30 GENERAL DISCUSSION OF CLAYS. 



that all of the particles become sufficently soft to per- 

 mit their adjustment into a condition of greater com- 

 pactness, leaving no interspaces, or in other words, 

 the clay becomes impervious. This condition is 

 spoken of as virtification, or complete sintering. The 

 particles of the clay are no longer recognizable, and 

 the maximum shrinkage has been reached. With a 

 further elevation of the temperature the clay mass 

 fusses completely, and becomes viscous or flows. 



We therefore can recognize three stages in the burn- 

 ing of the clay, i. e., incipient fusion, vitrification and 

 viscosity.* 



The points of incipient fusion and viscosity may be 

 within 75 degrees Fahr. of each other as in calcareous 

 clays, while in some fire clays they may be as much as 

 500 or 600 degrees apart, and furthermore the point 

 of vitrification does not necessarily lie midway be- 

 tween the two. 



Most clays show a difference of from 200 to 400 

 degrees Fah^. between the points just mentioned, and 

 it can be easily understood the farther apart these 

 two points, the safer will it be to burn the clay, for it 

 is not always possible to control a kiln within a range 

 of a few degrees of temperature, and therefore in burn- 

 ing a mass of ware to vitrification if this point lies too 

 near that of viscosity, there is danger of overstepping 

 it and reaching the latter. 



The fusibility of a clay depends on : 



1. The amount of fluxes. 



2. Size of the grain of the refractory and the non- 

 refractory constituents. 



3. The condition of the fire, whether oxidizing or 

 reducing. 



"These three terms have been suggested by H. A. Wheeler, Vitrified 

 Paving Brick, 1895. 



