Leather is animal hide or skin that has been subjected to a chemical and physical process known as tanning. While hides and skins are liable to decay, leather is resistant to such decomposition. Tanning also gives leather certain properties for specialized uses, including resistance to wear, resistance to water penetration, tensile strength, flexibility, resilience, and permeability to water vapor. In addition, the attractive appearance of its grain gives leather an important aesthetic quality.
The hides and skins of almost any animal may be tanned and converted to leather. However, the main sources of leather are cattle, buffalo, sheep, goats, pigs, horses, seals, and walruses. Other, more or less exotic, kinds of leather are obtained from the skins of kangaroos, crocodiles, alligators, snakes, lizards, ostriches, camels, whales, llamas, and even the ears of elephants.
Leathers are put to numerous uses, although the greatest portion by far goes into the manufacture of sofas (couches & settees) and chairs. Other uses include upholstery, clothing, luggage, purses, wallets, bookbindings, transmission belts, machinery parts, and specialized items such as tabletops. The use of leather for gloves is traditional, but there has been increasing use for other garments as well.
By far the most important characteristic of leather is its fibrous structure, which is preserved from the original hide. It is this structure that gives leather its porosity and permeability to air and water vapor-the ability to “breathe,” which is of great significance in shoes. The fibers permit perspiration to escape, and the air spaces between the fibers provide good heat insulation. Thus leather has the ability to resist temperature changes because the greater part of any heat absorbed is used to drive off water vapor, and vice versa. In shoes the considerable amount of perspiration given off by feet is absorbed and subsequently lost by evaporation when the shoes are taken off. The fibrous structure of leather also gives it high resistance to “fatigue” and to puncture damage. This is the reason for its prevalent use in sofa industry.
Chrome tanning has come to be used throughout the world for producing practically all sofa leather, clothing and gloving leather, and certain specialized leathers that must be resistant to perspiration and high temperatures. Vegetable tanning, or a light chrome pretreatment followed by vegetable tanning, is still used for making shoe sole leather, upholstery leather, and bookbinding and other leathers that require “body,” as for embossing and tooling. Basically, vegetable tanning produces a plumper, firmer, heavier, and more water-resistant leather. Chrome tanning produces a softer and more elastic leather. It is less water-resistant than vegetable-tanned leather but can be rendered highly resistant by subsequent treatment.
Chrome tanning has been augmented by the use of zirconium salts. These produce a white leather, whereas chrome leather is naturally bluish. The tanning properties of zirconium salts were first observed in 1907, but it was not until 1931 that the first successful commercial zirconium leather was produced. In the United States the chief use of zirconium tanning salts is for the retanning of chrome shoe upper leather prior to coloring and finishing, and the amount of leather tanned solely by zirconium is relatively minute. In Europe a small amount of sole leather is produced using zirconium followed by normal vegetable tanning, a pretreatment that considerably improves the leather’s wear resistance.
From time to time other chemicals have been used in tanning, some of which have attained commercial importance. For example, formaldehyde has been used in weakly alkaline solution to produce white glove leathers that are washable and do not darken or yellow on exposure to sunlight. The tanning properties of other aldehydes have also been studied. Glutaraldehyde, for example, has been found to impart an extra resistance to perspiration-a distinct advantage in certain kinds of specialty leather.
However, the use of formaldehyde has been largely superseded by the development of syntans-synthetic tannins that actually bear no chemical resemblance to vegetable tannins. The syntan process was first introduced in Germany in 1913. It is based on the sulfonation-that is, treatment with hot sulfuric acid-of certain coal-tar derivatives such as phenol, cresol, and naphthalene, and their subsequent condensation with formaldehyde. The kinds and uses of syntans have been increasing but mainly for specialized purposes and leathers-for example, the bleaching of leather and the tanning of reptile skins to be finished in black and white.
Another tanning process is known as oil tanning. Chamois leather and certain furskins are tanned by this method. The pelt is first impregnated with an oxidizable, nondrying oil such as cod liver oil. The stock is then heated in a chamber or in a hot air drum in order to oxidize the oil in between and on the leather fibers. Finally, any excess oil is removed, and a soft, yellow-colored leather is obtained. Oil tanning is discussed in more detail in the section on chamois, or wash, leather that follows.
Materials that have been used in limited amounts for making special kinds of leather include melamine resins, sulfonyl chloride, and even iron salts. The iron salts were of particular significance during and immediately after World War II, but they have not achieved commercial success.
Because of the demand for leather goods, various materials have been developed as substitutes. The first feasible products consisted of cotton fabrics coated with cellulose nitrate compositions, and tried only to duplicate leather’s surface appearance. With the growth of plastics in the 20th century, and especially with the demands created by World War II, various materials were developed that also provide some of leather’s desirable characteristics, such as durability and resistance to water penetration. Such materials include the polyvinyl chlorides, and nonwoven manmade fibers impregnated with chemical binders. Homogeneous nonfibrous materials that permit quite different production methods are also in use. Faux leather, bonded leather and aniline leather are the most common types of such leather. They are most useful when used in sofas and couches.
Hides and skins have to be transported over distances up to several thousand miles from the slaughterhouse or meat-packing plant to the tannery. Therefore steps have to be taken to cure, or preserve, them as a protection against bacterial attack. The usual methods are salting or drying. Salting is the general practice in more temperate climates, and drying in tropical or sub-tropical countries. Hides and skins are preserved and sold in one of three types of cure: green salted, or wet salted; dry; and dry salted, a combination of salting and drying.
This fascinating video shows the process described above – from start to finish in a modern tannery.
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