Although, owing to the many differences in the practice of tanning, no definite time can be stated for the various operations mentioned above, it may be observed that the usual period required for tanning such hides as are used for the soles of men's boots is from six to twelve months • and that from fifteen to eighteen months are required in preparing of the thickest kinds, which are termed butts or backs. Occasionally hippopotamus hides are tanned, requiring an enormous time for the process; and instances have been known of wild boar hides remaining seven years in tan, to produce leather two inches in thickness.
Before describing the processes for thinner leather, it will be neces sary briefly to notice some among the numerous plans for shortening the time involved in the tanning of thick hides. Several schemes have been devised for forcing a tanning solution through the pores of the hide by mechanical pressure. Mr. Spilsbury obtained a patent for this purpose some years ago ; but it failed for it appears that tannate of gelatin was formed on the outer sides of the skins in the form of long masses of slime ; while the leather had lost much in weight, was very porous, and unequally tanned, in consequence of the tan-liquor pene trating most readily the thinnest or weakest parts of the hide. A process patented by Mr. Drake consisted in sewing two skins together (after they had received a slight tanning in the ordinary way), so as to form a water-tight bag, which was filled with tan-liquor. The bag thus teemed was comp reeeed between two vertical gridiromlike frames or racks, by which it was prevented from bulging st the sides, and the liquor was confined to a thin vertical stratum. The aqueous portion of the tan-liquor percolated through the hides ; and this penetration of the leather was facilitated by heating the room so as to promote enpontson from the exterior surfaces of the bags or skins. In another somewhat similar plan, contrived by Mr. Cu; the hides were sewed up ha the form of bare, and supported by a casing of canna. In the process of Mr. Chaplin, the bags were laid lo an inclined position, and tamed periodically to equalise the action of the tan. In another plan, which has been tried under several forms, the tanning liquid is applied to both sides of the hides, which are placed in an air-tight vessel, and Is forced into their pores by hydrostatic pressure, the air being pre viously pumped out. The operation may be repeated as often as necessary, with infusions gradually Increasing in strength ; air being allowed to fill the pores of the hide between each immersion. Mr. Cagswell'il plan combo(' in laying the hides upon a quantity of sawdust, contained in shallow boxes, of which any required number may be arranged In a suitable framework, about twelve inches above one another. The hides are not laid Ilia, but have their edges a little raised, so that their upper surfaces form shallow troughs capable of holding a layer of the tinning solution, which must be replenished from tone to time as it filters through the hide and the sawdust, or other soft porous substance upon which it is laid. The spent liquor
runs oil from the bottom of the box or trough, which is somewhat Inclined for that purpose, Into a vessel or channel provided for it. Mount Herepath & Cox's process is founded upon the principle of washing a sponge, by alternately allowing it to imbibe water, and then forcibly expressing it. The weak infusion, or ooze, is forced out of the pores of the hide before it ie subj(cted to a stronger, so that the fresh ooze may be at le to act more efficiently. This is effected by connecting a number of hides together by strings, so as to form a continuous belt, and passing them between rollers turned by steam or other power, while they are being removed from one solution to another. In order to produce a tolerably uniform belt or continuous sheet of hides, they are either placed alternately head to bead and tail to tail; or, if laid across the belt, with the bends and tails toward, each side alternately. The lower roller is about thirty inches in diameter, and is covered with horse! air cloth ; and the upper roller, which is pressed against the lower one with any determinate degree of force by means of weighted levers, is only about eighteen inches in diameter, and is covered with woollen cloth. By this process a strong hide may be tanned more quickly than by the old method. Much attention has lately been paid to Mr Preller'e process, by which leather is now made at Bermondsey. The hides are first washed ; then unhaired; then partially dried ; and then covered uniformly with a paste or compo sition. This paste consists of starchy or glutinous matters, such as barley meal, rice flour, or starch, combined with an oily matter, such as butter, milk, oil, or grease. The paste is smeared on the flesh side of the hide, which is then put into a cylinder or drum, and rotated for some hours with steam occasionally admitted. The hides being taken out and dried, they are pasted and rotated a second and even a third time. At the end of this process they are found to be affected all through by a kind of substitute for tanning. It is said that an ox hide may be tanned in this way in two days and a half, and a calf skin In sixteen hours. It seems to be generally admitted that the leather thus made is light, pliable, strong, and durable; it is found useful for certain special purposes, such as driving bands for machinery, picks for biome, and for enamelling. A plan has been brought forward by Mr. Clarkaon for so applying a solution of chloride of soda and alumina, and then one of tannin, as to give is surface of leather to the hide, suffi cient for many useful purposes.