sulphate of potash and chromic acid. The question of the proper
exhaustion of bichromate baths is an important economic one. Now we must
remember that this chromic acid (CrO_{3}) oxidises our wool or fur, and
must oxidise it before it can of itself act as a mordant by being
reduced in the process to hydrated chromic oxide, Cr_{2}O_{3} + 3
H_{2}O. [2 CrO_{3} (chromic acid) = Cr_{2}O_{3} (chromic oxide) + O_{3}
(oxygen).] It is this hydrated chromic oxide in the fibre that yields
with the Hæmatein of the logwood your logwood black dye. Mr. Jarmain
finds that it is not safe to use more than 3 per cent. (of the weight of
the wool) of bichromate; if 4 per cent. be used, the colour becomes
impaired, whilst if 12 per cent. be employed, the wool cannot be dyed at
all with logwood, the phenomenon known as "over-chroming" being the
result of such excessive treatment. I think there is no doubt, as
Professor Hummel says, that the colouring matter is oxidised and
destroyed in such over-chroming, but I also think that there can be no
doubt that the wool itself is also greatly injured and incapacitated for
taking up colour. Now the use of certain coal-tar black dyes in place of
logwood obviates this use of bichrome, and thus the heavy stress on the
fibre in mordanting with it. It also effects economy in avoiding the use
of bichrome, as well as of copper salts; but even thus, of course, other
problems have to be solved before it can be finally decided which is
best.




LECTURE IX

DYESTUFFS AND COLOURS