CHAMOIS OR WASH-LEATHER.
This leather, which is remarkable for its soft felty texture, which it retains even after wetting, although perfectly porous and free from greasiness in its finished state, is prepared by the action of oil on the raw skin. Wash-leather was formerly manufactured from sheep- and calf-skins, and from those of the chamois, and various deer (hence the name), from which, after liming, the grain was removed (frized) with a sharp knife, either with the hair, or after unhairing. The flesh-splits of sheep-skins are now generally employed for ordinary wash-leather, and of course no such process is needed, though buff-leather for belts and military purposes is still so manufactured. The skins receive a thorough liming, which, where softness is desired, is so conducted as very thoroughly to remove the cement-substance (coriin) from between the fibres; and this removal is frequently carried still further by a short bran-drench, which also secures the complete absence of lime. After the usual beam-work, the skins are pressed or wrung out to remove surplus water, and while still moist are oiled on a table and folded in cushions. Fish-, seal-, or whale-oil is generally used, and vegetable oils do not seem to answer even in mixture, with the exception perhaps of olive-oil. The skins are next stocked for 2-3 hours, shaken out, and hung up for 1/2-1 hour to cool and partially dry. They are then again folded in bundles, and stocked for a short time, taken out, oiled again, and returned to the stocks; and this process is repeated, until the skins lose their original smell of limed hide, and acquire a peculiar mustard-like odour, and the water at first present has been entirely replaced by oil. The later dryings are frequently conducted in a heated room, and when the oiling is complete, the skins are piled on the floor, and the oxidation of the oil, which has already commenced during the fullings and dryings above described, is completed by a sort of fermentation, in which the skins heat very considerably. During this process, they are carefully watched, and if the heat rises so high as to endanger the quality of the leather, the pile must be turned over, so as to cool the skins, and bring those which were originally outside to the centre. When the fermentation comes to an end, the skins are no longer susceptible of heating, and are of the well-known yellow or chamois colour. Where this colour is objectionable, the oxidation is sometimes completed by hanging the leather in a heated room instead of by piling. It is now necessary to remove the surplus oil, and this in France is done by oiling with any sort of oil, throwing into hot water, and wringing or squeezing. The oil obtained in this way forms the moëllon or dégras so much prized for currying purposes. The unoxidised oil still retained by the skins is removed by washing with soda or potash lye. In England and Germany, the whole of the uncombined oil is removed in this way, and is recovered from the lye, in which it exists in a partially saponified state, by neutralisation with sulphuric acid. It forms the "sod" oil of commerce. About half the oil employed is obstinately retained by the skin, and cannot be removed even by boiling with alkalies, while no gelatin is obtained by boiling water, to which the chamoised skin is much more resistant than ordinary leather. The nature of the tanning process does not seem to be well understood. It is generally stated that the fibres of the skin are unaltered, but are merely coated with the oxidised products of the oil. It is hard, however, on this hypothesis to understand their extraordinary indifference to water, even at a boiling temperature, which speedily converts kid and other tawed leathers into a solution which gelatinises on cooling; and it seems more probable to the present writer that some actual chemical combination is formed. Lietzman ('Herstellung der Leder,' p. 164) supposes that the whole of the gelatigenous tissue has been removed by liming and bating, and that only the very indifferent yellow elastic fibres (see [p. 21]) remain. This view, however, is quite untenable, in consideration of the very small proportion of these fibres originally present in the skin. Müntz, in his researches (see [p. 17]), showed that the fibres insoluble in boiling water scarcely exceeded 3 per cent, of the dried pelt. Dry gelatigenous fibre has a considerable resistance to heat, and it is possible that the action of the oil may consist in preventing the absorption of water. This, however, will not explain its resistance to alkalies. Cotton or other vegetable fibres moistened with oil, readily undergo oxidation, with so much evolution of heat as sometimes to cause spontaneous combustion; but the oxidation products are easily and completely removed by alkaline solutions, leaving the fibre in its original state, as indeed is noted by Lietzman (loc. cit.).
