Sample of indigo dye, 1999.
© Science Museum / Science & Society Picture Library
This dye has been used since Neolithic times in Europe. Until the end of the 19th century, it was extracted from woad (Isatis tinctoria) and Dyer's Knotweed (Polygonum tinctorum) Woad was widely cultivated until the end of the middle ages, after which time it was used to make a woad vat for dyeing with indigo from India. The plant is covered with water and left to ferment in vats for 10-15 hours. The resulting yellow liquid is then beaten by hand or machinery. The colour of the liquid deepens from green to blue, and gradually the indigo separates out as flakes. It is allowed to settle, and the liquid is then drawn off. The indigo pulp is then boiled to remove impurities, filtered and presed. Finally, it is cut into cubes and dried. It is now posible to make indigo synthetically.