Indian Forest Records. 
10 
[VOL. III. 
previously noticed and the use of tin, zinc and aluminium in the extrac¬ 
tion was patented as far back as 190*2. 
Another patent 1 consists in subjecting the tannin extracts during 
preparation to the action of electricity with alternative currents, using 
electrodes of aluminium, tin and zinc. 
Another joatent, 2 on the principle of using nascent hydroxides and 
hydrogen, was taken out in October 1909. In this patent tannin extracts, 
more particularly Mangrove tannin extracts, are decolourised by means 
of nascent hydroxides of metals produced by electrolysis in a bath having 
different metals as anode and kathode, solutions of the salts of these 
metals being also added to the bath. The precipitated hydroxides carry 
down the colouring matter in the form of a lake while nascent hydrogen 
has also a clarifying effect; aluminium and zinc are most suitable metals 
for the electrodes. 
For the treatment of Mangrove bark, malletto hark and Quebracho 
wood, the use of “ activated aluminium ” 3 has been recommended. “ The 
activated aluminium ” (prepared by treating it with caustic alkali solu¬ 
tion and rinsing it with water and then treating it with mercury chloride 
and again washing) is well mixed with tannin extracts, which are agi¬ 
tated and allowed to cool and settle, after which they are concentrated. 
The leather made with the extracts thus treated becomes but little darker 
under the action of light, and is free from the red shade usually charac¬ 
teristic of leather prepared by using untreated extracts. 
It will be gathered from the above that a great variety of processes 
has been devised and various patents have been taken out for the 
decolourising of the tannin extracts. It seems that almost the whole field 
of original research on the subject has been covered. But the writer 
has employed a method for the decolourisation of dark and red coloured 
tannin extracts which, as far as he is aware, has not yet been adopted. 
This method consists in the use, in small quantities, of freshly precipitated 
nickel hydroxide for the fractional precipitation of the tan liquors, 
which takes up most of the colouring matter and impurities and settles 
down as a precipitate, from which it can be recovered and used over and 
over again. He hopes to revert to the subject in a future publication, 
1 Jour. Soc. Chem. Ind„ Vol. XXI, 1902, p. 1462. 
2 Loc. cit., Vol. XXIX, 1910, p. 365. 
8 Loc. cit., Vol. XXVIII, 1909, p. 1321. 
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