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.
Before tannage, the approximate composition of a freshly flayed hide is:
Water 64 %
Protein 33 %
Fats 2 %
Mineral salts 0.5%
Other substances (pigments, etc.) 0.5%
The 33% which is protein consists of:
1) Structural proteins, or Elastin (yellow fiber woven in the collagen fiber) 0.3% Collagen (which tans to give leather) 29 % Keratin (protein of the hair and epidermis) 2 % 2) Non-structural proteins, or Albumens or globulins (soluble, non-fibrous proteins) 1 % Mucins or mucoids (mucous materials associated with fibers) .7%
While all mammalian skin is made up of these constituents, the figure for keratin will vary widely, depending on the amount of hair present; the figure for fat will also vary. The division between albumen and mucins is debatable.
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. See: HAIR .
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. See: LIMING ;UNHAIRING ; 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 (sometimes known as theHYALINE LAYER ), 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 the living skin, the collagen fibers and cells are embedded in a watery jelly of proteinlike substance, called the GROUND SUBSTANCE .
The living collagen fibers are formed from this substance, which ranges in constitution from the blood sugars to substances which are almost collagen. The latter fibers have been called "interfibrillary" proteins, also known as nonstructural proteins, or pro-collagens. These are essential for the growth of the skin and also render the fiber structure nonporous. When the skin is dried (as in some forms of curing), they dry to a hard, gluelike substance, which cements all of the corium fibers together and makes the skin hard and horny. In producing a leather which is to be soft or supple, it is essential that these inter-fibrillary proteins te removed.
The corium fibers are composed of ropelike bundles of smaller fibrils, which in turn consist of bundles of sub-microscopic micelles. These in turn are made up of very long, threadlike molecules of collagen twisted together. All together, this gives a very tough, strong, flexible, three-dimensional structure, forming a network on which many of the qualities of leather depend. It is this structure which makes leather unique for, as of today, it has not been possible to produce it artificially. It is also unquestionably the basis for the remarkably high tensile strength of leather.
The skin also contains small arteries and veins which convey blood to the living tissues, as well as the nerve structures necessary for the sense of touch.
While all mammalian skins are structured along this basic pattern, they vary tremendously in size, e.g., from the hide of an elephant or ox to the skin of a rabbit or mouse, and they also vary considerably in shape and thickness. In addition, some animals have but little hair or wool and a thick epidermal layer—e.g., the pig—while others, such as the sheep, have a heavy fleece with curly wool and curly hair follicles but a relatively thin epidermis. The state of development of the animal is also important. A calf, for example, has finer structured hair than a full grown cow, consequently, leather made from the skin of a calf is relatively smooth and very fine-grained, while that of a cow is rougher and has a very pronounced grain pattern.
The skins of certain animals (at certain times of their lives) also contain considerable quantities of fat in globular cells, which lie approximately in the center of the dermis. Notable examples are the pig and sheep. Sheepskin may actually contain fat of this type, that is, in the interior of the skin and not merely on the flesh layer, amounting to 25% of the weight of the skin. Such excessive growth of fat cells disrupts and weakens the dermal fiber structure to such an extent that some sheepskins can be split into two layers along the line where the fat is located.
In general, the younger the animal at time of slaughter 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. See:LEATHER DEFECTS . 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 the covering materials used by bookbinders. 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 (See: LEATHER DRESSINGS ), it is probably the most permanent covering 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 tawing, as long ago as 2000 B.C., and that tanning practices there were well established by 1600 B.C.
Leather has been used for covering books since at least as early as the 3rd century A.D. (See: COPTIC BINDINGS ), and its use in craft bookbinding continues to this day. Although virtually every conceivable type of skin has been used, calf, deer, goat, pig, and sheepskin, as well as horsehide (RUSSIA LEATHER ) have been used most often, at least in Europe and the United States. Pigskin, as well as goatskin and deerskin, were often alum-tawed; however, vegetable tannage was and is the predominant method of preparation. Goatskin has been used to cover books in Europe for more than a thousand years and has been used extensively in Europe since the 16th century. See: VEGETABLE TANNING ;VEGETABLE TANNINS . See also: ALLIGATOR LEATHER ;AMERICAN RUSSIA ;BABY CALF ;BUCKSKIN ;BUFFALO ;CABRETTA LEATHER ;CALFSKIN ;CHAMOIS ;CHEVRETTE ;
CHEVROTAIN ;CORDOBAN LEATHER : CORDOVAN LEATHER ;COWHIDE ;DEERSKIN ;DOESKIN ;GOATSKIN ;HAIR SHEEP ;HOGSKIN ;KANGAROO SKIN ;LAMBSKIN ;MOROCCO ;
PARCHMENT ;PIGSKIN ;SEALSKIN ;SHEEPSKIN : VELLUM ;WALRUS HIDE .
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