Indigo
2 of the 7 encyclopedias on this shelf carry an entry for Indigo. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
a blue dye contained in the leaves of about 250 different plants. It is obtained chiefly from the green leaves of the indigo plant of Asia, a member of the pea family. The cultivation of indigo was encouraged in South Carolina by the British. Production rose to 1,000,000 pounds a year, but the Revolution cut off the industry and it has not been revived. India leads in the production of indigo with an annual export valued at $6,000,000. Central America and Manila also raise indigo for export. Great Britain imports annually not less than $3,000,000 worth of indigo from India alone. The United States buys quite as much for use in dyeing calico and other cloth. An indigo field looks something like a pink clover field. In the preparation of commercial indigo the green plant is piled in a vat and kept under water twenty hours until fermentation takes place, when the water turns green. The water is then drawn off to a second vat on a lower level, in which a second fermentation takes place. In the meantime the liquid is churned violently until the green turns to blue. The liquid is then allowed to settle. The clear water above is drawn off, the blue pasty fluid at bottom is drawn into a third vat, where the indigo is clarified and partly dried. It is shoveled up into large sacks to drain, and is afterward packed in small boxes about three inches square and set in the sun to dry. It is then ready for shipment. The wonder is that even by keeping the vats constantly full such quantities of indigo can be produced. The way in which indigo is prepared to dye cloth so that it will not rub off is an art in itself. Of late vegetable indigo has been supplanted to a degree by an artificial blue obtained from coal tar . See Dyeing ; Aniline ; Mordant ; Calico ; Dyestuffs In 1742 George Lucas, governor of Antigua, sent the first seeds of the indigo plant to Carolina to his daughter, Miss Eliza Lucas (afterward the mother of Charles Cotesworth Pinckney). With much perseverance after several disappointments she succeeded in growing the plant and extracting the indigo from it. Parliament shortly after placed a bounty on the production of indigo in British possessions, and this crop attained a rapid development in Carolina. In 1754 216,924 pounds, and in 1777 1,107,660 pounds were produced. But the war with the mother country, the competition of indigo-culture in the East Indies, the unpleasant odor emitted, the swarms of flies attracted by the fermentation of the weeds in the vats, and, above all, the absorbing interest in the cotton crop, caused the rapid decline of its culture, and in the early part of this century it had ceased to be a staple product, although it was in cultivation in remote places as late as 1848.--Harry Hammond.
Collier's New Encyclopedia (1921)
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 21½ 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.