biting or caustic mordant (plural mordants) - Any substance used
to facilitate the fixing of a dye to a fibre; usually a metallic
compound which reacts with the dye using chelation. - 1964, In
dyeing two mediums are required, the colouring matter and the
mordant which fixes the dye in the wool. — L.F. Salzman, English
Industries of the Middle Ages, p. 208. - Any corrosive substance
used in etching. a substance used to facilitate dye fixing to
subject to a mordant