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
_Classification._--In classifying the different dyestuffs and colouring
matters it is, of course, necessary to consider first the properties of
those colouring matters generally, and secondly the particular reason
for making such classification.
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