PAINTS, DYES, AND VARNISHES 



the sulphur has ceased to burn; like the brown it is 

 unstable, as the substance burns on exposure to the air. 

 Following the green comes the blue, which is formed 

 when the temperature has reached about 700 C., or a 

 bright-red heat; when the temperature gets higher the 

 color changes to a violet. With still higher tempera- 

 tures, first a red, then a white variety is formed. These 

 changes are due to oxidation; when the white ultra- 

 marine is heated with reducing agents, such as carbon, 

 the colors are re-formed in the reverse order to that in 

 which they first appeared." 



The ultramarine as it comes from the furnace must 

 be "finished" by grinding, and there are other manip- 

 ulations subordinate to the process described. The 

 essential thing is that it is possible now to make this 

 most useful pigment for such a small amount that 

 it is one of the cheapest of painters' colors. 



A blue pigment that is a close rival of ultramarine is 

 Prussian blue (known also as Berlin blue, or Chinese 

 blue) which is a ferric ferrocyanide a complex com- 

 pound of iron, carbon, and nitrogen, the last two ele- 

 ments combined in the form of the radical known as 

 cyanogen. It was discovered accidentally by the 

 Prussian color-maker, Diesbach, early in the nineteenth 

 century. Diesbach was engaged in the manufacture of 

 a red lake pigment, when, happening to use an alka- 

 line solution which had been employed in some treat- 

 ment of ox blood, he found that quite by chance he had 

 produced a beautiful blue pigment. Following up this 

 accidental discovery, he soon was able to produce the 

 pigment, Prussian blue, and place it upon the market 



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