Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/73

Rh of a decided reddish orange. The orange-yellow variety, when very finely ground, becomes less red and more inclined to yellow. Some of the palest cadmium yellows contain white pigments or flour of sulphur, added to reduce their depth of color. Yellow cadmium is prepared in several ways. A slightly acid solution of any cadmium salt is prepared and through it is passed a current of sulphuretted hydrogen gas. The product thus obtained has a pure chrome yellow shade. A lemon yellow shade may be obtained by dissolving 1 pound of cadmium sulphate in 4 gallons of water and adding $1 1/4$ gallons of the ordinary yellow ammonium, sulphide.

Cobalt yellow is a compound of the nitrates of cobalt and potassium. It is prepared by precipitating cobalt nitrate with sodium carbonate, dissolving the precipitate in acetic acid and adding a strong solution of potassium nitrate. On allowing the mixture to stand for some time the color is gradually precipitated, and is collected, washed and dried, when it is ready for use.

Cobalt yellow is a pure yellow color, and is almost transparent whether used in water or oil painting. Orange ochre also called Spanish ochre is a very bright yellow ochre, burnt, by which operation it acquires warmth, color, transparency and depth.

Mars orange is an artificial ochre similar to the above. It is made by taking equal weights of ferrous sulphate and alum, and adding a solution of carbonate of soda, thereby precipitating the iron and alumina. The precipitate, which forms a yellow pigment, the so-called Mars yellow, is collected, washed well with water, dried and converted into orange, by slightly calcining.

Oxford ochre is a native of the neighborhood of Oxford, England; it is semi-opaque, of a warm yellow color, and of a soft argillaceous texture, absorbent of water and oil, in both of which it may be used with safety, according to