How Leather Is Made
Leather is one of the oldest manufactured materials. Leather articles more than 3,000 years old have been recovered from Egyptian tombs. The use of skins for clothing is also mentioned in the Bible. It is thought that at first skins were preserved with wood smoke and then lubricated with fats and oils from animals.
The Egyptians, Greeks, and Romans all produced leather. They were familiar with the use of lime to unhair skins, and used a number of tanning agents, including tannins, oils, and aluminum compounds. Oak bark was considered the best tanning agent, but tanning with oak bark and water took over a year.
Tanning was one of the first industries in the American colonies. The first American tannery was set up in 1623 in Plymouth, Mass. As the years passed, the number of tanneries increased until there was one in almost every town. Native hides and the bark of native trees were used. As tanneries became more efficient, their number decreased, until today there are about 500 in the United States.
The United States is the world's leading producer of leather, processing close to 100 million hides and skins annually. Other leading countries in leather production are Great Britain, West Germany, and France.
How Leather Is Made
Leather is produced by treating suitably prepared animal hides or skins with certain chemicals called tanning agents. These react with collagen, a fibrous protein that makes up most of the skin, and render it insoluble in water, immune to bacterial decay, and permanently stable. This process converts the hide or skin into tanned leather, while maintaining the basic structure and the natural beauty of the skin. Leather is further treated by oiling and finishing to improve its flexibility and appearance.
Depending on the hide or skin used, the method of tanning, and subsequent treatment, leather can be firm and solid, like sole leather, or soft and pliable, like the leather used for the upper part of shoes. Leather can also be dyed, embossed, rolled, or otherwise treated to give it any surface appearance desired.
All leathers have in common the desirable characteristics of strength, resistance to abrasion, flexibility, resistance to shrinking or stretching, and, perhaps most important, the quality of allowing the passage of water vapor. This last quality, also called "breathing," is an important factor in the comfort provided by leather garments, footwear, and gloves. It has not yet been equaled by any synthetic leather substitute.
The chief use of leather, accounting for about 85 percent of the leather manufactured, is for shoe soles and uppers. The soles are made of heavy leather from the hides of large animals, and the uppers of light leather from the skins of smaller animals or from hides split edgewise. Heavy leather is also used in machinery belting and hydraulic packings in industry and for luggage, upholstery, and harness. Light leathers are made into a variety of products, such as gloves, garments, handbags, billfolds, and bookbindings.
The chief raw materials of leather manufacture are hides and skins and tanning agents. In addition, large quantities of various chemicals are used in the preparation of skins for the tanning process and for the treatment of tanned leather.
Hides and Skins. Hides are the skins of larger animals, such as cattle, horses, and water buffalo. Generally, leather made from hides is classed as heavy leather, but light leathers are sometimes made from split hides.
"Skin" is the term applied to the skins of smaller animals, such as calves, goats, kids, sheep, lambs, and pigs. Also in this category are the skins of aquatic animals, such as seals and sharks, and those of ostriches and camels. Skins are used to produce a wide variety of light and fancy leathers.
Tanning Agents. Any chemical that acts on collagen to make it stable can be used as a tanning agent. Tanning agents may be derived from vegetable or mineral sources or may be produced synthetically. The chief commercial types are tannins, which are complex organic chemicals extracted from a great variety of plants, and basic salts of chromium.
Vegetable tannins occur in most plants. Commercial tannins are obtained from the barks of oak, hemlock, mangrove, wattle, algarrobilla, and willow; the wood of quebracho, oak, chestnut, and eucalyptus; the leaf of sumac; the cup of the valonia acorn; Turkish or Aleppo galls (galls are growths resulting from insect stings); and the pods of divi-divi and tara. These tannins are usually extracted with water, concentrated by vacuum evaporation, and supplied to tanners in liquid or solid form.
The different plants mentioned yield tannins of different properties, which influence the leather made with them. Tanners know from experience the blend of tannins needed to produce various qualities in leather. Vegetable tannins are used chiefly in the manufacture of heavy leather.
Mineral tanning agents are salts of metals such as chromium, aluminum, or zirconium. The most important commercially is basic chromic sulfate, a complex alkaline compound. It is used in chrome tanning, the process of tanning light leathers. Oil tanning agents are derived from fish and other marine animals and are used to produce extremely soft and absorbent chamois leathers. Synthetic tanning agents, usually called syntans, are produced by treating certain organic acids known as cresols, or naphthalene or other hydrocarbons, with sulfuric acid and formaldehyde. Syntans are generally used together with other tanning agents. Many of the aldehydes, a class of organic compounds, have tanning properties. Two of these, formaldehyde and glutaraldehyde, are used commercially.
