Indian Red

Synonyms:

Indian Red, Persian Red, Indian Red Ochre, Indischrot

Indian red is a variety of red ochre, or red hematite, containing about 95 percent of ferric oxide, and having a slightly purplish hue. It varies somewhat in quality, and often requires sifting through a fine silk sieve, followed by washing over, in order to fit it for use as a pigment. Most of the Indian red imported from India is a natural product, but some has been prepared by calcination. Some so-called Indian red is imported from Ormuz in the Persian Gulf; some is an English hematite from the Forest of Dean.

A recent recipe for making an artificial Indian red directs that a mixture of 75 parts of green vitriol be taken and dried at a moderate heat previous to mixing it with 18 parts of calcined magnesia and 7 parts of common salt, all the ingredients being in fine powder. The mixture is then ignited, preferably under reduced pressure, and the residuum thoroughly washed with water. By the introduction into the original mixture of a little aluminium sulphate the purple hue of the product may be enhanced; indeed, it is quite possible in this way to obtain the pigment known as Mars violet.

Indian red, when genuine, is a perfectly permanent pigment in all media, and is without action upon other colours. It was extensively employed by the older masters of the English Water-Colour School, in association with true ultramarine, with Prussian blue, with indigo, or with indigo and yellow ochre, to produce the lilac greys of stormy clouds. The indigo in some of these greys having often perished, the Indian red (and the yellow ochre where employed) remains intact, giving a hot and frequently foxy red to spaces which were originally cool in hue, and comparatively neutral. This change has been incorrectly attributed to an action exerted upon the indigo by the Indian red. But as indigo disappears when used alone, or when a thin wash of it on a sheet of gelatine is placed over, but not in contact with, a wash of Indian red, the current explanation of the phenomenon in question cannot be true. Greys made with light red or Venetian red show similar alterations of colour. Colcothar, or jewellers' rouge, the red oxide of iron obtained as a residue when green vitriol (ferrous sulphate) is calcined, has sometimes been called Indian red, and substituted for the native oxide.

Those portions of the above-named residue which have been more strongly heated generally present something of the purplish red hue which belongs to the true native Indian red. And this peculiar hue may be imparted to ordinary rouge by moistening it with a weak solution of potassium chlorate, drying, and then calcining the mass once more. It generally contains basic ferric sulphate, and then should be looked upon with suspicion, for it may seriously damage the indigo and other organic pigments with which it is associated. If a small pinch of Indian red be boiled with hydrochloric acid, thrown on a filter, and the filtrate tested with barium chloride solution, the genuineness of the pigment will be proved by the absence of any white precipitate of barium sulphate.