98 CLAYS AND CLAY INDUSTRY. 



ing being to stick the grains together. This is termed incipient 

 fusion, but the softening has not been sufficient to prevent identi- 

 fication of the coarser grains in the clay. With a further variable 

 increase in the temperature, depending in amount on the clay, 

 and ranging from 27. 7 C. (50 F.) to 111.1 C. (200 F.) or 

 sometimes even more, an additional amount of shrinkage occurs, 

 and most of the particles become sufficiently soft to allow them 

 to settle into a compact impervious mass, thus closing up all the 

 pores in the clay. This condition is termed vitrification, and a 

 piece of vitrified clay when broken shows a very smooth fracture 

 and sometimes a slight luster, since all the particles except the 

 coarse quartz grains have been welded into a dense solid mass. 

 This condition, since it represents one of the closest compactness 

 of the clay particles, also> represents the maximum of shrinkage. 

 If the heat is raised still further the clay softens so that it can 

 no longer hold its shape and flows or gets viscous. 



We can, therefore, recognize three stages in the burning of a 

 clay, 1 viz. : 



Incipient fusion. 



Vitrification. 



Viscosity. 



It is sometimes difficult to recognize precisely the exact attain- 

 ment of these three conditions, for the clay may soften so slowly 

 that the change from one to the other is very gradual. 



The difference in temperature between the points of incipient 

 fusion and viscosity varies with the composition of the clay. In 

 many calcareous, clays these points are within 27.7 C. (50 F.) 

 of each other, while in refractory clays they may be 377-7° C. 

 (700° F.) to 4444° C. (8oo° F.) apart. The glass-pot clays, 

 which are refractory, but still burn dense at a comparatively low 

 temperature, approach the last mentioned condition quite closely. 



It is of considerable practicable importance to have the points 

 of incipient fusion and viscosity well separated, because in the 

 manufacture of many kinds of clay products the ware must be 

 vitrified or rendered impervious. If, therefore, the temperature 

 interval between the points of vitrification and viscosity is great, 



1 H. A. Wheeler, Vitrified Paving Brick, p. 12, 1895, Indianapolis. 



