woad, name for a perennial plant ( Isatis tinctoria ) of the family Cruciferae (mustard family) and for a blue dye obtained from its leaves. The plant is believed to be native to S Russia, but was in cultivation (and escaped) throughout Europe in early times. The pigment was obtained by fermentation and oxidation of a colorless glucoside, indican. Indican is also present in the leaves of the unrelated indigo, the other major blue vegetable dye plant. Although the dye obtained from indigo is superior in vividness of color, fastness, and ease of processing, woad growers and distributors of the Renaissance prohibited the sale of indigo in Europe for more than a century. In 1392 the Saxon town of Erfurt, Germany, had gained enough wealth through the woad trade to establish its own university. By the mid-17th cent., however, woad had been largely replaced by its successor—partly because of the low prices of indigo imports from the New World. Both woad and indigo have been eclipsed by the synthetic aniline dyes perfected in the late 19th cent. Woad was also extensively used for brilliant blue paint pigments. The ancients used it medicinally for ulcers and other ailments, and the early Britons painted their bodies with it. Woad is classified in the division Magnoliophyta, class Magnoliopsida, order Capparales, family Cruciferae.
See J. B. Hurry, The Woad Plant and Its Dye (1930, repr. 1974).