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.
The most skilful connoisseurs distinguish forty-three varieties of Bengal indigo. The most important are the following:—
1. Superfine blue, light or floating.—Color bright blue; light, friable, and spongy; adherent to the tongue, soft to the touch, showing coppery reflections when rubbed by the nail; paste uniform and pure.
2. Fine blue.—Like the preceding, but the color a little less vivid.
3. Violet blue.—A little less light and friable. Has a violet blue.
4. Superfine violet.
5. Superfine purple.
6. Fine violet.
7. Good violet.
8. Red violet.
9. Ordinary violet.
10. Good soft red.
11. Good red.
12. The indigoes, fine coppery, good coppery, ordinary coppery, and low coppery.
The Indigoes of Oude and Coromandel.—These are made in the interior of Hindostan. Those of the best quality correspond to the middling Bengal indigoes, and are met with in square masses, having an even fracture, but are more difficult to break; the inferior qualities are heavy, of a sandy feel, having a blue color, bordering on green or gray, or even black; often in large squares, and covered with a slight crust or rind of a greenish color. They are the most difficult to break of all the indigoes of commerce.
Madras Indigoes.—They have a grained fracture, and are of a cubical figure. The superior qualities have no rind. The qualities are fine blue, mixed violet blue, and ordinary. They are all lighter, and less rich in coloring matter than the Bengal indigoes.
Manilla Indigoes.—These occur in cubical blocks, flat squares, or in irregular pieces. They are light, with a fine paste, and of a clear blue. They effervesce with acids, showing the presence of carbonate of lime incorporated in their paste. They are consequently poor in coloring material, and are hence almost exclusively used as a bluing material in washing fabrics.
American Indigoes. Guatemala.—These indigoes are produced now altogether in Hunduras, although they still retain in commerce the name of Guatemalan. They are generally found in small pieces, irregular in form and size, and come in envelopes of skin containing about half as much as the Bengal chests. Putting aside the difference in exterior form, these indigoes approach very closely to those of Bengal. The same qualities are found, only they are more frequently mixed. The clear blue is more rare, and, when it is found, it is poorer in coloring matter. In purchasing these indigoes it is necessary to beware of the reds, which often contain a strong proportion of the brown extractive matter. It is not rare to find among the Guatemalan indigoes beautiful specimens of the blue violet, equal to the richest Bengal variety. Unfortunately, this superior variety is generally mixed with inferior kinds, as to have less value. The American indigoes are classified as follows:—
Guatemala floro.—Bright blue, paste uniform, soft and light. This variety, in Bancroft’s time, was the most esteemed of all indigoes.
Guatemala sobresaliente.—Less light, the paste firmer and the blue less beautiful.
Guatemala corte, or copper-colored.—Paste less firm and heavier, coppery red.
Caraccas.—These resemble very much the Guatemala varieties. The qualities are designated by analogous names, but they are, in general, less esteemed than the preceding.
Mexican.—They hold an intermediary rank between the Caraccas and Mexican.
Brazil.—These indigoes are in small rectangular parallele-piped masses, or in irregular lumps of a greenish gray color externally, and having a smooth fracture, a firm consistency, and a copper-colored tint of greater or less brilliancy.
The indigoes of Africa and Egypt.—These have only been manufactured within the last twenty years; they are in flat squares. The paste is fine and quite light, and the color pure blue or bordering on violet. The varieties are distinguished as fine blue and good violet and red.
Indigoes of the Isle of France and Senegal. Rare in commerce, but of good quality.
The indigoes of the inferior qualities, characterized by a salt-like color, bordering more or less upon green; by a coarse, uneven, and very dense paste; by not adhering to the tongue, and by not showing a coppery color when scratched,—can never be employed to advantage, notwithstanding their low price. The purchaser of these qualities must be guided solely by the results of analysis; for an article is found in commerce whose richness in indigotine does not exceed twelve to fourteen per cent. The presence of so high a proportion of foreign matter prevents the chemical change which the indigo ought to undergo in the dyeing vat; and this foreign matter, added to the deposits of the dyeing vats, causes great loss of the coloring matter. These indigoes should be used as little as possible, especially in the cold vats used for dyeing cotton and linen. The middle varieties of the Bengal and Guatemala indigoes, and, above all, the red varieties, produce in the cold vats the most advantageous results. The lower qualities above spoken of present less inconvenience in the hot vats used for dyeing wool; and it is for this purpose that they are generally used. In considering the previous observations, the wool manufacturer may arrive at this conclusion: that while he can, with less loss than the maker of cotton fabrics, make use of the lowest qualities of indigo, he will obtain the best results from the middle qualities of the reddish Bengal indigoes.
The skilled dealers in indigo recognize not only the above distinctions, founded upon the country of production, color, and physical qualities, but they observe whether the article has any of the following defects, which are designated by certain well-understood terms: such as whether the indigo is sandy,—when brilliant points are observed in the interior, which are in reality particles of sand; spotted, that is to say, of unequal tint, and marked by small blackish points; ribboned, marked by transversal bands of a paler, and sometimes red color; burnt, the pieces having a scorched appearance, due to rapid drying, and separating into small black fragments under the pressure of the hand; crumbly, when in pieces of irregular figure, proceeding from fractures of the squares; cold, when the indigo does not adhere to the tongue. The above classification is presented with a full knowledge that these distinctions are by no means recognized in the ordinary commerce in this article. It is not, however, without interest as an illustration of the minute attention given to this subject in Europe, where a higher manufacture requires a nicer investigation of the qualities of materials employed.