Powdered malachite (even the dark green varieties) is always rather light in colour, and for this reason is not much used. Furthermore, the mineral is rather hard (3·5), and is consequently difficult to grind; in addition to which the mineral is fairly expensive, on account of its employment as a source of copper, particularly fine pieces being also used as ornaments or for making works of art. Moreover, like all copper compounds, it is very sensitive to the action of sulphuretted hydrogen, and liable to discoloration in course of time. (F) BROWN EARTH PIGMENTS Numerous minerals are adapted for the manufacture of brown pigments. On the basis of chemical composition, they may be classed in two groups; those consisting of ferric hydroxide, and those in which the brown colour is due to organic substances. The first group comprises the minerals which have already been mentioned in connection with the red earth pigments, bole and brown ochre (umber), Terra di Siena, Cologne earth and a number of other earths rich in ferric hydroxide belonging to this category. The second, or organic group, includes compounds that are very rich in carbon and are therefore of a very dark colour, the shades ranging from light brown to black, _e. g._ the true umbers and asphaltum. _Umber_ As already mentioned, the term “umber” was formerly applied to brown varieties of ochre, whereas at present it is extended to certain masses of brown-coal character, often interspersed with iron ochre and sometimes containing manganese. Umber generally consists of fairly dense, earthy masses, which are dried and ground--after crushing and levigation, if necessary. Valuable varieties are Cappagh brown and Caledonian brown, both with a reddish tinge. It is thus evident that “umber” now implies two different kinds of materials, organic masses and iron-manganese compounds, which can also be used as oil paints. These umbers can also be extensively shaded by burning, the final colour being particularly influenced by the amount of manganese compounds present. The carbonaceous umbers (Cassel brown, Carbon brown) are combustible, and mostly leave behind a merely small residue of ash. An important property of these umbers is their partial solubility in alkalis, a peculiarity which is utilised for the preparation of brown wood stains. _Asphaltum_ Asphaltum forms very friable dark brown to black masses, which, in contact with a light, easily ignite and burn with a bright, but very smoky, flame, disengaging a peculiar, “bituminous” smell, and leaving only a very small quantity of ash. Extensive deposits of asphaltum are found at the Dead Sea, the Pitch Lake on the island of Trinidad, in Dalmatia, and many other places, where, however, it is in an impure condition and frequently contains large quantities of sand. In many localities the rock is impregnated with asphaltum, which makes it dark brown to black in colour and gives rise to a bituminous odour when rubbed.