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Asphalt, asphaltum, or mineral pitch, has long been used as a pigment. The best known is that from the Dead Sea (Lacus asphaltites). Other abundant sources of this carbonaceous mineral occur in Trinidad: Caxitambo and Berengela, in Peru; Val de Travers, Neufchâtel; Avlona, in Albania, etc.
Asphalt is rather a mixture of minerals than a single mineral; it is, moreover, very variable in the nature, character, and proportion of its constituents. Essentially it consists of a number of liquid, semi-solid and solid, colourless hydrocarbons (related to the paraffins), associated with certain ill-understood dark-brown or black substances, which constitute the useful part of the raw material. The best varieties for artistic use are those which contain the smallest proportion of the above-described hydrocarbons, for to the presence of these the treacherous character of asphalt as a pigment is due. On this account the hardest, most earthy and most brittle kinds should be chosen, and the crushed samples should always be submitted to a temperature of at least 250° C. before being ground in oil or turpentine. The operation of roasting native asphalt - keeping it over a slow fire 'till it will boil no more and becomes nearly a cinder' - was recommended by Williams in his 'Essay on the Mechanic of Oil-Colours' (1787), and furnishes a perfectly satisfactory and safe product.
If carefully-selected asphalt be submitted to either of the processes named above, and then be moistened with spirits of turpentine, and ground in drying-oil (prepared with borate of manganese), a paint is obtained which neither cracks nor moves on the canvas like the unpurified material. Its fixity is further ensured by mixing it with a little copal varnish, and more particularly by associating it with a denser pigment, such as umber or flake-white. It is superior to the imitative asphalts made from coal-tar, now largely sold in lieu of the original and genuine product. The disadvantages attending the use of these coal-tar browns and of ordinary asphalt are two-fold. Not only are they treacherous on account of their easy fusibility, but they are liable to stain contiguous pigments by reason of their solubility in oil or varnish. When used successfully by the older artists they were always introduced sparingly, or were largely commingled with more solid paints.
'Mummy,' as a pigment, is inferior to prepared, but superior to raw, asphalt, inasmuch as it has been submitted to a considerable degree of heat, and has thereby lost some of its volatile hydrocarbons. Moreover, it is usual to grind up the bones and other parts of the mummy together, so that the resulting powder has more solidity and is less fusible than the asphalt alone would be. A London colourman informs me that one Egyptian mummy furnishes sufficient material to satisfy the demands of his customers for twenty years. It is perhaps scarcely necessary to add that some samples of the pigment sold as 'mummy' are spurious. Mummy was certainly used as an oil-paint at least as early as the close of the sixteenth century.
Asphalt, after having been heated to drive off the hydrocarbons previously alluded to, cedes to ammonia a considerable quantity of a dark-brown colouring-matter, which in this way may be made available for water-colour painting. The ammoniacal solution is either evaporated slowly, to the consistence of a thick syrup, after the addition of a little gum and glycerin, or it is precipitated with acetic acid, and the precipitate (after washing) is mixed with gum and glycerin, and then partially dried until it has acquired a suitable degree of consistency. But the water-colour paint thus made is not permanent.
Merimée's process for preparing asphalt for use as an oil-colour cannot be recommended. He introduces shellac, white wax, and Venice turpentine into the mixture, as well as a large proportion of boiled linseed-oil. This preparation constitutes a very treacherous pigment.
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