skins hides tanning tannin bark water acid tanned skin hide
LEATHER consists of the hides and skins of certain animals, prepared by chemical and mechanical means in such a manner as to resist influences to which in their natural condition they are subject, and also to give them certain entirely new properties and qualities. Skins in an unprepared moist condition are readily disintegrated and destroyed by putrefaction, and if they are dried raw they become hard, horny, and intractable. The art of the leather manufacturer is principally directed to overcoming the tendency to putrefaction, to securing suppleness in the material, to rendering it impervious to and unalterable by water, and to increasing the strength of the skin and its power to resist tear and wear.
Leather is made by three processes, or with three classes of substances. Thus we have - (1) tanned leather, in which the hides and skins are combined with tannin or tannic acid ; (2) tawed leather, in which skins are prepared with mineral salts ; (3) shamoyed leather, consisting of skins combined with oils or fatty substances.
Hides and ,S'kins. - 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, &c.). 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 Equid& - 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, &c., and the skins of the numerous species of deer and antelope, dogs, kangaroo, and other Australian marsupials, porpoises, alligators, and occasionally boas.
Strlecture of ,Ski n. 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 flit scales, and is renewed from the inner portion of the epidermis known as the rete 71121COS21711 or Malpighian net. The skin proper (coil um, dermis, or texas), 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 in terspaces 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 gelatigenons 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 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, &c.) and in strong hydrochloric acids, but insoluble in water. ft 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 Araterials. - 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 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, 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; tile 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. - ln early times the bark of the common oak:, Quercsis Robur, 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 inure rapidly than was formerly the rule. The most useful bark is obtained from coppice wood of about twelve years' growth, although iu 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 ct nt. 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, Qucrcus 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 balks; the red oak and white oak are of less consequence.
Mimosa Bark. - Under this general name a large amount of bark collies 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 23 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. pycnantha), the silver wattle of Tasmania (A. leucophylla), and A. eyanophylla. The red colour of mimosa bark produces a (lark leather against which there is a prejudice, and the material has therefore to be used sparingly hi 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 ca7zdensis, 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. Willow 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.
Wood.--Quebracho wood (Aspidospermum Quebrach,o), a wood rich in tannin, obtained from the river Plato, 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 Adgilops, 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. Belleriea and 7'. Chebula. 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-mvi (q.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 fine white colour, as in the ease 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 Rims, R. eopallina and R. glabra, growing principally in Virginia, the Carolinas, Georgia, and Alabama. These are, however, mmdi 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.
Relracts. - 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 CATECIM, VOL v. p. 220). Gambir, which comes almost exclusively from Singapore, is the inspissated juice of the leaves of Unearia Gambir, a tall shrub belonging to the natural order Rubiace.T. 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 highs price precludes its use in tanning. These extracts are used in tanning only in conjunc tion 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 au 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 he 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 lie 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 strengthened till it reach the maximum richness in tanning principle, while the tanning material as it descends is in like proportion deprived of its extractive constituents, till in the end nothing further soluble remains. From the last pit the bark, &c., are turned out as " spent tan, " usually to be burned in a special form of tan-burning furnace for raising steam. The use of leaches or infusions was first insisted on by Seguin about the end of the 18th century, and the adoption of his suggestion led to the shortening of the time occupied in tanning heavy leather by about one half.
Testing Tan Liquors. - The methods by which the tanning value of any substance is determined are numerous, but few of them are at once capable of simple application and minutely accurate. One of the commonest plans for ascertaining the strength of the tan liquor technically called ooze, or wooze, is by means of a kind of hydrometer called a harkometer. It is graduated to the standard of pure water; and, when it is placed in a specimen of ooze, the strength of the latter is judged of by the position of the stem above or below the water-mark. But, as bark or other tanning material may contain several soluble substances besides tannin, the barkometer obviously cannot be relied on. Some tanners judge of the strength of ooze by its astringency to the taste. Seguin, who in the end of the 18th century was the first to insist on the advantage of tanning with previously prepared infusions, proposed the use of a solution of gelatin as a test of the presence of the tannin. In trying the quantity of tannin by Seguin's process, 480 grains of the bark in coarse powder should be acted on by half a pint of boiling water. The mixture should be frequently stirred, and suffered to stand twenty-four hours ; the fluid should then be strained through a linen cloth, and mixed with an equal quantity of solution of gelatin, made by dissolving glue, jelly, or isinglass, in hot water, in the proportion of a drachm of glue or isinglass, or six table-spoonfuls of jelly, to a pint of water. The precipitate should be collected by passing the mixture of the solution and infusion through folds of blotting-paper, and the paper exposed to the air till its contents are quite dry. Every 100 grains of precipitate contains 40 grains of tannin nearly. As, however, some kinds of tannin produce larger precipitates of gelatin than other kinds, and as the composition of tanno-gelatin varies with the strength both of the solution of gelatin and of tannin, this method is not reliable. Sulphate of cinehonin is said to afford a better test ; a solution of this, acidulated with a few drops of sulphuric acid, will, it is said, precipitate tannin completely from the solution. Ldwenthal's method, founded on the fact that solutions of tannin in presence of sulphuric acid are readily oxidized by permanganate of potash, is very useful for the comparative determination of the value of different tanning substances. A given weight of tanning material is infused, and the solution is brought up to a definite volume. One half of this measured quantity of tannin solution is mixed with definite quantities of a standard indigo-carmine solution and sulphuric acid, and to this mixture permanganate solution is added from a graduated tube till the colour of the indigo is completely discharged, when both tannin and indigo are oxidised. A parallel experiment is next made with similar measured amounts of indigo and sulphuric acid solutions, but without any tannin infusion. The difference between the quantity of permanganate required to discharge the colour in the two experiments gives the standard for calculating the amount of tannin in the solution to be tested. Another good method of testing the value of tanning material is to digest a piece of dry prepared hide or skin in a known quantity of the infusion, until the whole of the tannin and other matters be separated. The skin is then taken out, slightly washed, dried, and weighed, when the increase of weight is supposed to be the weight of tannin and of the other matters required. An apparatus devised by MM. Muntz and Ramspacker has recently been introduced for facilitating this last test. It consists essentially of a small vessel sufficient to hold a measured quantity of a tanning infusion, the specific gravity of which is carefully ascertained. That vessel is so arranged that strong pressure can be brought to bear on it by means of a screw acting on an india-rubber surface, thereby forcing the liquid through a piece of skin which covers the lower part. The skin absorbs the whole of the tannin during the passage through it of the infusion, and by ascertaining the specific gravity of the escaped liquor the percentage of the tannin material in the infusion can be readily deter- mined. This apparatus has been extensively introduced in practice in England and on the Continent.
