CLAYS OF NEW YORK 



551 



farther, the combined water begins to pass off at a dull red heat, 

 and the clay shrinks to an additional extent, becoming not only 

 harder bnt denser, till it reaches a condition approaching imper- 

 viousness, and a hardness of about 6. {See scale of hardness, p. 

 855) This condition of hardness commonly indicates the beginning 

 of fusion, not of the whole clay mass, but of the more fusible con- 

 stituents, which soften slightly and bind the whole together. It 

 is called the stage of incipient fusion. In clays that have been 

 burned to this condition, the clay particles are commonly still 

 recognizable. 



With an increase of the temperature ranging from 50° to 

 200° F., or sometimes even more, an additional amoumt. of shrinkage 

 occurs, and most or all of the particles have become sufhciently 

 soft to allow their adjustment to the most compact condition, leav- 

 ing no interspaces, or, in other words, making the burned clay im- 

 pervious. This is spoken of as vitrification, and brick or other clay 

 products burned to this stage are vitrified or completely sintered. 

 The particles are no longer recognizable, and the maximum shrink- 

 age has been reached. With a farther rise in temperature the clay 

 becomes viscous or flows. 



We can therefore recognize three stages in the burning of a 

 clay: 



Incipient fusion ^ 



Vitrification 



Viscosity 



The three stages are not by any means sharply marked, they do 

 not show the same difference in temperature, nor does incipient 

 fusion begin at the same temperature in all clays. 



In general we can say, that other things being equal, the greater 

 the percentage of total fluxes, the lower the temperature of in- 

 cipient fusion, vitrification and viscosity. 



The difference in temperature between incipient fusion and 

 viscosity varies with the composition of the clay. In calcareous 



1 These three terms have been suggested by H. A. Wheeler. Vitrified pav- 

 ing 'brick, p. 12. 1895. 



