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.