Sole Leather or Heavy Leather Tanning. - The hides of oxen are received in the tan-yard in four different conditions. These are - (1) market or slaughter icicles, which, coming direct from local abattoirs, are soft, moist, and covered with dirt and blood ; (2) wet salted hides ; (3) dry salted hides ; and (4) sun-dried or "flint" hides, - the three last forms being the condition in which the imports of foreign hides are made. The first operation in the tannery is to clean the hides, to free them from salt, and to bring the hard dry hides to the uniformly soft flaccid condition in which all market hides are obtained. The treatment at this stage requires skill and attention to prevent the more soluble constituents of the hide from dissolving out in the washing and soaking processes, and also to secure the complete softening of the entire substance, upon which the successful tanning greatly depends. In the case of market hides cleaning and softening are principally effected by washing and soaking in spent lime-water, while for dry hides and dry salted hides brine is essential. The softening of these materials is helped and rendered thorough by working them for some time in the stocks (fig. 1) after they have been well soaked. After being thus brought as nearly as possible into a uniform condition, all hides are treated alike. The first operation to which they are subjected is depilation,which removes, not only the hair, but also the scarf-skin. This is effected variously in different countries. In England the most common plan is to throw the hide or skin into a strong watery ley of slaked lime, with lime in excess. By this, in a few days, more or less according to the proportion of lime present, the hair is easily detached, the hair-sheath having been dissolved. The hair was formerly taken off by making a sour liquor from fermented vegetable matter, in which the hides lay for several days; they were also smoked in a damp state for the same purpose ; but both those methods are now abandoned. They are still sometimes, especially on the Continent, sweated, that is, they are laid in heaps and kept wet and warm, a plan which is still adopted in England for skins. In America the sweating is performed cold ; the hides are hung up wet in a damp underground cellar, and are kept moist for ten days or a fortnight. In either of these sweating processes incipient putrefaction takes place sooner or later, when the hair and scarf-skin are easily removed ; but the fatty matter remains, and in some cases prevents the hide from taking the tan.
There have been numerous other methods proposed and patented for unhairing skins, few of which have been received with much favour. Among the agents proposed may be mentioned caustic soda, sulphide of sodium and sulphide of calcium, borax, sugar, and charcoal - substances which it is obvious must act in very different manners. Lime and alkaline solutions not only loosen the hair and scarf-skin, but also "plump" the corium or true skin, that is, they swell it and render it consequently porous and more permeable to the tanning solution. Lime further forms with the fatty matter of the flesh side calcareous soap, thus neutralizing the fat which would otherwise interfere with the tannin. Some tanners, especially Americans, who work the so-called acid process, plump their hides by the use of sulphuric acid, hanging them six or eight hours in a solution containing 7-01--uth of acid. The plumping i3 sometimes done as a preliminary operation, and again others add the acid to the colour pits, or the first pit into no lime is secured in the weak acid liquors of the handling pits.
In the case of limed stock the hides, at the proper stage, are withdrawn from the pits and stretched over an unhairing beam (fig. 2), when with a working knife (fig. 3, a) a workman partly scrapes partly shaves off the hair and scarf-skin. Another workman in a similar way with a fleshing knife (fig. 3, b) removes the fatty compounds and flesh from the flesh side. For these operations several machines have been adapted, working mostly with revolving knives or cutters, under which the hides or skins pass in a fully extended state. Such machines are, however, only applied to the smaller skins. The next step in the preparation of the hide is to remove from it as thoroughly as possible all traces of lime. This is partly accomplished by going over the hide on the beam with a scudding knife, pressing the combined lime and interfibrous matter out of the tissue. For more complete neutralization of lime in the larger hides the influence of the weak acid of the colouring pits is trusted to. Harness hides are washed by some means in pure water, the most convenient and generally adopted method being to place them in the clash wheel (fig. 4), in which they revolve and tumble about whilst fresh water is continually being poured on them within the revolving wheel.
The hides now come to be trimmed and prepared for tanning in the shape in which they are intended ultimately to be sent into the market. An entire untrimmed hide (fig. 5) is termed a crop; a side is half a crop, the dividing line of the two sides being shown at EF; a butt is the back portion ABCD, and a bend is half a butt ABFE. G, 0 are belly pieces, and H, H the cheeks, both together being the offal. When the shoulder (the upper part of the butt) is removed, what remains is a short butt.
The actual tanning now commences, and the operations involved may be divided into a series of three - (i) colouring, (2) handling, and (3) the laying away. The colouring consists in exposing the hides in a series of pits containing oozes which are almost entirely deprived of tannin, but in which some amount of gallic and acetic acids have been developed, and which, moreover, contain a large proportiou of the colouring matter extracted from the tanning substances. In these pits (also called suspenders) the hides are suspended over poles laid across the pit, and they are moved daily from one to another of a series of four or six, this stage usually occupying about a week. As the hides are moved forward in the series they are exposed to a liquor containing a small and steadily increasing proportion of tannin, and this, it may be said, holds good till the hide reaches the last lay-away pit, in which the tanning is completed. The objects attained in the colouring pits are the superficial colouring or dyeing of the hide, some amount of plumping from the acids of the ooze, and a dissolving out of remaining traces of lime, principally by the acetic acid to which the hide is exposed. After colouring, the hides pass on to the handlers or handling pits, n round or series of which may consist of from four to twelve, according to the mode of working. In the handlers the hides arc spread out horizontally; and in the first series they arc "handled" once a day or more frequently if convenient. The handling consists of lifting the hides out of the pit by means of a tanner's hook (fig. 6), piling them on the side till they drain, and returning them into the pit, the hide on the top in one handling going to the bottom in the next. Tins operation is continued throughout the series ; only as the hides advance the necessity and advantage of frequent handling decreases, while the strength of the tan liquor in which they are handled increases. The whole handling stage consumes on an average about six weeks. Finally, the hides are carried over into the layers or lay-aways. In these the stock is exposed to the strongest tanning liquors, and between the hides thin layers of the tanning bark or mixture are strewn. The object of this interstratification is to separate the mass of hides so as to secure the more ready permeation of the entire mass by the liquor, and also to feed and strengthen the ooze itself as its tannin is absorbed by the hides. In these layers the hides are allowed to rest for about six weeks, after which the pits are cleared out, charged with fresh ooze, and filled with the hides and tan as before. These processes may be repeated three or four times before the tanning is completed. When the process is deemed complete, each hide, on being taken out, will be found to be converted into leather, and a portion of its gelatin which has been dissolved from its interior is, by combination with a portion of tannin from the strong solution, deposited upon its surfaces, where it is found in the form of a yellow deposit, technically known as bloom, or pitching, which disguises the under colour of the leather just as if it were covered with yellow paint. This, prejudice says, must be on its surface, or it is not saleable, but it is so much quality and weight lost to the consumer, as he pays for it on the outside of his leather to be worked off in the dressing and currying operations. By some tanning agents - mimosa, for example - there is little or no bloom deposited.
