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
is so harmless to the fibre, and is easily driven off on steaming, etc.
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