Woad, a chest-height member of the mustard family, appears unremarkable. It has long, blue-green leaves, a strong cabbage-like odour, yellow flowers and flattened, lantern-like fruits. However, as the plant ages, a clue to its use appears; it develops a dark blue tinge. Woad is native to central and western Asia but widely introduced across Europe as a major fabric dye; woad fruits have been found at Iron Age sites across Europe.
To use woad as a dye, harvested plants must be processed; only then are chemical precursors contained in woad converted to the blue dye indigotin. Young woad leaves are harvested, crushed and moulded into cricket ball-sized spheres before being dried and stored. Eventually, woad balls are pulverized and piles of damp woad powder left to ferment. During this smelly process bacteria convert indigotin precursors into indigotin. However, indigotin is insoluble and needs further treatment to make it useful for dying fabrics. Indigotin is dissolved in alkaline solution to make a yellowish dye, indigo white. When fabrics are immersed in woad vats they emerge yellow-green, but quickly turn blue as they dry.
In modern terms, woad production was unsustainable. For high yields, woad needed fertile land and large amounts of fertilizer. The wealth it promised encouraged farmers to set aside large areas of rich land to woad, often at the expense of food production. During periods of political, social and environmental instability, there was serious risk of famine. The sixteenth-century English herbalist John Gerard was direct about the impact of woad on people's lives: 'it serueth well to die and colour cloth, profitable to some few, and hurtfull to many'. In October 1585, Queen Elizabeth I gave vent to concerns that short-term woad profits were damaging long-term food security; she banned new land being sown with woad.
Until the eighteenth century, woad was the primary source of indigotin in western Europe but it is not the only source. A tropical legume, Indigofera tinctoria, was a particularly important indigotin source in eastern and southern Asia, having been used in India for at least 5,000 years. Woad use declined as Indigofera became the preferred source of indigotin. However the use of both plants was overtaken in 1883 when the German chemist Adolph von Baeyer discovered how to synthesize indigotin. In 1897, about 19,000 tonnes of plant-derived indigotin was produced annually; a century later a similar amount was produced using Baeyer's industrial process.
Balfour-Paul J 2011. Indigo: Egyptian mummies to blue jeans. British Museum.