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.
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