The bath is generally
heated to the boiling-point, and kept there for some time. A large
number of these coal-tar colours show a tendency of going so rapidly
and greedily on to the fibre that it is necessary to find means to
restrain them. This is done by adding a certain amount of Glauber's
salts (sulphate of soda), in the solution of which coal-tar colours are
not so soluble as in water alone, and so go more slowly, deliberately,
and thus evenly upon the fibre. It is usually also best to dye in a bath
slightly acid with sulphuric acid, or to add some bisulphate of soda.
There is another point that needs good heed taking to, namely, in using
different coal-tar colours to produce some mixed effect, or give some
special shade, the colours to be so mixed must possess compatibility
under like circumstances. For example, if you want a violet of a very
blue shade, and you take Methyl Violet and dissolve it in water and then
add Aniline Blue also in solution, you find that precipitation of the
colour takes place in flocks.
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