are faster to rubbing and milling.

The mordanting bath may be kept standing and used again for fresh lots
of wool, in which case it is only necessary to add 2-1/2 lb. of
bichromate of potash and 1 lb. sulphuric acid to the bath for each
additional lot of wool that is being dealt with. Old mordant baths
work rather better than new ones, but the use cannot be prolonged
indefinitely, there comes a time when the bath gets too dirty to use
and then it must be thrown away.

During the operation the bichromate of potash becomes more or less
decomposed and there is formed on the wool fibre a deposit of chromic
acid and chromic oxide, this deposit forms the mordant that in the
subsequent dye-bath combines with and fixes the colouring matter, the
haematoxylin of the logwood, and develops the black on the wool.

In place of sulphuric acid, hydrochloric acid can be used with (p. 085)
some advantage as regards the proportion of bichromate decomposed, and
therefore an increase in the amount of chromium oxide deposited on the
wool.

This gives a deep blue black, somewhat wanting in bloom. The following
recipe gives a much bloomier black, but is rather more expensive to
dye.

_Chrome Logwood Black_.--Mordant by boiling in a bath containing 3 lb.
bichromate of potash and 7 lb. tartar. Dye and otherwise treat as in
the last recipe; 4 lb. of tartaric acid used in place of the tartar,
gives rather brighter and bloomier shades. The use of so-called tartar
substitutes is not to be recommended, they give no better results than
does sulphuric acid and are much dearer to use.

A somewhat greener shade of black than is yielded by either of the
above two recipes is the following:--

_Chrome Logwood Black_.--Mordant the wool in a bath containing 4 lb.
oxalic acid and 3 lb. bichromate of potash, afterwards dyeing as in
the first recipe.

All the above recipes give blacks of a bluish tone, which on the whole
have a good bloomy and solid appearance. Often what is called a jet
black is wanted, this can be obtained by following the recipe given
below.

_Chrome Logwood Jet Black_.--Mordant the wool by any of the methods
given above. The dyeing is done in a bath made from 40 lb. logwood and
5 lb. fustic, working as described in the first recipe. Using these
properties a good jet black is obtained, which is quite satisfactory
on the score of solidity and fastness. It is not advisable to exceed
the quantity of fustic here given, or otherw