The theory of the formation of the bloom is this. As soon as ooze has penetrated into a hide it loses its tanning material, but by capillary attraction is detained ; this exhausted ooze acts by maceration on the finer and more soluble interstitial gelatin, and dissolves it. In handling,. about one-twelfth of this flows out ; the remaining eleven-twelfths accompany the hide into the next stronger solution, of which only one-twelfth is absorbed directly, and a small portion is slowly exchanged by endosmosis and exosmosis. The small portion of strung solution which passes into the pores of the hide contributes to tan the hard fibrous portions not dissolved, and the small portion of weak solution passing out of the hide by exosmosis gives up its dissolved gelatin to the tan of the stronger solution outside to form tannate of gelatin, which partly adheres to the surface as bloom, and partly falls to the bottom of the pit as pitching.
From the time when the raw ox hide is taken in hand till the leather is fully dried, not less than a year is consumed in the case of the best qualities of sole leather. It was formerly the practice in England, as it still is on the Continent., to tan by the process of stratification, for which purpose a bed of bark is made upon the bottom of the pit ; upon this is laid the hide, then bark, then a hide, and s2 on until the pit is full ; water is sometimes pumped in, and the pit left for some months ; it is then emptied, and the same hides returned with fresh bark and water for a few months longer ; this is repeated again and again, until the tanning is completed, the time varying from one to four years for heavy leather.
The devices and processes which have been proposed and to some degree put in operation with the Yiew of shortening the time occupied in tanning are beyond all enumeration. In scarcely any ease have tiro -abridging processes proved successful in practical working, so far us the production of good leather is involved ; and now the opinion appears to be completely established that, for the thorough tanning of heavy leather, a slowly operating influence and consequently long time are essential. The devices for the hastening of tanning have for the most part turned upon some plan for forcing the t.ui liquor into and through the pelt, or for alternate soaking an 1 squeezing of the hides. Among the plans which have been tried on a commercial scale may be enumerated tanning by the application of hydrostatic pressure to force the liquor through the hides, a method which failed simply because the pressure was equal on both sides. The vacuum tanning principle is another which has been extensively tried, only to issue in disappointment. It consists in hanging the hides in a la or cylinder so constructed that the air eau be exhausted by an air pump, after which tan liquors are forced into the vessel, air readmitted, and again withdrawn. hides, however, ]oiled with water swell little under diminished atmospheric pressure, and the practical difficulty of procuring and maintaining a vacuum in tan pits is very greatt. More promising results have been obtained by setting up in tan pits the physical process of endosmosis and exosmosis. This is done by sewing up hides two and two as bags which, being filled with solution differing in specific gravity from the tan liquor in which they are immersed, thereby set up transfusion through the hide. This process failed chiefly through the hardness of the leather it yielded. A plan of sewing hides into 1,ag-; and suspending them filled with strong tan liquor, which as the fluid exuded was renewed, was also tried for some time. .Again, it has been attempted to keep the hide suspended stationary in the pits and move the liquors instead of carrying over hides from one pit into another. A more recent device, which may not yet be fully tested, consists in keeping up the strength of the liquor by a continuons circulation through pipes from the stronger into the weaker infusions. By this system of circulation, instead of the oozes in which hides are immersed becoming weaker and weaker the longer they rest in the liquor, the ooze is kept up at least to its original strength, and it mry imbed, if desirable, be increased in proportion as the tannin combines with the hide.
Heavy hides for sole leather, belting, and similar purposes d) not require to am lergo any elaborate dressing or currying. When finally r:frnoved from the tan pits they are piled grain to grain and flesh to flesh to drain, care being taken that no tan liquor is allowed to lurk in the pile, which is covered over from the light. When sufficiently drained, they are brushed or scoured to free them from adhering impurities, and removed to the drying loft, where, after lightly rubbing over with oil, they are hung on poles to dry. In the loft steam-heated pipes keep a dry atmosphere during winter, and enable the attendants to regulate and control the drying of the leather. The leather when dried in this condition is rough tanned, and for finishing as sole leather it has to be struck out or " pinned " and compressed by rolling. For striking or pinning by hand the hide is dampened with water, thrown over a beam, and worked all over the grain side with a striking pin (fig. 3, c). This operation smoothes and levels the grain, removes smaller wrinkles, and to some extent compresses and solidifies the leather. Striking machines (fig. 7) are now very generally used for the operation.
These consist of a drum or cylinder having a parallel series of projecting knives, or plates of gun-metal, set angularly across its surface. Underneath the drum is a brass bed, fixed on a yielding cushion, which can be pressed up or eased by means of a foot lever, according as the leather operated on is thick or thin. The drum is made to revolve at a very rapid rate, the blunt edges and external angles of the knives thereby striking the surface of the leather with great violence, and thus the grain is struck out, smoothed, and compressed in a very rapid and efficient manner. Finally, the leather is rolled and compressed on a level zinc-lined wooden bed by a heavy hand roller, such as is shown in fig S, or on the platform of one of the numerous forms of machines designed for that purpose.
The yield of leather from a given weight of dry hide varies very much according to the different styles of tannage and materials used. As a mean outcome, it may be said that 100 lb of green hide, tanned with from 300 to 400 of oak bark, will yield 40 to 50 lb of leather ; 100 lh of green hide, however, when deprived of hair, flesh, and moisture, will weigh only 18 lb, and, taking 100 lb of dry hide, which, fleshed and unhaired, weighs 85 lb, the yield of leather will be from 180 to 200 lb according to tannage.
XIV. -- .12 The percentage of tannin alone absorbed from different tanning agents has been found to be for hemlock, 64.2 ; pine, 90.8 ; chestnut, S5.2 ; oak, 76.9 ; oak, three years in pit, 70.2. Heavy leathers, being sold by weight, are subject to adulteration, and have fictitious weight given them without any benefit to the material, but rather the opposite, by impregnation with such salts as sulphate of magnesium or chloride of barium, or with glucose, the last being the most frequently used adulterant.
