Mordant dyes are so called because to apply them necessitates the use of mordant. This group of dyes includes natural dyes: Logwood, Fustic and Madder (now replaced by synthetic Alizarine) and a large group of synthetic dyes with a widely differing constitution.The mordant dyes can be applied to fibres by three different methods:

a. By mordanting the fibre with a suitable metallic salt and then applying the dye.

b. By dyeing the fibre and subsequently after-treating it with a suitable metallic salt so as to form an insoluble lake. This is the basis of 'after-chrome' method used in particularly fast dyeing of black and brown colour on wool.

c. By the simultaneous application of the dye and mordant. In wool dyeing as 'metachrome' or 'monochrome' process and is extensively used for dyeing of brown and khaki colours.

USE: Since the dyes used in this process vary widely, not only by the methods of application and dyeing of different fibres, reliable recipes and instructions are to be followed carefully