matter or dye-stuff, which in some cases, as the Alizarine Blue,
Alizarine Yellow, etc., leads to a destructive effect, and, therefore,
the production of weak shades, so that it is not possible always to
use an oxidising mordant. When tartar, argol, oxalic acid, lactic
acids and other assistants of an organic nature are used, then a
different effect is obtained, the bichromate is completely decomposed,
and a deposit of chromium oxide formed on the wool. This does not
exert any action on the colouring matter, and hence this mordant is
known as the non-oxidising mordant. It may be pointed out that when
wool is mordanted with potassium or sodium bichromate and sulphuric
acid (oxidising mordant) it has a deep yellow colour, while when
mordanted with bichromate or other chrome salt, and the organic
assistants enumerated above (non-oxidising mordant), it has a green
colour, and one sign of a well-mordanted wool is when it has a good
bright tone free from yellowness.

Of the organic assistants tartar is undoubtedly the best in general
use, and, although slow in its action, leaves a good deposit of oxide
of chrome on the wool in a suitable condition to develop the best
results on dyeing. Argols are only an impure tartar. They can only be
used when dark shades are to be dyed. Oxalic acid does not work as
well as tartar, and there is not so much chrome oxide deposited on the
wool, while there is a slight tendency for a small proportion of this
to be in the form of chromic acid. Of late years lactic acid and (p. 117)
lignorosine have been added to the list of assistant mordants; both
these give excellent results, they lead to a more complete and more
uniform decomposition of the bichromate, and therefore the mordanting
baths are more completely exhausted, so that rather less bichromate is
required; the shades which are obtained are in general fuller and
brighter. Examples of the use of these assistants will be found among
the recipes given below.

With fluoride of chrome either oxalic acid or tartar is used, and a
deposit of chromium oxide is formed on the wool, the general effect
being the same as when bichromate of potash is used with oxalic acid
or tartar.

Alumina is applied either in the form of alum or of sulphate of
alumina, argol or tartar being used as the assistant, oxide of alumina
being deposited on the fibre.

When ferrous sulphate (copperas) is used then tartar is almost
invariably used as the assistant morda