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This earth is found in several localities; the best variety has come for some time past from Cyprus. A considerable number of Cypriote specimens, of several nuances, some excellent, were shown in the Colonial and Indian Exhibition of 1886. It differs chemically from the yellow and brown ochres in several particulars, notably in the presence of a considerable quantity of one of the higher oxides of manganese (Mn3O4, or MnO2), and in the small proportion of water which it contains. Samples from English localities are poor in iron; one Derbyshire specimen gave Mr. G. H. Hurst no less than 30 percent of barium sulphate. An analysis of my own, made with a choice sample from Cyprus, showed the following percentages:
This sample had the peculiar greenish hue so much prized by artists. It should be stated that a part of the manganese probably existed as Mn3O4.
Before being used as a paint, this brown mineral is finely ground, washed with water, and then dried at 100° C, or at a slightly higher temperature. When, by a stronger heat, the whole of its water has been expelled, the umber acquires a reddish hue, and is then the pigment known as burnt umber. This change of colour is due to the passage of the brown ferric hydrate into the red ferric oxide, and to an increase in the proportion of the red-brown manganese oxide present.
Raw umber in powder, after having been purified, soon acquires a very slight reddish hue on exposure to light and air; it is a good plan to place the undried, finely-ground mineral on trays in the sunshine before completing its desiccation and mixing it with oil or other medium.
Raw umber is permanent when used with each or any of the painting media: the slight yellowish or dull aspect which it acquires in oil may be traced to the augmented translucency of the paint, and to the yellowing of the associated oil. Umber is without action on other pigments. A very few samples of umber, used as a water-colour, have been observed to fade slightly after from five to ten years' exposure to sunlight. But this deterioration is due to the presence of traces of brown peaty acids, or 'humus' substances, which occasionally occur in the umbers from certain localities.
Raw umber possesses a semi-opaque, citrine-brown colour; it works and dries well in oil. Associated with transparent blues, it yields soft, quiet green hues; it is invaluable both in figure and landscape painting. As a priming or first painting-ground, it is apt, like most dark pigments, to become more conspicuous in time, owing chiefly to the translucency which the superimposed painting gradually acquires.
Raw umber is not subject to adulteration, but a ferruginous brown coal has been occasionally substituted for the true mineral. The great variation in quality shown by the umbers of commerce is due, in great part, to the difficulty of securing, even from the same mine, continuous supplies of the same excellence.
Burnt Umber. It has before been pointed out that raw umber, from which burnt umber is prepared by calcination, is not an ordinary ochre, but owes its colour in great measure to the presence of a considerable amount of some compound of manganese. The exact constitution of raw umber is not, however, known, although the slight change in hue which occurs when it is roasted negatives the idea that it contains any considerable proportion of the ordinary ferric hydrates. If these were present in notable proportions, roasting would certainly redden raw umber much more than is actually the case.
Burnt umber differs in quality and hue from the raw earth mainly in being more translucent, and of a warmer brown. Although some Continental authorities affirm that this pigment darkens and becomes purplish in course of time, I cannot regard it as otherwise than perfectly permanent, and as exerting no action on stable pigments.
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