"All these ingredients are ground together into a 

 homogeneous mass; this is a point of great importance. 

 The mixture is loosely packed into crucibles fitted 

 with flat lids, which are luted on by means of mortar. 

 When the mortar-luting is dry the crucibles are piled in 

 ovens large enough to hold 400 to 500 crucibles. . . . 

 After all the doors and openings into the oven are made 

 up, it is fired to a bright-red heat for several hours, the 

 length of time varying considerably and depending upon 

 a number of factors such as the state of the weather, the 

 composition of the mixture, etc. Experience is the only 

 school in which the ultramarine-maker can learn how to 

 regulate the time required. 



"After the heating, all the apertures are carefully 

 closed, so as to exclude air, and the furnace allowed to 

 cool for four or five days; the oven is then opened, 

 and the crucibles withdrawn and opened, the con- 

 tents turned out, and the badly burnt pieces care- 

 fully separated; the good portions are ready to be 

 finished. 



"The changes which go on during the heating of the 

 mixture are both curious and interesting. The mixture 

 when first put into the crucibles is of a grayish color, 

 but during the process of burning it passes through a 

 series of color-changes brown, green, blue, violet, red, 

 and white. The brown appears with the blue flames, 

 due to the burning of the sulphur; it is a fine chocolate- 

 brown but is very unstable; on exposure to the air it 

 enters into combustion. Many efforts have been made 

 to preserve it, but these have been fruitless. The 

 green, which is the next change, begins to form when 



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