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. The method usually employed to
mordant woollen fabrics consists in boiling them with weak solutions of
the metallic salts used as mordants, often with the addition of acid
salts, cream of tartar, and the like. A partial decomposition of the
metallic salts ensues, and it is induced by several conditions: (1) The
dilution of the liquid; (2) the heating of the solution; (3) the
presence of the fibre, which itself tends to cause the breaking up of
the metallic salts into less soluble basic ones. Thus it is not really
necessary to use basic aluminium sulphate for mordanting wool, since the
latter itself decomposes the normal or neutral sulphate of alumina on
heating, an insoluble basic sulphate being precipitated in the fibres of
the wool. (4) The presence of other added substances, as cream of
tartar, etc. The best alumina mordant is probably the acetate of
alumina ("red liquor"), and the best iron mordant, probably also the
acetate ("iron liquor") (see preceding lecture), because the acetic acid