Polychromy was known to them, but it is very difficult to decide whether they coloured in tempera or in fresco, with wax or by some other means--perhaps even in oil. The incrustation of clay walls with baked and painted, and even glazed, tiles it is difficult to explain. In many instances, as at Vurka, each cone has its own colour, and, by a proper arrangement, squares, imbrications, diapers, and networks were produced; but at Babylon and Nineveh the tiles show clear marks of painting. Diodorus (probably after Ktesias) gives us a description of the interior circular wall of the royal palace at Babylon, and says: ‘It was decorated with all sorts of coloured human and animal forms baked in clay, much resembling nature. The whole represented a hunt. The figures were more than four yards high. Semiramis was to be seen by the side of her husband Ninus, she killing a panther, he piercing with his dart a lion.’ The principles of decoration with the Assyrians far surpass mere ornamentations in geometrical patterns. Dramatic life of a higher kind is introduced, showing greater artistic power than the reliefs in stone. The outlines are neither black nor red, the treatment is tasteful, the colours a tender blue, brown, white or yellow, and the ground a lightish green. These paintings are not mosaics, as at Vurka. The baking and glazing must have taken place in the following manner. The unbaked tiles were arranged and numbered in a horizontal position on the ground, then they were painted and placed vertically on the wall as a tapestry decoration. When the wall was finished it must have been heated, so as to change the light and fluid colours into a kind of enamel, giving the clay-wall a thin terra-cotta coating.