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.

For the modern process of manufacture, good, even and well-flayed hides are selected, and unhaired either by sweating, or by a very short liming, which must be assisted by rockers or some mechanical mode of moving the hides, so as to get them unhaired in the shortest possible time and with the least injury to the fibre. Sodium sulphide (see p. 147) may be employed with great advantage. The fleshing and scudding are performed as usual, according to the mode of unhairing adopted. The hides are then very commonly rounded, and the bellies tanned in the usual manner; but sometimes the whole hide is made into crown leather.

As crown leather is naturally almost white, it is usual at this stage to colour the hide with bark or other liquors. As in this case simply colouring and not tanning of the grain is required, high-coloured liquors, made by steaming materials with much colour and little tannin, are preferable. For this purpose wood extracts, such as chestnut, quebracho, or oak-wood are said to be very suitable, and beech, pine or alder bark may also be used. In practice, chestnut and hemlock extracts, and occasionally cutch are employed; but the last named is not to be recommended. A chestnut liquor of 7¹/₂° Tw. or 5° B. (34° Bark.), with constant handling or in a paddle-tumbler, will give a satisfactory colour and grain in 1-2 hours. This rapid colouring is preferable to the slower process, which occupies 24 hours in weaker liquors. If sweated, the hides are now plumped with sulphuric acid, but only to a very moderate extent. This process is best performed in a paddle-tumbler; about 3¹/₂ oz. of sulphuric acid are required per hide, and a time of 6-12 hours according to the water employed. The liquor may be several times used, strengthened with the necessary quantity of acid. Limed hides do not require further swelling. The hides are washed through clean water, and hung up to dry somewhat.

The hides are next spread on a table, flesh-side uppermost, and covered with a layer of the tanning paste nearly ¹/₄ in. thick. The composition of this paste may be varied according to the relative prices of different materials, and the amount of hard fats must be regulated according to whether or not appliances are provided for heating the tumbler. A good mixture is 7 parts common wheat-flour, 7 of horse-grease, 1 of salt, and 1-2 of tallow. If too soft, more tallow may be employed. The salt is first added to the horse-grease, then the melted tallow, These fats are added little by little to the flour till a uniform paste is obtained. Another good mixture is 27 parts wheat-flour, 25 of bone-grease, 4 of tallow, and 4 of salt. Another recipe gives 28 lb. fine white flour made to a paste with 13-14 pints water and then worked up to a uniform mass with a tepid mixture of 28 lb. beef tallow and 28 lb. hard horse-fat (Pferdekammfett). These mixtures are all for use in warmed drums; a specimen of one used in a factory where the mixture was simply trodden in cold into the leather in open tubs is as follows:—7 parts flour, 9·4 of horse-fat, 2·8 of fish-oil, 7 of ox-brains and 0·7 of salt. The hides are next folded in bundles and placed in the drum; or in stocks, which are occasionally used for the purpose. If a drum be used, it must be of large diameter, 8-9 ft., provided with pegs inside, and should make about 25 revolutions per minute, so as to work the hides with considerable force. Much more care is needed in warming the drum, than is required in ordinary stuffing, and this is best accomplished by warmed damp air. This may be arranged by the use of an air-pump, which draws air through water warmed by exhaust steam, and forces it through the hollow axles of the drum (or drums); or a simple aspirator consisting of a cask filled with water may be connected to one axle, so that as the water runs out it will draw air through the drum from the opposite axle, which is connected with a cask half filled with hot water through which air is allowed to bubble. Probably the same effect could be reached in a still simpler and cheaper manner by the use of a steam-jet blower, such as KÖrting's. In any case the drum must be warmed to a temperature of 82°-104° F. (28°-40° C.). Warm dry air may also be used, but is not so suitable, as it dries the hides too much. The hides are tumbled 8-12 hours, hung up till half-dry, and the process is repeated. For very heavy hides, 4 tumblings may be required. In the later tumblings, a lower temperature, 95° F. (35° C.), may be employed, and the time extended to 15 hours.

The currying of crown leather is very simple. It is set out on flesh and grain, and boarded to raise the grain. Mossner, before currying, washes 2 hours in water and brushes with tepid soda solution (1 in 60). The yield of weight is small, only amounting to about 30-40 per cent. of the raw hide employed, and hence the price per lb. must be considerably higher than that of tanned leather to yield a profit. The above information is mostly drawn from articles by W. Eitner ('Der Gerber,' iv. 1 et seq.) and Franz Kathreiner ('Gerber Zeitung,' 21st December, 1875).