irect from the
raw state, it must be boiled for several hours to extract its natural
impurities. For dark colours water alone may be used, but for light
and bright colours a weak solution of carbonate of soda, 5%; or of
caustic soda, 2%, should be used.

MORDANTS

_Alum._ Alum (1/4 weight of cotton) is dissolved in hot water with
carbonate of soda crystals, or other alkali (1/4 weight of alum); work
cotton in the solution, steep for several hours or overnight. Then
well wash. Aluminium acetate solution as for silk (page 56) may be
used. After drying, the cotton may be passed through a fixing solution
of some alkali, for examples see page 50. Before mordanting with
alum, the cotton is often prepared with tannic acid.

_Iron._ Iron is usually employed as a "saddening" agent, i.e. the
cotton after dyeing is steeped in a cold solution of the mordant. A
further use is in dyeing black, when the cotton, after being prepared
with tannin, is steeped in a cold solution of Iron. This process by
itself gives a dark colour before any dye is used.

_Tin._ Tin is rarely used alone as a mordant for cotton but brightens
the colour in combination with other mordants.

_Chrome._ Chrome is used for browns and other colours with Catechu.
After boiling in a solution of the dye stuff, boil a short time in
chrome solution, this oxidizes the colouring matter of the Catechu.

_Copper._ Copper is sometimes added in small quantities to the dye
bath for brown or yellow to vary the shade.

_Tannin (Tannic acid)._ Cotton and linen strongly attract tannin and
when prepared with it they are able to retain dyes permanently. Cotton
saturated with tannin attracts the dye stuff more rapidly, and holds
it. Tannic acid is the best tannin for mordanting as it is the purest
and is free from any other colouring matter; it is, therefore, used
for pale and bright shades. But for dark shades, substances containing
tannic acid are used, such as _sumach_, _myrobalans_, _valonia_,
_divi-divi_, _oak galls_, _chestnut_ (8 to 10 per cent tannin),
_catechu_.

Cotton and linen are prepared with tannin after they have been through
the required cleansing, and, if necessary, bleaching operations. A
bath is prepared with 2 to 5 per cent of tannic acid of the weight of
the cotton, and a sufficient quantity of water. For dark shades, 5 to
10 per cent should be used. The bath is used either hot or cold. It
should not be above 60 deg.C. The cotton is worked in this for