Green earth is a valuable pigment for all kinds of painting, on account of its extreme permanence. It may be applied directly over lime without suffering any change, whereas most of the cheap green colours are destroyed in like circumstances. This behaviour renders green earth specially valuable in fresco work, although it is also largely used as an oil colour. Augite is of frequent occurrence in volcanic districts; and in such localities, deposits of green earth are certain to be found. The test for the suitability of a green earth consists mainly in treatment with dilute hydrochloric acid. If the mineral assumes a handsome green tone, it will generally form a useful pigment. The test may be supplemented by applying the colour to a fresh coating of whitewash, under which conditions it should remain unaltered. ARTIFICIAL GREEN EARTH (GREEN OCHRE) A product sometimes put on the market as green earth or green ochre has nothing beyond its name in common with green earth properly so called, except a certain similarity in colour. This pigment is prepared by mixing yellow ochre to a thin pulp with water and adding about 2% (of the weight of ochre) of hydrochloric acid. After a few days, a solution of 2 parts of yellow prussiate of potash is added, and if the liquor still gives a precipitate when tested with ferrous sulphate, this last-named salt is added so long as such a precipitate continues to form. The deposit is washed, and dried in the ordinary way. When the right proportions have been taken, a pigment is obtained that coincides fairly in point of tone with true Verona earth. It is, however, inferior in point of permanence, the Berlin blue present being somewhat unstable and decomposing very quickly when brought into contact with lime. The reaction taking place in the production of so-called “artificial Celadon green” is that the hydrochloric acid used dissolves ferric oxide from the ochre, the addition of the yellow prussiate of potash then forming a blue precipitate of Berlin (Paris) blue which, in conjunction with the yellow of the ochre, gives a green-coloured mixture. MALACHITE GREEN Although the pigment sold under this name is nearly always an artificial product, it cannot be omitted from a work dealing with the earth colours, because, in former times, it was prepared exclusively from the mineral malachite. Owing to the fact that artificial malachite green is one of the cheapest of colours, the troublesome work involved in the mechanical preparation of the native pigment has been almost entirely abandoned, and the malachite itself is now utilised to greater advantage as a source of copper. Malachite green (or mountain green) is found in nearly every case where copper ores exist, and is still--though very rarely indeed--prepared, in a few places, from the mineral, the dark-coloured lumps being picked out because the lighter-coloured ones would furnish much too pale a colour.