A colouring matter which requires, as some
do, to be applied in an acid bath, ought not to be applied
simultaneously with one that dyes best in a neutral bath. Numerous
descriptions of methods of using coal-tar dyestuffs in hat-dyeing are
available in different volumes of the _Journal of the Society of
Chemical Industry_, and also tables for the detection of such dyestuffs
on the fibre.
Now I will mention a process for dyeing felt a deep dead black with a
coal-tar black dye which alone would not give a deep pure black, but one
with a bluish-purple shade. To neutralise this purple effect, a small
quantity of a yellow dyestuff and a trifle of indigotin are added. A
deep black is thus produced, faster to light than logwood black it is
stated, and one that goes on the fibre with the greatest ease. But I
have referred to the use of small quantities of differently coloured
dyes for the purpose of neutralising or destroying certain shades in the
predominating colour. Now I am conscious that this matter is one that is
wrapped in complete mystery, and far from the true ken of many of our
dyers; but the rational treatment of such questions possesses such vast
advantages, and pre-supposes a certain knowledge of the theory of
colour, of application and advantage so equally important, that I am
persuaded I should not close this course wisely without saying a few
words on that subject, namely, the optical properties of colours.
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