247. CASSEL EARTH,
Terre de Cassel, or, corruptly, Castle Earth, is specially an oil pigment, similar to burnt umber but of a more russet hue. It is an earth containing bitumen, a substance which, with pit-coal, lignite or brown coal, jet, petroleum or rock oil, naphtha, &c., is looked upon as a product of the decomposition of organic matter, beneath the surface of the earth, in situations where the conditions of contact with water, and almost total exclusion of atmospheric air, are fulfilled. Deposited at the bottom of seas, lakes, or rivers, and subsequently covered up by accumulations of clay and sand, the organic tissue undergoes a kind of fermentation by which the bodies in question are slowly produced. The true bitumens appear to have arisen from coal or lignite by the action of subterranean heat; and very closely resemble some of the products yielded by the destructive distillation of those bodies.
Rich as is the tone of colour of Cassel earth, it is apt to lose this in some measure on exposure to light. Mérimée remembers to have seen a head, the brown hair of which had been painted partly with the earth alone, and partly with a mixture of the earth and white; yet the hair where the white was employed was darker than that painted solely with the brown, the white having fixed the colour. To compensate for its thus fading, it should be mixed with pigments that are permanent, such as umber and lamp black. Like all bituminous earths, it needs the strongest drying oil. By calcination, a greater degree of intensity may be imparted to the colour, and perhaps a little more solidity. In landscapes it is of much service for the most vigorous portions of foregrounds and the trunks of trees, as well as for painting cavernous rocks or deep recesses in architecture. Compounded with burnt lake and a little Prussian blue, it gives a black the most profound.
248. CHALON'S BROWN
is a water-colour pigment, transparent and inclining to red; deep, full, and very rich. On exposure to light it becomes less russet, but is otherwise strictly stable.
249. COLOGNE EARTH,
incorrectly called Cullen's Earth, is a native bituminous earth, containing less bitumen than Cassel earth, and therefore drying more quickly. Darker than that variety, it is less transparent, and covers better. In its general qualities it resembles Vandyke brown, except that in combination with white, it affords a range of cooler brown tints. Useful for the shadows of buildings, it does not wash so well as sepia, and is preferred occasionally on that account. By some it has been called durable, by others branded as fugacious. According to Bouvier, brown hair represented by this colour has been known to disappear in six months, all the brown vanishing, and nothing remaining but a few black lines of the sketch. As it is similar in composition to Cassel earth, the safest course would be to mix it with umber, and not to employ it alone. Calcined, it acquires a reddish hue.
250. INDELIBLE BROWN INK.
Although this cannot be classed as a pigment, yet, being very useful in water-colours, it may be proper to describe its qualities. The ink is a rich brown fluid, and, as its name imports, is indelibly fixed on the paper as soon as it is dry; thus allowing the artist to work or wash over it repeatedly, without its being disturbed. If diluted with water to its faintest tint, it still continues to retain its indelible properties undiminished. It is generally used with a reed pen, and employed chiefly in architectural details and outlines.
Various brown inks, principally solutions of bistre and sepia, were adopted in sketching by Claude, Rembrandt, and many of the old masters. In modern times, a beautiful transparent brown for water-colour artists, known as Liquid Prout's Brown, has been extensively employed. This contains less fixative than the indelible ink, and is the vehicle with which nearly all Samuel Prout's drawings were executed.