ith the metallic
oxide or mordant that they form a colour and dye a fibre. On the (p. 115)
other hand the azo mordants are in general colouring matters, and can
be used to dye wool without the aid of a mordant, the latter only
serving to make the colour faster to light, acids, milling, etc., and
it often has no material effect on the shade or tone of colour being
dyed. Alizarine Yellow G G, Gambine Yellow, Anthracene Yellow, Chrome
Violet, are examples of such dyes.

There are, however, some dyes (such as the Chromotrops, Azofuchsine,
Anthracene Acid Browns, etc.) on which the mordant has a marked
effect.

The methods adopted in practice for the application of this class of
dyes are many and varied. The mordants used are alum, alumina
sulphate, acetate of chrome, chrome alum, fluoride of chrome, ferrous
sulphate and tin chloride, while, in addition, along with these true
mordanting materials, assistant mordants are used, such as argol,
tartar, tartaric acid, lactic acid, lignorosine, oxalic acid and
sulphuric acid.

The mordanting may be done either before or after the dyeing, the
first plan being that commonly adopted with the phenolic colours,
while the second method may be used and is the best to use with
azo-mordant dyes. Sometimes the mordanting and dyeing may be done in
one bath, but this method is one which leads to a loss of colouring
matter and often to the production of colours which are loose to
rubbing, and cannot, therefore, be recommended.

#Mordanting.#--This operation is carried out in the same way in all
cases. The goods are entered into the bath at a temperature of about
150 deg. F. The heat is raised to the boil, and is then maintained for
one and a half hours, after which the mordanted wool is lifted and well
rinsed, when it is ready for the dye-bath. As mordanting materials
bichromate of potash and fluoride of chrome are chiefly used when
chrome mordants are required, sometimes chrome alum. With these (p. 116)
are used sulphuric acid, oxalic acid, cream of tartar or argol, lactic
acid, etc.

Which of these are used depends entirely on the results which are to
be got and the dye-stuff to be used, more particularly is this the
case when bichromate of potash is the mordanting material. When
sulphuric acid is used as the assistant along with the bichrome, then
there is formed on the wool fibre a deposit of chromic acid and
chromium oxide, and this exerts an oxidising effect on the colouring