Indigo

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 percent 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 percent 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.

An identical experiment in an ordinary glazed frame resulted in a reduction of tone from 10° to 1°, while the residual hue was a greenish grey.

The effect, if any, of the several iron reds, such as light red, Venetian red, and Indian red, in accelerating the fading of indigo in sunshine has been tested in many ways. Provided the three red pigments above-named be free from soluble salts, and especially from sulphates, they are equally innocuous; the indigo disappears with or without them at the same rate. In one instance only a wash of Venetian red with indigo lost its indigo completely three weeks before the destruction of the blue in the parallel experiments was complete, which was the case in six months. It is impossible to assert that this result was not due to a smaller amount of indigo having been present in this particular case, but I believe it to have been due to this sample of Venetian red having been an imitative one prepared from colcothar.

In comparing under the same conditions the relative stability, when exposed to light, of different indigoes, it appears that 'Bengal refined' is superior to the best 'raw Bengal,' and even to the indigotins obtained by the green vitriol and glucose processes.

The superiority, when exposed to light, of refined indigo - that is, of indigo which has been purified by the treatment with acid and the other solvents already named - although observed in recent experiments, seems to have been ascertained in the time of De Mayerne. One of the authorities (Elias Feltz, of Constance) quoted by De Mayerne affirms that this pigment may be rendered safe by steeping it in vinegar and exposing it to the sun for several days; the vinegar is then to be poured off, and the paste when dry ground in oil. Another plan recommended (in a note, dated 1642) by Feltz consists in grinding the indigo with a little calcined alum; this plan is, however, objectionable for several reasons. The treatment of the indigo with the solvents previously named, which are adapted to remove its natural impurities, is the only legitimate plan. It is scarcely necessary to add that there is abundant evidence to prove that in De Mayerne's day indigo was generally regarded as an unstable colour.

Under all circumstances no indigo should be employed in painting unless it be completely free from acids.

Although it is not possible to ascertain the exact richness in indigotin of commercial indigoes by any tests save those in use by professional chemists, yet it is easy to learn a good deal about any particular sample by means of a few simple experiments. A good sample should appear homogeneous, and should float on water. Dried at 100° C, it should not lose more than 6 parts per 100 of moisture. When rubbed with a hard smooth substance it should show a coppery lustre. One hundred grains when burnt should not leave more than 10 grains of grey ash. It should dissolve perfectly in four times its weight of fuming sulphuric acid. Starch, gypsum, clay, chalk, steatite, and Prussian blue are amongst the adulterants of indigo.

Indigo dissolved in four times its weight of fuming oil of vitriol (kept cool) forms a liquid from which, after slight dilution and filtration through asbestos cloth, potassium carbonate precipitates a fine blue compound, which has been used in water-colour painting under the name of indigo carmine. It is not a safe pigment.

The artificial or synthetic indigo, although characterized by freedom from impurities, both organic and inorganic, does not offer any advantages over the natural product, if properly purified, when employed as a paint in water-colour or oil. It may be added that those derivatives of indigotin, in which some of the hydrogen atoms have been substituted by bromine or chlorine, are no more permanent than the original dyestuff. This is the case with that form of dibrom-indigotin (the punicin of the late H. E. Schunck), which occurs naturally in, or rather was obtained from, certain marine molluscs (Purpura lapillus and Murex brandaris), and which has been known for many centuries as Tyrian Purple. The very minute yield of the natural pigment would have made its general use impracticable, but now that the identical substance can be freely made from indigo, it is found that its lack of permanence should exclude it from the 'selected' palette of the artist of to-day.

In examining old water-colour drawings, it will often be found that the parts protected by the mount or frame show the indigo used in compounding the greys of clouds and the greens of vegetation perfectly intact, while it has completely left the exposed parts. In many cases where indigo is supposed to have withstood long exposure to light, it will be found that the blue used has been Prussian blue, modified by admixture with other pigments. But in many old pictures and drawings it will be found that the unnatural bluish hue of the foliage represented is due to the complete loss of rapidly evanescent yellows, rather than to the entire stability of the indigo with which the greens have been compounded. Nor must it be forgotten that indigo was not the only blue in such mixed greens, blue carbonate of copper and even lapis-lazuli having been extensively employed at that time. The latter pigment is perfectly permanent except in the presence of acids and alum.