Indigo
a vegetable dyestuff, yielding a beautiful and very durable blue dye.
Many tropical and sub-tropical plants contain a substance, probably a glucoside, which, on being extracted and allowed to ferment, produces impure indigo. The plants from which it is chiefly obtained are the various species of Indigofera, Isatis tinctoria, Polygonum tinctorium, etc. Commercial indigo is by no means pure indigo-blue; it contains indigo-gluten, indigo-brown, and indigo-red, together with insoluble impurities. Indigo prepared in Java by Sayer's process contains from 66 to 71 per cent. of indigo-blue, and only 2½ per cent. of ash, while ordinary commercial indigo seldom contains more than 65 per cent. of indigo-blue, with not less than 15 per cent. of ash. Indigo is tasteless, odorless, and of an intense blue color, passing into purple.