The oldest group of coal-tar dyes are the basic dyes, of which Magenta,
Brilliant green, Chrysoidine, Bismarck brown, Auramine are typical
representatives. For a long time these dyes were only used for dyeing
wool and silk; for cotton, linen, and some other vegetable fibres they
have little or no affinity, and hence cannot dye them direct. However,
it was found out that if the cotton be prepared or mordanted (as it is
called) with tannic acid or with any substance containing that compound
they could be used for dyeing cotton.

The mordant used, tannic acid, has the property of combining with the
dyes of this group to form insoluble coloured tannates. Now tannic acid
has a certain amount of affinity for cotton, if the latter be immersed
in solution of tannic acid or any material containing it some of the
latter is taken up and more or less fixed by the cotton fibre. Tannic
acid is a vegetable product found in a large number of plants, and plant
products, such as sumac, myrabolams, divi-divi, galls, oak bark,
gambier, cutch, algarobilla, valonia, etc., which are commonly known as
tannins, or tannin matters, on account of their use in the conversion of
animal skins or hides into leather, which is done in the tanning
industry.

By itself the tannin-colour lake, which may be formed on the cotton
fibre by immersion first in a bath of tannin and then in a dye-bath, is
not fast to washing and soaping, but by taking advantage of the fact
with such metals as tin, iron, antimony, etc., it combines to form
insoluble tannates; the tannic acid can be fixed on the cotton by
immersion in a bath containing such fixing salts as tartar emetic, tin
crystals, copperas, antimony fluoride, and antimony oxalate. The dyeing
of cotton with the basic colours therefore resolves itself into three
operations:--

(1) Tanning with tannic acid or some tanning matters.
(2) Fixation with tartar emetic or other fixing agent.
(3) Dyeing with the required colour or mixture of colours.

(1) =The Tanning Operation.=--The practice of tanning or mordanting
cotton with tannin is variously carried on by dyers. Some steep the
cotton in the tannin bath over night, others immerse it from two to
three hours in a lukewarm bath, while some enter it in a boiling bath,
which is then allowed to cool down and the cotton is lifted out. The
last is perhaps the quickest method, and experiments have shown that it
is as good as any other method, if the quantity of tanni