Ch. 25] 



DRYING SHRINKAGE 



TABLE 4 



Bonding Strengths for Clay Minerals 



[After Grim and Cuthbert (1945)] 



469 



1 Optimum H2O content is the amount of tempering water necessary for maximum 

 compression strength in mixtures of 8 percent clay and 92 percent standard testing 

 sand. 



2 Green compression strength. 



3 Dry compression strength. 



In ceramic processes, it is necessary to have only sufficient dry and 

 green strength so that the ware can be handled without deformation 

 and stacked in a kiln before firing. In general any clay material with 

 a fair amount of clay mineral (±50 percent) and non-clay material 

 that is not sorted or coarser than sand will have adequate strength. 

 Strength can be given to a clay deficient in this property by the ad- 

 dition of some organic binder or a strong natural clay like a montmoril- 

 lonite clay (bentonite) or a ball clay. Ball clays are very fine-grained, 

 light-burning clays that are composed largely of kaolinite but usually 

 contain some organic material which is partly responsible for their 

 strength. 



As noted for montmorillonite, the strength of a clay may be related 

 to its exchangeable-base and soluble-salt composition. The addition of 

 soda ash to some clays increases their strength, particularly the dry 

 strength. 



Drying Shrinkage 



Economic use of a clay requires that the drying shrinkage be uni- 

 form, small in amount, and develop without cracking the formed piece. 



