A kind of deep blue, one of the seven prismatic colors.
2.
(Chem.) A blue dyestuff obtained from several plants belonging to very different genera and orders, such as, the woad, Isatis tinctoria (family Cruciferae), Indigofera suffroticosa, Indigofera tinctoria (family Leguminosae), Indigofera Anil, Nereum tinctorium, Polygonum tinctorium Ait. (family Polygonaceae), etc.; called also natural indigo. It is a dark blue earthy substance, tasteless and odorless, with a copper-violet luster when rubbed. Indigo does not exist in the plants as such, but is obtained by decomposition of the glycoside indican. Note: Commercial indigo contains the essential coloring principle indigo blue or indigotine, with several other dyes; as, indigo red, indigo brown, etc., and various impurities. Indigo is insoluble in ordinary reagents, with the exception of strong sulphuric acid.
Chinese indigo (Bot.), Isatis indigotica, a kind of woad.
Wild indigo (Bot.), the American herb Baptisia tinctoria which yields a poor quality of indigo, as do several other species of the same genus.
... the huts of the natives, a plant with tuberous roots, exactly like cassava (yucca). It is called cumapana, and is cooked by being baked on the ashes. It grows spontaneously on the banks of the Cassiquiare.) We saw wild indigo around the missions of San Miguel de Davipe, San Carlos, and Mandavaca. No doubt can exist that several nations of America, particularly the Mexicans, long before the conquest, employed real indigo in their hieroglyphic paintings; and that small cakes of this substance were sold at the great ... — Equinoctial Regions of America V2 • Alexander von Humboldt
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