28 GENERAL DISCUSSION OF CLAYS. 



combined water begins to pass off, and it may be just 

 as variable as the air shrinkage. In fine grained clays, 

 as those from near Prattville, the shrinkage from 

 burning was found to be comparatively uniform, 

 while on the other hand moderately fine grained 

 kaolin from Rock Run shrank more rapidly as it ap- 

 proached the temperature of vitrification. 



Sometimes the clay instead of shrinking during the 

 burning appears to expand ; and this is especially the 

 case with very quartzose ones, for the quartz has the 

 property of expanding at high temperatures. This 

 expansion of siliceous clays may sometimes be respon- 

 sible for the presence of cracks in the burned ware. 



As the addition of quartz to diminish the shrinkage 

 also tends to decrease the tensile strength of the clay, 

 there will be a certain limit beyond which it must not 

 proceed. 



Organic matter and combined water tend to in- 

 crease the shrinkage in burning, but lime has the opo- 

 site tenlency. 



Clays containing a large amounfof feldspar will, in 

 stead of showing a steady shrinkage up to the temper- 

 ature of complete vitrification, often exhibit a tempo- 

 rary increase of volume when the fusing point of the 

 feldspar is reached. 



The shrinkage of most clays in burning doesi 'Ot 

 proceed regularly and steadily up to the temperature 

 of vitrification, for some clays attain their maximum 

 density at a comparatively low temperature, below 

 that at which thev vitrify. Thus the plastic clay of J. 

 C Bean, near Tusca;loosa, attains its maximum 

 shrinkage at cone 5, but does not vitrify until cone 27. 



Between the point] at which the moisture seems to 

 pass ofE and that at which the combined water begins 

 to escape, the clay shrinks little or none at all, and 



