s as logwood, fustic and alizarine, require more attention. With
these, alumina, iron, and chromium mordants are used as chief mordants,
either alone or in combination with one another, and with other bodies.
The principal point is to obtain a good deposit of the mordant on the
cotton fibre, and this is by no means easy.

There are several methods by the use of which a deposit is formed of the
mordant, either in the form of metallic oxide (or, perhaps, hydroxide)
or of a basic salt. In some cases the cotton is passed through alternate
baths containing, on one hand, the mordanting salt, _e.g._, alum,
copperas, etc., and, on the other, a fixing agent, such as soda or
phosphate of soda. Or a mordanting salt may be used, containing some
volatile acid that on being subjected to a subsequent steaming is
decomposed. Both these methods will be briefly discussed.

* * * * *

_Methods of Mordanting._

The cotton is first steeped in a bath containing Turkey-red oil, and is
then dried. By this means there is formed on the fibre a deposit of
fatty acid, which is of great value in the subsequent dyeing operations
to produce bright and fast shades. After the oiling comes a bath of alum
or alumina sulphate, either used as bought, or made basic by the
addition of soda. The result is to bring about on the fibre a
combination of the fatty acid with the alumina. Following on the alum
bath comes a bath containing soda or phosphate of soda, which brings
about a better fixation of the alumina.

These operations may be repeated several times, especially when a full
shade having a good degree of fastness is desired, as, for instance,
Turkey-reds from Alizarine. This method of mordanting is subject to
considerable variations as regards the order in which the various
operations are carried out, the strength of the baths, and their
composition. A great deal depends upon the ultimate result desired to be
obtained, and the price to be paid for the work.

Iron is much easier to fix on cotton than is either alumina or chrome.
It is usually sufficient to pass the cotton through a bath of either
copperas or iron liquor, hang up to dry or age, and then pass into a
bath of lime, soda or even phosphate of soda. The other mordants require
two passages to ensure proper deposition of the mordant on the fibre.

Following on the mordanting operations comes the dyeing, which is
carried out in the following manner. The bath i