The process for obtaining indigo from the Nerium is altogether the same, but hot water has been generally applied to the dried leaves. For woad, hot water must be employed, and also lime water as a precipitant, on account of the small proportion of indigo in the plant. Dilute muriatic acid is digested upon the woad indigo to remove the lime, without which no dye could be precipitated. According to the warmth of the summer and the ripeness of the plant, from 2 to 5 ounces of indigo may be obtained from 100 pounds of the dried woad, or upon an average 4 ounces to the hundred weight.
The indigo found in European commerce is imported from Bengal, Coromandel, Madras, the Mauritius, Manilla, and Java in the Eastern hemisphere; from Senegal, Caraccas, Guatimala, Brazil, (South Carolina and Louisiana in small quantity), and formerly from the West India islands, especially St. Domingo. Its quality depends upon the species of the plant, its ripeness, the soil and climate of its growth, and mode of manufacture. The East Indian and Brazilian indigo comes packed in chests, the Guatimala in ox-hides, called surons.
The organ which affords the indigo is confined entirely to the pellicle of the leaves, and exists in largest quantity at the commencement of maturation while the plant is in flower. The indigofera is remarkable for giving a blue tinge to the urine of cows that feed upon its leaves.
According to some manufacturers, the plants should be cut down in dry weather, an hour or two before sunset, carried off the field in bundles, and immediately spread upon a dry floor. Next morning the reaping is resumed for an hour and a half, before the sun acts too powerfully upon vegetation; and the plants are treated in the same way. Both cuttings become sufficiently dry by three o’clock in the afternoon, so as to permit the leaves to be separated from the stems by threshing. They are now thoroughly dried in the sunshine, then coarsely bruised, or sometimes ground to powder in a mill, and packed up for the operations of manufacture.
In the spring of 1830 I subjected a variety of specimens of indigo to comparative analyses, by dissolving a few grains of each in strong sulphuric acid, diluting the solutions with an equal volume of water, and determining the resulting shade of colour in a hollow prism of plate glass, furnished with a graduated scale. The following are the results, compared to the shade produced by a like weight of absolute indigo.
I. East India Indigos; prices as at the last October sales.
| No. | Price. | Real indigo in 100 parts. | Characters by the Brokers. | ||
|---|---|---|---|---|---|
| s. | d. | ||||
| 1 | 3 | 9 | 42 | Broken, middling violet, and coppery violet spotted. | |
| 2 | 3 | 6 | 56 | ·5 | Ditto, a little being coppery violet and copper. |
| 3 | 3 | 3 | 46 | ·0 | Ditto, middling red violet, dull violet and lean. |
| 4 | 4 | 3 | 54 | ·5 | Large broken, and square, even middling red violet. |
| 5 | 4 | 2 | 75 | ·0 | Much broken and very small, very crumbly and limy, soft, good violet. |
| 6 | 4 | 9 | 60 | ·0 | Square and large broken, 1⁄2 middling violet, and 1⁄2 good coppery violet. |
| 7 | 5 | 3 | 70 | ·0 | Large broken, very good; paste a little limy, good violet. |
| 8 | 6 | 6 | 60 | ·0 | Square and large broken, soft, fine paste, fine violet. |
| 9 | 6 | 0 | 66 | 2⁄3 | Square, ditto, good red violet. |
| 10 | 7 | 0 | 75 | Square, ditto, fine purple and blue. | |
| 11 | 2 | 3 | 37 | ·5 | Middling ordinary Madras. |
| 12 | 3 | 6 | 60 | ·0 | Good Madras. |
| 13 | 4 | 3 | 58 | ·0 | Very fine ditto. |
| 14 | 2 | 0 | —— | Low, pale Oude. | |
| 15 | 2 | 4 | 27 | 3⁄4 | Middling, ordinary Oude. |
| 16 | 3 | 3 | 54 | Good Oude. | |
| 17 | 1 | 9 | 29 | Lundy, very low quality. | |
II. American Indigos; wholesale prices at present. (March 1830.)
| Indigo. | No. | Price. | Parts in 100. | ||
|---|---|---|---|---|---|
| s. | d. | ||||
| Caraca flor. | 1 | 6 | 6 | 54 | 1⁄2 |
| Guatimala | 2 | 5 | 5 | 33 | 1⁄2 |
| —— | 3 | 3 | 3 | 19 | |
| —— | 4 | 4 | 4 | 32 | 1⁄2 |
| —— | 5 | 5 | 5 | 50 | |
| —— | 6 | 5 | 5 | 50 | |
| —— | 7 | 5 | 5 | 35 | |
| —— | 8 | 4 | 4 | 46 | |
| —— | 9 | 4 | 4 | 33 | 1⁄2 |
| —— | 10 | 5 | 5 | 50 | |
Properties of Indigo.—It possesses a dark blue colour, passing into violet-purple, is void of taste and smell, dull, but by rubbing with a smooth hard body, it assumes the lustre and hue of copper. It occurs sometimes less and sometimes more dense apparently than water, which circumstance depends upon its freedom from foreign impurities, as well as upon the treatment of its paste in the boiling, pressing, and drying operations. It is insoluble in water, cold alcohol, ether, muriatic acid, dilute sulphuric acid, cold ethereous and fat oils; but boiling alcohol and oils dissolve a little of it, which they deposit on cooling. Creosote has the property of dissolving indigo.