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What is Leather?

The outer covering from an animal (usually a mammal) tanned, or otherwise dressed and prepared in such a manner as to render it usable and resistant to putrefaction, even when wet. Leather is a unique and flexible sheet material that is somewhat analogous to textiles, and may in fact be considered to be the first and only natural fabric.

The unique characteristics of leather are due largely to its structure, which is an interwoven, three-dimensional network of fibers inherent in the natural raw materials—hides and skins. This raw material is principally a fibrous protein called collagen and is composed of one continuous network of fibers.

In the raw skin, at least four distinct structures can be distinguished: 1) the thin outermost layer termed the EPIDERMIS ; 2) the grain layer or dermal surface; 3) the juncture between the grain layer and the dermis or corium; and 4) the major portion of the skin (the DERMIS or corium), which is the part converted into leather. In addition, there is the flesh layer, or hypodermis, which is the structure adjacent to the body tissues.

Beginning with the outer surface of a skin, there are:

• 1) the hairs, embedded in the skin, each in a sheath of epidermis known as the hair follicle and each with a hair root at its end, fed by a tiny blood vessel. Chemically the hairs consist of the protein keratin, and penetrate deeply into the papillary layer of the dermis. Most animals have hair of two types, primary and secondary. The positions which these hairs occupy relative to each other as they enter the surface of the skin, together with their different thicknesses, determine the characteristic marking or grain of the dermal surface, i.e., the grain pattern of the leather, which is exposed upon removal of the hair and other epidermal structures;

• 2) the epidermis, which is a protective, hard-wearing layer of keratinous cells. Those on the outside are dead and, upon drying and shrinking, fall off the skin. On the underside, adjacent to the skin proper, they consist of soft, jellylike living cells, which have little resistance and are readily attacked and degraded by bacterial action or enzymes, as occurs with stale skins or in enzyme unhairing. They are usually disintegrated by alkalis, especially sodium sulfide or hydrosulfide. 

• 3) the sudoriferous (sweat) glands, which are also lined with epidermal tissue and discharge sweat from the skin through the pores in the grain surface. and the sebaceous glands, which are located at the side of the hair follicles and discharge into them an oily, waxy substance, which protects the hair. (The gland is operated by a muscle, called the erector pili, which also causes the hair to stand upright);

• 4) the skin proper or dermis (corium), consisting of a network of collagen fibers, very intimately woven and joined together. In the grain layer these fibers become thin and tightly woven and are so interlaced that there are no loose ends on the surface beneath the epidermis. Thus, when the epidermis is removed, a smooth layer is revealed, which gives the characteristic grain surface of the leather. Toward the center of the dermis the fibers are coarser and stronger, and the predominant angle at which they are woven indicates the properties of the resultant leather. If the fibers are more upright and tightly woven, the leather will be firm and hard, with little stretch, while if they are more horizontal and loosely woven, the leather will be softer and stretchier. The dermis is also the strongest part of the skin; and

• 5) the flesh of the dermis, i.e., that layer next to the body wall of the animal, where the fibers have a more horizontal angle of weave, and fatty (adipose) tissue may also be present.

In general, the younger the animal the thinner and smaller the skin, the smoother and finer the grain structure, and the less likelihood of damage due to disease, scratches, insects, etc.. The more natural the animal's feeding and living conditions, the better the quality of the resultant leather; overfeeding, for example, produces greasier, weaker skins, while starvation results in thin, weak, misshapen skins showing skeleton markings The skin of the female is usually finer grained than that of the male, and has a looser fiber structure, especially in the flanks, giving a somewhat softer, stretchier leather. The less hair or wool there is on the animal the tougher and stronger the resultant leather, especially in the grain layer. Heavily wooled Merino sheep, for example, are inferior in this respect to goats and pigs.

Leather occupies a unique position among materials. Its structure gives it a very desirable softness and strength, while its chemical nature gives it the property of adhering well to paper, hoard, linen, etc. Its outstanding characteristics include its durability (when properly prepared and cared for), suppleness, porosity, beauty, temper and feel, in addition to its strength and softness. In terms of permanence, when properly tanned, stored, and maintained; it is probably the most permanent material known at this time.

The manufacture of leather predates recorded history. There is evidence that some leather samples found in Northern Germany may have been produced perhaps 12,000 years ago. Leather artifacts believed to date from the Neolithic and European Bronze Ages have been discovered, and it is an established fact that the Egyptians knew the art of vegetable tanning, as well as alum tanning, as long ago as 2000 B.C., and that tanning practices there were well established by 1600 B.C.

Information from Stanford University: http://palimpsest.stanford.edu/don/dt/dt2021.html