A. B. Ferric oxide 68·75 63·85 Zinc oxide 1·29 1·23 Copper oxide 0·50 0·88 Sulphuric acid 9·80 13·59 Water 15·52 18·45 Clay and Quartz 4·14 2·00 The preparation of the ochre is a simple matter, consisting in collecting the mass and sorting out the loose, earthy portions of a pure yellow colour from the denser and darker parts. The former are dealt with separately, usually by a simple process of levigation, for the sole purpose of getting rid of the earthy matter, quartz sand in particular. The denser varieties require much more work, but yield a far superior product, which, by suitable treatment, can be converted into the finest grades of ochre. The first operation consists in a very careful crushing, and as the pieces are often very hard, they are treated in ordinary or stamp-mills, edge-runners being also employed with advantage. The product reduced by any of these means is passed through a number of sieves, to separate the fine particles from the coarse; and the finest dust is burnt. This last treatment causes a considerable loss in weight, both the accompanying water and most of the sulphuric acid being volatilised. However, since, as already stated, all varieties of ochre can be obtained, the process is consequently very remunerative notwithstanding the loss in weight it involves. _Yellow Earth._--From the particulars given in the general description of the earth colours, yellow earth may also be regarded, to some extent, as an ochre, but one containing a large proportion of foreign substances. It might, however, be more accurately termed a clay contaminated with a considerable amount of quartz sand and a certain proportion of ferric oxide. The method of preparation is on the same lines as for ochre, but burning is never practised, nor is the treatment so careful as for the better grades of ochre, the low price of the colour making this unremunerative. CHAPTER VI RED EARTH COLOURS The number of minerals that can be directly used as red earth pigments is comparatively small, and by far the greater proportion consist of ferruginous colours, a few of which are obtained by the mechanical treatment of native iron ores or clays coloured red by ferric oxide, the majority, however, being formed by burning certain materials of another colour. To these belong nearly all the materials mentioned in connection with the ochres and the brown iron colours, together with a few by-products of the chemical industry.