This section is from the "A Complete Dictionary of Dry Goods" book, by George S. Cole. Also available from Amazon: A complete dictionary of dry goods and history of silk, cotton, linen, wool and other fibrous substances,: Including a full explanation of the modern processes ... together with various useful tables.
Indigo. A well-known and exceedingly valuable blue dyeing substance. It has been in use in Europe since the First century, being mentioned by Pliny as indicum. When it made its appearance in England it was termed indico. As a commercial material, indigo is entirely obtained from the vegetable plant called indigofera, which grows from a height of 4 to 5 feet, producing an abundance of leaves. It is in the leaves that the indigo-yielding principle chiefly resides, and these are most gorged with it at the period when the flower-buds are about to open. It is then that the plant is cut down, in India the same stalk yeilding in one year the second and even the third crop of stems and leaves. The method of preparing indigo in Bengal where the best quality is at present made, is to take the fresh green stems and leaves and beat and bruise them, after which they are fermented in vats of water, depositing a blue substance which is collected and dried in the form of the square cakes seen in commerce. Indigo sells in India at the rate of 97 cents to $1.80 per pound, depending on the quality and the state of the market. The different qualities of indigo which come to this country are classified according to their sources, as follows: Bengal, Gautemala, Java, Mexico and Brazil. The indigo imported into the United States in 1887 amounted to 2,961,453 pounds; in 1888, 3,466,665 pounds; in 1889, 3,161,084 pounds; in 1890, 3,550,871 pounds.
Indigo once and for a considerable period formed one of the staple exports of this country, while now it is one of the heaviest articles of import. It has been manufactured in America from the earliest days of its discovery; it was commonly used in Mexico when that land first became known to European nations, and was afterwards extensively cultivated by the Spaniards in Central America. In the early days of the West India colonies it was an article of great export. The island of Jamaica was very successful in its cultivation. Indeed, such attention did the West India planters pay to the dye that their indigo was sought after in Europe in preference to that of old India. The dye rose to such eminence as an article of trade, that the British Parliament in keeping with its usual hoggishness, in the year 1745, with the view of securing a large revenue, laid the heavy tax of 75 cents per pound on all the indigo manufactured in their West Indian colonies. From this date the cultivation declined. It was a case of goose and golden egg, being literally killed out of the country by over-taxation, and, although the tariff was repealed finally and a bounty of 12 cents per pound was offered by the home legislature the industry never regained its former position. In the Colonial days of America, from 1747 to 1792, the Southern States produced vast quantities of first-quality indigo. It was to Carolina and Georgia what tobacco was to Virginia - their principal agricultural product. In 1790 the Southern Colonies supplied Europe with the same amount of indigo that Bengal did - about 600,000 pounds. About the year 1800 the export of this dye stuff almost ceased, owing to the impetus Whitney's cotton gin had given the cotton industry. In 1802 indigo began to be imported into America from Bengal, the cultivation of cotton having driven out the once flourishing indigo industry in the short space of ten years.
Indigo is the most important of all coloring matters both as regards the large quantity and monetary value of what is produced and sold, as well as the permanency of the dye colors which it yields. Indigo is distinguished from nearly all other coloring matters by its complete insolubility in water. The only real solvent for it is acetic acid mixed with a little sulphuric acid, from which water precipitates it unchanged, but this solvent is inapplicable for dyeing. Complete solubility is an essential condition for dyeing, and a means was found to obtain satisfactory solutions of indigo by circuitous methods which involve the temporary destruction of its blue color and a change of its chemical composition. By various deoxidizing agents, indigo-blue is changed into a white substance (called indigo-white) which dissolves readily in all alkaline liquids, forming a colorless or slightly yellow solution. On exposure to the air the solution yields the insoluble blue indigo, and permanently dyes any fabric or fiber that has been saturated with it. This is the only case in which such a method of dyeing is followed, and on that account it possesses much interest. The number of successive dips that a piece of cloth undergoes varies according to the shade of blue which the dyer desires. The more "dips" the more permanent the color becomes and the darker the shade.