Upper Leather. - Under this head are included the thin, soft, and pliable leathers which find their principal, but by no means exclusive, application in making the uppers of boots and shoes, which may be taken as the type of a class of leathers. Upper leathers are made from such hides and skins as East Indian kips, light cow hide, calf skins, horse hide, and also from split heavy hides. The preparatory dressing of such skins, and the tanning operations, do not differ essentially from those already described. In proportion to the thinness of the skin treated, the processes are more rapidly finished and less complex, while at the same time the skins absorb a large percentage of tanning extract. The lime used for unhairing must be removed in the preliminary stage, with greater thoroughness than is essential in the case of hides for sole leather ; and for this purpose the skins are washed in the dash wheel, and undergo a process of bating or grainering. A quantity of pigeon's dung is dissolved in water, and in this the hides are steeped for a week or ten days, with occasional removals and strikings. The theory of this process is obscure, but it has been explained on the supposition that the uric acid of the clung removes the excess of lime, and that the ammonia generated by the putrefaction of the mixture tends to form au ammoniacal soap with any remaining fat of the hide ; but as the gelatin of the hide exists in two states,--one the principal, hard, or fibrous portion, and the other (which is more soluble) contained between the fibres, and more affected by agents and putrefaction - this softer portion is removed by grainering, and the leather, when tanned, is light and porous, and more readily permeable by water. Small skins are not fished one by one out of the colouring and handling pits, but the whole contents of the pits are tied together, so that when the upper skin is seized it is thrown over a. sparred cylinder erected between each pair of pits, and, the wheel being set in motion, the entire string of skins comes up over its surface and is passed into the neighbouring pit with the utmost rapidity. Such an apparatus is used for handling all small pieces and fragments, as for example the cheeks and bellies of heavy hides.
The time occupied in tanning an upper leather, say an East Indian kip, with a mixed mimosa, sumach, valonia, and terra tannage, may be about three months. In the fine turning of calf and kip skins on the Continent, for which French and German tanners are famous, the duration of the operation may be from four to eight months.
Splitting. - In the preparation of most kinds of upper leather, the hides are split into two, or three, and sometimes more portions. In the case of a single split the portions form a grain and flesh side; when three sections, or slices, are made they result in grain, middle, and flesh splits. Some tanners split their hides in the green condition, others after colouring, and in many instances the splitting is done, after the leather is fully tanned, by the currier, as a regular part of his operations, this being particularly the case with imported tanned East India kips, and other fully tanned leather of foreign origin. Splitting machines will be alluded to in connexion with the operations of currying.
C? ryl rg. - Leather as it leaves the tannery is a comparatively rough, harsh, and intractable substance, and the duty of the currier is to dress and otherwise fit it for the use of the shoemaker, coaehbuilder, saddler, and the numerous other tradesmen who work in it. The currier has to smooth the leather, so to pare it down as to reduce inequalities of thickness, to impregnate it with fatty matter in order to render it soft and pliable, and to give it such a surface-dressing, colour, and finish as will please the eye and suit the purposes of its consumers. The operations of currying are complex and varied, each particular class of goods receiving a treatment in many respects peculiar to itself. The fact also that machinery is used by some curriers for nearly every mechanical operation, while others adhere to the old manual system, renders it almost impossible to • give in brief an outline of operations which will be consistent with the practice of any considerable number of curriers.
Regarding currying as principally a handicraft, the following may be taken as an outline of the range of operations for the preparation of a waxed calf leather, the commonest form of upper leather in use. The leather is first made pliable by soaking in water, after which it is shaved on the flesh side, and a tolerably smooth surface is produced. This operation is carried on at a beam, or strong frame of wood, supporting a stout plank faced with lignum vita, and set vertically, or nearly so. The knife (fig. 9) is a double-edged rectangular blade, about 12 inches by 5 inches, with a straight handle at one end, and a cross handle at the other in the plane of the blade. The edges of this knife are first made very keen, and are then turned over so as to form a wire edge by means of the thicker of the two straight steel tools shown in fig. 10. The wire edge is preserved by drawing the thinner steel tool along the interior angle of the wire edge from time to time as required, for which purpose the man holds this smaller tool between his fingers, together with the beam-knife. The skin being thrown over the plank, the man presses his body against it, and leaning over the top holds the knife by its two handles, almost perpendicularly to the leather, and proceeds to shave it, shifting it from time to time so as to bring all the parts under the action of the knife, and frequently passing a fold between his fingers to test the progress of his work. The skin is then placed in hot water, and removed to a mahogany or stone table, to which the wet flesh side adheres, and is worked with a tool called a stretching-iron, or slicker S (fig. 10), consisting of a flat, rectangular piece of iron, copper, or smooth hard stone, fixed in a handle. With this tool a man scrapes the surface of the skin, exerting a strong pressure with both hands, and dashing water upon it from time to time, by which means lumps and inequalities are made to disappear, the leather is equalized and extended, and the bloom is brought to the surface. The superfluous moisture and the superficial bloom are now slicked out, and a stuffing, or dubbing, of cod oil and tallow is rubbed into both sides of the skin, but chiefly the flesh side, by means of a brush, or with the woolly side of a piece of sheep skin. The skin is now dried in a loft, and, as the water only evaporates, the dubbing sinks into the pores. When dry enough for the purpose, the skin is boarded, or worked with a graining board or pommel C (fig. 10), the effect of which is to bring up the grain, or give a granular appearance to the leather, and also to make it supple. The pommel is a piece of hard wood, grooved like a crimping-board, and attached to the band by means of a strap, whence the word pommel, from the French paunielle, or palm of the hand. The leather passes through various manipulations, each having its distinct name ; thus graining consists in folding the skin with the grain sides in contact, and rubbing strongly on the flesh side ; bruising, or rubbing the extended skin on the grain side ; whitening, or passing a knife with a very fine edge over the skin at the beam, so as to clean the flesh side preparatory to waxing, which is done just before the skins are sold ; for at this point the currier stores his skins, as they can be kept best in the state of finished russet, as it is called, previous to waxing. Waxing consists of two parts: the first is the hying on the colour, or blacking of oil, lampblack, and tallow, which is well rubbed in on the flesh side with a hard brush ; then, secondly, the skin is black-sized with stiff size and tallow, laid on with a sponge or a soft brush, and thoroughly rubbed with a glass slicker, a finishing gloss being given with a little thin size. The curried skin is now said to be black on the flesh, or waxed, in which state it is used for the upper leathers of men's boots and shoes. In the case of any of the numerous varieties of grained leather which are blackened and dressed on the grain side, the finishing operations are different. These are hard dried after slicking, and the operation of stuffing or dubbing is omitted. They are grained in the dry state, often by machinery, then boarded to soften them, and next blackened on the grain side with a solution of copperas. The flesh side is whitened or ileed and the grain is treated with sweet oil or some similar oil, and finally glazed with a thin solution of gelatin or of shellac.
