important as alum. The temperature
of the mordanting bath must be raised very gradually to boiling point
or the wool will dye unevenly. A general method of dealing with
copperas is to boil the wool first in a decoction of the colouring
matter and then add the mordant to the same bath in a proportion of 5
to 8 per cent of the weight of the wool, and continue boiling for half
an hour or so longer. With some dyes a separate bath is needed, such
as with Camwood or Catechu. Great care is needed in the using of
copperas, as, unless it is thoroughly dissolved and mixed with the
water before the wool is entered, it is apt to stain the wool. It also
hardens wool if used in excess or if boiled too long. A separate bath
should always be kept for dyes or mordants containing iron. The least
trace of it will dull colours and it will spoil the brilliancy of
reds, yellows and oranges.

Copperas is mostly used for the fixing of wool colours (Fustic, etc.)
to produce brown shades; the wool being boiled first in a decoction of
the dye for about 1 hour, and then for 1/2 an hour with the addition
of 5 to 8 per cent of copperas. If used for darkening colours,
copperas is added to the bath after the dyeing, and the boiling
continued for 15 to 20 mins.

TIN

(_Stannous chloride, tin crystals, tin salts, muriate of tin._)

Tin is not so useful as a mordant in itself, but as a modifying agent
with other mordants. It must always be used with great care, as it
tends to harden the wool, making it harsh and brittle. Its general
effect is to give brighter, clearer and faster colours than the other
mordants. When used as a mordant before dyeing, the wool is entered
into the _cold_ mordant bath, containing 4 per cent of stannous
chloride and 2 per cent oxalic acid; the temperature is gradually
raised to boiling, and kept at this temperature for 1 hour. It is
sometimes added to the dye bath towards the end of dyeing, to
intensify and brighten the colour. It is also used with cochineal for
scarlet on wool in the one bath method.

CHROME

(_Potassium dichromate. Bichromate of Potash._)

Chrome is a modern mordant, unknown to the dyer of fifty years ago. It
is excellent for wool and is easy to use and very effective in its
action. Its great advantage is that it leaves the wool soft to the
touch, whereas the other mordants are apt to harden the wool. The wool
should be boiled for 1 to 1-1/2 hours with bichromate of potash in the
proportion of