THE TECHNOLOGIST. [Oct. 1, 1864 



102 ON CHEMISTRY APPLIED TO THE ARTS. 



the term skin is applied to those of small animals, such as the sheep r 

 goat, seal, &c. 



I will now endeavour to give you an idea of the preparation which 

 hides undergo to fit them for the art of tanning. These operations are 

 four. The first consists in washing off the dirt from the hide, softening 

 it, if a dried one, or removing the salt, if salted. The second has for 

 its object the removal of the hair, which is effected by two or three 

 different methods. The most usual plan is to place the hides in large 

 vats, containing a weak milk of lime, for two or three weeks, care being 

 taken to remove and replace them every other day, after which time the 

 hair is sufficiently loosened to be removed. A second plan consists in 

 piling up the hides, allowing them to enter slightly into a state of putre- 

 faction, and then placing them in weak milk of lime, so as to complete 

 not only the loosening of the hair but also the swelling of the hide, for 

 lime also possesses that property. Another process, which is called the 

 American plan, is to hang the hides in pits for two or three weeks, 

 keeping them at a temperature of 60® and constantly wet, when the hair 

 can be easily removed. Weak alkalies are sometimes substituted with 

 advantage for lime in the above processes, and this plan is certainly the 

 best, as it does not leave in the hide any mineral residue, as is the case 

 with lime, either in the form of an insoluble soap of lime or of car- 

 bonate, both of which are highly objectionable in the subsequent process 

 of tanning, as they act on the tannic acid of the tan, facilitating its 

 oxidation, and thereby rendering it useless. Depilation of hides is some- 

 times effected by the employment of weak organic acids ; thus the 

 Calmuck Tartars have used from time immemorial sour milk for that 

 purpose. In some parts of France, Belgium, and Germany, the unhair- 

 ing of the skins is also effected by an acid fluid, produced by the fer- 

 mentation of barley meal, which gives rise to acetic and lactic acids. To 

 carry out this process, generally speaking five vats are used. In the first 

 the hides are cleaned ; in the second they are softened, and the hair and 

 epidermis prepared for depilation ; and the third, fourth, and fifth are 

 used to swell and give body to the hide. This operation, which is called 

 white dressing, does not work so well as lime for heavy hides, as it swells 

 them to such an extent as to render them unfit to prepare compact 

 leather. When the hair can be easily pulled off, the hides are placed on 

 a convex board, called a beam, and scraped with a double-handed con- 

 cave knife, which not only removes the hair, but a large amount of fatty 

 lime-soap and other impurities from the hides. The third operation 

 consists in fleshing the hides, by shaving off all useless flesh, fat, and 

 other matter by means of a sharp tool. The fourth operation is called 

 swelling or raising the hide, the purpose of which is the following : — 

 First, the removal of any lime or alkali which may remain in the hide ; 

 and secondly, to swell or open the pores of the hide, so as to render them 

 better adapted to absorb the tannic acid of the tanning liquors. This is 

 effected by dipping the hides in weak spent tanning liquors, or liquors 



