470 grim. CERAMICS AND CLAY COMPOSITION [Ch. 25 



Clays which require large amounts of water to develop the plastic 

 state will show high drying shrinkage. 



In general montmorillonite, when present in more than small 

 amounts, increases shrinkage greatly. Clays composed of kaolinite 

 and illite tend to have moderate drying shrinkage unless the com- 

 ponent clay minerals are exceedingly fine-grained (minus 1 micron). 



Non-clay mineral components such as quartz and feldspar reduce 

 the drying shrinkage when they are present in a clay. Shrinkage 

 characteristics are in general improved by the presence of moderate 

 amounts (25 to 40 percent) of such components. It is common practice 

 to add non-clay material to reduce the high shrinkage of some clays. 



To be of commercial value a clay must form ware that will not be 

 sensitive to considerable variation in drying conditions. The ware 

 must not check or crack during drying, even though there is a large 

 variation in time, temperature, or relative humidity during the drying 

 operation. Actual tests are necessary to determine this point, but, in 

 general, clays with very high clay mineral content (particularly if the 

 clay mineral is very fine-grained) and montmorillonite are difficult to 

 dry satisfactorily. 



Firing Characteristics 



Firing characteristics of a clay are: its shrinkage during firing; its 

 color after burning; the temperature range during which it vitrifies; 

 its resistance to heat or refractoriness; and the strength, texture, and 

 other properties of the fired ware. Small quantities (a few percent) of 

 certain components of clays, notably alkalies, alkali-earths and iron, 

 may exert a controlling influence on the firing characteristics of a clay. 

 The influence of certain of these minor components may depend on 

 whether the component is present alone or mixed with some other con- 

 stituent. Thus the effect of iron on color varies somewhat with the 

 presence of lime and alkalies in a clay. Furthermore, the conditions 

 under which a clay is fired, the rate of firing, and oxidizing or reducing 

 conditions also influence the firing properties. 



It follows from the foregoing that variations in firing characteristics 

 are exceedingly difficult to predict, and that actual firing tests are 

 necessary for the evaluation of the burning properties of a clay. How- 

 ever, a few general relationships seem to be established. 



Kaolinite clays tend to be light-burning (white if pure) and re- 

 fractory. The color darkens, usually to a shade of red, and the re- 

 sistance to heat decreases as the kaolinite is mixed with increasing 

 amounts of illite and montmorillonite. Obviously iron in the non-clay 

 minerals (limonite, pyrite, etc.) will also cause a change in color, and 



