A natural dark blue dye obtained from Indigofera tinctoria plants native to India, Java, Peru, 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 coloring component, indigotin, is extracted as a colorless glycoside, but turns blue with oxidation. Synthetic indigo was first produced in 1880 by Adolf von Baeyer. 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 watercolor media. The exposed pigment can fade rapidly in strong sunlight. Indigo is still used to dye jeans, where its fading and uneven coloring have become favorable characteristics.
Synonyms and Related Terms
2,2'-biindolinyliden-3,3'-dion; Indigofera tinctoria; Natural Blue 1; CI 75780 (natural); Vat Blue 1; CI 73000 (synthetic); Pigment Blue 66; indigotin; indicum (Pliny); indigo (Esp. Fr., Dan., Ned., Port., Sven.); Indigo (Deut.); anil (Esp.); Indiko (Gr.); indaco (It.); aneel; anile; ai (Jap.); rams (Tibetan); blue ynde; blue inde; anneil; India blue; intense blue; rock indigo; stone blue; indigo carmine; intense blue; indico; indicoe; indego; nil
Soluble in nitrobenzene, phenol, chloroform, glacial acetic acid. Insoluble in water, ethanol, acetone, ethyl acetate, pinene.
Absorption max = 599 (in xylene).
ISO R105 Lightfastness Classification = 3-4
Microscopically, indigo has fine, translucent dark blue, rounded particles that are weakly birefringent and appear red under Chelsea filter.
Hazards and Safety
Discolored by reducing agents and bleaches.
Fisher Scientific: MSDS
° H.Schweppe, "Indigo and Woad", Artists Pigments, Volume 3, E. West FitzHugh (ed.), Oxford University Press: Oxford, 1997.
° Pigments Through the Ages: Indigo