INDIGO: a vegetable dyestuff of much value, obtained from several plants native to India and America. The fresh plant juice is colorless, but when the plant is steeped in water and fermentation sets in, the coloring matter dissolves in the water, forming a yellow solu tion, which is drawn off from the rest of the vegetable matter and agitated and beaten to bring it freely into contact with the air for about two hours. This treatment causes the indigo to form and settle down as a blue precipitate, which is cut while soft into cubical cakes and dried by artificial heat. To hasten the formation of the indigo, a little lime water is sometimes added to the yellow solution.
Indigo is used in the manufacture of inks and for laundry purposes. The best quality has the deepest purple color, will float upon water, is glossy, and when rubbed by the nail produces a bright coppery or purple-red streak. When the streak is dull and wrinkles, the quality is poor. Com
mercial indigo of good quality contains about 50% of pure indigo. The common varieties are very numerous, some merchants recognizing sixteen distinct grades.
Brown and red indigo are also manufactured.
Artificial indigo is now produced in enormous quantities from Coal Tar. See also BLUING.