Indigo - Coloring Matters Or Dyes

Indigo has been called the "King of Dyestuffs" in recognition of the permanency and strength of its deep blue color, and the supremacy it has maintained over all rivals from the time of its first use in India thousands of years ago even to the present day, although an artificial indigo is now coming into use. Curiously enough the blue coloring matter is not present as such in indigo-plants. It is derived from a substance called indican (C₂₆H₃₁N0₁₇) which is extracted by water from the leafy shoots, and, under the influence of an enzym which accompanies it, gives rise to a compound resembling glucose and to indigo blue (C₁₆H₁₀N₂O₂). A substance which thus decomposes into a sugar and some other compound is known as a glucoside. Indigo blue is insoluble in water and can therefore be separated along with certain impurities by filtration. The pasty mass retained is dried in cakes to form the indigo of commerce. The insolubility of indigo blue in water presents a peculiar difficulty to its use as a dye, yet at the same time gives it a great advantage when once it is incorporated with a fiber. The difficulty is overcome by taking advantage of the fact that indigo blue may be readily changed (in various ways which increase the proportion of hydrogen) into a colorless substance called indigo white (C₁₆H₁₂N₂O₂) which is soluble in dilute alkaline solutions and has the fortunate property of quickly changing back to indigo-blue on exposure to the air. The means commonly employed by dyers to change the indigo-blue is to add indigo to vats containing lime-water in which bran or molasses or some other substance is undergoing fermentation. When the indigo is all transformed and dissolved, a piece of white woolen or cotton soaked in the solution and then exposed to the air soon takes on a permanent blue color.

A considerable number of plants have been found to contain indican, and several different species are cultivated in India and other warm countries for the manufacture of indigo. Of these plants the most important one is the dyer's indigo shrub (Fig. 275).

Fig. 275.-Dyer's Indigo Shrilb (Indigofera tinctoria, Pulse Family, Leguminosoe). Flowering branch; a, flower, enlarged; b, standard (uppermost petal), back view; c, wing (side petal), inner view; d, e, keel-petal, inner and outer views; f, flower with corolla removed; g, pistil, h, fruit, natural size; i, seed; k, same, cut vertically. (Berg and Schmidt.)- Shrub growing 2 m. tall; leaves downy beneath; flowers reddish yellow, fruit dry. Native home, Southern Asia.