(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.