C16H10N2O2 . A natural dark blue dye obtained from Indigofera tinctoria plants native to India, Java and other tropical areas. The use of indigo was first mentioned in Indian manuscripts in the 4th century BCE. It was exported to Europe in Roman times but did not become plentiful until sea routes opened up in the 17th century. The natural material is collected as a precipitate from a fermented solution of the plant. The colouring component, indigotin, is extracted as a colourless glycoside, but turns blue with oxidation. Synthetic indigo was first produced in 1880 by Adolf von Baeyer and became commercially available in 1897. Made from anthranilic acid, the synthetic colorant is chemically identical to natural indigo and has almost entirely replaced the natural dyestuff. Indigo is a fine, intense powder which may be used directly as a pigment in oil, tempera or watercolour media. It is listed among the pigments and dyes used for colouring stucco marble in the seventeenth and eighteenth century. Synonyms: indigotin; India blue; intense blue; rock indigo; stone blue; indigo carmine; intense blue; indico; indicoe; indicum (Pliny); indego; nil; indigo (Fr.); Indigo (Ger.); anil (Sp.); indaco (It.); aneel; anile; ai (Jap.); blue ynde; blue inde; anneil; Natural Blue 1; CI 75780 (natural); Vat Blue 1; CI 73000 (synthetic); Pigment Blue 66. Trench (2000); Wittenburg (1999); Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]