Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/399

 LEATHER 381 chiefly from the East Indies, the Cape, North Africa, South America, Mexico, Asia Minor, and the hilly regions of Europe. Seal skins, obtained from the arctic regions, are an important material, while hog skins are of value for the purposes of the tanner almost exclusively for making saddle leather. Among the skins which are only occasion ally or locally used mny be enumerated walrus, rhinoceros, hippopotamus, and elephant hide, yielding very thick leather used for buffing wheels in cutlery manufacture, etc., and the skins of the numerous species of deer and antelope, dogs, kangaroo, and other Australian marsupials, porpoises, alligators, and occasionally boas. Structure of Skin. All hides and skins are externally clothed more or less with wool, hair, bristles, or scales. The skin itself has a thin superficial horny and cellular layer, the cuticle or epidermis, into which neither nerves nor blood-vesaels penetrate. This layer is, during the life of the animal, continually in progress of peeling off in the form of small fl.it scales, and is renewed from the inner portion of the epidermis known as the rete mucosum or Malpighian net. The skin proper (corium, dermis, or cutis), which io the only portion of the hide of use for the tanner, consists of a denss plexus of fibrous bundles, knit together and interwoven in every direction, the interspaces being filled up with an albuminoid substance. The bundles of fibres terminate on the upper surface of the corium in separate masses, producing the irregularly papillated appearance seen in the &quot; grain &quot; of leather, and hence that surface is distinguished as the grain side in contradistinction to the flesh or under side. Chemically the connective tissue or fibrous portion of the corium consists of gelatigenous tissue or collagen, which, according to Reimer, is similar in composition to the fibroin of silk. It is in soluble in cold water, weak acids, and alkalies, but with boiling water it dissolves, forming gelatin, and it is also soluble in concentrated acids and alkalies. It combines with tannic acid, forming the essential basis of leather, and it similarly combines with oils and fats. The inter- fibrous binding albuminoid material called by the same authority coriin is soluble in alkaline solutions (being withdrawn from the skin by treatment with lime water, Arc.) and in strong hydrochloric acids, but insoluble in water. It is precipitated from solutions by tannin, with which it combines. Many competent authorities main tain that the distinction between the fibrous and non- fibrous portions of skin is only one of physical condition. Tanninrj Materials. Tannin or tannic acid is a product of the vegetable kingdom, abundantly formed in a very large number of plants, and secreted in such diverse organs and members as the bark, wood, roots, leaves, seed-pods, fruit, (fee. The tannin obtained from various sources is not precisely the same in its chemical relations and reac tions. Dr Stenhouse was the first to insist on the principal distinction which possesses practical interest to the tanner. He pointed out that tannin-producing bodies may be divided into two classes, the first class comprising such as by their decomposition develop into gallic acid, and by destructive distillation yield pyrogallic acid. Of these gallotannic acid, obtainable from galls, is the type. The other principal tanning materials which yield gallotannic acid are sumach, valonia, divi-divi, and myrobalans. The second class embraces tannins which do not resolve themselves into gallic acid or yield pyrogallic acid, and of this class oak bark, mimosa bark, and gambier yield characteristic types. All varieties of tannin, however, agree in possessing a powerfully astringent but not bitter taste, and a distinctly acid reaction ; they yield with solu tions of salts of peroxide of iron a deep blue-black or green- black solution, and particularly they combine and form insoluble compounds with gelatin and with the gelatigenous tissue which constitutes the principal portion of animal skins. By the action of ether, containing a little water, on gall-nuts, pure gallotannic acid may be procured. The ethereal solution separates by repose into two layers, the lower one, which is of an amber colour, being a solution of tannin in water, while the upper layer contains gallic acid, mixed with other substances. On gently evaporating the aqueous solution, nearly pure gallotannic acid is procured, to the extent of from 35 to 40 per cent,, from galls. Obtained in this way, it is a shining, porous, uncrystalliz- able mass; it is soluble in water, and then exerts the properties of an acid. By exposure to air it absorbs oxygen and gives off carbonic acid, two new products, gallic acid and ellagic acid, being formed at the expense of the tannin; the latter is insoluble. Gallotannic acid may be precipi tated from its solutions by sulphuric and some other acids; by boiling the precipitate with sulphuric acid for a few minutes in a dilute solution of the same acid, gallic acid is formed, and crystallizes in cooling. Gallic acid also exists ready formed in gall-nuts, sumach, valonia, tea, and other substances. It does not combine with gelatin, and is therefore useless in tanning. Some tanners, however, imagine the gallic acid of the waste liquor to be useful in swelling or raising the hides, preparatory to removing them to a stronger liquor. Tannin is in no case isolated for use as a tanning agent. It is only brought in contact with skins and hides by the medium of infusions, decoctions, or extracts of the various tanning materials in which a percentage of tannin is present mixed with colouring and other extractive material. The substances enumerated below comprise the principal tanning materials in use throughout Europe and America. Oak Bark. In early times the bark of the common oak, Qucrcus JRobur, was almost the only tanning material used by British tanners, and it still is the substance from which the highest quality of heavy tanned leather is prepared, although with it the process is necessarily tedious. Throughout the country there are still a few tanners of sole leather whose boast is that they use nothing but oak bark. The entire supply of British oak bark is estimated at from 200,000 to 300,000 tons annually. This quantity, with the additional import of 30,000 tons from the Continent, is altogether inadequate to meet the demands of the tanners, apart from the necessity which is now felt for forcing the tanning operation much more rapidly than was formerly the rule. The most useful bark is obtained from coppice wood of about twelve years growth, although in inner bark of longer growth a large proportion of tannin is secreted. The amount of astringent matter in coppice bark may be taken to average from 8 to 10 per cent., but the statements of the quantities of tannin in different samples vary within wide limit?, as much as 18 per cent, being in some cases found. There is no doubt that the peculiar excellence of the sole leather of England is due in great measure to the superior oak bark produced there. Oak bark imparts firmness and solidity to leather, while other sorts give softness ; thus the peculiar softness of French curried leather is referred to the bark ot the evergreen oak, with which the better kinds are tanned, while the other tanning materials next to be named give each its peculiar quality with respect to colour, scent, toughness, or the power of resisting moisture and decay. Other species of oak also yield tanning materials of much import ance, and are extensively used. The cork oak, Qucrcus Subcr, of south Europe and north Africa, in addition to its well-known external layer (the cork of commerce), possesses a fibrous inner bark which is richer in tannin than ordinary oak bark. It is much employed in France, and is imported also to some extent into the United King dom, la the United States several varieties of oak yield staple tanning materials. Of these the principal are the rock or chestnut oak, yellow oak or quercitron, both important sources of tan baiks; the red oak and white oak are of less consequence. Mimosa Bark. Under this general name a large amount of bark comes to the English market from Australia, principally from Melbourne and Adelaide, and from Tasmania. It is obtained from a large number of trees belonging to the genus Acacia, widely distri buted throughout Australasia, and the various barks are rich in tannin, which ranges from 15 up to 32 per cent. The qualities imported into England are the richer kinds. They amount to about 30,000 tons annually, and may be assumed to contain on an average 28 per cent, of astringent matter. These richer barks are the produce of A. harpophylla, a Queensland tree, the black wattle (A. molissima the gold wattle (A. pucnan(ha), the silver wattle of