Malachite

Synonyms:

Malachite, Green Verditer, Green Bice, Mountain Green, Green copper Carbonate, Vert De Montagne, Berggrün, Malachitgrün

This green copper mineral was employed as a paint by the ancients. It often accompanies the blue carbonate, and occurs in many European, Asiatic, African, and American localities. The mines at Ekaterinburg and Nischne Tagilsk in Russia, and at Burra-Burra in South Australia, furnish malachite of fine colour; it also occurs abundantly in Namaqualand. Its variations in depth of colour are due less to impurities than to differences in its state of aggregation. Its specific gravity is about 4. Its composition is that of a hydrate and carbonate of copper. It may be represented by the formula CuCO₃,CuH₂O₂. It contains, therefore, less of the copper carbonate than azurite, or blue verditer.

Malachite. Ref.: Wikimedia Commons, User "Robert_Lavinsky"

Malachite requires no other treatment than careful grinding to fit it for use as an artists' pigment. The raw material must, however, be carefully selected, and all visible impurities, such as ochreous veins and deposits, and azurite, completely removed. An artificial malachite was prepared and largely used in the seventeenth century, and is still often substituted for the mineral; but it is inferior in colour and stability to the native form.

Malachite as an oil-paint has often proved to be permanent, although it may seem to acquire a dull, brownish hue, owing to the darkening and yellowing of the oil; sometimes, however, it becomes somewhat olive in colour. In admixture with cadmium yellow it is liable to blacken. It is so easily injured by impure air when unprotected by any hydrofuge, that it is quite inadmissible as a water-colour. In old tempera paintings it is sometimes found to have stood well; but the sulphur from the egg-medium and from the size has not infrequently browned it.

Malachite is sometimes adulterated with baryta white; sometimes a mixture of that pigment and an arsenical green is substituted for it. The former falsification may be detected by boiling the sample in hydrochloric acid, when the malachite dissolves, leaving the baryta white as a sediment. To detect an arsenical green, a small portion of the sample should be mixed with powdered charcoal, gently warmed at first in a long narrow test-tube to drive off moisture, and then strongly heated; a dark sublimate of metallic arsenic will form on the cooler part of the tube.