ciple and mordant may be brought about in dyeing wool
with these bodies, we may either mordant the wool first, and then
apply the dye-stuff, or we may impregnate the wool with the dye-stuff
first, and then fix or develop the colour afterwards, or, lastly, we
may carry on both operations in one process. Each of these methods
will now be discussed, and their relative advantages pointed out.

The mordanting method is one of the most generally useful. It consists
in first causing a combination of the metal with the wool fibre. (p. 071)
This is carried out by boiling the wool in a solution of the metal,
such as bichromate of potash, chrome alum or chrome fluoride when
chrome is to be used as a mordant, with alum or sulphate of alumina
when alumina is required to be deposited on the fibre, and with
copperas when iron is to be the mordant. It is best to add a little
oxalic acid, cream of tartar, or tartaric acid to the mordanting bath,
which addition helps in the decomposition of the metallic salt by the
wool fibre, and the deposition of the metallic oxide on the wool. With
bichromate of potash, sulphuric acid is often used, much depending
upon the character of the mordant required. Some dye-stuffs, such as
logwood for blacks, work best when the wool is mordanted with chromic
acid, which is effected when sulphuric acid is the assistant mordant.
Other dye-stuffs, such as fustic, Persian berries and Alizarine
yellow, are best dyed on a basic chrome mordant, which is effected
when tartar or oxalic acid is the assistant mordant used, or when some
other form of chrome compound than bichrome is employed.

The actual mordanting is done by boiling the wool in a bath of the
mordant, the quantity of which should be varied according to the
particular mordant that is being employed and to the quantity of
dye-stuffs which is to be used. It is obvious that for a fixing deep
shade of, say, Alizarine on the wool, a larger quantity of mordant
will be required than to fix a pale shade; sometimes this point is
overlooked and the same amount of mordant employed for pale or deep
shades. The best plan of carrying out the mordanting is to enter the
wool in the cold bath or at a hand heat, and then raise to the boil
and continue the boiling for one hour; of course the goods should be
kept turned over during the process to facilitate the even mordanting
of the wool. A great deal of the success of dyeing with the dye-stuffs
now under consideration d