16 Indian Forest Records. [Yol. III. 
out here that the colour of tanning materials contained in the mangrove 
bark is extremely sensitive to alkalies. 
The alkalis change the extract to deep dark red and the acids turn 
it into light yellow. 
Jf, is therefore essential that distilled water should he used in the 
treatment of all harks, and especially for the mangrove hark, which is 
particularly sensitive to alkales. 
The addition of mineral or even of organic acids to neutralise the 
alkali contained in water is also very injurious, as the presence of these 
acids has the effect of precipitating most of the tannin out in the 
mangrove bark extract in the form of insoluble “ reds/’ 
Considering the sensitiveness of the mangrove tannin, the writer 
recommends that the distilled water to be used in its extraction, should be 
previously made acid, not by the addition of any organic or mineral 
acids, but by passing it over some light coloured tannin contained in 
various available leaves and fruits. 
(3) Iron contact. —Great care is to be exercised in avoiding the contact 
of tan liquors with iron in any shape, as this causes the blackening 
of the extract. One of the chief causes of the blackening of the 
Burma tannin extract has been the neglect of this precaution. 
(4) The scales on the bark of mangrove are very hard and have a black 
fracture. They seem to contain the dark coloured end-oxidation pro¬ 
ducts of mangrove tannin. It was also found that in the hollow spaces 
between the scales and the inner bark, a good deal of ferruginous dust 
is deposited through various agencies. This dust and the black mate¬ 
rial contained in the scales themselves, if permitted to enter the leaching 
vats, will exert a very injurious effect on the colour of the extract. 
The mangrove hark, should therefore he peeled and all earth brushed 
off its surface hy means of copper wire brushes using a minimum 
quantity of water. The scales are of course to be rejected, and the bark 
thus cleaned should be immediately used. 
Any neglect of these apparently small details results in lowering the 
quality of the extract to a great extent. 
(5) The tan liquors should not be allowed to be exposed for any 
length of time to atmospheric action, ds soon as they are sufficiently 
concentrated they should he, as described in Chapter I, passed through 
refrigerators and then through decanting centrifugals to the vacuum pans . 
233 
