TANNING 231 



S4. TANNING. 



The process by which hides are converted into leather is known as 

 tanning. The operation may be divided into three stages : 



I. The unhairing of the hide. 

 II. The expansion of the hide. 

 III. The introduction of the tannin. 



I. With regard to the unhairing, not only must the hairs be removed, 

 but also the two superficial layers of the hide, known respectively as 

 the epidermis and the mucous membrane. Of several methods in use, 

 the best known are the sweating and the liming or slackening pro- 

 cesses. When hides are kept damp putrefaction sets in, and the 

 sweating process consists in placing the hides at a moderate temperature 

 in a chamber saturated with moisture. They are allowed to putrefy 

 just sufficient to enable the hairs and the two superficial layers to be 

 easily scraped off with a knife. The putrefaction is entirely accom- 

 plished by bacteria, which are always present both on the hides and in 

 the surrounding atmosphere. They attack the hides just as they do all 

 other moist organic substances, which are left exposed to their action. 

 In the liming process, the hides are soaked in baths of milk of lime, at 

 first in weak, and later in progressively stronger solutions, until finally 

 they become saturated with this substance. This process is of course a 

 purely chemical operation ; but it has been found that some bacteria, 

 which are present in the baths, retard the introduction of the milk of 

 lime into the hides, so that even in this process the fermentations set 

 up by bacteria cannot be ignored. Of the bacteria that are present in 

 the sweating process, we know next to nothing ; but as we are dealing 

 with a putrefactive process, the causal agents will have to be sought for 

 among the saprophytic bacteria. 



II. The second stage is the preparation of the hide for the introduc- 

 tion of the tannin. In the case of those hides that have been unhaired 

 by the liming process, the lime with which they are saturated must be 

 removed. This is done by soaking the hides in what is known as a 

 "pickle" or "bate," which consists of a mixture of barley, husks, bran, 

 excrement of dogs, fowls, etc., in which fermentation has already set in. 

 Lactic acid is developed as a result of this fermentation, and this it is 

 which, acting on the insoluble lime, changes it into calcium lactate. 

 Now this latter substance is soluble in water, and can therefore be 

 easily removed from the hides. During this process, however, a change 



