Cotton, for instance, receives scarcely
any tinge from the same bath that will dye woollen a deep scarlet. Wool
is that which appears to have the strongest affinity to colouring
matter; next to it is silk; then linen; and cotton the weakest, and is
therefore the most difficult of all to dye perfectly. Thus, if a piece
of linen cloth be dipped into a solution of madder, it will come out
just tinged with the colour; but if a piece of the same be previously
dipped into a solution of alum or copperas, and dried previously to
being dipped in the madder, the alum will become so far impregnated with
the colouring principle, that the cloth will receive a perfect dye, and
be so fixed that it cannot be separated by any common means. Thus it
will be observed, that the art of dyeing permanent colours depends on
this intermediate principle, which is termed a mordant. These mordants
are very numerous; and on a knowledge of them appears to rest the
principal secret of dyeing. The following mode is, however, a very
convenient one for makig experiments on fixing the colouring principles
of any vegetable extract: To have several pieces of cloth, woollen,
cotton, silk, and linen, dipped in the different mordants, and by
keeping a small vessel filled with the colouring solution on a fire in a
state a little below boiling, by cutting small pieces of each, and
immersing them in the colour, and examining and comparing with each
other.
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