LEATHER is manufactured from the hides and skins of various animals, the principal of which are cattle, calves, sheep, goats and pigs. The skins of many rarer animals such as the walrus, seal, antelope, deer; the reptiles (snake, lizard and alli gator) ; and the ostrich, camel and elephant are also converted into leather for special purposes. The object of tanning (or the manu facture of leather) is the conversion of the putrescible skin into a material which under ordinary conditions of use does not putrefy, and which can be wetted and subsequently dried without becoming hard or horny. A characteristic difference between dried skin and leather is that the former becomes soluble in water at a tempera ture of F whilst leather is unaffected by such immersion.
Treatment of Hides and Skins.—Hides and skins must be treated to prevent putrefaction; i.e., must be cured in some suitable way. The chief methods are drying and salting. Ordinary domestic stock is simply treated with a small quantity of salt which acts as a temporary preservative during the period of transit from the slaughter house to the tanner. Imported hides and skins are received in one or other of the following conditions—(a) Sun. Dried, (b) Dry Salted, (c) Wet Salted. Sun dried skins are pre pared for shipment by laying them on the ground in the shade, or by hanging them on poles, until they have become thoroughly dry. Such skins are sometimes treated with a weak arsenical solu tion before drying in order to lessen the risk of putrefaction and of insect damage. Dry salted skins are lightly sprinkled with salt prior to drying. In countries where salt is subject to taxation and where salt is scarce, the "salting" process consists in the appli cation of some native saline earth, which may be composed of a large number of salts. Wet salted skins are prepared by either applying salt to the flesh side or by first completely immersing the hide or skins in a strong brine and then subsequently re-salting with fresh salt applied to the flesh side. The moisture is so reduced by the de-hydrating action of the salt that the goods are no longer liable to drip, but have been converted into a semi-dry condition. The methods of sun drying and of salting depend for their efficacy upon the removal of the natural moisture from the hide or skin or the replacement of it by brine, since bacterial action, which results in putrefactive damage, cannot occur in the absence of moisture or in strong salt solutions.
Sources of Supply.—The following figures give the estimated average slaughterings in various countries— Histology of Skin.—A knowledge of the histology and com plex chemical constituents of animal skin is essential to a proper understanding of the intricate reactions taking place in manu facturing commercial leather. (See SKIN.) The skin may be said to consist of living cells and the products of cell activity, e.g., fibres, dead cells, etc. Skin consists of 3 well defined layers, distinct both in structure and origin. There are (I) a thin outer layer of epithelial cells, the epidermis, (2) a thick layer called the derma and (3) a subcutaneous layer or adipose tissue known com mercially as "flesh." The epidermal tissue is completely removed during the preparation for tanning. The derma is the layer con verted into leather. The hair is embedded in a pocket or follicle of epidermis, the hair follicle and hair being penetrated by a projec tion coming from the derma and known as the hair papilla.
Variation of Fibre Structure.—The variation of fibre structure determines the resulting tensile strength and pliability of the leather produced from it and the satisfactory preparation of skins for tanning depends upon the separation of these white fibres without damage. The derma consists of connective tissues and fat cells in varied proportions in different skins, determining the general character of the finished product ; where large groups of fat cells are present interspersed between the collagen fibres the result will be the production of soft, spongy leather because of the empty spaces left when the fat cells have been removed, by the preparatory processes to which the skin is subjected. In con sidering fibre structure from the tanner's standpoint the presence of connective fibres, which consist of yellow fibres as distinct from the white fibres of the collagen, and which are of different chemical composition and are composed of the protein elastin, is of great importance. The presence or absence of these in the finished leather has an important influence on its physical character, par ticularly as to the grain the leather possesses, and also influences stretch and elasticity. These elastins are removed to a pre-deter mined degree in order to effect desired variations of character.