Asphaltum Brown (Bitumen)
As a natural product, which can be used as a painters’ colour without any special preparation, asphaltum (bistre, bitumen) may also be classed among the earth colours. Chemically, it is composed of hydrocarbons of various kinds, and is thus similar to tar; in fact, asphaltum may also be regarded as a natural tar resulting from the decomposition of various organic substances. Many deposits of this mineral are known, and two of them are particularly celebrated: those of the Dead Sea, in Syria, and the Lake of Asphalt, in Trinidad. Both deposits consist of craters filled with water on which the asphaltum floats in large cakes.
Several kinds of asphaltum are met with in commerce, ranging in colour from brown to black. The preparation of the material as a pigment is confined to grinding the mass, which is always of a low degree of hardness. Being readily soluble in oil of turpentine and then furnishing the most beautiful brown tones when laid on thinly, the pigment is usually sold in this condition, although it is also ground in oil for the same purpose.
Finally, it may be mentioned that various useless materials can be transformed, by suitable treatment, into brown pigments closely resembling Cologne earth and applicable to the same uses. Such pigments can be prepared from brown-coal slack (from inferior brown coal) or bituminised wood—a variety of brown coal looking like charred wood—by treating these materials with a lye made from wood ashes and lime, and washing and drying the residue.
CHAPTER VIII
GREEN EARTH COLOURS
Although the number of green-coloured minerals is large, but few of them are suitable for painters’ colours, because they occur so rarely in Nature that their employment for this purpose is out of the question, more especially since a very large number of green pigments can be obtained by artificial means. The most important of the earth colours in this category are Celadon green, or green earth, and malachite green—the latter, however, less so, because the substance of which it is composed can be prepared artificially.