A mordant is a colourless chemical substance used in certain photographic toning processes that has the specific property of absorbing and binding dye molecules to the photographic image, allowing the silver image to be converted into a dyed image of a desired colour. The term is borrowed from the broader field of textile dyeing, where mordants have been used for centuries to fix dyes permanently to fabric fibres that would not otherwise absorb or retain them, and the underlying chemical principle is essentially the same in both applications.
In photographic toning processes that employ a mordant, the procedure typically involves two distinct stages. In the first stage, the developed silver image is chemically converted into a mordant substance through the application of a bleaching and conversion bath that transforms the metallic silver of the image into a colourless or pale mordant compound - typically a metal salt or similar chemically reactive substance - that retains the precise tonal structure and detail of the original silver image while replacing it with a material capable of absorbing and binding dye. In the second stage, the mordanted image is immersed in a dye bath containing the chosen colourant, which is selectively absorbed by the mordant in proportion to its local density, reproducing the full tonal gradation of the original image in the colour of the dye rather than in silver.
Mordant based toning processes offer the photographer considerable creative flexibility in terms of the final colour of the image, as a wide variety of dye colours can in principle be applied to the mordanted image, allowing warm browns, cool blues, rich greens, or virtually any other hue to be introduced into the print. The intensity and saturation of the final colour can also be influenced by the concentration of the dye bath and the duration of the dyeing step. Some mordant toning processes also allow selective toning, where different areas of the print can be treated with different dyes to produce a multicoloured toned result.
Mordant based toning should be distinguished from the more widely practised direct chemical toning processes - such as selenium, sepia, and gold toning - in which the silver image is chemically converted directly into a differently coloured compound without the intermediate mordant stage. While direct toning processes are generally simpler and more straightforward to carry out, mordant based toning offers greater colour variety and the distinctive visual character of a dye based rather than metal based image colour.