alizarin (əlĭzˈərĭn) [key], or 1,2-dihydroxyanthraquinone, mordant vegetable dye obtained originally from the root of the madder plant ( Rubia tinctorum ), in which it occurs as a glucoside. The term also includes a group of synthetic dyestuffs prepared from coal-tar derivatives. A method for the synthesis of alizarin was first discovered (1868) by Karl Graebe and Karl Liebermann, German chemists. With salts of metals the compound forms brilliant lakes, although by itself it is a poor dye. Turkey red is produced with an aluminum mordant, other shades of red with calcium and tin salts, dark violet with iron mordants, and brownish red with chromium. Purpurin, also used in dyeing, occurs with alizarin in madder and is produced synthetically.