e dyed on to cotton with
a metallic mordant is that to which the term "mordant dyes" is now
given. This includes such dyes as logwood, fustic, madder, alizarine,
and all the dyes derived from anthracene. Many of these are not really
dyes, that is, they will not of themselves produce or develop a colour
on to any fibre when used alone; it is only when they combine with the
mordant oxide which is used, and then the colour varies with the
mordant. Thus, for instance, logwood with iron produces a bluish black;
with chrome, a blue; with alumina, a reddish blue. Alizarine with iron
produces a dark violet; with alumina, a scarlet; with chrome, a red;
with tin, a bright scarlet. Fustic gives with tin and alumina, bright
yellows; with chrome, a dark yellow; with iron, an olive, and so on with
other members of this group, of which more will be said later on.

_Dyeing with Eosines._

At one time a fairly large quantity of cotton was dyed with the eosines,
owing to the brightness of the shades given by them; but the
introduction of such direct dyes as the Erikas, Ceranines, etc., has
thrown the eosines out of use.

The method adopted for the production of eosine pinks and scarlets on
cotton involves three operations: (1) impregnating the cloth with sodium
stannate; (2) fixing oxide of tin by a bath of weak sulphuric acid; and
(3) dyeing with the eosine.

=(1) Preparing with Sodium Stannate.=--A bath of 8 deg. Tw. is prepared, and
the cotton is allowed to steep in this bath until it becomes thoroughly
impregnated, after which it is taken out and wrung.

=(2) Fixing the Tin Oxide.=--A bath of sulphuric acid of 2 deg. to 4 deg. Tw. is
prepared, and the cotton is sent through it, after which it is washed
well with water, when it is ready for dyeing.

Stannate of soda is easily decomposed by acids; even the carbonic acid
present in the air will bring about this change. The tin contained in
the stannate is deposited on the cotton in the form of stannic oxide,
or, more strictly, stannic acid. As this is somewhat soluble in acids,
it is important that the sulphuric acid bath be not too strong, or there
will be a tendency for the tin oxide to be dissolved off the cotton, and
then but weak shades will be obtained in the final operation of dyeing.
Further, owing to the decomposition of the stannate by exposure to the
air, it is important that the substance should be used while fresh, and
that only fresh baths should be used.

=(3) D