=Indian Red= consists of ferric oxide. It was originally obtained from a very pure hæmatite, occurring in India, by grinding and levigating. It can, however, also be artificially made, in shades varying between bright red and dark brownish-red, by heating pure ferric oxide. This valuable pigment is extremely brilliant and durable. The darkest brownish-red shades can be obtained from Indian red by mixing it with varying quantities of litharge, and heating the mixture very strongly in a covered crucible. CHAPTER XX. OTHER RED MINERAL PIGMENTS. =Chrome Red or Chrome Vermilion.=—Under the most varied names, chrome orange, Persian red, Derby red, Chinese red, Indian red, chrome vermilion, etc., there occur in commerce numerous pigments, orange to dark red in colour, which can all be made from neutral lead chromate. When this salt, _i.e._, ordinary chrome yellow, is treated with smaller or larger quantities of a strong base, such as caustic potash or caustic soda, “basic” lead chromates are formed, which exhibit a more intense red shade the more lead they contain. These compounds can be made either by adding caustic potash to the solution of potassium chromate used for making the chrome, or the precipitated chrome yellow may be treated with caustic potash. The latter process readily yields good results, so that it is generally followed. Recent researches have shown that differences in the shade of chrome red are due to the varying size of the single microscopic crystals of which it consists. When chrome reds of the most different shades are ground the powder has almost the same colour. The larger are the crystals the deeper is the shade of the red, so that the art of making deep chrome red lies in working so as to produce large crystals.