TANNING

chemical treatment of raw animal hide or skin to convert it into leather. A tanning agent displaces water from the interstices between the protein fibres and cements these fibres together. The three most widely used tanning agents are vegetable tannin, mineral salts such as chromium sulfate, and fish or animal oil. See also leather. The oldest system of tanning relies on the chemical action of vegetable material containing tannin, or tannic acid, on the protein constituents of skin. Vegetable tanning seems to have been practiced in prehistoric times. In historic times, the Hebrews tanned with oak bark, and the Egyptians, with babul pods. The Romans used bark, certain woods, and berries. The Arabs tanned with bark and roots, and in the Middle Ages they reintroduced the art into Europe via Spain. By the 18th century the value of materials such as oak bark, sumac, valonia, and hemlock was well established. The procedure, essentially unchanged in modern times, involves soaking hides in vats of increasingly strong liquors, or liquid extracts of vegetable tannin. Tanning with chromium salts, introduced at the end of the 19th century, was probably the first change in the chemistry of leather production in at least 2,000 years. Two methods are used. In the double-bath method the hides are first bathed in a mild chromic acid solution. In the second bath, sodium thiosulfate and another acid react with the chromic acid to produce basic chromium salts, which are deposited on the fibres of the skins. In the more common single-bath method the hides are soaked in revolving drums filled with increasingly strong chromium sulfate solutions. Aluminum and zirconium salts are also used in tanning. Oil tanning is an ancient method that is used for such soft, porous leathers as chamois and deerskin, which can be repeatedly wetted and dried without detrimental effects. Fish oil is sprinkled onto the hides and pounded in with mechanical hammers. The hides are then hung in ovens, and the oxidized oil adheres to the skin fibres. The two main raw materials of tanninganimal hide and vegetable tanninare available almost everywhere. As a result, tanning has spread throughout the world. It remains one of the first industries to be established in areas or nations undergoing industrialization.