For almost every operation in currying efficient machinery has now been adapted, the use of which not only modifies the operations of the currier, but also enables him to split up hides and to finish his splits as imitations of any kind of leather he may desire to copy. In machine currying the tanned hides, duly damped, are struck out in a " stoning " machine. It consists of a strong oscillating aria or bar having a blunt steel blade fixed on its end, which works back and forward over a concave bed on which the hide is laid, and which by its scraping and striking action on the softened leather smooths and equalizes the grain, and produces a compact uniform surface on it. From the stoning machine the hide may pass to the splitting machine, of which there are numerous forms, the American union splitter with a fixed knife being the oldest and best known. A much inure perfect machine, however, is the belt or band splitting machine. In this machine (fig. 11) the knife or cutter a is all endless band of steel which revolves at considerable speed with its cutting edges close to the sides of a pair of rollers through which the leather is fed and pressed against the knife. The lower of these rollers is made of short segments or rings, each separately capable of yielding to some extent so as to accommodate itself to the inequal thicknesses of various parts of a hide. The thickness of the slice of leather to be cut is ganged to the utmost minir;:eness by means of the hand screws b, b, which raise or lower the upper roller. The knife edge of the cutter is kept keen by rubbing against revolving emery wheels c as it passes round. So delicately can this machine effect its work that slices of leather uniform throughout and as thin as paper can be easily prepared by it, and with its help it is no uncommon practice to divide a comparatively thin East Indian kip into three useful splits. Another machine now largely used by carriers is the scouring machine (fig. 12), a level table or platform freely movable iu all directions, having mounted over it a reeiproeating frame in which are fixed brushes and pieces of slate or thin stone. These, with a small jet of water, scour and brush the entire surface of the leather lying on the platform, effectually scouring out bloom and all soluble impurities. Other machines are washing and stuffing drums and whitening machines. In the latter the leather is pared and equalized by the cutting action of a small cylinder armed with oblique cutting edges. The cylinder, moved to and fro with a pendulum motion, and revolving at the enormous rate of from 2000 to 3000 revolutions per minute, pares and shaves the leather on the same principle as the lawn-mower cuts grass. Embossing or graining cylinders, boarding cylinders, glassiug machines, and emery wheels for fluffing the flesh side of levant leather are also among the mechanical adaptations for currier's use.
Patent or Enamelled Leather. - Leather finished with a brilliant, smooth, and glossy surface, used for dress boots and shoes, dress belts, and fine harness, is known under a variety of names, as lacquered, varnished, japanned, and enamelled leather, &c. Such leather is finished principally from tanned calf skins, and in more recent times from seal, goat, and sheep skins, but lighter ox hides and horse leather are also japanned for special purposes. The finishing of leather in this style involves two processes - (1) the grounding or preparation of a smooth surface, and (2) the varnishing and polishing. The grounding material used by French and German finishers, who greatly excel in the production of such leather, consists of a thick syrupy mixture of lamp-black with a varnish of boiled linseed oil, umber, and litharge. This is uniformly spread over the surface of the leather, which has been previously stretched and tacked on a wooden frame. The first coating is dried in the air, then exposed in the japanning stove to a heat of about 170° Fahr., and afterwards rubbed smooth with pumice stone. This process of coating, drying, and smoothing is repeated several times, and leather is next varnished with a compound of boiled oil, Berlin blue, litharge, and some dryer, thinned either with oil of turpentine or petroleum spirit. Two or three thin coatings of such varnish are given, the surface being carefully polished after each ; but the composition of the varnishes, S:c. and the number of coatings applied, vary much in the hands of different manufacturers. Coloured enamel leathers receive two preliminary coatings of oil, rosin, and spirit of turpentine, which are sun-dried ; they are then brought up with several coatings of oil, varnish, and the special colouring substance, and finished with a thin coat of copal varnish.
Seal Leather. - The tanning of seal skins is now an important department of the leather industry of the United Kingdom, in -which this branch has been specially developed. The skins form one of the items of the whaling industry, which principally centres in Dundee, and at that port, as well as at Hull and Peterhead, they are received in large quantities from the arctic regions. A considerable number are also imported at Greenock from the coast of Newfoundland. The skin of these seals is light but exceedingly close in texture, and yields a very strong tough leather. The skins are prepared, split, and tanned in the same way as other light leathers, tanning with mixed oak bark and sumach usually occupying about six weeks. Seal leather is generally finished on the grain side as "levant" seal with a large coarse grain, and in that form it is principally used by bootmakers. A proportion of seal leather is finished as enamel and japanned leather.
R-assia Leather was originally, as the name implies, a speciality of Russia, where it was made from the hides of young cattle, and dressed either a brownish-red or a black colour, for upper leather or for bookbinding, dressing cases, purses, and similar objects. Russia leather is now made throughout both Europe and America, the best qualities being obtained from Austria. Horse hides, calf, goat, and sheep skins, and even splits, are now finished as russia leather ; but most of these are decidedly inferior in quality, and, as they are merely treated with birch bark oil to give them something of the odour by which an ordinary observer recognizes russia leather, they scarcely deserve the name under which they pass. Genuine russia leather is tanned like other light leathers, but properly in willow bark, although poplar and spruce fir barks also are used. After tanning, scouring, and setting out, the hides are treated on the flesh side with an empyreumatic oil obtained by the dry distillation of birch tree bark and buds, to which the peculiar smell of the leather is due. The red colour commonly seen in russia leather is given by dyeing with a preparation of brazil wood, rubbed over the grain side with a brush or sponge. Black-coloured russia leather owes its colour to repeated stainings with acetate of iron. The leather of genuine quality is very water-tight and strong, and, owing to its impregnation with empyreumatic oil, it wards off the attacks of insects.
Morocco and Tithe Leathers. - Originally morocco leather was a product of the Levant, Turkey, and the Mediterranean coast of Africa, where the leather was made from goat skins tanned with sumach, and finished either black or various bright colours. Such leather was peculiarly clear in colour, elastic, and soft, yet firm and fine in grain and texture, and has long been much prized for bindings, being the material in which most of the artistic work of the 16th century binders was executed. Now, in addition to genuine morocco made from goat skins, we have imitation or French moroccos, for which split calf and especially sheep skins are employed, and it may be said that, as the appearance of morocco is the result of the style of graining, which can be artificially produced on any leather, and of the finish, morocco can be made from all varieties of thin leather. The Germans distinguish between saffian and morocco, including under the former term leather tanned with sumach, and dyed bright colours without previous stuffing with fats, while as morocco proper they reckon leather which may be prepared with mixed tannage, is stuffed, and afterwards is finished black. Saffians are, according to this classification, the leathers principally used for bindings and fancy purposes, morocco being more especially devoted to shoe work.
