As will be seen from the chemical analysis of raw umber, it contains quite a high percentage of manganese dioxide, which constituent exerts a strong drying influence on linseed oil, even when it is not incorporated with the oil by heat. Umber, whether raw or burnt, that is put up in containers for the trade where it is liable to be held in storage for long periods, should not be mixed and ground in boiled linseed oil. Not only on account of its liability to form skins, but for the reason that when used in large portions with white to obtain certain effects on exterior painting, it would if not used judiciously by omitting much of the dryers, exert the same bad influence on the life of the paint, as if an excess of dryer had been used. If the dry pigment is in fine powder and of soft texture, it is best ground on 20 or 24-inch esopus stone mills, running at no higher speed than that suggested for grinding sienna, as too high a temperature in the mill is very apt to darken its natural color. If it is harsh in texture, buhr stones are required to grind it down to proper fineness without needing many runs. Raw Turkey umber of good average quality will require its own weight of raw linseed oil to produce a smooth paste, while American raw umber usually requires thirty-five pounds of raw oil to sixty-five pounds pigment for one hundred pounds of paste.
For use as a japan or coach color, none but the best grade of raw Turkey umber of the olive hue should be selected, and it is best to temper the very quick drying of the color somewhat by the addition of raw linseed oil, this addition depending upon the strength of the japan. The usual proportions being 46 per cent by weight of pigment, 50 per cent japan and 4 per cent oil.
For artists' tube color only the very warmest olive toned raw Turkey umber should be selected and ground very fine in poppyseed oil, on stone mills of small diameter, not permitting too high a temperature, so as not to dull the natural tone. Here as in the ordinary oil color, oil and pigment will be about equal in weight. While raw umber is not used to any great extent ground in water, it is all the same necessary to have it on the list of distemper colors, as it is desirable for producing certain effects in fresco or distemper painting. To produce one hundred pounds of finished paste will require a mixing of sixty pounds water with fifty pounds pigment, the loss being caused by the evaporation of the water during the grinding process.