B4 Observations on the Processes of Tannlug. 



Skins, after being depilated and cleansed, are in this 

 country generally subjected to other processes ot" prepara- 

 tion before they are nnpregnatcd with the tanning prin- 

 ciple. 



The large and thick hides w hich have undergone incipi- 

 ent putrefaction are introduced for a short time into a 

 strong infusion of bark, when they are said by manufac- 

 turers to be cn/oi/rrd ; and after this they are acted upon 

 bv water imprconated with a little sulphuric acid, or ace- 

 tous acid formed bv the fermentation of barley or rye. In 

 this Cttse thev become harder and denser than before, and 

 fitted, after being tanned, for the purpose of forming the 

 stouter kinds of sole leather. 'J'he acids are capable of com- 

 bining both with skin and with tannin ; and it would ap- 

 pear that, in this process, a triple combination must be 

 effected on the surface 61' the skin, though from theory 

 one should be disposed to conclude, that the interior part 

 could be little modified in consequence of the colourmg, 

 and the action of the acids. 



The light skins of cows, the skins of calves, and all 

 smaller skins, arc treated in a very different way, being 

 submitted for some days bo the action of a lixivium, called 

 the graincr, made by the infusion of pigeons dung in water. 

 After this operation thev are found thinner and softer than 

 before, and more proper for producing flexible leather. 

 When the infusion of pigeons dung is examined, after being 

 freshly made, it is found to c(mtain a little carbonate of 

 ammonia ; but in a short time it undergoes fermentation, 

 when carbonic acid and hydrocarbonate are, given out by it, 

 and a small quantity of acetous acid formed. The alkali ]n 

 the grainer may probably have some action upon the skin;, 

 it may be supposed to free it from any oils or calcareous 

 soap that lemained adhering to it : but the great effect pro- 

 bably depends upt)n the complicated process of fermenta- 

 tion, during which the skin loses its e!asti(ftty, and becomes 

 soft ; and u. i§ fbimd by tanners, that dung which has un- 

 dergone fermentation is wholly unfit for their use. 



i have tried icvcral experiments on dif^'crRnt substances, 

 as substimtes for the p)^:;eons dung used in the grainer, but 

 without gaining successful results. Very weak solutions of 

 carbi)nale of potash and carbonate of aj>nnonia seemed to 

 soften considerably small pieces of skin that had been depi- 

 lated by lime; but when they were tried by Mr. Purkis, ii^ 

 the processes of manufacture, the cfl'ec^s were less distinct. 

 In the western counties of England the excrement of dogs 



