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 dy

(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.