CHAPTER XXVII. 



THE PART PLAYED BY BACTERIA IN TANNING. 



157. The Fermentation of the Plumping- Soak. 



THE preservation of animal hides by simple drying is feasible only when they 

 are to be stored for some time an<l brought on the market in a dry state. To 

 accelerate this drying, the fleshy side is in many instances rubbed over with 

 arsenic or with common salt. A few investigations into the utility of the last- 

 named substance were made by F. HAENLEIN (I.). The brittle solidity and 

 fragility acquired by the hides in drying prevent their utilisation for practical 

 purposes, and the more so because putrefaction sets in directly they are wetted. 

 To remedy this defect is the object of tanning, i.e. the conversion of hides into 

 leather. Bacterial activity here comes into play at the outset of this operation, 

 micro-organisms being necessary even in the preliminary treatment known as 

 unhairing. As is well known, leather consists neither of the outer skin (epi- 

 dermis) nor of the next layer of mucous cells (mucous membrane), but of the 

 third layer (composed of closely interwoven cells), which, on this account, has 

 been termed the leather (or true) skin (corium). Several methods of exposing 

 this layer and simultaneously removing the hair exist, one of them being the 

 so-called sweating process. In this operation the cleansed, soft hides are 

 maintained at a moderate warmth in a chamber saturated with moisture the 

 sweating pit. Putrefaction quickly sets in, but is only allowed to proceed so 

 far as to loosen the hairs and enable them to be scraped off, along with the 

 epidermis and mucous membrane. A second method of unhairing is by liming 

 or slackening, the hides being repeatedly steeped in an initially weak, but 

 progressively stronger, milk of lime. In the absence of investigation on the 

 subject, it is still uncertain whether an active part is played in this operation 

 as is undoubtedly the case in the sweating process by bacteria, certain species 

 of which, capable of resisting the action of the milk of lime, have been discovered 

 in the liquor by J. VON SCHRODER and W. SCHMITZ-DUMONT (I.). The result of 

 both operations is substantially the same, viz., the hair, epidermis, and mucous 

 membrane are loosened, and can then be removed. 



In one respect, however, limed hides differ from those slackened by sweating. 

 In the former case, calcium carbonate has been deposited in the intercellular 

 spaces in the hide, which is thus rendered somewhat brittle and less pervious to 

 the tanning liquor. This carbonate precipitate is removed by steeping the hides 

 in a pickle or " bate " consisting principally of a mixture of bran, barley, groats, 

 and the excrement of various animals (dogs, fowls, pigeons, <fec.) in a state of acid 

 fermentation. The chief, if not the sole raison d'etre of this process, which has 

 been gradually developed through tentative experiments alone, has only been 

 brought to light in recent years. The first progressive step consisted in the 

 discovery, by AUG. FREUND (I.) in 1871, that lactic acid is the chief product of 

 the spontaneous acid fermentation of wheat bran. This explained the favourable 

 result of the bating of lime-slackened hides, the calcium carbonate being con- 

 verted into the soluble calcium lactate, and thus washed out of the hides. 



Pickling, however, effects a second and remarkable alteration in the hide, 



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