Indian Red in Oil is listed by some grinders in one shade only, and here a medium shade is selected, while others quote light and dark shades. Indian red is made artificially from green copperas, and Leech, Neal & Co., of Derby, England, practically monopolized the sale of Indian reds in this country for a long time, but of late years several dry color makers succeeded in making inroads on the sale of these reds and also on some of the strong, bright red oxides.
Indian reds, like all the oxide of iron pigments, should be selected by the color grinder by comparing them in making rubouts, testing for fineness, tone, shade and tinting strength, but in testing Indian reds extra precaution is necessary in order to avoid trouble after the oil color is in stock or in store, packed for the market, to see before mixing that any free sulphuric acid is neutralized, and also that it is as free from alkali as possible. When Indian red is being levigated before it is being dried and packed in casks or other containers for the market, an alkali, usually milk of lime, is added to neutralize all traces of acid, but if lime is added in excess it is liable to saponify a portion of the oil on mixing, and so tending to turn the paste into a liver-like mass or at least to make it necessary to add extra quantities of oil to keep the paste in good condition. Any color grinder who may intend to bid on proposals for supplying Indian red, dry or in oil, to the Naval Supply Fund should carefully read over the United States Navy Department specifications, which are very exacting on the acid and alkali feature, in order to save himself from eventual loss through rejection. Chemically pure Indian reds contain from 96 to 98 per cent sesquioxide of iron, balance being silica and calcium sulphate, and can be mixed and ground at the rate of 83 to 84 pounds dry pigment and 16 to 17 pounds raw linseed oil. Boiled oil is not well adapted for grinding Indian red that is put up in containers for sale in paste form, as it is usually too limpid to hold the heavy pigment in suspension. Raw linseed oil free from foots and somewhat aged is best. All of these reds based on oxide of iron are best ground in oil on mills of large diameter if the size of batches warrants it. Any diameter, from 20 inches to 36 inches, will be proper, providing the mill is run at a speed conforming to this.
Mars Red, properly speaking, is an artificial color made by calcining Mars yellow at red heat. Mars yellow should be a precipitate of copperas and alum, but French ocher is sold under that name, and French burnt ocher as Mars red. This burnt ocher will require a mixing of 74 pounds of dry pigment and 26 pounds raw linseed oil for 100 pounds marketable paste.