preservattotnt of fish nets. 9 



loss represents, of coui'se. the substanoe taken u\) by the gelatin or 

 tannin. 



Since to the leather tanner the chief item of interest is the amount 

 of substance actually taken up by hides, <2:elatin, as a measure of the 

 tannin*; power of materials, has been supplanted by hide powder, 

 which is clean oxhide dried and ground in a suitable mill. The 

 liquor is drawn through this, and the difference in the weight of 

 dried residue is noted and recorded as tannin. Account is also 

 taken of the difference between weights of the liquor and dried resi- 

 due, this difference representing water. There is also given the per- 

 centage of matter that does not dissolve in cold water, but wdiich was 

 filtered off of the liquor. Thus an analysis of a tanning material 

 intended to represent its value for leather tanning gives four figures, 

 of -which the following is a typical example (Cunningham, 1902) 

 of Burmah cutch or catechu : 



Per cent. 



Tannins 46. G 



Nontannins 20. 



Insoluble 11. o 



Water 21. 9 



Total 100. 



There are some very significant differences between the treat- 

 ment given to hides in making leather and that given to nets to 

 preserve them and in the behavior of the treated materials. Hides, 

 once they have taken up tannins, never let go, even though they 

 are boiled or soaked in water; nets, however, lose their tanning 

 materials rapidly in water, so that they must be tanned repeatedly 

 if the method used for treating them is the same as that used for 

 treating leather. It is thus obvious that the tannin does not com- 

 bine in nets with any gelatin or other similar substance, but rather 

 is merely deposited in the fibers. There is another important dif- 

 ference : Hides are tanned in a cold decoction of tan bark or ex- 

 tract, while nets are usually treated in hot liquors. 



When catechu, for example, is dissolved in cold water, there 

 always remains a residue, which appears in the analysis given 

 above, as " insoluble." If the solution is heated to boiling, nearly 

 all of this apparently insoluble material dissolves, only some pieces 

 of leaves, sticks, and sand remaining. If the liquor is cooled off, 

 the insoluble material again separates from solution. There is 

 thus a comparativel}^ large amount of substance insoluble in 

 cold water but soluble in hot water. Upon examination, this sub- 

 stance, catechin, is found to be an active tanning material, and for 

 the purpose of net preserving is better than that which is soluble 

 in cold water, because it Avill not dissolve out in use. Cunning- 

 ham tried to fix the tannin in nets by first impregnating them with 

 glue (which is crude gelatin) and then treating them with catechu. 

 The effort resulted in failure, apparently because the glue did not 

 penetrate the fibers. 



The coloring matters in l)arks and extracts are independent of 

 the tannins and have no tanning power. Pure tannin (gallotannic 

 acid from nut galls) is a light grayish yellow flaky powder, easily 

 soluble in water. 



