the cadmium yellow, which is made by the action of sul- 

 phuric acid upon the metal, is preferred. 



Mars yellow, which is quite a favorite with artists, is 

 a mixture of oxide of iron with calcium or aluminum 

 sulphate, while aureolin is a double nitrate of potassium 

 and cobalt. There are also yellows made from arsenic, 

 mercury, antimony, and thallium that are sometimes 

 used for pigments; but none of these has any great 

 value commercially as compared with the chrome yel- 

 lows enumerated. 



The familiar yellow seen in the gilding of picture- 

 frames, which rivals gold in luster, is a bisulphide of tin 

 (SnS 2 ). It is known as "mosaic gold," and is made by 

 heating together tin filings, sulphur, and ammonium 

 chloride, the relative proportions being varied con- 

 siderably by different manufacturers. In the heating- 

 process care must be taken not to raise the temperature 

 too high. It is to prevent this that the ammonium 

 chloride is used. This salt volatilizes at a relatively 

 low temperature, and so long as this volatilization is 

 going on, the general temperature of the mass contain- 

 ing it will not be raised high enough to injure the 

 product. By this process a pigment may be made, 

 the luster of which closely rivals the pure metal it is 

 made to imitate. 



A cheaper and inferior mosaic gold can be made by a 

 wet process, in which the bisulphide of tin is precipitated 

 from the solution of a tin salt by the action of sulphur- 

 etted hydrogen. When prepared in this way, however, 

 the pigment not only lacks luster, but is of a distinctly 

 inferior color. 



[284] 



