Aquatint by Fumagalli showing indigo being made. Until the end of the 19th century, indigo was extracted from woad (Isatis tinctoria) and Dyer's Knotweed (Polygonum tinctorum). 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. Men are shown working with the indigo carrying it in sacks to the uppermost vat and tending to the drying racks.
© Science Museum / Science & Society Picture Library