PHYSICAL PROPERTIES OF CLAYS. 27 



the clay, the quantity of organic matter, the size of the 

 grain, and the amount of combined water. 



The amount of .water absorbed, and the texture in- 

 fluence the air-shrinkage which begins as soon as the 

 water commences to evaporate from t'he clay. It has 

 already been mentioned that a clay keeps on losing in 

 weight after the shrinkage has ceased, and this fact is 

 well shown by the following experiments on some Ala- 

 bama samples. 



The clay was from property of J. C. Bean, Sec. 31, T. 20, 

 R. 11 w. 



After moulding, the clay weighed 35.698 grams. 



At end of 24 hours the shrinkage was 11 J 



per cent, and the weight 30.891 ' 



At end of 48 hours, shrinkage 12 per cent., 



weight 29.588 " 



At end of 6 days, shrinkage 12 per cent., 



weight 29.460 u 



At end of 8 days, shrinkage 12 per cent., 



weight 29.140 " 



At end of 12 days, shrinkage 12 per cent., 



weight 29.093 " 



Throughout this period the clay was kept exposed to a temper- 

 ture of 70 Fahr. 



The shrinkage is generally equal in all three direc- 

 tions, and consequently only the linear shrinkage is 

 given. The greater the shrinkage of a clay the more 

 danger there is of its cracking and warping in burn- 

 ing, and when there is any apprehension that this may 

 occur, an attempt is made to prevent it by t'he addition 

 of grog (burned clay) which diminishes the shrinkage. 



Coarse grain clays having larger pores permit the 

 water to escape more rapidly, and hence can be dried 

 more quickly than fine grained ones, from which the 

 water can not very readily escape. If the 

 drying of fine grained clays is hastened, the surface 

 shrinkage is more rapid than that of the interior and 

 cracking ensues. We might perhaps expect that on 

 account of their greater porosity ; the fine grained 

 clays would absorb more water, and consequently 

 shrink more in drying, but the Alabama clays do not 

 always bear out this fact. 



The fire shrinkage generally commences when the 



