he
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.
A further reason is that from the solution of acetate of iron or
alumina, basic acetates are very easily precipitated on heating, and are
thus readily deposited in the fibre.

_Mordanting and Fixing Basic Colours._--Now let us ask ourselves a very
important question. Suppose we have a colour or dyestuff, such as
Magenta, which is of a basic character, and not of an acid or phenolic
character like the colours Alizarin, Haematein (logwood), or carminic
acid (cochineal), and we wish to fix this basic dyestuff on the tissue.
Can we then use "red liquor" (acetate of alumina), acetate of iron,
copperas, etc.? The answer is, No; for such a process would be like
trying to combine base with base, instead of base with acid, in order to
form a salt. Combination, and so precipitation, would not take place; no
lake would be formed. We must seek for an acid or acid body to use as
mordant for our basic colour, and an acid or acid body that will form an
insoluble precipitate or colour-lake with the dyestuff. An acid much
used, and very valuable for this purpose, is tannic acid. The tannate of
rosaniline (colour principle of Magenta) is a tolerably insoluble lake,
which can be precipitated by Magenta from a solution of tannate of soda,
the Magenta being capable of displacing the soda. But tannic acid,
alone, does not form very fast lakes with Magenta and the other basic
dyestuffs, and so a means of rendering these lakes more insoluble is
needed. It is found that tannic acid and tartar emetic (a tartrate of
antimony and potash) yield a very insoluble compound, a tannate of
antimony. Perchloride of tin, in a similar manner, yields insoluble
tannate of tin with tannic acid. These insoluble compounds, however,
have sufficient acid-affinity left in t