Oct. 1, 1864.] THE TECHNOLOGIST. 



ON CHEMISTRY APPLIED TO THE ARTS. 103 



which have lost the tannic acid, hut which contain more or less of gallic 

 acid, for not only do all tanning matters contain gallic acid, hut its pro- 

 portion is greatly increased during the operation of tanning, hy a process 

 of fermentation which goes on during that operation, and which converts 

 tannic acid into gallic acid and a peculiar sugar. 



The Tanning of Hides. — The old process of tanning consisted in 

 placing layers of wet tan and of hides alternately, and after two or three' 

 months removing the whole from the pit and replacing the old hy fresh 

 tan. These operations were repeated until the hides were tanned, which 

 took from eighteen months to two years, owing to the difficulty of the 

 tannic acid reaching the interior of the hide. Of late years the process 

 of tanning has been greatly shortened hy treating the hark with water, 

 and steeping the hides in the liquor, first weak and afterwards strong. 

 By this means good leather can he obtained in the space of eight or ten 

 months. More rapid tanning, but probably giving inferior leather, is 

 effected by employing, in conjunction with, or as a substitute for, bark, 

 a decoction of divi-divi, valonia, myrobalan, catechu, terra japonica, or 

 gambier, &c. Many efforts have been made of late years to apply the 

 laws of hydraulics, as well as several physical and physiological princi- 

 ples discovered by eminent philosophers, with the view of shortening 

 the period of tanning ; but as I believe that none of them have received 

 the general sanction of the trade, I shall confine myself to giving you an 

 idea of the most successful ones. The first attempt to accelerate the 

 process of tanning consisted in forcing the tanning fluids into the sub- 

 stance of the hide by means of hydraulic pressure. Mr. Spilbury, in 

 1831, employed a process which consisted in packing the hides into 

 sacks, and plunging them into a tanning liquor, and as the fluid perco- 

 lated through the skin into the interior of the bag the air was allowed 

 to escape. By this means a certain amount of time was saved in bring- 

 ing the tanning liquor in contact with the various parts of the skin. 

 Mr. Drake soon followed in the same direction, his plan being to sew 

 hides together, forming bags, which he filled with a solution of tan ; and 

 to prevent the distension of the skins by the pressure of the liquid 

 within, they were supported in suitable frames ; as the pores became 

 gradually filled with tannin, artificial heat was applied to increase the 

 percolation of the fluid. Messrs. Chaplin and Cox's process is also very 

 similar to the above, the difference being that the tanning fluid is placed 

 in a reservoir, and allowed to flow into the bag of hides through a 

 pipe, the fluid being thus employed at pressures varying according to 

 the height of the reservoir. The bag of hides is at the same time 

 plunged into a solution of tannin to prevent excessive distension. 

 Messrs. Knowles and Dewsbury have recourse to another principle to 

 compel the percolation of the tanning liquor through the hide. To effect 

 their purpose they cover vessels with hides, so as to form air-tight en- 

 closures, and, having placed the tanning fluid they employ on the hides, 

 the vessels are exhausted of air, and atmospheric pressure then forces 



