Mordant dyes can be defined as those dyes that requires a mordant for application as a dye. This upon combining with a mordant imparts insoluble colour on the substrates. The question now is what is a mordant? A mordant can be a substance of organic or inorganic nature that can combine with the coloring matter and is used for fixing in the production of the color.

Most of the natural dyes in use are mordant dyes. The Colour Index classification and naming system of mordant dyes would be like this- mordant + base colour + number. Commonly used mordant dye are Hematein (natural black 1), note the numbering system. Other popular mordant dyes are Eriochrome cyanine R (mordant blue 3), Celestine blue B (mordant blue 14) and Alizarin red S (mordant red 3).

Chromium salts like sodium or potassium dichromate are commercially used for synthetic mordant dyes. The diagrams below shows C.I. Mordant Black 1 with a chromium (III) ion.

• Mostly mordant dyes have hydroxyl and carboxyl groups. To certain extent they can possess Amino groups.
• They are negatively charged, i.e. Anionic.
• Mordant dyes stain by ionic interaction like other ionisable dyes.
• They ensure light-and-wash-fastness.
• Prevents color bleeding.
• Has the ability to brighten or changes some dye colors.

The mordant dyes are generally applied to fibers by three distinct methods:

• In the first method Mordanting of the fiber is done with an appropriate metallic salt and then the dye is applied.
• In the second method the fiber is dyed first and then it is treated with a suitable metallic salt to form an insoluble lake. This is called 'after-chrome' method and is used in fast dyeing of black and brown colour on the substrate wool.
• The third method is the simultaneous application of the mordant and the dye. As the processes of 'metachrome' or 'monochrome' in wool dyeing.