BLUE, one of the seven primary colors, of the same shade as the clear sky and the turquoise, and located in the solar spectrum between the green and the violet. The blue pigments commonly employed by artists are few in number, including native and artificial ultramarine, cobalt, indigo and Prussian blue. Genuine ultramarine, prepared from the min eral lapis lazuli, and ordinary cobalt blue, sold for artists' work, are permanent colors. They are used either alone, or mixed with other pigments, chiefly for skies and distances in landscape, and by themselves, or to make up grays and other mixed tints in figure painting. Owing to the exceptionally high price of real ultramarine, the artificial color, which is of doubtful permanency, is usually substituted for it. Prussian blue and indigo are highly useful colors, since it is only these that yield dark blues, and only from them, mixed with yellows or browns, that strong greens can be obtained. It is unfortunate accordingly that both are more or less fugitive. All the blues above named are used both in oil or water color painting, but indigo less than the others in oil, since it is most apt to fade in that medium.
A number of different names are used in commerce for what is essentially the same pig ment, or for pigments closely resembling one another. The following statement gives some explanation of these: Cobalt blues are mix tures of cobalt with earthy or metallic bases, which have been subjected to the action of heat, and have received the following names: Cobalt blue, cerulean blue, royal blue, Du mont's blue, Saxon blue, Thenard's blue, Leith ner's blue, Hungary blue, Zaffre or enamel blue, Vienna blue, azure blue and Paris blue. The
last name is also applied to a Prussian blue, and azure is also given to a variety of ultra marine blue. Smalt is a powdered cobalt glass used in illumination and flower painting. Arti ficial ultramarine is also called French ultra marine, French blue, new blue and permanent blue. Coarse qualities of this color are largely used by house painters. Intense blue is a refined indigo. Prussian blue (sesqui-ferro cyanide of iron) is otherwise named Berlin blue, Paris blue and ferrocyanide of iron. The name Paris blue is also given to a cobalt color. Antwerp blue is a variety of Prussian blue made lighter by the addition of an aluminous base, and not so permanent. Blue ochre (hy drated phosphate of iron) is a subdued per manent blue, but not much employed. Blue verditer is a hydrated oxide of copper which changes and ultimately blackens by time. It is used in distemper work and paper staining. Blue was adopted as their distinctive color by the Scottish Covenanters in the 17th century and is the usual color of the uniforms of the soldiers of the United States army for dress uniforms. A dark shade of this color is generally worn by the sailors of most countries, whence the term navy blue is de rived. The dyestuffs yielding a blue color are the following: Alizarin blue; aniline blue; an thracene blue; chemic blue, a solution of in digo; Coupier's blue; dahlia blue; diamine blues; ethyline blue; indigo blue: now made synthetically; logwood blue; methylene blue; night blue; resorcine blue; sulphur blues; and Victoria blue.