Preparation for Tanning
The skin or hide of an animal is composed of three layers. The outer layer, called the hair side, consists of a very thin layer of epidermal tissue, with hair growing through it. The central layer, called the derma or corium, is composed mostly of a network of collagen fibers. The outer part of this layer contains the roots of the hair. The cavities that remain after the hair is removed form the grain of the leather. The inner layer is not part of the true skin, but is made up of fatty tissue from the animal's flesh. This side of the skin is called the flesh side. The derma is the part of the skin that is converted into leather, and the preparatory processes are concerned with making this portion accessible and responsive to tanning agents.
Curing. The first concern in dealing with skins taken from slaughtered animals is to prevent them from decaying. This is accomplished by a dehydrating, or water-removing, process known as curing. Before the curing, the hides are trimmed to a standard shape.
In the United States, treatment with sodium chloride, or common salt, is the usual method of curing. In countries where sodium chloride is scarce, skins are stretched on frames and dried in the air. They may also be rubbed with native salts, such as sodium sulfate. The following commonly used methods for curing cattle hides are good examples of American curing practice.
In the pack-salt cure, hides are piled on a. sloping concrete floor, flesh side up. An amount of rock salt equal in weight to the hide is spread over each hide, and the pack of hides is built to a height of about 3Y2 feet (105 cm). The hides are allowed to stand for 30 days while they lose moisture and absorb salt.
In the agitated brine cure, the hides are first washed in perforated drums and then immersed in vats of salt brine for 24 hours. They are stirred constantly by a paddlewheel mechanism. At the end of the period the skins are stacked on racks, and they are drained for 48 hours.
In the pit brine cure, hides or skins are first washed and trimmed to remove as much adhering flesh as possible. They are then piled in pits, and over the flesh side of each hide is spread an amount of rock salt equal to half the hide's weight. The packs are built to a height of about 4 feet (120 cm) and are covered with burlap, and the pits are flooded with brine. The skins remain in the pits for 48 hours and are then removed and drained for 24 hours.
The cured and drained skins are folded, hair side out, tied into bundles, and shipped to the tannery.
Washing and Soaking. At the tannery the hides and skins are washed in rotating drums filled with water and are then soaked in vats with cool water running through them. The soaking, which lasts for from 10 to 20 hours, restores moisture removed from the skin tissues by the curing process. It also removes salt, cleanses the skins, and plumps, or swells, the fibers. The plumping and moisture restoration are extremely important in determining how well the skins will react to tanning.
Liming and Unhairing. The function of liming is to loosen the hair and epidermal tissue. The skins are soaked in vats containing a strong solution of calcium hydrate, sometimes strengthened with sodium sulfide. This strong alkaline solution is called the lime liquor. It may also contain arsenic sulfide, dimethylamine, and various enzymes. (Enzymes are chemical substances, produced by all living cells, that aid chemical reactions involving organic substances.) Soaking in lime liquor may last only a few hours, or the process may continue for two or three days, depending on the type of liquor, the kind of skin, and the type of leather to be made.
After liming the hair and epidermal tissue are scraped off by unhairing machines, which have rollers with dull knife blades. Usually one or two passages through the rollers is sufficient to unhair a skin. The skins next pass through fleshing machines, which remove the last of the fleshy tissue. Then the skins are washed in circulating water for several hours.
After unhairing is completed, the lime remaining in the skins and hides must be removed to restore the skins to a neutral state. For most hides this process, called deliming, is carried out with weak organic acids, such as lactic acid. After this treatment the hides are ready for tanning. However, skins and split hides that are used to make light leathers must undergo two additional processes, bating and pickling, before they are ready for tanning.
Bating. This process involves soaking the skins for light leather in a warm solution for several hours. The bating solution contains ammonium chloride to neutralize the lime remaining in the skins. It also contains proteolytic enzymes, which act on certain proteins in the skin. The effect of these enzymes is to clear the color of the grain of the skin, thus enhancing its beauty. The enzymes used are derived from bacteria, fungi, or the pancreas tissue of animals. Bating also reduces the swelling of the skin resulting from previous processes. Bating is considered to be complete when the grain side of the skin retains an indentation made with the thumb.
Pickling. The purpose of pickling is to bring the skins into the acid state necessary for the reactions in mineral tanning to take place. The duration and methods of pickling vary widely, but a typical pickling solution for calfskin contains 1,000 pounds (450 kg) sulfuric acid for each 1,000 gallons (3,790 liters) of water. The salt is important because it prevents the swelling of skin tissue from acid absorption. After being drained and washed, skins are ready for tanning.
There are many variations in tanning processes, and special modifications are made by different tanneries. Generally the two most commonly used tanning methods are vegetable tanning and chrome tanning.
