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(Ferrous Sulphate, copperas, green vitriol.)

Iron is one of the oldest mordants known and is largely used in wool
and cotton dyeing. It is almost as 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.

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