PAINTS AND VARNISHES 377 



obtained from coal tar products) are used. Such colors, 

 however, fade as a rule within a short time after being ex- 

 posed to light. Some of the metallic pigments are changed 

 by the action of light, air, and sulphur fumes. Lead, for 

 instance, is turned black by the fumes of burning coal. 



When paints are mixed, the color of the mixture is deter- 

 mined, not by the mean average of the individual colors, but 

 by the nature of colors which are reflected, i.e., not absorbed, 

 by both pigments. If all colors are absorbed then the 

 body is black, while if no colors are absorbed the body is 

 white. 



A great many artificial lights, such as yellow gas light, 

 are lacking in certain colors. Consequently the reflected 

 light from them will differ from the reflected sunlight, and 

 the color will be correspondingly different. 



421. Chrome Yellow. Chrome yellow (PbCrO 4 ) is a 

 solution or a powder of dichromate. Zinc sulphate added 

 to the yellow lightens the color. The basis of all yellow pig- 

 ments is chrome yellow, which is formed by precipitating a 

 chromate of lead, zinc, or barium. The shades may be modi- 

 fied by the addition of lead, barium, or calcium sulphate in 

 a grinding mill. Lead chromate has a great covering power, 

 but is blackened by sewer gas (hydrogen sulphide). It 

 should not be mixed with any substance that contains 

 sulphur. 



422. Red Lead. For protecting surfaces of metals, etc., 

 a red paint, called red lead (Pb 3 O 4 ) is used. It is made 

 by heating lead in air to a molten state to convert it into 

 litharge (PbO) and by then slowly heating the litharge until 

 it acquires the desired color. 



