Indigo (a vegetable dyestuff, known to the ancients by the name of indicum, from its being brought into Europe from India. The same name appears to have been applied to India ink also, but in this case usually qualified by the epithet nigrum. So little was known of the real nature of this substance, which for centuries had been employed in painting and dyeing, that as late as the year 1705 it was spoken of as a mineral in letters patent issued in Halberstadt, Germany. The use of indigo in dyeing was probably introduced into Italy as early as the 11th century. With the establishment of direct trade with India by sea, supplies of it were more easily obtained, and after the- discovery of America a similar product was brought from the new world. Francisco Hernandez speaks of it as in use by the Mexicans, the pigment being called mohuitli and tleuohuilli, signifying the same as the Latin name for it, coeruleum. In the beginning of the 17th century the importations of indigo from the East Indies into Holland assumed no little importance. In 1631 there was brought by seven vessels 333,545 lbs., estimated to be worth $500,000. Its introduction caused great complaint by the Germans on account of its superseding the indigenous woad.
Its use was prohibited by the diet in 1577, and it was denounced under the name of the devil's dye as a pernicious, deceitful, corrosive substance. The people of Nuremberg, who cultivated woad, enacted a law compelling the dyers to take an oath annually not to use indigo, and this they were still obliged to do long after the dye was in universal use. By the French government the use of indigo was forbidden in the province of Languedoc in 1598, and the law was long enforced. A similar outcry was raised against it in England in the reign of Elizabeth, and in 1581 it was condemned by act of parliament, and persons were authorized to search for and destroy it and logwood also in any dye house. This law remained in force nearly a century. - Indigo is a product of numerous plants belonging to the order leguminosae, and indigenous to the tropical regions of Asia, Africa, and America. The genus indigofera contains about 220 species, several of which yield the indigo of commerce. I. tinctoria is the one most cultivated in the East, and I. anil is the most employed in America; these two species are found naturalized in the southern states as remains of former cultivation.
I. Caroliniana and I. lep-tosepala are indigenous species from North Carolina southward, and are said to be useful in domestic dyeing. Indigo is also yielded by a few other genera of leguminosae. These plants contain the coloring principle in their leaves, in the form of a colorless substance which is brought out and rendered apparent by its oxidation as the leaves dry, or is developed by submitting green leaves to a process of fermentation and oxidation. The I. tinctoria is cultivated both in the East and West Indies. It is a shrub, though sometimes cultivated as an annual, 4 to 6 ft. high, with pinnate leaves and rose-colored papilionaceous flowers. The seeds are sown, in March and April in a light soil, and harrowed in. Weeds are removed, but after a few showers the plants cover the ground, keeping out all other vegetation. Before they have reached their full height the plants should be cut, always early in the morning, and carried the same day to the factory. Here they are laid at once in a stone cistern 20 ft. square and 3 ft. deep. Hurdles are placed upon them, and heavy beams are laid across these and secured to the sides of the vat, the object being to keep the plants down when they swell. Water is then admitted so as to cover the plants.
Fermentation soon commences, and may be allowed to go on for 10 to 14 hours, according to the condition of the plants, the temperature, and the weather. The liquor is in commotion as if boiling; frothy bubbles rise to the surface, and their color, first white, becomes grayish blue and then deep purple, and finally a copper-colored scum covers the surface. When the agitation subsides the liquor is drawn off into a lower vat; and the beams and hurdles being removed from the upper one, the steeped plants are taken out to be dried for fuel, and the vat is prepared for another charge. Several men enter the lower cistern and beat up the liquid with their hands or with paddles till the coloring matter begins to appear in small atoms. This may require an hour and a half. The appearance of a precipitate as fine as small sand, leaving the water clear, indicates favorable progress; the beating is then discontinued, and the vat is left a few hours for the indigo to subside. The liquor is then run off from an upper vent, and after this the indigo from a lower one; or the latter is sometimes left in part as the water is drained away, and is then gathered up by a person entering the vat.
Lime and gum have been employed to hasten the precipitation, but their use is considered objectionable from their supposed injurious effect upon the quality of the indigo. The pulpy precipitate is next freed by standing in another cistern from more of the water mixed with it, and is then passed through a strainer into a boiler, in which it is heated to ebullition, and by some kept boiling for five or six hours. Being freed from scum, it is drawn off into a vat, from which, after subsiding, more water is taken off the top, and the rest is removed to the dripping vat, a wooden case having its floor perforated with holes and covered with a woollen cloth. The liquor passes through this filter, and the operation is completed by subjecting the residue to the action of a press, forming it into a cake, which is cut by a wire into 64 square blocks. These are laid out upon hurdles to dry in the shade, and left for several days or weeks in the drying house before packing. By the other method the leaves separated from the stems are dried in the sun, and then stored. When a large quantity is collected they are infused with six times their bulk of water, and stirred for two hours till the leaves all sink.