The preparation of skins for morocco leathers must be conducted with much care. The skins, being usually hard and dry when received, are first soaked and softened by milling in the stocks and working on the tanner's beam. They are next limed, unhaired, fleshed, and trimmed in effect as already described in the section on sole leather, and they are pured or bated in a preparation of dog's dung. After undergoing the influence of this preparation, the skins are washed and slated with a knife-edged piece of slate to remove from their surface fine hairs and adhering dirt, and then they are put into a drench of bran and water, heated to about 185° Fahr., after which they ought to be perfectly free from deleterious impurities and ready for tanning. Several processes are adopted in tanning, but that most approved is based on the original Eastern practice, which consists in first treating the skins with an already used sumach infusion. Next they are, in pairs, sewed up as bags, grain side outwards, and these bags are filled with concentrated sumach liquor and a proportion of powdered sumach, and by the exudation of the liquor through the skins, partly aided by pressure, the tanning is quickly completed. After ripping out, the skins arc thrown into vats containing sumach liquor, to tan the edges and shanks, which are not reached by the liquor in the bags. The fully tanned skins are now struck out on the beam with the striking pin, and hung in the loft to dry, when they are ready for the finishing processes. A large proportion of the goat skills imported into western Europe from the East Indies, whence they are exported in enormous quantities, are received in the fully tanned condition, and ready for the morocco finishing operations, after a short treatment with sumach liquor. For finishing, the leather is first damped in soap-suds, and shaved on the flesh side to equalize the thickness of the leather, and next on a table worked over repeatedly with slickers, which renders the skin firm, smooth, and uniform. The skins are next blacked on the grain side with a solution of acetate of iron, and from this point the methods of finish diverge in an endless manner according as it is desired to finish the leather as " kid," "levant," " peebled," "bright," or "dull," &c. The bright-coloured moroccos are dyed in two different methods, the dyeing being done as a preliminary to the finishing operations. In the case of genuine moroceos, the skins are dipped and drawn through small troughs containing the dye liquor ; two skins are taken, placed flesh side to flesh side, and so worked through the liquor by hand, the operation being repeated as often as necessary to bring up the requisite strength of colour. Imitation morocco, on the other hand, is usually dyed by-stretching the skins on a table and brushing the dye liquor over the grain side. After the dyeing the skins are shaved and dressed, the dyed surface is rubbed over with an emulsion of white of egg, linseed oil, and dye liquor, and afterwards grained and glassed, or finished smooth and glossy, according to the purpose for which the leather maybe required. In recent times aniline colours have been very largely employed in the dyeing of all bright leathers.
In the tanning of sheep and lamb skins the general operations outlined above in the ease of goat skins are necessary. Previous to tanning, the prepared skins are submitted to hydraulic pressure, to expel the oleaginous matter with which sheep skins are richly impregnated. Sheep skins tanned, generally with beech bark in the United Kingdom, and uncoloured are known as basils. Roan Leather is sheep skin tanned in sumach, coloured and dressed throughout in the same manner as imitation morocco, excepting that it is finished smooth and glassed. Skivers are split grain sides of sheep skins tanned in sumach, and similarly finished, - the flesh split being shamoyed for inferior qualities of shamoy or wash leather. Skivers from their thinness are quickly tanned through in a sumach liquor, and in no case are they sewed into bags, as is most commonly the case with entire sheep and goat skins. The splitting machine used for split sheep skins has two rollers, the lower one of gun-metal and solid, and the upper made of gun-metal rings, while between the two rollers, and nearly in contact, is the edge of the sharp knife, to which an oscillating movement is given by a crank. When a skin is introduced between the two rollers, it is dragged through against the knife edge and divided, the solid lower roller supporting the membrane, while the upper one, being capable of moving through a small space by means of its rings, adjusts itself to inequalities in the membrane ; where this is thin the rings become depressed, and where it is thick they rise up, so that no part escapes the action of the knife. Skivers are finished white, or in colours in variously lined or diced patterns, and in imitation grain, and are principally employed for hat and other linings and various purposes in which they meet little strain or tear and wear.
Danish Leather is tanned sheep and lamb skins principally, but goat and kid skins also are used. The tanning medium is willow bark, and the leather, bright in colour and highly elastic, is used for strong gloves. The same name is also applied to tawed lamb skins, dressed and finished on the flesh side.
such as cigar cases, pocket books, &c.
Kangaroo Leather. - The Australian colonists have turned their attention to the preparation of leather from the skins of the kangaroo, wallaby, and other marsupials native to their continent. These skins are both tanned and tawed, the principal tanning agent being the mimosa bark, which abounds in Australia.. The leathers they yield are of excellent quality, strong, and elastic, and rival in texture and appearance the kid of European tanners. The circumstance that the animals exist only in the wild state renders this a limited and insecure source of leather.
Under the term tawing is embraced the preparation of leather by the action of mineral substances on hides and skins. In the process of timing the substance principally employed is alum or some of the simple ahnninous salts, although many other inorganic salts have been proposed, some of which have given considerable promise of practical success. The system of tawing is principally applied to thin and light skins of sheep, lambs, kids, and goats, although in former times much heavy leather was tawed for military ladts, heavy gloves, machine belts, &c., for most of which purposes, however, surnach-tanned or similar leathers are now found more applicable and durable. The products obtained by tawing are of a pure white colour, whence the MUM white leather is frequently applied to goods of this class. The most important departments of the tawing industry are the calf kid manufactures for boots and shoes, and glove kid or glacj leather tawing, the products of which are exclusively devoted to glove making. A large number of white tawed sheep skins are also used by druggists and perfumers as tie-over leather for bottles, and for linings by bootmakers, &c.
eggs (or about 1A- gallons of er,, yolk), pint of olive oil, and 12 to 16 gallons of water. In this mixture, at a temperature of not more than 100° Fahr., the skins are worked for about forty minutes, by which action the towing is completed. After the withdrawal from the drum the skins are allowed to drain, dried rapidly by artificial heat, damped, staked out by drawing them over a blunt steel tool, and then wetted and shaved down on the beam to the required thickness. Next they receive, if necessary, a second treatment with the tawing mixture. The dyeing or colouring follows, which in the case of calf kid is always black, the colour consisting of a compound of bichromate of potash, stale urine, logwood extract, and copperas. It is applied either by brushes on a table., or by dyeing the leather in small vats as in the parallel case of morocco leather. The dyed leather is washed with pure water, dried, grounded with a curious moon knife, stretched in all directions, ironed, and oiled on the flesh side with a mixture of oil, wax, Rt.c.
