-^2 THE SCIENTIFIC ASPECTS OF TANNING. 



The butt at this stage of the process is like a sponge, ready to 

 imbibe any quantity of Tannic which may be presented to it. 

 Various expedients have been adopted to help on the process, 

 some of them showing no little ingenuity and contrivance : some 

 hang the butts on revolving wheels, presenting continually fresh 

 surfaces to the action of the liquor 3 whilst others place them in 

 tumblers, like large casks, where, excluded from the air, they 

 rapidly abstract all the Tannic acid in a few revolutions ; but the 

 majority simply suspend the butts on poles in deep pits, where 

 they can be more readily moved. 



In olden time the hides were piled one on the other in the pits, 

 with a layer of finely ground bark between each hide ; when the 

 pit was full water was added, and this process was repeated unti^ 

 the hide was tanned. This made good tough leather, but the 

 time taken in the process was often 2 years. Now separate tanks 

 or pits are used, in which the strong liquors are brewed, and the 

 clear solution presented to the skin. The old system would give 

 liquors which nominally registered 3 to 25 degrees by the hydro- 

 meter, the modern plan giving often liquors from 12 to 70 degrees, 

 the process of tanning heavy butts being now shortened from 8 

 to 12 months. As the leather progresses in the tanyard Ellagic 

 acid begins to settle, — at first, not unlike the healthy bloom of the 

 peach, which afterwards gives a brownish yellow deposit, com- 

 pletely covering the butt, and is even found in the pores of the 

 skin. Light and heat both promote the formation of this bloom, 

 so that liquors exposed to the sunlight rapidly become turbid with 

 the quantity. Whilst speaking of the liquors it would be well to 

 notice another condition in which they are sometimes found, 

 which we know as "Ropey." They become very thick, almost 

 coagulated, and very sour to the taste. We believe it to contain a 

 large amount of Ellagic acid and Vinegar. Great negligence and 

 bad management are the cause. 



Our remarks hitherto have especially had reference to the heavy 

 trade, or the tanning of sole leather. Another branch, which is 

 tnown as the light trade, having to do especially with the treat- 



