PRESERVATION OF FISH NETS. 13 



FIXING ()1{ MOHDANTTNO OF TAXNFI) NFTS. 



'JMie next step, thoivfore, is to cm use the preser\ in«j^ tannin to stay 

 in the net, and to do tliis it is necessary to know soniethinfji^ of the 

 chemistry of the tannins. It has already been shown that tannins 

 combine with <rehitin to form a permanently insoluble substance, 

 this combination beiii"!: the essential principle in converting hides 

 into leather. (lelatin exists naturally in hides, but there is nothing 

 naturally occurring in cotton corresponding to gelatin with which 

 the tannin can iovm an insoluble compound. 



Tannin is readily oxidized ; that is to say, it readily takes up and 

 combines with the oxygen of the air, and when so combined is in- 

 soluble and much darker in color. AVhy, then, does not the tannin 

 oxidize and stay in nets? In fact it does, to a certain extent, as any 

 fisherman who has used " barked " nets knows the nets never regain 

 their original whiteness. But the oxidation takes time, and in prac- 

 tice the net is put into the w^ater before much of the tannin is 

 oxidized, whereupon the unoxidized tannin promptly washes out. 

 To store the net for a time after barking would do little good, for 

 in a dry condition the oxidation proceeds vevy slowdy or not at all. 

 Repeated wetting and drj'ing was credited by Bull in his comments 

 on the paper by Aase referred to above, with the eventual thorough 

 impregnation of nets by the old-time, cold-l)arking method. It is 

 probaiile that by holding the nets w^et for a long time after barking 

 and allowing free access of air the tan might be fixed permanently 

 in the fibers. 



There is, however, no need to do this, for once the chemistrj^ of 

 the process is understood the same end may be achieved by quicker 

 chemical means. There are a number of chemicals which contain 

 oxygen in large quantities, and which yield up this oxygen easily. 

 One such chemical is potassium bichromate. If a solution of potas- 

 sium ])ichromate be added to a solution of catechu, a dark-colored, 

 insoluble compound is formed — the combination of tannin with 

 oxygen, the oxygen coming from the potassium bichromate. Ad- 

 vantage is taken of this principle in dyeing; catechu or some other 

 natural wood dj^e is applied to the fibers and then darkened and 

 fixed by a solution of potassium bichromate. This process is used 

 primarily in the leather and textile industries for the coloring it 

 gives, but no modification is needed to use the method for preserving 

 nets. 



The bichromate acts not only on the tannins that are soluble in 

 cold water, but upon the catechin that is soluble in hot water. 



The oxidation of tannins is greatly accelerated by alkalis — ^lye, 

 soda, potash, etc. — but these all have a very objectionable character- 

 istic in that they cause to dissolve in cold water the tanning sub- 

 stance which in its natural state is soluble only in hot water. These 

 alkalis are therefore not used. 



cunxixoham's experiments. 



The very thorough work done by Bull in Norway and Cunning- 

 ham in England has made it possible to preserve nets at much less 



