MORDANTS (Fn., Mordants; GER., Beizen).
In dyeing and tissue-printing, a very important part is played by "mordants." If we examine the action of colouring matters upon organic fibres and tissues, we find that they may be divided into two great classes. One group—the so-called substantive colours—are capable of attaching themselves at once to the article to be dyed, on simple immersion or steeping. Thus, if a piece of clean white silk or wool, or the human akin, be moistened with a solution_ of magenta, or, in fact, with the majority of the aniline colours, it is permanently dyed, and cannot be rendered white again by mere rinsing in water. The other group of tinctorial matters, which, till lately, at least, were by far the more numerous, have received the name of adjective colours. If yarn or cloth be steeped in solutions of these bodies, it may when taken out seem dyed ; but on squeezing or wringing, and still more on washing in pure water, the colour is entirely removed, leaving the cloth merely somewhat soiled. It is true the distinction between these two classes is not absolute ; many dyes which are substantive upon animal fibre prove merely adjective when cotton or linen. Still, the classification, though not scientifically correct, is practically useful, and, as such, may be retained.
In order to cause the adjective colours to attach themselves to the fibre, so as to yield a full, thoroughly-dyed shade of a reasonable degree of permanence, recourse is had to the intervention of some third body, which is called a "mordant," and which enables the colour and the tissues to com bine. Even in the case of substantive colours, mordants are very frequently called into requisition, because they brighten and modify the shade which would he obtained if the colour alone were employed.
One and the same colouring matter, if employed with different mordants, may give totally different shades—a fact which comes into great prominence in the so-called " madder-style " in calico-printing. If, for instance, there be printed upon different portions of a piece of calico : (1) strong acetate of alumina, (2) weak acetate of alumina, (3) strong acetate of iron, (4) weak acetate of iron, and (5) a mixture of the acetates of alumina and iron, and the cloth be then dyed in the madder beck, or, as is now the ease, in a solution of artificial alizarin, there are produced : (1) full red, (2) pink, (3) black, (4) purple [violet], (5) chocolate. Other dyes can also be made to produce
varied effects, according to the mordant selected, though not to such an extent.
The properties which a good mordant should possess are various. If for general use, it should not alter the colour of the fibre. This is the great advantage possessed by the compounds of alumina and tin ; they leave the fibre white as they found it, and hence may be sucoessfully used for all pure and bright shades, such as the prismatic colours. Other mordants, e. g. the preparations of iron, and certain chromium compounds, alter the colour of the fibre, or darken—technically called " saddening "—the colour of the dye. Thus cotton yarn, steeped in a solution of iron and exposed to tilt:Lair, takes a rusty or buff shade ; hence the preparations of iron cannot be employed for dyeing any colour with which such a ground would be incompatible. Iron, further, is a powerful saddening agent ; in contact with tannin in its various forms, the dye-woods, cochineal, and the madder colours, it produces blacks, deep-olives, dark-browns, &c. Hence its use is necessarily very much restricted.
Mordants must, as a matter of course, be soluble, so that they may be presented to the fibre in a liquid state. Doubtless, many substances, when in a very fine state of suspension, are capable of combining with the fibre ; but in this case, there is greater difficulty in producing perfectly even shades. It is, however, necessary that the mordants, or at least their essential constituents, should be easily rendered insoluble, as soon as they have combined with the fibre. Were this not the case, they could be washed away, and would prove useless.