The liquor is then drawn off, beaten, and further treated as in the process already described. - The Asiatic commercial indigo is brought from the several ports of India, and from Java and Manila. It differs much in quality and in shades of color. The best Bengal indigo shipped from Calcutta is the superfine or light blue, in cubical cakes, so light as to float upon water, friable, soft, of clean fracture, and of beautiful copper color when rubbed with the nail. Other qualities are of shades of violet, red, and copper color. The African indigoes from Egypt and Senegal are fine blues, but generally contaminated with earthy matters. The best American qualities, as some of those from Guatemala and Caracas, are equal to the best Bengal. These countries furnish a considerable portion of the indigo of commerce. The southern portion of the United States exported annually in the early part of the present century about 134,000 lbs. of indigo, worth 62 cts. per lb. Up to the time of the civil war it was cultivated in Florida and South Carolina, where the yield was about 60 lbs. to the acre, and the crop required attention from July to October. In 1871 the United States imported 1,994,752 lbs., about equal portions coming from Bengal and from Central and South America. A very superior quality is now produced at Bogota. - The coloring matter of indigo, called pure indigo or indigo blue, usually constitutes nearly 50 per cent. of the commercial article, which may be obtained by dissolving out what is soluble in boiling water, then that which alcohol will remove, and finally what hydrochloric acid will take up.
The residue is pure indigo and any silica that may be present. Various methods are adopted by different chemists for determining more exactly the proportions of indigo blue in samples of indigo. Some reduce the coloring matter by deoxidizing agents to indigo white, which is supposed to have been its original condition in the plants, and then precipitating and collecting this. Thus Dr. Dana dissolves the indigo by boiling in caustic soda with cautious addition of protochloride of tin; the insoluble portion being then separated, bichromate of potash recovers and throws down the indigo blue, which when washed with hydrochloric acid is collected and weighed. Others adopt the plan of first taking up impurities by a succession of appropriate solvents; these impurities are chiefly resinous and gummy matters. The pure indigo has also been obtained by another process dependent on its property of volatilizing at the temperature of about 550° F., and condensing in needle-shaped and prismatic crystals. The operation is hastened by mixing the indigo with water and twice its weight of plaster of Paris to a paste, which is spread on an iron plate.
Heated over a spirit lamp, the steam and vapor of indigo separate together, and the latter collects in beautiful velvety crystals, upon the surface of the mass. The pure substance melts nearly at the temperature at which it sublimes, and is also charred and decomposed at about the same. It also ignites and burns with a bright flame, giving off much smoke. The crystals have a beautiful and intense copper color, and when in thin plates they present by transmitted light a splendid blue. Their composition is represented by the formula C16H10N2O2. The substance resists in a remarkable degree the action of the ordinary solvents (sulphuric acid excepted), unless it be first deoxidized, when it readily dissolves in alkalies. The conversion into colorless indigo, though called oxidation, appears to be rather an accession of two atoms of hydrogen, which, according to Liebig, unite with one of the oxygen present, giving to the body the composition of a hydrate, having the formula C16H10N2O + H2O or C16H12N12O2; the indigo blue in this case being an oxide of the same body, C16H10N2O. The facility with which the change is effected, and the readiness with which the indigo regains its blue color and insolubility by exposure to the air, admirably adapt the substance for use as a dye.
It is applied in the solvent state to the fabric steeped in the liquid; and when the cloth is exposed to the air, the insoluble substance is developed with its characteristic color and fixed in the fibres. - Sulphuric acid dissolves indigo blue without changing its color to red, the usual action of acids upon vegetable blues; and when the substance is digested for three days with 15 parts of concentrated sulphuric acid, a deep blue pasty mass is obtained, which dissolves completely in water, and under the Dame of sulphindylic acid, or more properly hyposulphoindigotic acid, is often used in dyeing, and also in the manufacture of the blue inks. Many other beautiful and highly interesting bodies result from this chemical change, and still more from the oxidation of indigo blue; and still another series from its treatment with the alkalies. These have received much attention from eminent chemists, and are particularly treated in Dumas's Traite de chimie appliquee aux arts, vol. viii., in Brande's " Manual of Chemistry," and in Muspratt's "Chemistry." - Indigo has been somewhat used in medicine, but is not at present recognized as a remedy of value. It sometimes produces nausea and vomiting. It colors the stools bluish black, and also passes into the urine.
Indican has occasionally been found in the urine when no indigo has been taken. The dose is from 30 to 120 grains.