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. 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._)