The geological characteristics of Cologne earth enable one to conclude that, where similar conditions prevail, materials of analogous nature may be discovered. This earth is found embedded in a deposit of brown coal, in which it forms pockets, and occasionally large bodies. Now, brown-coal deposits of enormous extent occur in very many localities, as for instance in Upper Austria and in Bohemia; and many of these mines are sure to contain pockets of brown-coal mould, which have perhaps been overlooked, but might very well be utilised in the preparation of colours of very similar character to Cologne earth. The preparation of this material is very simple. The earth coming from the deposits is put through a simple levigation treatment which leaves, as residue, lumps of semi-decomposed wood, mineral admixtures, sand, etc. The levigated earth is sold in the form of cubes. Cologne earth comes into the market under various other names, such as: umber, Cassel brown, Spanish brown, etc. The fiery brown which was so greatly preferred by the famous painter Van Dyck, and named Vandyke brown after him, was of very similar composition to Cologne earth, and is said to have been obtained from a deep brown peat earth. The Vandyke brown of the present day, however, is almost invariably a ferric oxide pigment, toned to the proper shade by suitable calcination. 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