TANNING 233 



for from eight to ten weeks, after which they are transferred to another 

 pit, where they remain for another period of from eight to ten weeks. 

 This process is repeated from three to five times till the hides are 

 impregnated with tan. The thinner hides not intended for sole leather 

 are treated in essentially the same way, except that they are placed in 

 what is called " bark-liquor," an extract of tanning substances, instead 

 of being placed in the tan-pit. 



The changes which take place at this phase are not essentially of a 

 fermentative nature, for the actual tanning is purely a chemico-physical 

 operation. It is, however, important to note that the quality of the 

 leather is controlled by the microorganisms that are found in the 

 tan-pit or bark-liquor. As the tanning proceeds the liquor gets sour, 

 owing to the development in it of an acid, through the agency of these 

 microorganisms. Now the tanner watches the development of this 

 souring very narrowly, because if absent or improperly regulated the 

 leather becomes inferior in quality, and the reason for the introduction 

 of spent tan from the previous tanning into the tan-pit lies in this, that 

 he knows by experience that this spent tan facilitates the souring. The 

 spent tan, in fact, contains multitudes of lactic acid bacteria, and it is 

 the multiplication of these, and the consequent formation of lactic acid, 

 that causes the sourness of the tan-pit. One of these has been isolated. 

 It is called Bacillus corticalis, and was found in bark-liquor. The 

 individuals are short rods — O'7-l'O /^ broad and 15-2 -0 /^ long. The 

 species thrives best at 30°-40° C. and stops multiplying below 5° C. 

 It ferments sugars {e.g. dextrose, saccharose, and lactose), producing 

 lactic acid and a considerable quantity of gas : it does not act on 

 tannin itself. This species cannot be the only one causing sourness in 

 the tan-pit, but the amount of investigation on this subject has hitherto 

 been very meagre. It must also be borne in mind that there probably 

 are other species in the tan-pit, with properties inimical to the tanner, 

 in which case it would be highly desirable if the tanner could work 

 with starters and regulate the souring of the tan-pit in the same way as 

 the modern butter- maker regulates the souring of milk. 



§5. EETTING. 



This is the term applied to the steeping of flax, hemp, etc., for the 

 purpose of loosening the fibre from the other portions of the plant, by 

 the softening of the non-essential parts. The fibre alone by this 

 process escapes the softening, and can therefore be easily separated. 



