Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/70

62 ammonium sulphide and ammonium oxalate to the filtrate.

The purity of zinc white in oil may be tested by burning out the oil by means of a blast lamp, on an iron spoon or ladle. Take of the zinc white a piece about the size of a pea, place it in the center of the spoon and direct the blast on it until it is burned white and perfectly dry. Crush the white cinder which is left to a line powder and drop this into a glass of diluted sulphuric acid, 1 part of acid to 10 parts of water. If the powder be fine and very little dropped in at a time, it will, if pure, dissolve completely before reaching the bottom and without effervescence. If there be any effervescence it indicates the presence of whiting, which will precipitate as sulphate of lime, which is, however, sparingly soluble, barytes is insoluble, and a considerable adulteration of terra alba is not readily soluble; clay is insoluble.

Gypsum mixes well with either water or oil, and, being neutral in its properties, it can be mixed with all other pigments without affecting them or being affected by them. It is used very largely by paper stainers and makers of wall paper, who prefer it to barytes on account of its having more body when used for that class of work. It is used in finishing of cotton goods, in paper making, and for a variety of other purposes where a cheap white pigment is required.

Whiting is sold under a variety of names, such as Spanish white, Paris white, English white. Whiting is the carbonate of calcium, purified by washing. It is prepared by grinding chalk under water to a very fine powder by passing it through several mills. The powder is run into tanks in which the coarser and heavier particles settle, while the liner chalk passes on to other tanks in which it settles. When the settling tanks are full, the chalk or whiting is dug out and dried. When partially dry it is cut into masses of a cubical shape and dried. When dry it is ground.

Paris white is a finer quality of whiting, but the grinding