Just such peculiar _quasi_-acid, or phenolic substances are
Alizarin, and most of the natural adjective dyestuffs, the colouring
principles of logwood, cochineal, Persian berries, etc. Hence these
substances will be combined and carried down with such precipitated
basic salts. The complex compounds thus produced are coloured substances
known as lakes. For example, if I take a solution containing basic
sulphate of alumina, prepared as I have already described, and add to
some Alizarin, and then heat the mixture, I shall get a red lake of
Alizarin and alumina precipitated. If I had taken sulphate of iron
instead of sulphate of alumina, and proceeded in a similar manner, and
added Alizarin, I should have obtained a dark purple lake. Now if you
imagine these reactions going on in a single fibre of a textile
material, you have grasped the theory and purpose of mordanting. The
textile fabric is drawn through the alumina solution to fill the pores
and tubes of the fabric; it is then passed through a weak alkaline bath
to basify or render basic the aluminium salt in the pores; and then it
is finally carried into the dye-bath and heated there, in order to
precipitate the colour lake in the fibre.
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