The finishing processes consist in staking during drying to retain softness, and in whitening and smoothing the flesh (or sometimes both sides) on the fluffing wheel. Skins for gloves, &c., are bleached like linen, by sprinkling and exposure to the sun; or more rapidly by treatment with a weak solution of potash permanganate, and subsequently with sulphurous, or very dilute sulphuric acid, to remove the brown manganous oxide formed (Barreswil, Dingl. Polyt. Jour., 161, 312). Gaseous sulphurous acid from burning sulphur may also be used for bleaching. The "dyeing" of chamois leather is generally done with ochres and similar colouring matters, and may be removed by washing. Treatment with egg-yolk in water, or with an emulsion of olive-oil with a little soap, and rubbing, or stretching, will restore softness to chamois leather which has become stiff by washing.
[CHAPTER XVIII.]
CROWN LEATHER, OR PRELLER'S LEATHER.
The process of manufacture of this leather, which has obtained a firm position as the most suitable material for certain classes of belting, picker-straps, &c., was discovered about 35 years since by Theodor Klemm, a cabinet-maker in Wurtemburg and founder of the present well-known firm of leather manufacturers, Gebrüder Klemm of Pfullingen. Klemm, at that time in poor circumstances, sold his patent in Paris to an Englishman, Preller, who started a manufacture of it in Southwark and adopted a crown as his trade-mark. Since this time the manufacture has spread, first to Switzerland and then through Germany; but in England, to the writer's knowledge, it is confined to one or two firms.
The process of manufacture of crown leather is in principle intermediate between that of calf-kid (see [p. 223]), and the pure oil-tanning, if we may call it so, of which the chamois leather (see [p. 210]) is typical. It depends on impregnating the raw hide with a mixture of fats and albumens, to which salt or saltpetre is added to prevent putrefaction. The process as described in the original patent was as follows,—The hides were unhaired by liming or painting (with sulphides), and cleansed as usual, no plumping lime being given. After unhairing they were allowed to dry some little time in the air till no longer plump, and were then worked in a tumbler drum, without water, till uniformly soft. They were then spread on a table and brushed over on the flesh-sides with a mixture of 23 parts of ox-brain, 61/2 of butter, 28 of soft fat, and 4 of salt or saltpetre, with 26 of barley-flour and 121/2 of milk, of which the leading 4 ingredients were first to be mixed and the flour stirred in, the milk being last added. The hides were then returned to the tumbler, which was provided with tubular axes, through which a portion of exhaust steam was admitted to warm the drum. After tumbling some hours, the drum was opened, and the hides were examined. If the tanning was not complete, the hides were hung in the air for a time to dry, and the process was repeated till a cut showed that the mixture had completely penetrated the hide.
From Eitner's researches it appears that the essential tanning ingredients of the mixture above described are the fat (and butter which acts simply as fat) and the albuminous matter of the milk (casein), brains (albumen, &c.), and flour (gluten); the starch serving at most to assist in the emulsification of the fats. Eitner treated crown leather with dilute potash solution to remove the albumen and fats, and after washing and drying obtained a material like an insufficiently stocked chamois leather. On again stuffing with a quantity of fat equal to that removed, but without the albumen, the leather became dark and quite greasy, so that by sharp bending oil could be pressed out. Good results may be obtained in crown leather manufactured with fats and flour only, without the use of milk or brains, so that it is obvious that the same purpose is served by either vegetable or animal albumenoids. The most important point in the purposes for which crown leather is employed is toughness, and this is given by the unaltered hide-fibres, which are merely preserved by the coating of oily matter with which, like those of chamois leather, they are surrounded. The albumen serves the purpose of filling the spaces between the fibres, and giving solidity and firmness, so that the belts may keep their shape, and not stretch inordinately. It also serves to make the leather waterproof, and fit it for water-bags for military purposes (as it gives no taste to the water) and for hose-pipes. The albumen, which much resembles the hide-fibres in composition, is like them preserved by the fats.