This method serves both for an assay of the sample and the production of a standard sample of pure indigotine. The operation may be carried on upon a larger scale for the manufacture of a standard sample.
In the processes given I have not referred to the qualitative analysis or testing for all the kinds of adulterations, but have given only valuation of the coloring power of indigo.
I have had occasion to search indigo for Prussian blue, an occasional adulterant. This is ascertained by caustic potash, which becomes in part an oxide if Prussian blue is present. This acidulates with muriatic acid, and, tested with sulphate of iron, proves, by formation of Prussian blue, the presence of the ferrocyanide of potash in the solution, and hence Prussian blue in the indigo. Lime and clay are the usual adulterants, and oxide of iron is often present accidentally or from the clay adulterants. Starch and flour are rarely used, as they add little to the weight.
Before proceeding to a consideration of the practical applications of indigo in manufacturing, we must pause to make some general observations upon the commerce in indigo.
The first European impulse given to this commerce was made by the Spanish and Portuguese. They not only imported indigo from the Indies, but established its fabrication in their colonies. To them we owe its production in Guatemala, Caraccas, and Brazil. The French exported from the Island of San Domingo, only, in 1774, 2,350,000 pounds weight of this commodity. British influence was exerted in favor of the development of this article in the American colonies, and, in 1773, in the space of twelve months, over a million pounds of indigo were exported from South Carolina. The production in India was at that time of little importance. It was not until 1783 that the attention of the English was directed to the culture of indigo in India for European consumption, that produced by the natives being all consumed in their own manufactures. In the hands of the English this product rapidly rose to be the most important of India, in a commercial view, except that of rice. The small cost of a factory, and the comparatively small capital required for this production, caused the indigo culture to be preferred to sugar planting. The importation and sale of this commodity at the East India House, in 1792, amounted to 581,827 lbs., while the importation into Great Britain from other parts of the world amounted to 1,285,927 lbs. In 1806 the importation from the East Indies, and sales at the East India House, amounted to 4,811,700 lbs., and produced in sterling money £1,685,275. In the year 1862–63, the export from India, and the destination of supplies, were as follows:—