It has been asserted that this substance was not known in Europe until the time of the discovery of the passage to India round the Cape of Good Hope. But Dr. Bancroft has shown that indigo was brought by merchants from India to Alexandria, and thence to Venice, when that city was the entrepôt of Europe and the East. It doubtless contributed to the excellence which the Italian states first attained in the wool manufacture. The drug was called endigo in Venice, and it is from that city that we have derived its name and use. It was imperfectly known in England under its Spanish name in the sixteenth century, for we find in Hackluyt “Voyages” his instructions to a traveller who was going to Turkey to ascertain “if anile, that coloureth blue, be a natural commodity of those parts, and if it be composed of an herbe.”
The general introduction of indigo into Europe was impeded by legislative enactments, prompted mainly by those employed in industries which it threatened to displace. These were chiefly the producers of and dealers in woad, formerly used exclusively for dyeing blue, and the corporation of woad dyers. When dyers from Italy and Flanders attempted to introduce the superior dyes of indigo, the woad interests were sufficiently powerful to induce the Elector of Saxony to denounce the use of the new dyestuff. It was pronounced in the Diet of the Empire as “a corrosive color,” and “fit food only for the devil,” fressende teufels. Similar propositions were made in England and France, in which latter the free use of indigo was not permitted until 1737.
Although indigo as known in the arts is a product of vegetable origin, we must not omit to notice that one source of its production is the human body. It was discovered some years since that the blue color sometimes found in diseased urines, and in certain suppurations, is due to indigo. Dr. Schunck, in some papers read before the Royal Society, has shown that it is a frequent constituent of urine secreted by persons in a healthy state, and that, in fact, it is produced generally when persons do not take sufficient exercise; and he has several times succeeded in producing it by taking in his food a rather large excess of sugar. He has found this substance also in the urine of beef cattle. It must also be observed that the chemical actions of indigotine with oxidizing agents, showing indigo to have a very close relation to aniline and carbolic acid, both products derived from coal-tar, have produced in the minds of chemists the conviction that indigotine, like alizarine, the coloring principle of madder, will one day be artificially produced from coal-tar.
The plants which are known to furnish indigo are quite numerous, being not less than sixty; they do not all belong to the same family, and none of them contain the coloring principle already formed. The most important belong to the leguminous family, from which most of the vegetable dyes are derived, and to the genus indigofera. The species cultivated and most esteemed are Indigofera tinctoria, I. disperma, I. anil, I. argentea.
The principal source of the indigo of commerce is the Indigofera tinctoria. The accompanying figure is a correct representation of the plant, and we may dispense with a description of its botanical characters, observing only that the plant has a half woody stem, and rises to the height of from three to five feet. The plants exhale a strong odor towards evening in the fields where they are cultivated. The leaves have a disagreeable taste, and rapidly putrefy in water. The plant originated in Campaja, or Guzerat, but is cultivated in Hindostan, China, Java, and in the East Indies generally. It was carried by the Spaniards to South America and the West Indies, and it can be acclimated in all hot countries. The Indigofera argentea, or indigo plant of Egypt, furnishes the indigo produced in that country and Arabia.
The culture of the plant and the production of commercial indigo is carried on a vast scale in Lower Bengal. We have before us a large map, placed at our disposal by an India merchant of Boston, showing the location of each of the hundreds of factories of that important centre of production. These factories have been developed by British enterprise; and India thus receives some slight compensation for the ruin of her cotton manufacture by the same influence.
The propagation of the indigo plant in that country is made by sowing in a thoroughly tilled silico-argillaceous soil. The seed of the plant is sowed annually in the spring or autumn, according to the variety used, some germinating more slowly, and requiring to remain in the ground longer than others. The time of putting in the seed is also governed by the nature of the soil and its position in respect to neighboring rivers. In the lowlands subject to inundations, the indigo ought to be all cut at the period of the rains and inundations, which would destroy the crop in a brief time. Besides, during the rainy period the planter has at his disposal sufficient water to commence his operations of fabricating the indigo, which is the suitable time for beginning the cutting of the plant. The time of cutting the indigo plants is therefore regulated by the elevation of the land and danger from floods. The high lands are always sowed several weeks after those subject to inundations.
The Chinese prick out the young plants in parallel rows, always preserving the land quite clear of weeds. By taking away the blossoms of the plant before their development, they increase the growth of the leaves, and, consequently, the return of indigo; for it is in the leaves principally that the coloring material is found.
In certain localities the planters break off the leaves which have acquired a bluish green tint. But more frequently the whole plant is cut down close to the ground in the months of June or July, when the flowers begin to open. The portion of the plant which remains pushes up quite rapidly, and furnishes a second, and even third, and sometimes, though rarely, a fourth cutting. The quality of the product diminishes according to the number of the cuttings.