Leather

I

Introduction

Leather, 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. See also Fur Industry.

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.

II

Curing

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.

III

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.

---

---

IV

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.