Cotton dyeing does not, as a general rule, afford a good means
of assaying extracts, as it is generally done under conditions which
do not admit of complete exhaustion of the dye bath, but it might
often with advantage be resorted to as an additional trial throwing
further light on the degree of oxidation or development of the
coloring matter. Printing trials are apt to give fallacious results
unless the proportion of mordant is carefully adjusted to the amount
of coloring matter present, and several trials with different
proportions would be necessary to prevent erroneous conclusions. For
the trials with bichromate of potash on wool I would recommend pieces
of cloth weighing about 150 grains, and the most suitable proportion
of bichromate of potash is 3 per cent. of the weight of the cloth. The
requisite number of pieces (equal to the number of samples to be
tested) should be thoroughly scoured and then heated in the bichromate
solution at or near the boiling point for not less than 11/2 hours,
after which they should be well washed and then dyed separately in the
solutions of equal weights of the extracts at the same temperature and
for the same length of time; 15 grains of extract is a suitable
quantity for a first trial under these conditions. These trials can
then be repeated with different relative proportions of extract in
order to ascertain what weight of a sample would give the same depth
of color as 15 grains of the standard example.
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