MORDANTS (Fr., from Lat. mordere, to bite), materials used in dyeing and calico printing for the purpose of fixing the colors. Their action is in accordance with a twofold attraction for the coloring matter and the material of the fabric, serving as a bond of union. Substances which produce precipitates by acting upon the dyestuff, so that they may be produced within the fibres of the fabric, are also called mordants. In the strictest sense, however, they are not true mordants, but more properly speaking are components of the dyeing material, the mordant being a material that has the property of fixing the dye which has already been produced. It acts by altering the texture of the fibre in such a way as to cause it to retain the particles of the color. The principal mordants are the aluminic, stannic, and ferric salts, in which the affinity of the base and acid is comparatively weak, so that the precipitated coloring matter may be formed without much difficulty. The action of a mordant generally depends much upon the temperature at which the operation is conducted, as must be apparent from a consideration of the effects of heat upon chemical affinity, the affinity between some substances being much more affected by alterations of temperature than that between others. These reactions are also greatly modified by the nature of the fabric operated upon, and in general can only be well ascertained by experiment. Dyeing, therefore, like nearly all industrial processes, requires for its perfection the combination of theory with practice. (See Calico Printing, and Dyeing.)