True Umber
Umber, properly so called—also known as Turkish, Cyprian or Sicilian umber, from the country of origin—derives its name, according to some authorities, from the province of Umbria (Italy), where a brown earth is found, though others ascribe it to the Latin “umbra” (shade) because of the pigment being used for painting shadows.
True umber is an earthy mass of fine texture and liver-brown colour, merging into chestnut in some of the lumps. Chemically, it consists of a double silicate of iron and manganese combined with water, a portion of these metals being usually replaced by alumina. The greater hardness (1·5) and higher specific gravity (2·2) of true umber in comparison with Cologne earth (which is quite arbitrarily termed “umber”), form a ready means of differentiation between the two.
According to Viktor Merz, the umber found in Cyprus consists of: ferric oxide, 52%; manganese oxide, 14·5%; and alumina, 3%; and is, possibly, merely a mixture of clay with hydroxide of iron or manganese. An umber examined by Klaproth contained 13% of silica, 5% of alumina, 48% of ferric oxide, 20% of manganese oxide and 14% of water.
The tone of umber can be modified, in the direction of red, by calcination, but this process is seldom employed, the dark brown shade of this colour being the one most appreciated.
In some parts of northern Germany, Thuringia in particular, the iron mines contain smaller or larger pockets of ferric hydroxide, of a fine earthy texture, from which umber is prepared, by levigation and calcination. The product is sold under various names: chestnut brown, wood brown, mahogany brown, bistre flea brown, roe brown, according to the shade of the calcined pigment.
A mineral (“siderosilicate,” according to Von Walterhausen) composed of ferric silicate, and approximating in this respect to terra di Siena, is found in the neighbourhood of Passaro (Sicily) in deposits of tuff. It forms masses which are transparent at the edges and are usually liver-brown to chestnut in colour. The hardness of the mineral is 2·5, the sp. gr. 2·713, and the average chemical composition: silica, 34%; ferric oxide, 48·5%; alumina, 7·5%; and water, 10%.
The foregoing are only a few examples of brown or red-brown earth colours. In all these minerals the pigmentary principle is iron, in combination either with oxygen alone (ferric oxide), with oxygen and water (ferric hydroxide), or silica compounds (ferric silicate), and always associated with certain quantities of other metallic oxides, especially alumina and manganese oxide. Although but few of these minerals have gained any special reputation as pigments, there is no doubt that similar minerals, which are certain to occur in or near many deposits of iron ores, could equally well be used for that purpose. There is no need to emphasise that the discovery of such a mineral would be a very valuable find, and that the products obtainable therefrom could be utilised to great advantage.
The testing of a mineral for its suitability as pigment is a very simple matter, all that is required being to subject a small quantity to the same treatment that is applied to the earth colours on a large scale. For this purpose a few pounds of the mineral are levigated, and the residue is dried. To ascertain the tones obtainable by calcination, small samples—of about 100 grms.—are placed in crucibles, and gradually heated in a furnace. When the masses have attained a sufficient temperature, the samples are taken out of the furnace, at intervals of ten minutes, and left to cool. It will then not be difficult to decide whether the mineral is at all suitable for the purposes of the colour-maker; and if so, these tests afford at once an indication of the temperature and time the mineral must be heated in order to obtain pigments of definite tones.