CLAYS OF NEW YOKK 



547 



It sometimes happens that the clay, instead of shrinking dur- 

 ing the burning, appears to expand, and this is specially the case 

 in very quartzose clays, for the quartz has the property of expanding 

 at high temperatures. If the clay contains a large amount of 

 quartz, the expansion of the latter will not only tend to decrease 

 the shrinkage but may even counteract it and cause the clay to 

 expand. This may sometimes account for the presence of cracks in 

 the burned ware. 



As the addition of quartz to diminish shrinkagei also tends to 

 decrease the tensile streng-th of the clay, there will be a certain 

 limit in each case beyond which the addition of quartz must not 

 proceed, otherwise the clay will not hold together in molding or 

 drying. 



Organic matter and combined water tend to increase the shrink- 

 age in burning, but lime has the opposite tendency, some calcareous 

 clays even appearing to swell. 



Clays containing a large amount of feldspar will, instead of 

 showing a steady shrinkage up tO' the temperature of complete 

 vitriiication or sintering, often exhibit a temporary increase of 

 volume when the fusing point of the feldspar (about 2300° F. for 

 orthoclase) is reached. 



The shrinkage of most clays in burning does not proceed regu- 

 larly and steadily to the temperature of vitrification, for some reach 

 their maximum density at a comparatively low temperature, far 

 below that at which they vitrify. 



Between the points at which the moisture has ceased coming 

 off and that at which the combined water begins to escape, the 

 clay shrinks little or none at all; consequently the heat can be 

 raised rapidly iii this interval, but above and below these two 

 points it must proceed slowly to prevent cracking or warping of 



the ware. 



Method of counteracting' shrinkage 



As many clays shrink to such an extent in drying that they 



crack, it is often found necessary to add materials that will prevent 



