Leather

Chemical Composition of Skin.

The following occur naturally in skin—Water, proteins, fatty matter, carbohydrates and min eral matter. The proportions vary according to the kind of animal, and, in a given species, according to the age, sex, mode of feeding, climatic conditions, etc. ; further in a given kind of skin they vary from place to place. The composition of the skin as received by the tanner differs from that in the natural state because foreign matter is picked up by the animal while it is alive, and is intro duced during curing and also because soluble matter may be washed out during the curing process. The composition is further modified by bacterial and other changes occurring in the interval between flaying and receipt by the tanner. The skin also contains enzymes and bacteria.

Collagen forms about 95% of the protein matter in the derma of calf, steer and cow hides, i.e., the layer which is converted into leather. The fatty matter consists of numerous other substances besides glycerides. The amount in hides is usually small and dur ing the tanning process it is for the most part converted to lime soaps which are removed during scudding. If not properly elim inated it may give rise to greasy leather. The naturally present mineral matter consists of sodium chloride ; in addition there are chlorides, sulphates, phosphates, silicates of sodium, potassium, calcium and magnesium. The mineral matter introduced is re moved in the wet work. The chief carbohydrate is glycogen.

The first operation preliminary to tanning consists in washing and soaking the hides or skins.

Washing and Soaking.

The method ordinarily practised in the case of freshly butchered hides or skins which have only been very lightly salted and which have not subsequently been dried, is to carry out the soaking in a pit, This vessel which is also commonly employed in liming consists of a brick or cement chamber sunk to ground floor level and of a capacity of usually r,000 gallons The skins are immersed in water in this vessel for varying periods dependent upon their condition, to remove the small amount of salt, and to effect the removal of blood, the extraction of albumens and other nitrogenous matters soluble in water, and the softening of adhering dung so that the latter can be removed by mechanical means. The soaking of dried skins is very much more difficult. In this case the proteins have been coagulated by the temperature to which the skins have been sub jected in the drying and some liquefaction of the collagen has taken place with the result that the fibres have become attached to each other by a hard film of gelatinous matter; and in addition, even when dried at moderate temperatures, the natural greases of the skin have formed a water-resisting film on the surface.

When the soaking is done in water only as in the case of comparatively fresh goods, they should be immersed for as short a time as possible and preferably in two or more successive changes of clean water. In the case of highly dried goods the use of some chemical agent to effect the swelling of the fibres in as short a time as possible without detrimental influence is now almost universal. The chemical agents so employed may be divided into two classes (a) alkaline (b) acids. Probably the most commonly employed alkaline agents are sodium sulphide and sodium hydroxide ; the goods being soaked in a comparatively weak solution of one or other of these until a sufficient amount of swelling and water absorption has been effected to bring the skins back into a condition of pliability. The soaking operation is usually supplemented by working the goods over an inclined beam by hand with a dull unhairing knife to stretch the skin in every direction and thereby facilitate the separation of the fibres; this latter operation is termed "breaking" and is com monly applied to sheepskins. Hides and goatskins are subjected, instead, to a "dry drumming" process. This consists in placing the skins when moderately soft in a large wooden circular drum, which is mechanically rotated and which is provided with shelves or pegs in the interior for the purpose of lifting the skins up during rotation and allowing them to fall from a point midway between the top and the bottom of the vessel, whilst the drum is rotated some 8 or ro revolutions per minute. The weight of skins falling upon each other assists in the separation of the fibres by the constant kneading action so produced. After this mechanical treatment the goods are then usually subjected to a further im mersion either in water or in a weak solution of the soaking agent, the operations being repeated until the skins have acquired the desired degree of flaccidity and flexibility comparable with that of a freshly flayed skin. Weak solutions of acetic, formic, sulphurous and boric acids are also used. For sheepskins, the wool of which is detrimentally affected by strong alkaline solutions, sodium bi sulphide or sulphurous acid are commonly employed. The quality and yield of the finished leather is materially affected by this soaking process. Loss of skin material should be avoided.