COMMERCIAL INDIGOES.
The following description of the indigoes of commerce is taken principally from Schutzenberger’s excellent treatise on coloring materials. It coincides very nearly with that given by Napier from Dumas and Chevrueil. Indigoes are classed, according to their origin, into three groups.
1. Indigoes of Asia (from Bengal, Oude or Coromandel, Manilla, Madras, and Java).
2. Indigoes of Africa (Egypt, Island of France, Senegal).
3. Indigoes of America (Guatemala, Caraccas, Mexico, Brazil, and the West Indies).
The three varieties in most esteem are those of Bengal, Java, and Guatemala.
Indigoes of Java.—These are distinguished by the great purity of their coloring material. They contain the minimum of extractive organic matter. If, in spite of this, they do not give a high yield of indigotine; this is owing to a mixture of silicious mineral substances with their paste. The paste is soft. It adheres strongly to the tongue, and its density is feeble. They are generally of a pure blue, light or ash colored in the kinds which are less rich, and of a magnificent violet blue in the superior qualities. The last take a beautiful copper color when scratched by the nail. They are placed in the very first rank among all indigoes in respect to fineness and beauty, if not in richness in the blue coloring principle. Their purity, complete absence from carbonate of lime, and the small quantity of foreign organic materials which they contain, cause them to be much sought for, for the preparation of carmine of indigo. The consumption of the Javan indigoes in this country is so small as not to be appreciated.
Bengal Indigoes.—These are the indigoes par excellence, for in them are found the most varied qualities, from the most beautiful and rich to the most ordinary. The superior qualities are of a deep violet blue, with a fine and uniform paste; they adhere to the tongue, are easily pulverized, and take a beautiful coppery tint when scratched by the nail. The fresh fracture shows a magnificent purplish blue reflection. Their yield in indigotine does not surpass seventy-two per cent.
After these come the reddish-violet indigoes with a purplish hue, and a fracture more uniform and shiny. They are also more dense and hard than the superior qualities. The reddish hue does not proceed from the greater or less amount of coloring material contained, but from the presence of a greater quantity of brown and red extractive matter. These qualities are not to be despised, for the kinds which give the best results in the dyeing vat are found in these indigoes. It would seem, in fact, says the author whom we are following, “that the browns and reds of indigo play an important part in vat dyeing, that they are able to become dissolved and to fix themselves upon the tissues at the same time as the indigotine, and thus operate to reinforce the hue. The fact is, that dyers generally prefer the reddish indigoes to the other varieties.” Among the Bengal indigoes there is found a clear blue variety, less rich in coloring matter, but also more exempt from organic substances. The impurity is constituted by mineral matters. It is less dense, adheres strongly to the tongue, and does not take a coppery hue, like the other varieties, when scratched by the nail.
The worst qualities of the Bengal indigoes, as in all the species, are the clear blues, shading on to gray or green. This coloration denotes a great quantity of extractive matter different from the indigo brown which characterizes the red varieties, and completely inert. These indigoes are hard, dense, adhere little or none to the tongue, and do not show coppery reflections when scratched.