CHEMISTRY APPLIED TO THE ARTS. 





corresponding to the pattern. Mordants and dye- 

 stuffs are then successively forced through, and 

 act only on the parts opposite the holes. 



LEATHER 



consists of the skin of mammals so prepared that 

 it is not stiff and brittle when dry, and when wet 

 does not easily putrefy. There are three different 

 methods of making leather namely, Tanning, 

 Tawing, and Shamoying ; and there are thus 

 three essentially different kinds of leather tanned 

 leather, white leather, and shamoy, or wash leather. 



The skins undergo a preliminary treatment, 

 which is of nearly the same character for all the 

 three processes. This consists in cleaning the 

 skin, freeing it from hair, epidermis, blood, fat, 

 portions of flesh, &c. by soaking in water, scrap- 

 ing, and paring. The hair is loosened in some 

 cases by treatment with lime ; as it is difficult to 

 remove the lime entirely from very thick skins, 

 these are allowed to undergo a slight superficial 

 decomposition, after which the hair and epidermis 

 are easily scraped off. Very thin skins are liable 

 to be injured by either of these processes, and 

 from them the hair is removed by the application 

 of a substance called ' rusma/ which is a mixture 

 of nine parts of lime and one of orpiment, which 

 very rapidly loosens the hair. The skins, after 

 being cleaned, are subjected to the process called 

 ' swelling.' This consists in placing the hides in 

 a tank containing water and bran. The bran 

 undergoes fermentation, producing acetic and 

 lactic acids ; these remove any lime which may 

 have been left ; while, by absorbing water, the 

 skins are rendered soft. The skins are now ready 

 to be tanned, tawed, or shamoyed. 



Tanning. This consists in the absorption by 

 the skin of tannin, a substance mentioned in the 

 paper on CHEMISTRY, but which we must now 

 describe somewhat more fully. 



Tannin is a generic name given to a consider- 

 able number of substances of vegetable origin. 

 They all give a dark colour, either blue-black or 

 olive-green, with ferric salts ; are easily oxidised, 

 especially in the presence of an alkali, forming 

 dark-coloured substances : all combine with the 

 fibrous tissue of skin to form leather, and all unite 

 with gelatine to form an insoluble substance. It 

 has been supposed that this insoluble gelatine 

 compound is the same substance as leather, but 

 this is certainly not the case ; skin contains no 

 gelatine, although, by prolonged boiling, the 

 fibrous tissue of the skin can be converted into 

 gelatine. The most important sources of tannin 

 for tanning purposes are oak-bark; sumach, the 

 bark of plants belonging to the genus Rhus ; 

 catechu, an extract prepared from various plants ; 

 dividivi, from Casalpinia coriaria; valonia, the 

 cups of the acorns of Quercus agilops; nut-galls, 

 excrescences produced by insects on the leaves 

 and leaf-stalks of Quercus infectoria. 



The prepared skins are tanned in two different 

 ways tanning in the bark, and tanning in the 

 liquor. Tanning in the bark is carried on in pits 

 lined with oak-wood. A layer of spent-tan is 

 placed at the bottom, then an inch of fresh tan 

 (oak-bark sometimes mixed with valonia flour), 

 then a hide, then another layer of bark an inch 

 thick, then another hide, and so on. All cavities 

 are filled up with bark, and a layer about a foot 



deep of spent-tan is placed on the top. Water is- 

 then introduced till it stands a little above the 

 uppermost hide. v The hides remain in this pit 

 undisturbed for about eight or ten weeks, if pure 

 oak-bark is used ; if valonia has been added, a 

 shorter time is required. The hides must be taken 

 out of the pit before all the tannin has been ab- 

 sorbed from the liquid, and before too much acetic 

 acid has been formed by fermentation, and placed 

 in a second pit, those hides which were formerly 

 at the top being put at the bottom. Here the 

 same arrangement of hides and tan is made as ia 

 the first pit, only rather less tan is used. In this 

 pit the hides remain for three or four months, and 

 then in a third similarly arranged pit for four or 

 five months. Very thick hides sometimes require 

 a fourth, or even a fifth treatment, occupying alto- 

 gether two years or more. The thicker the hide 

 the longer is the time required to tan it Thus r 

 the skins of cows are tanned in about a year ;. 

 those of calves, in four or six months. 



Perfectly tanned leather is recognised by exam- 

 ining a section made with a knife ; the surface 

 should be uniform in character, without fleshy or 

 horny streaks. 



The second mode of tanning is tanning ia 

 liquor, or quick-tanning. It is used for all except 

 thick hides intended for sole leather, which are 

 tanned in the bark. 



The liquor is a cold infusion of oak-bark or 

 other material yielding tannin. The hides are 

 first placed in very dilute liquor, in order that there 

 may be no superficial tanning, which would render 

 it difficult for the liquor to penetrate into the inte- 

 rior of the skin. They are moved about in the 

 tank to bring new solution in contact with them,, 

 and when they are taken out, they are pressed and 

 rolled, to remove the spent liquor, and enable them 

 more easily to absorb new. The hides are moved, 

 into successively stronger liquors, remaining eight 

 hours in the first or weakest, sixteen in the second,, 

 twenty-four in the third, forty-eight in the fourth,, 

 and in the last and strongest until the tanning is 

 completed. Leather tanned in this way is often, 

 placed between layers of bark in the pit for a few 

 weeks, to make it denser. In this way calves' 

 skins can be tanned in eight days ; thicker skins 

 require considerably longer, four or five weeks 

 being sometimes necessary. Various plans have 

 been suggested for hastening the tanning process^, 

 such as forcing the liquor through the skins under 

 pressure, passing them between rollers in the 

 liquor, &c. ; but none of these has been extensively 

 adopted. 



After the leather is fully tanned, it is ' curried,' 

 or prepared for use. The currying varies very 

 much with the different kinds of leather and pur- 

 poses to which it is to be applied. It consists 

 essentially of smoothing the leather by paring, 

 scraping, and rubbing, and of rendering it supple 

 by rubbing into it oil and tallow, or the mixture of 

 fatty acids and soap which occurs as a waste pro- 

 duct in the process of ' shamoying.' 



Russia leather is tanned in the usual way, and 

 then rubbed on the flesh-side with birch-tar, and 

 dried and washed on tke hair-side with solution* 

 of alum. It is then rolled in various directions- 

 with a grooved roller, which gives it the peculiar 

 surface-marking, stained with infusion of Pernam- 

 buco wood, hammered and rolled, and finally 

 oiled and rubbed with flannel. 



347 



