Indigo has been used either as a pigment or a dye from very early times in India and in Egypt. It is referred to under the name of indicum by Pliny; later on the Byzantine writers called it azorium Romanum. 'Indigo bagadel' - that is, indigo of Bagdad - is named as early as 1228 in the Marseilles tariffs; in the early English accounts relating to painting works (1274) it is called 'indebas.' In the fourteenth century it was designated as 'ind,' 'inde,' and 'ynde.' 'Endego' and 'indico' were used in the sixteenth and seventeenth centuries. It was first largely imported from India into Europe in the seventeenth century by the Dutch.

A large number of different plants yield true indigo. This pigment was once obtained in considerable quantity from a crucifer, Isatis tinctoria, the dyers' weed or woad, the 'pastel' of the French; but the chief source is now Indigofera tinctoria, a leguminous shrub, probably of Indian, or at least Asiatic, origin.

Indigo (C16H10N2O2) does not exist ready-formed in the plants which yield it, but occurs in the form of a colourless compound, or glucoside, which, by combining with water, splits up into a sugar and indigo. It is prepared from the freshly-cut plants, or from the dried foliage, by maceration in water and fermentation, followed by boiling (sometimes lime-water is first added); the dark precipitate which forms is thrown on to cloth-strainers, and finally pressed and dried. The mineral impurities which commercial indigo contains are derived partly from the plant itself, partly from the water used in preparing it, and partly from the lime-water above mentioned; moreover, it is sometimes adulterated. Indigo is easily oxidized by a very large number of substances rich in oxygen, yielding a yellow product called isatine; it is converted into a colourless body (C16H12N2O2) by many reducing agents.

Indeed, several of the processes of purifying indigo depend upon the reduction of the blue colouring substance, or 'indigotin,' into 'white indigo,' and the subsequent precipitation of the blue matter by exposure to the oxygen of the air. Green vitriol is the commonest reducing agent, and is used in association with lime. The purified indigo prepared by this process, though of fair colour, does not, however, work so well as a paint as the best Bengal indigo treated successively with acid, alkali, acid, and alcohol. Indigotin, if quite pure, has a somewhat purplish cast in thick water-colour washes; this hue is observable with this substance whether obtained by sublimation or by Fritzche's process with grape-sugar, caustic soda, and alcohol.

The impurities in commercial indigo constitute from 20 up to 70 or 80 per cent. of its total weight - the average is about 50. They consist mainly of mineral matter, indigo red, indigo brown, and nitrogenous compounds. Much of the mineral matter may be removed by digestion in hydrochloric acid, followed by treatment with boiling water. Sodium hydrate solution dissolves the indigo brown, while strong alcohol takes away the indigo red, which amounts to nearly 4 parts in 100 of the original indigo. After treatment with these three solvents, the residual purified indigo is of an intense and very beautiful hue. Java indigo is generally of very good quality, that from Bengal comes next, and then the indigo from Oudh, Kurpah, and Madras. Japanese indigo is generally poor.

Indigo frequently receives no purifying treatment previous to its being ground into a fine powder suitable for admixture with oil or with gum and the other media of water-colours. The necessity of choosing the purest and finest samples of the commercial dye-stuff is of course evident, but it is better in every case to adopt the processes of purification named in the preceding paragraph, No sample of purified indigo should leave, after being burnt, more than 3 per cent. of ash.

This rich and transparent blue is, unfortunately, gradually oxidized and browned when exposed to light. In thin washes of water colour it disappears rapidly in the sun's rays, much more slowly when submitted to diffused daylight. The following figures approximately represent the reduction in force of a sample of indigo as a moist water-colour when exposed to sunlight:

Other trials with other samples gave in some cases less unfavourable results.

Indigo in cake is sometimes less affected by sunlight than the moist preparations. As an oil-colour, indigo loses from one-third to one-half of its intensity when exposed to sunlight for five years, its hue being at the same time altered, in different specimens, either to a greyish or a greenish blue; the change is more conspicuous when the indigo has been mixed in tint with flake or other white. Locked up in copal or amber varnish it is more slowly changed. The fading is due to oxidation.

Indigo may be replaced advantageously by ultramarine mixed with a trace of viridian, or by a good Prussian blue, either being associated with a little ivory-black.

Several pigments, such as aureolin, true Naples yellow, and all the chromates, have a very marked effect upon indigo.

In order to ascertain whether the fading of indigo as a water-colour, on exposure to sunshine, was increased by the presence of alum in the paper, a series of comparative experiments were made. A pale tint of indigo was spread upon (1) paper free from alum; (2) paper washed with alum solution; (3) paper containing a trace of alum; (4) paper which had been washed with weak ammonia-water after having received an alum-size. After six months' (April to September inclusive) exposure to sunlight, all the four specimens showed complete extinction of the blue pigment, the disappearance of the colour from No. 2 having, however, been a trifle more rapid than in the other cases. Honey and glycerin, owing to their hygroscopic character, appear to hasten the fading, so does water-vapour; for a dried slip of indigo-washed paper sealed up in a glass tube loses its colour less quickly than one in its ordinary moist condition, when both are exposed side by side to sunshine. A tightly-framed water-colour drawing presents, of course, a close analogy to the second or more unfavourable set of conditions. When a medium tint of indigo on paper was exposed for four years to sunlight in a tube containing air kept dry by a water-absorbing agent, its original depth of colour was perfectly preserved.