Vegetable Tanning. Vegetable tanning is used to make heavy leathers, such as sole leather, from hides. The chief tannins used are those from chestnut and quebracho trees, but many others are blended into tanning solutions, affecting the type and color of the leather produced. Vegetable tannins penetrate the derma or corium of the hides, and are adsorbed by the collagen; that is, they adhere to the surfaces of the collagen structures to make it stable. This procedure may be illustrated by the tanning of sole leather.
First, individual hides are cut into halves, called sides. These are hung on frames in vats and rocked continuously for nine days. The strength of the tanning solution is increased daily. Next, the hides are piled in vats containing a mixture of tanning agents from quebracho, oak, chestnut, and other vegetable sources. The strength of this tanning liquor is increased in five steps over a period of three to six weeks.
The two sides are wrung and rinsed and then whirled around in a rotating drum containing more tanning solution and some fish oil. The object of these processes is to induce the hides to absorb as much tannin as possible. The next step in tanning is a five-day soaking in a fairly strong tanning solution. This process, known as tempering, serves to distribute the tannin uniformly throughout the leather stock.
Next, the leather stock, hung on frames, is passed, at seven-minute intervals, through vats of hot water, weak alkaline solution, weak acid, a second weak acid, and cold water. In this series the alkali dissolves excess tannin from the grain of the stock and the acid neutralizes any excess alkali and prevents darkening of the stock by oxidation. After tanning, the stock is rigid and somewhat brittle.
Chrome Tanning, Chrome tanning is used to manufacture light leathers and, sometimes, to prepare hides for vegetable tanning. It is faster than vegetable tanning and does not make the leather stock as rigid. The usual agent is basic chromium sulfate. This salt combines with the collagen of the skin.
There are two main methods of chrome tanning. The two-bath method has the advantage of producing a softer leather, but it is more cumbersome and is largely being replaced by the one-bath process.
In the one-bath process a chrome liquor is made from a solution of chromic sulfate, formed by the action of sulfuric acid on sodium dichromate. The solution is treated with a chemical reducing agent, such as glucose or sulfur dioxide, which gives it the necessary alkaline, or basic, quality.
The chrome liquor is slowly poured into a revolving drum in which skins have just been pickled, and the drum continues operating until tanning is complete. Sodium bicarbonate is added to the solution to bring the chrome salt out of solution so that it can combine with the leather. Tanning is considered complete when a test piece of leather held for one minute in boiling water does not curl or shrink.
After tanning, the skins are washed to remove the excess tanning solution and are treated with a mild alkali to neutralize any free acid. The skins are then washed again to remove salts formed by the neutralization reaction.
Glutaraldehyde Tanning. Recent research has shown that glutaraldehyde has excellent tanning properties. It tans rapidly, in either an alkaline or acid solution. It is used to produce high-quality leathers for garments and shoe uppers. The leathers for uppers produced by glutaraldehyde tanning are very resistant to alkaline cleaning agents and are useful for work shoes to be worn in dairies, cement plants, and service stations.
Glutaraldehyde can also be used for simultaneous tanning with chrome-tanning agents and can be used to retan chrome-tanned or vegetable-tanned leathers.
The preparatory and tanning processes deprive the skins of their natural oils, and these must be replaced to make the leather suitable for use. Other finishing processes include drying of the leather, dyeing, and various mechanical and chemical operations to improve the leather's properties. Smoothing, embossing, and other surface treatments are also carried out.
Stuffing, or fat liquoring, is the process of restoring oil to the leather to make it flexible. The leather is paddled in vats containing a mixture of raw oils and sulfated oils, or oils treated with sulfuric acid, until it takes up some of the oil. The process of dyeing can be carried out simultaneously with this process, or it can be done before or afterwards. The dyes used are generally synthetic coal-tar derivatives. Heavy leathers are usually not dyed, and are oiled to a lesser extent than chrome leathers. Various chemicals may be added to the leather at this time, depending on the specific properties desired.
Drying is begun by passing the leather through rollers with blades that squeeze out moisture and smooth the leather. Light leathers are then "pasted" onto sheets of glass or enameled metal and allowed to dry for several hours in this flat state. Heavy leathers are hung on racks and allowed to dry in warm circulating air for about five days.
After they are dry, heavy leathers are dampened and then jacked, or rolled under heavy pressure, by glass rollers. They are then dried, brushed, and waxed. Light leathers, after drying, are dampened and then staked, or flexed over an edge to soften the fibers. Then they are sprayed with various chemicals that provide a uniform surface coating but do not interfere with the appearance or breathing of the leather. The surface may be embossed or otherwise decorated. The flesh side of light leather is buffed with fine sandpaper to render it attractive. Suede is a type of leather used flesh side out, this side being buffed and brushed to enhance its appearance and produce a soft nap.