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The modern commercial leather-making process involves three basic phases: preparation for tanning, tanning, and processing tanned leather. As a preliminary step, a hide must be carefully skinned and protected both in storage and transportation before reaching the tannery. A hide will begin to decompose within hours of an animal’s death; to prevent this from happening, the hide is cured by a dehydrating process that involves either air-drying, wet or dry salting, or pickling with acids and salts before being shipped to a tannery.
At the tannery the hide is soaked to remove all water-soluble materials and restore it to its original shape and softness. Hair is loosened usually by a process called liming, accomplished by immersing the hides in a mixture of lime and water; the hair and extraneous flesh and tissue are removed by machine. The hide is then washed, delimed, bated (the enzymatic removal of nonfibrous protein to enhance colour and suppleness), and pickled (to provide a final cleansing and softening).
The tanning process derives its page_seo_title from tannin (tannic acid), the agent that displaces water from the interstices of the hide’s protein fibres and cements these fibres together. Vegetable tanning, which is the oldest of tanning methods, is still important. Extracts are taken from the parts of plants (such as the roots, bark, leaves, and seed husks) that are rich in tannin. The extracted material is processed into tanning liquors, and the hides are soaked in vats or drums of increasingly strong liquor until they are sufficiently tanned. The various vegetable tanning procedures can take weeks or months to complete. The end result is a firm water-resistant leather.
Mineral tanning, which uses mineral salts, produces a soft, pliable leather and is the preferred method for producing most light leathers. Use of this method can shorten the tanning period to days or even hours. Chromium salt is the most widely used mineral agent, but salts from aluminum and zirconium are also used. In mineral tanning the hides are soaked in saline baths of increasing strength or in acidic baths in which chemical reactions deposit salts in the skin fibres.
Oil tanning is an old method in which fish oil or other oil and fatty substances are stocked, or pounded, into dried hide until they have replaced the natural moisture of the original skin. Oil tanning is used principally to make chamois leather, a soft porous leather that can be repeatedly wetted and dried without damage. A wide variety of synthetic tanning agents (or syntans), derived from phenols and hydrocarbons, are also used.
After the basic tanning process is completed, the pelts are ready for processing, the final phase in leather production. The tanned pelt is first thoroughly dried and then dyed to give it the appropriate colour; common methods include drum dyeing, spraying, brush dyeing, and staining. Blended oils and greases are then incorporated into the leather to lubricate it and to enhance its softness, strength, and ability to shed water.
The leather is then dried to about 14 percent moisture, either in the air or in a drying tunnel or by first stretching the leather and then air or tunnel drying it. Other less frequently used methods include paste and vacuum drying. The dried leather is finished by reconditioning with damp sawdust to a uniform moisture content of 20 percent. It is then stretched and softened, and the grain surface is coated to give it additional resistance to abrasion, cracking, peeling, water, heat, and cold.
The leather is then ready to be fashioned into any of a multitude of products. These include shoes and boots, outer apparel, belts, upholstery materials, suede products, saddles, gloves, luggage and purses, and recreational equipment as well as such industrial items as buffing wheels and machine belts.
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