A mordant is any substance which can be fixed to the fibre and which can be dyed later on mostly hydroxide or basic salts of chromium aluminium and iron are used as mordant. A dye which imparts different colours in the presence of different mordant is referred to as a mordant dye. For ex. alizarin is a mordant dye when mordanted with aluminium salt solution. It imparts rose red colour to fabric but the same fabric is dyed blue when it is mordanted with barium salt and it dyes violet is mordant with Ferric salt.