Glove Kid. - ln the preparation of kid leather for gloves the tender skins of young kids alone are used for the best qualities, but for a large proportion of such leather young lamb skins are also tawed. The genuine kid leather is for the most part produced in France, specially at Annonay and Paris, while lamb kid is more particularly a product of Germany, Austria, and Denmark. In all stages of the preparation of this leather the utmost care and attention are requisite, and it is specially of consequence that the operations preparatory to tawing should receive thorough attention. The unhairing is best effected by steeping the skins in a mixture of lime and orpiment, and, while the general sequence of unhairing, fleshing, bating with clog's dung, scudding, washing, and treating with the bran drench is the same as in the case of other skins, much more attention is bestowed on each stage in order to maintain the smoothness of grain, and to obtain a thoroughly clean elastic pelt, than is absolutely needful for any other variety of leather. The tawing mixture consists for each 100 lb of skins of about 28 lb of flour, 3 lb of alum, nearly 1 It of common salt, and 230 eggs. These substances are made into the consistency of a cream with water, and placed either in a vat or in a revolving drum. In the former case, the skins are trodden with the feet, while in the latter they are tumbled about. The tawed skins are hung over poles, grain side inwards, and dried rapidly ; when hard dry they are heaped in a damp place to soften a little, then damped by passing them through water, next trodden out by foot on a ridged or barred floor, staked or stretched over a blunt knife, partly dried, and again staked and dried thoroughly. For dyeing, the skins are first washed out in warm water to free them from superfluous alum, and then again " fed " with yolk of eggs and salt. For bright colours such as soft greys, lavenders, and yellows, the skins are plunged into small dye vats of the proper dye colours ; but for all the darker colours the skins are stretched out on a table and the dye stuffs applied with a brush. In the latter case the leather is first grounded with some alkaline solution, then dyed mostly with logwood, brazil wood, fustic, Prussian berries, or preparations of indigo, - aniline colours being now little used for glove dyeing. The dye is mordanted by a wash of the sulphate of either zinc, copper, or iron, which operation also clears and develops the colour. After dyeing it only remains to free the leather from superfluous moisture, dry it, and then with slight damping stake or stretch it out once or twice, which finishes the preparation of this valuable class of leather.
The "feeding" of kid leathers with yolk and sometimes oil producesapartial shamoying, softening the texture andgiving the leather that peculiar suppleness to which much of its value is due. The flour added, by means of the gluten it contains, is supposed to facilitate the absorption of the alumina and thus hasten the timing.
Hungarian Leather consists of hides and heavy skins partly tawed and partly shamoyed. In the preparation of this leather it was formerly the practice to shave off the hair with a sharp knife, but now the hides are unhaired either by sweating or liming. After tawing with alum and salt the leather is stuffed by first heating it over a charcoal fire and impregnating the hot leather with tallow, or, as is now common, by working it in a drum with a hot mixture of cod oil and tallow. This leather, being comparatively cheap, is much used on the Continent for common saddlery purposes, for which it is blackened, and it is also serviceable for machinery belts.
A leather has been patented by Professor Knapp, in which the active tanning or timing principle is a basic salt of the cxide of iron. It was intended principally for sole leather, but it does not appear to have met with practical success, and its manufacture is understood to have been abandoned.
Ifeinzerling's Chrome-tanned Leather. - Quite recently a large amount of attention has been devoted to a system of tanning or timing by means of chromium compounds patented by Dr Heinzerling, a German chemist. The oxidizing power of chromate salts, and the deoxidizing effect which organic matter has upon these salts, have long been recognized, and the knowledge of this action and counter-action has led to many unsuccessful attempts, in the past, to use ehromates in tanning. It is claimed, however, that the difficulties have been overcome by Pr Heinzerling's process, which consists practically in the use of bichromate of potash, chloride of potassium or chloride of sodium, and sulphate of alumina. These are mixed together in one large stock tank, from which is drawn by means of a system of piping communicating with each pit the quantity required to make the necessary strength of liquor ; this at first, as in tanning by bark, is very weak, but is strengthened systematically every few days according to the thickness of the hides being tanned. The quantity of chromic acid used ordinarily amounts to from about 2i to 5 per cent. of the weight of leather produced. The price of bichrome at present is 54d. per lb, so that 100 lb weight of leather would cost for this agent from ls. to 3s.
Light skins such as sheep skins and calf skins are tanned in less than a week, ox and buffalo hides in about a fortnight, and walrus hides, over 2 inches thick, in six weeks. After being tanned, the hides, which are at this stage of a yellowish tint like sumach-tanned leather, are dipped in chloride of barium, which converts the soluble chromates on the surface into the insoluble chromate of barium. If any particular shade of colour is desired it is then put on, and in general hides are coloured like ordinary leather. After being coloured the leather is allowed to get nearly dry, when it is immersed in pure paraffin wax and resin dissolved together in certain proportions. These materials, with chloride of potassium or chloride of sodium and sulphate of alumina, go to give the necessary substance, weight, and waterproofing to the leather. The hides are afterwards dried and brushed clean by brushing machinery. Thus finished, the leather differs very little in appearance from ordinary leather.
Dr Heinzerling claims as the meritorious and original features of his process the combined use of chromate compounds and fatty matters. The stuffing with fat or paraffin of chrome leather, ho maintains, in the first place, reduces chromic acid to chrome oxide, and secondly the oxygen thus liberated in the substance of the hide oxidizes the fatty into acid bodies, which, uniting with the chrome oxide, form a third insoluble compound mordanted in the fibre of the leather, rendering it at once supple and waterproof.
The leather has been reported on by Mr David. Kirkcaldy, London, as considerably stronger than the best bark-tanned leather he was able to procure. After steeping samples of it in cold. water six days it has been found that the total quantity of tanning material extracted amounted to from '014 to '135 per cent., while first-class bark-tanned leather similarly treated yielded 619 per cent. By boiling chrome leather in water for half an hour, the loss ranged from '005 to '054 per cent.
The process seems to offer the moans of utilizing classes of hides, such as sheep skins, and very heavy hides, as those of the walrus, hippopotamus, Sze., in a way which has not hitherto been found practicable by other processes. Sheep skins in chrome-tanning do not require to be pured and freed from their oleaginous constituents, and when finished by this process are no longer porous, but become waterproof. They can be shaved and whitened like calf skins, and may be used for shoe purposes.
