Leather

V

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.

VI

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.

VII

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.

VIII

Finishing

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.

IX

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.

Contributed by: Tanners Council of America, Inc.

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain