leather, animal skins and hides that have been treated with chemicals to preserve them. To make them suitable for use as clothing, footwear, handbags, furniture, tools, and sports equipment.
The term hide is used to designate the skin of larger animals (e.g., cowhide or horsehide). Whereas skin refers to that of smaller animals (e.g., calfskin or kidskin). The preservation process employed is a chemical treatment called tanning. Which converts the otherwise perishable skin to a stable and non-decaying material.
Tanning agents include vegetable tannins, mineral salts, and fish or animal oils. Although the skins of such diverse animals as ostriches, lizards, eels, fish, and kangaroos have been used. The more common leathers come from seven main groups: cattle, including calves and oxen; sheep and lambs; goats and kids; equine animals, including horses, mules, and zebras; buffalo; pigs and hogs; and such aquatic animals as seals, walrus, whales, and alligators.
Animal skin drying process
The hides of mammals are composed of three layers. Epidermis, a thin outer layer; corium, or dermis, the thick central layer; and a subcutaneous fatty layer. The corium is used to make leather after the two sandwiching layers have been removed. Fresh hides contain between 60 and 70 percent water by weight and 30 to 35 percent protein. Tanning agents dissolve fats and nonfibrous proteins and strengthen the bonds between the collagen fibres.
Read origins, processing and more interesting facts about leather making by Pritish Kumar
Origins of leather making
Leather making is an ancient art that has been practised for more than 7,000 years. Fresh skins were dried in the sun, softened by pounding in animal fats and brains, preserved by salting and smoking. Simple drying tanning techniques was developed by the Egyptians and Hebrews about 400 BCE. During the Middle Ages the Arabs preserved the art of leather making. By the 15th century, leather tanning was once more widespread in Europe. By the mid-19th century, power-driven machines for splitting, fleshing, and dehairing were introduced. Toward the end of the 19th century, chemical tannage—the use of oak, sumac, and hemlock tanbark and chrome salts—was introduced.
Modern leather making
The modern commercial leather-making process involves three basic phases:
- preparation for tanning,
- tanning, and
- processing tanned leather.
preparation for tanning
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, pickled.
Tanning
The tanning process derives its name from tannin. 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 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. It is the preferred method for producing most light leathers. Use of this method can shorten 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.
Chemical tanning of leather
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.
processing tanned leather
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. 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.
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. Also recreational equipment as well as such industrial items as buffing wheels and machine belts.
Artificial leather
Some of the earliest leather substitutes were invented in the 19th century. Nitrocellulose (guncotton) was developed by German chemist Christian Friedrich Schönbein in 1845. Later it was turned into collodion (pyroxylin) in 1846 by French scientist Louis-Nicolas Ménard. Collodion was used as a protective coating in wound dressings, and it was later applied to fabrics. Fabrikoid, created a pyroxylin-infused cotton fabric in 1910.
It resisted water and was used broadly in items such as upholstery, book bindings, linings, and automobile covers. Naugahyde, a fabric coated with leather fibres and rubber, was first used in women’s handbags in the early 1920s. Since the 1960s, fabrics coated or infused with polyurethane and polyvinyl chloride have been the preferred types of artificial leather, having both the look and the durability of real leather.
Artificial leather—known colloquially as faux leather, imitation leather, and pleather—has been viewed as having a number of advantages over real leather. It can approximate the appearance and durability of real leather at a lower cost. And its production is far less labour-intensive. In addition, animal-rights organizations have condemned the leather-making industry for its slaughter and abuse of animals.
This combination of practical and ethical considerations has driven the demand for artificial leather substitutes for handbags, shoes, clothing, and other fashion items. By 2015 the size of the artificial leather market was estimated at more than $50 billion.
