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Mordants






Any dye belongs to one of two classes. Substantive, giving colouring
directly to the material: and adjective, which includes the greater
number of dyes and requires the use of a mordant to bring out the
colour.

There are thus two processes concerned with the dyeing of most
colours; the first is mordanting and the second is the colouring or
actual dyeing. The mordanting prepares the stuff to receive the dye
(mordere, to bite).

The early French dyers thought that a mordant had the effect of
opening the pores of the fibre, so that the dye could more easily
enter; but according to Hummel, and later dyers, the action of the
mordant is purely chemical; and he gives a definition of a mordant as
"the body, whatever it may be, which is fixed on the fibre in
combination with any given colouring matter." The mordant is first
precipitated on to the fibre and combines with the colouring matter in
the subsequent dye bath. But, whether the action is chemical or merely
physical, the fact remains that all adjective dyes need this
preparation of the fibre before they will fix themselves on it. The
use of a mordant, though not a necessity, is sometimes an advantage
when using substantive dyes.

In early days the leaves and roots of certain plants were used. This
is the case even now in India and other places where primitive dyeing
methods are still carried on. Alum has been known for centuries in
Europe. Iron and tin filings have also been used. Alum and copperas
have been known in the Highlands long ages.

Mordants should not affect the physical characteristics of the
fibres. Sufficient time must be allowed for the mordant to penetrate
the fibre thoroughly. If the mordant is only superficial, the dye will
be uneven: it will fade and will not be as brilliant as it should be.
The brilliancy and fastness of Eastern dyes are probably due to a
great extent to the length of time taken over the various processes of
dyeing. The longer time that can be given to each process, the more
satisfactory will be the result.

Different mordants give different colours with the same dye stuff. For
example:--Cochineal, if mordanted with alum, will give a crimson
colour; with iron, purple; with tin, scarlet; and with chrome or
copper, purple. Logwood, also, if mordanted with alum, gives a mauve
colour; if mordanted with chrome, it gives a blue. Fustic, weld, and
most of the yellow dyes, give a greeny yellow with alum, but an old
gold colour with chrome; and fawns of various shades with other
mordants.

Silk and wool require very much the same preparation except that in
the case of silk, high temperatures should be avoided. Wool is
generally boiled in a weak solution of whatever mordant is used. With
silk, as a rule, it is better to use a cold solution, or a solution at
a temperature below boiling point. Cotton and linen are more difficult
to dye than wool or silk. Their fibre is not so porous and will not
hold the dye stuff without a more complicated preparation. The usual
method of preparing linen or cotton is to boil it first with some
astringent. The use of astringents in dyeing depends upon the tannic
acid they contain. In combination with ordinary mordants, tannic acid
aids the attraction of the colouring matter to the fibre and adds
brilliancy to the colours. The astringents mostly used are tannic
acid, gall nuts, sumach and myrobalams. Cotton has a natural
attraction for tannic acid, so that when once steeped in its solution
it is not easily removed by washing.





Next: Alum
Previous: Linen



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