Tanned Leather. Hides and Skins.— The skins of all mammalians may be made into leather, but in practice it is only from a few of the larger animals, readily obtainable in sufficient numbers, and reared and slaughtered for other objects, that commercial supplies are obtained. The term hides is by tanners restricted to the large and heavy skins of full-grown oxen, horses, and other large animals—all the lighter stock being known as skins (calf skins, sheep skins, goat skins, <tc.). Of all hides and skins used by the tanner, by far the most important and valuable are those obtained from oxen. Not only do these yield the most useful and valuable hides, but they are slaughtered in all civilized countries in enormous quantities; and, while in Europe the skins of cattle are only of secondary importance, the vast herds which roam practically wild in the plains of South America are valuable more on account of their hides and other products than as sources of animal food. Ox hides are imported into Europe and the United States of America in enormous quantities, and come principally from South America, the Cape, Australia, the East Indies, and North Africa. The main centres of the import trade in hides are Antwerp, Liverpool, Havre, and New York. For tanners’ purposes calf skins are distinguished from ox hides, and the kinds of leather into which they are manufactured are entirely distinct. Intermediate between the heavy ox hides and calf skins are East Indian kips, a medium weight skin which comes both raw and tanned from Calcutta and Madras in such large quantities as to form a distinct branch of the leather trade. Horse hides and the skins of the other Equidae — the ass, zebra, quagga, &c.—have in modern times become important raw materials of leather. The various breeds of sheep, on account of the vast numbers in which their skins come into the market and the numerous applications of sheep and lamb skins, come near in value to oxen as sources of leather. As a rule the importance of a breed of sheep for the purposes of the tanner is in inverse proportion to its value as a source of wool. Goat and kid skins come next in order of importance, the products they yield being beautiful in texture, of high value, and of varied usefulness. Goat skins are obtained 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 occasionally or locally used may be enumerated walrus, rhinoceros, hippopotamus, and elephant hide, yielding very thick leather used for buffing wheels in cutlery manufacture, <fcc., 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-vessels penetrate. This layer is, during the life of the animal, continually in progress of peeling off in the form of small flat scales, and is renewed from the inner portion of the epidermis known as the rete mucosum or Malpighian net. The skin proper (comum, dermis, or Cutis'), which is the only portion of the hide of use for the tanner, consists of a dense 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 “grain” 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 insoluble 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 interfibrous binding albuminoid material called by the same authority coriin is soluble in alkaline solutions (being withdrawn from the skin by treatment with lime water, &c.) and in strong hydrochloric acids, but insoluble in water. It is precipitated from solutions by tannin, with which it combines. Many competent authorities maintain that the distinction between the fibrous and non-fibrous portions of skin is only one of physical condition.

Tanning 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, &c. The tannin obtained from various sources is not precisely the same in its chemical relations and reactions. 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 solutions of salts of peroxide of iron a deep blue-black or greenblack 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, uncrystallizable 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 precipitated 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, Quercus Rοbur, 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 limits, 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 of 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 importance, and are extensively used. The cork oak, Quercus Suber, 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 Kingdom. In 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 barks; 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 distributed 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 (√f. pycηaηtha), the silver wattle of Tasmania (A. leucophylla), and A. cyanoρhylla. The red colour of mimosa bark produces a dark leather against which there is a prejudice, and the material has therefore to be used sparingly in mixtures. It is also said that mimosa tanning results in a somewhat hard brittle leather.

Hemlock Bark is the most important tanning material in North America. It is the produce of the hemlock spruce, Abies canadensis, which grows in vast forests throughout Canada and the northern and eastern States of the Union, the principal bark-producing States being Pennsylvania, Michigan, and Wisconsin. Hemlock bark is obtained by cutting down the trees; and, as no provision is made for renewing the denuded forests, the strain on the more accessible portions of the American forests is already beginning to make itself apparent. The bark contains 7 or 8 per cent, of tannin, and the leather it makes has a strong reddish-brown colour. A large and increasing amount of hemlock extract, an inspissated decoction of the bark having a specified richness in tannin, is now imported into Europe, principally to the United Kingdom.

Among barks used to a limited extent and for special purposes are larch and Scotch fir barks, used for tanning sheep skins into basils, &c. Willoιv bark is used in Russia for tanning russia leather. Mangrove barks ( Rhizophora Mangle}, which are exceedingly abundant in India, and rich in tannin, have been tried in the United Kingdom, but their use did not prove satisfactory.

IVood.— Quebracho wood {Aspidospermum Quebracho), a wood rich in tannin, obtained from the river Plate, has recently grown rapidly in favour as a tanning substance in the United States and France, and is now coming into notice in the United Kingdom.

