for any length of time, but of recovering its original 

 shade when placed in the dark. 



Cobalt blue, the third important blue mineral pig- 

 ment, is known commercially in two forms, "smalt," 

 and cobalt blue. Smalt is really a glass colored by 

 cobalt, and is a very old pigment. In recent years its 

 popularity as a pigment has declined as it is inferior to 

 artificial ultramarine. 



True cobalt blue is a favorite color with artists, 

 particularly with those who work in water-colors, on 

 account of its permanency and mixing qualities. It is a 

 compound of the oxides of aluminum and cobalt, with an 

 occasional trace of phosphoric acid. It may be made in 

 various ways, perhaps the best method being that of dis- 

 solving nitrate of cobalt in water, and to this solution 

 adding sufficient sodium phosphate to precipitate the 

 cobalt as phosphate of cobalt. After this precipitate is 

 washed with water it is mixed with a precipitate of 

 alum and sodium carbonate in the proportion of one part 

 cobalt to eight parts aluminum. This is then heated 

 to a red heat and kept at that temperature until the blue 

 color is fully developed, the process requiring from 

 one-half to three-quarters of an hour. 



The three blue pigments just described ultramarine, 

 Prussian blue, and cobalt blue have practically dis- 

 placed all the older blue metallic pigments, such as those 

 made from copper. Such pigments as mountain 

 blue, Bremen blue, blue verditer, which are all com- 

 pounds of copper; and lime blue, which is a mixture 

 of copper hydroxide and calcium sulphate, are now of 

 interest historically only. But of far greater interest in 



