[Illustration: Drawing of the Isatis tinctoria plant]
The leaves which surround the stem are collected in May or June of the second year, when they begin to turn yellow. The wasted and dried leaves are sometimes used directly for dyeing, but more generally the leaves, after being cut and dried, are carried to a mill, and then ground to a paste, after which it is formed into a mass or heap, and being covered to protect it from rain, is left to undergo a partial fermentation for about a fortnight. The heap is then well mixed and formed into balls, which are exposed to the sun and wind to dry, and thereby prevent the putrefaction which would otherwise take place. Being afterwards collected in heaps, these balls again ferment, become hot, and emit the odor of ammonia, which Hume tells us, in the History of England, gave such offence to Queen Elizabeth that she issued an edict to prohibit the cultivation of this plant. After the heat has continued for some time, these balls fall into a dry powder, in which form the woad is usually sold to the dyer. The best French woad comes from Provence, Languedoc, and Normandy. In Germany, the pastel of Thuringia is used almost exclusively; the packages have the trade-mark of three towers, with the numbers 4, 5. In this country, owing probably to the prejudices of practical dyers, who have generally come from England, the Lancashire woad is almost exclusively used. The very little imported of late years, ranging from two thousand to twelve thousand dollars annually in value, is used for mixing with indigo in the so-called woad vat, to be hereafter described.
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.