With regard to the astringent extracts, such as sumac, myrabolam,
divi, valonia, quebracho, oak, etc., it is the aim of the
manufacturer, whenever such extracts are intended for the purposes of
dyeing and printing, to obtain the tannin in a form in which it is
best calculated to fix itself upon the fiber. The case is somewhat
different when the same extracts are required for tanning. For this
purpose it is necessary that the extract shall have considerable
permeating power, and that the tannin contained in it shall readily
yield leather of the desired texture, color, and permanency. Extracts
specially suited for this purpose are by no means always the most
suitable for the dyer, and _vice versa_.
A brief description of the processes by which the astringent extracts
may be tested with particular reference to their fitness for definite
purposes concluded the paper.
With regard to the question as to whether experimental dyeing with
bichromate of potash should be employed as a test even in works where
all the dyeing was done with other mordants, he was decidedly of
opinion that it should always be resorted to as one of the tests,
inasmuch as it was the only simple and expeditious method giving a
fair idea of the actual wood strength and money value of the extract.
The test should, in such cases, be supplemented by dyeing trials with
the mordants used at the works, and, if necessary, also by a chemical
analysis.
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