chrome alum(n = noun.substance) - a violet-colored salt used in hide tanning and as a mordant in dyeing;
is a kind of mordant, salt

alum, alum root, alum schist, alum shale, alum stone, ammonia alum, ammonium alum, chrome, chrome green, chrome red, chrome steel, chrome yellow, potash alum, potassium alum, roche alum