Fruits.— Under this head are comprised valonia, myrobalans, and divi-divi, three substances which now play an important part in tanning both in Europe and America. Valonia consists of the imbricated acorn cups of a species of oak, Quercus Aegilops, which is indigenous in Asia Minor, Greece, and southern Turkey, and is mainly shipped from Smyrna. It contains as much as from 40 to 45 per cent, of tannin, and the average annual imports into the United Kingdom are now not less than 30,000 tons. Myrobalans are the dried immature fruit of species of Terminalia, principally T. Bellerica and T. Ghebula. They vary in size and appearance, but in general they are oval hard wrinkled nuts rather larger than a filbert. The amount of tannin they yield varies from 20 to 36 or sometimes as high as 40 per cent. Myrobalans grow abundantly throughout India and are largely and increasingly exported from the three presidencies to the United Kingdom.' Divi-divi (g.v.) contains as much as 50 per cent, of tannin. Though it is in considerable use, tanning with divi-divi is subject to several grave objections. The abundant mucilage of the pods ferments readily, and thereby sometimes causes a rotting of the hides. Leather tanned by divi-divi also draws moisture readily in presence of damp, while in a dry atmosphere it is hard and horny, and further this agent leaves finished leather very dark in colour.

Leaves. Sumach.— The leaves of various species of Rhus, under the name of sumach, or sumac, form materials of the first importance for the tanning of light skins in which it is essential to have a fine white colour, as in the case of bright morocco leather, &c. Of the species the most important is the Sicilian sumach, R. Coriaria, a shrub or low tree indigenous to Italy, Spain, France, and the Mediterranean coasts of Africa. It is cultivated with much care in Sicily, the leaves being gathered from shoots not more than a year old, which thereafter are cut down close to the parent stem. The same shrub is largely cultivated in the southern departments of France, in Spain, and in Portugal. The leaves gathered in June are dried and ground under edge rollers to a fine dust, in which condition the material comes into the market. It has a bright olive green colour with something of the odour of tea, and should contain in the best qualities from 25 to 30 per cent, of gallotannic acid, the same tannin which is present in galls. The leaves of the Venetian sumach, R. Cotinus, are similarly used in the eastern countries bordering on the Mediterranean. In America a large quantity of sumach for tanning is obtained from two species of Rhus, R. coρallina and R. glabra, growing principally in Virginia, the Carolinas, Georgia, and Alabama. These are, however, much inferior to European sumach, both on account of the colour they communicate to leather, and also from the smaller percentage of tannin they yield, although it is believed that with careful cultivation and proper attention the quality might be greatly improved.

Extracts.—Cutch and Gambir, vegetable extracts obtained from the East, are frequently confounded, and in commerce they indifferently pass under the name of terra-japonica and catechu (see Catechu, vol. v. p. 220). Gambir, which comes almost exclusively from Singapore, is the inspissated juice of the leaves of Uncaria Gambir, a tall shrub belonging to the natural order Rubiaceae. The extract comes into the market in the form of cubes about an inch in size, of a dull brown earthy appearance. In composition and properties it agrees with Cutch. The exports from Singapore amount to about 80,000 tons annually. Kino (q.v.) is exceedingly rich in a variety of tannic acid, but its high price precludes its use in tanning. These extracts are used in tanning only in conjunction with other materials; they not only hasten the operation, but, judiciously used, they tend to render the leather soft and mellow.

Galls.— Although galls are among the richest of all bodies in tannic acid, they do not form an important item among the materials of the tanner, being most valuable for other industrial purposes, and therefore too costly for use as tanning agents. The “knoppern” galls of Hungary, which are formed on the acorn cups of a species of oak, however, are to some extent used in Continental tanneries. For full information regarding galls, see vol. x. p. 43.

It is to be noted that most of the tanning substances above alluded to may be and are used in dyeing as well as for tanning.

Grinding and Leaching of Tanning Materials.— Bark, valonia, myrobalans, and other tanning bodies are reduced to a small and as far as possible uniform size by means of grinding or comminuting machinery. The main object in such machines is to produce uniformity of size with as little dust as possible, and the apparatus most commonly used is similar in principle to the ordinary coffee-mill, with breaking arms for the bark and segmental cutters for smaller materials. Various forms of disintegrator are also used, which produce their effect by violent concussion obtained by the revolution in opposite directions of two large and strong disks armed with projecting spikes on the sides of the disks facing each other. These disks are enclosed within a stout iron drum; and, as they revolve at a speed rising to three thousand revolutions per minute, some conception of the violence with which the tanning materials are struck and smashed may be formed. The tanning materials so prepared are next leached, latched, or infused for preparing the strongest tanning solutions for use in the “layers” or lay-away pits noticed below. In making these leaches or infusions, some tanners use hot (even-boiling) water, others use cold water alone; some employ only pure water, and by some the weak and exhausted oozes or woozes from the pits are strengthened up by renewed leaching. The sole object of the tanner is to obtain the greatest amount of the tanning principle contained in the materials operated on, and to take care that what he gets is not lost or wasted. The method of leaching commonly adopted in the United Kingdom is to pass the bark through a series of leachers or spender pits. New or fresh bark is put into the first of the series, and over it is pumped cold the well-strengthened ooze from the next leacher. In this first pit the ooze or infusion is brought up to the full strength required for the lay-away tan-pits, and after the infusion is pumped off the tan (now somewhat reduced in strength) is passed over into No. 2 leacher, where it is treated with liquor in its turn also somewhat lower in strength. In this manner the bark passes by stages through a series of pits, diminishing in richness in tannin at each stage, and in the same gradual manner being infused in a weaker and weaker liquor, till in the last of the series it is fully exhausted with pure warm water. Thus pure water is put in at one end of the range and fresh tanning material at the other; the water as it ascends is gradually...