Ferric oxide is distinguished by two properties which render it specially valuable to the colour-maker. When combined with water, its colour is no longer red, but a handsome brown; and, on the other hand, when heated, the colour passes through brown into a permanent dark violet. By suitable treatment of such minerals as consist mainly of ferric hydroxide, mixtures can be obtained which contain the oxide and hydroxide in variable proportions and give a whole range of shades between brown and red. The preparation of these colours is easy when very pure red ironstone is available. The somewhat expensive pigment, Indian red, is--when pure--really nothing but a very pure ferric oxide of Indian origin. Ferric oxide, however, often contains impurities which considerably influence the colour of the product. Owing to the fact that large quantities of ferric oxide are formed as by-products in certain chemical processes which are carried out on a very extensive scale, this oxide, which is very pure, can be advantageously used for making iron pigments, especially as its application for other purposes is very restricted, and it can therefore be had at a very low price. The following analyses show the composition of a number of red ironstones, Nos. 1, 2 and 3 being hematite from Froment, or Wetzlar, No. 4 from Wetzlar, Nos. 5 and 6 hematite from Whitehaven, No. 7 from Thuringia, No. 8 from Bohemia, No. 9 from Spain, No. 10 from N. America, and No. 11 from England. +-----+--------+-----------+---------+-------------+-----------+--------+ | | Ferric | Manganese | | | Alumina, | | | No. | oxide. | oxide. | Silica. | P{2}O{5}. | lime and | Water. | | | | | | | magnesia. | | +-----+--------+-----------+---------+-------------+-----------+--------+ | 1 | 94·00 | Trace | 2·00 | -- | Trace | 2·00 | | 2 | 80·95 | -- | 16·74 | 0·51 | 0·97 | 0·83 | | 3 | 73·77 | -- | 23·16 | 0·45 | 1·41 | 1·21 | | 4 | 92·45 | -- | 5·63 | 0·19 | 0·65 | 1·08 | | 5 | 96·27 | -- | 4·20 | -- | 0·59 | 4·40 | | 6 | 63·05 | 5·29 | 22·76 | -- | 0·49 | 4·40 | | 7 | 85·00 | 1·601 | 3·304 | 0·457 | 8·795 | 0·633 | +-----+--------+-----------+---------+-------------+-----------+--------+