker method. In agitated brine-curing, the method most commonly used, skins are placed in large vats called raceways that contain a disinfectant and brine maintained close to full salt saturation. After about 16 hours in the raceway, the skins are completely penetrated by the salt.

Soaking and Unhairing
The cured skins are soaked in pure water to eliminate salt, blood, and dirt, and also to replace moisture lost in the curing process. After the skins have soaked for a period varying from two hours to seven days, the flesh is removed mechanically from the inner surface. To loosen the hair, the skins are then immersed for one to nine days in a solution of lime and water containing a small amount of sodium sulfide. Following this operation the hair is easily removed by a dehairing machine, and the distinctive pattern known as the grain can be distinguished on the outer surface of the skin. To ensure clear, clean surfaces, any remaining flesh and hair is scraped off, usually by hand with a dull knife, by a process called scudding.

Deliming and Bating
The next operation involves deliming the skins by soaking them in a weak solution of acid, which reduces the swelling caused by the lime. Simultaneously, most types of skins are treated with a "bating" material consisting of enzymes to give a smoother grain and render the skin soft and flexible. The amount of bating varies greatly, from none at all for sole leather to a concentrated treatment for leather to be used in kidskin gloves. After the deliming and bating operations, the stock can be tanned.
Each type of skin may be treated by several tanning processes. The process is chosen according to the use for which the leather is intended. The two principal tanning processes are mineral, or chrome, tanning, and vegetable tanning. Chrome tanning often can be completed in a single day, whereas vegetable tanning requires many weeks or months. Vegetable tanning results in a firmer leather with greater water and stretch resistance. Chrome tanning shrinks the stock and produces a longer-wearing leather with greater resistance to heat. The processes are sometimes combined to derive some of the advantages of each.

Vegetable Tanning
In this process the tanning agent, which renders the skin immune to decay and prevents shrinkage, is a substance known as tannin. Tannin is extracted from the bark, wood, fruit, and leaves of trees. Chestnut wood, oak bark, and hemlock bark are the major domestic sources of the tannin used by the United States leather industry. Foreign sources, which provide more than 80 percent of the tannin supply, include the wood of the quebracho tree of South America, mangrove bark from the island of Borneo, wattle bark from South Africa, and myrobalan fruit from India.
In vegetable tanning the hides are suspended from rocking frames in a series of vats containing increasingly stronger tannin solutions, called liquors. After several weeks the hides are transferred to a "layaway" section, which consists of larger vats containing still stronger liquors. Each week more tannin is added to the liquor, until the hides have absorbed enough tannin to complete the process. The last stages of the process may be accelerated by the use of warm liquors. Flexible vegetable-tanned leathers to be used for belting, luggage, upholstery, or harnesses are less heavily tanned than the leather intended for shoe soles.

Mineral Tanning
The mineral tanning process is known as chrome tanning because the tanning agent used most frequently is a salt compound of chromium. Chrome-tanned leathers, which stretch more than vegetable-tanned leathers, are suitable for handbags, shoe uppers, gloves, and garments.
To prepare the stock for chrome tanning, the bated skins are pickled in a solution of salt and acid. The skins are then immersed in a basic chromium-sulfate solution within a large revolving drum that tumbles the skins. This type of liquor penetrates the skins so rapidly that tannage is accomplished in less than a day. The chrome process originally involved the use of two different liquors, both solutions of compounds of chromium, and required substantially more time. Known as the two-bath process, it is still used for some varieties of leather. Aluminum or zirconium compounds may be used in place of chromium in the production of white leather. Alum, formaldehyde, gluteraldehyde, and synthetic tannins (Syntans) are also used to impart special characteristics.
In the production of combination-tanned leather, the skin is first chrome-tanned and then retanned with vegetable tannins. The modified applications of both processes produce leather with some of the advantages of each type.

Lubrication and Dyeing
After tanning, all types of leather undergo various operations that differ according to the use of the desired product. Vegetable-tanned leather for shoe soles is first bleached a lighter color. Next, it is infused with such materials as epsom salts, oils, and glucose, and then lubricated with hot emulsions of soap, greases, and sometimes wax. Finally, the stock is run through rolling machines to give the leather a desired degree of firmness and a high gloss. Chrome-tanned leather intended for shoe uppers is split and shaved to the desired uniform thickness. It is then placed in a rotating drum for the dyeing process, which usually involves the use of several types of coloring materials to achieve color fastness and durability. Before or after dyeing, the leather is rolled in a "fat liquor," which contains emulsified oils and greases. More than 100 leather colors exist, ranging from traditional tans and browns to such exotic shades as fuchsia and turquoise.
After dyeing and fat-liquoring, the stock is stretched for drying. Workers paste the stock on frames made of glass or ceramics or "toggle" it on perforated metal frames. The frames are then conveyed through drying tunnels with controlled heat and humidity.

Heavy leathers are finished by coating the grain surface with a finishing compound, and finally by brushing it under a revolving, brush-covered cylinder. The grain surface of light leathers is buffed, or sandpapered, to correct imperfections in the skin. Buffing the flesh side of leather raises the nap and produces the popular leather known as suede. For smooth finishes, most light leather is seasoned, or treated with a mixture of such materials as waxes, shellac or emulsified synthetic resins, dyes, and pigments. Pigments are used sparingly to avoid a painted look. Glazing gives the grain a highly polished surface. Several coats of thick, oily varnish are required to give patent leather its characteristic high gloss.

Leather Substitutes
Today, many artificial substances are produced and sold as "leather goods." These modern synthetics include such plastics as polyvinyl chloride and nonwoven fibers impregnated with binders. These materials lack leather's porous quality, pliable nature, and resilience. However, the artificial materials cost less to produce than leather and have come to command a large share of the leather market, particularly in shoe soles.
By: Tanners Council of America, Inc.