The Heinzerling process is at work in various localities throughout Germany. For the United Kingdom and British colonies the patent rights have been acquired by the Eglinton Chemical Company of Glasgow, who, as manufacturers of biehromate of potash, have an indirect interest in the development of the system. Although the method. has yet scarcely passed the critical stage of practical experiment, the products appear to be gaining the favour of men of experience ; and, should the system meet the expectation of its originator and promoters, it cannot in the end fail greatly to cheapen many useful classes of leather.
The process of preparing leather by impregnating hides and skins with oil is probably the oldest system of leather manufacture. It is that which in earlier times was most largely followed, and among rude and semi-civilized people it is still commonly practised. Notwithstanding this, well-shamoyed leather requires the exercise of much care and numerous manipulative processes. Hides and skins of all classes are prepared by shamoying ; but sheep, goat, deer, antelope, and small calf skin are those usually treated, an enormous number of flesh splits of sheep being sharnoyed for common purposes. The extensive employment of deer skins in shamoying gives the product the name of buck Or doe leather, and from the use of the chamois skin of the Alps is derived the name of the process chamois or shamoy, while from the fact that it may be easily washed like cloth it is called wash leather. In former times a large number of ox hides were shamoyed, but now that is little practised.
Skins for shamoying are in the preliminary stages treated almost as for ordinary tanning ; but, beyond unhairing, the surface of the grain is shaved off in all except the small thin skins. They are afterwards treated with fresh lime solution, and repeatedly washed to bring the pelt to somewhat open and porous condition, drenched with bran to remove all lime, and rinsed in an acid liquor. The skins are next staked out and taken to the fulling machine or stocks, where, after being rubbed over individually with fish oil, they are hammered for about two hours to folyce the oil into the substance of the skin. They are then stretched, hung up for some time, again oiled, and fulled ; and these operations may be repeated from six to twelve times according to the thickness of the skins treated. After thorough impregnation the skins are dried, then heaped np in a heated room, where a process of oxidation is quickly set up. So soon as the skins assume a yellow colour and give off a peculiar odour, not at all like fish oil, the process is complete and the fermentation is stopped. It is now found that about one-half of the oil is oxidized within the skin and combined with the tissue to form leather, while the remainder is present only in the condition of mechanical impregnation. This uncombined oil is washed out with a warm potash solution, and the fat so recovered, known as degras, forms a valuable material for the dressing of common leather by curriers.
These substances, properly speaking, do not come under the heading of leather at all, seeing they are neither tanned nor tawed, but simply are dressed skins dried and prepared for their peculiar uses. Parchment is made front calf, goat, sheep, ass, and swine skins by the unhairing and dressing processes through which all skins pass preparatory to tanning. When they are thoroughly scudded and fleshed, the skins are stretched tightly in every direction over a frame, and in that condition shaved and equalized. on both sides with the currier's knife. After drying, the skins are ready for use as drum leather and for the other ordinary applications of parchment. The common kinds of vellum are made from sheepskin splits, of which two may be obtained from a single skin. To prepare these for use, the splits are, after stretching and drying, repeatedly rubbed over with powdered chalk and powdered stone to raise the fine even velvety surface peculiar to vellum. Common shagreen consists of the skins of various species of sharks and rays prepared in a similar manner to parchment ; and Persian shagreen is a kind of tawed parchment with an artificial grain embossed in it, by pressing into the substance while in a damp condition the small round seeds of a species of Chezzopodium. Shagreen is frequently dyed in bright colours, and used for ornamenting the surfaces of small articles, and the handles of daggers, swords, ke.
It is quite impossible to form any adequate estimate of the extent and value of such a trade as that in leather. The raw materials are obtained, in almost equal abundance, throughout the civilized world, and the manufaetnring operations are, to a greater or less degree, carried on in every centre of industry. Thus local wants are largely supplied by local producers, and much of this trade is never recorded in any statistical returns. There can be no doubt that leather takes rank among the foremost half dozen of human industries. Both in Europe and America there is a large international trade in the raw materials and manufactured products, while from the East Indies and the British colonies the largest supplies of various untanned and tanned hides and skins are now exported. Taking cattle hides alone, the import trade of the great manufacturing centres has been thus stated for 1879: - United Kingdom 1,162,400 Hamburg 640,130 Mcrseilles 652,967 Roston and New York (U.S.).. 3,153,026 Havre 853,741 Antwerp 958,41°2 Total for Europe and America 7,426,881 These came principally from the River Plate, Rio Grande, and Brazil, and are exclusive of 250,000 horse hides imported into Europe, and the whole of the enormous exports of the East Indies. The total number of hides, raw and tanned, exported from India in the year 1877-7S was 9,300,955, and the average for the four preceding years exceeded 7,250,000. A great proportion of these are tanned kips, shipped from Madras almost exclusively to the United Kingdom, - buffalo and the heavier flint dry hides going more largely to the United States. The imports of East Indian kips into the United Kingdom during 1880 amounted to 6,135,978, and in 1881 the number fell to 4,580,303. Of tanned goat and sheep skins, again, the annual export from Madras alone reaches nearly 10,000,000 skins, the numbers for 1880 having been 9,799,900 skins. The total number of sheep and goat skins, raw and rough tanned, imported into the United States in 1880 was 11,731,885, of which 6,332,635 were raw soft stock, 3,353,750 were raw hard East Indian skins, principally from Bengal, and 2,055,500 were tanned East Indian skins.
The following table shows the sources and number of hides imported into the United Kingdom during 1880: - River Plate, salted bides 819,745 West Indies 10,584 dry „ 31,057 West Coast of America 15,107 Rio &mle, salted „ 178,897 Cape kips and gnus 13,097 Cape „ „ 36,538 North America 5,983 dry „ 209,176 East India kips and buffalo ... 6,707,112 Australia . 196,140 Horse bides 98.200 Brazil 96,246 The following are the Board of Trade returns of articles connected with tanning imported into the United Kingdom, and exported, during the five years ending 1880 : - The principal leather markets of the United Kingdom are London, where there are quarterly fairs ; Leeds, with eight fairs yearly ; and Bristol, which has two leather fairs per year. In the United States the commerce centres principally in New York, Boston, and Philadelphia. On the European continent Paris, Marseilles, Vienna, and Berlin are the most important centres of the leather trade, with Antwerp and Havre as great marts for the sale of hides and skins. (J. PA.)
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