546 NEW YOEK STATE MUSEUM 



one anotiier, but this does not mean that all the mechanically com- 

 bined water has passed off, for there may remain spaces which hold 

 some. Consequently a clay will continue to lose weight after the 

 air shrinkage has ceased. This fact is shown by the following 

 tests made on samples of clay of 'New York state, 



A sample of soft, moderately plastic shale was mixed with water 

 and molded into briquets and then allowed tO' dry. 



Weight 

 When molded Air in 



shrinlsage grams 



When molded 41 . 220 



End of 12 hrs life 38 . 030 



End of 1 day 2^ 37.616 



End of 3 " 2fo 37.500 



End of 4 "... 2fo 37.496 



Coarse-grained clays commonly shrink less than the fine-grained; 

 they may at the samie time absorb as much water. Having larger 

 pores, they will permit the water to escape more; rapidly, and 

 hence can often be dried more quickly than fine-grained clays, 

 from which the water, on account of the smallness of the pores, 

 can not escape so quickly. Again, if fine-grained clays are dried 

 rapidly, the surface shrinks more quickly than the interior, and 

 cracking may ensue, more specially if the clay has a low tensile 

 strength, or if it is highly plastic. 



Air shrinkage begins as soon as the clay is molded and set out to 

 dry, at first taking place very rapidly, but with decreasing speed. 

 It is in nearly all cases co^mpleted before the brick or wares are 

 placed in the kiln. The final traces of moisture are nat driven 

 off, however, till the first stages of burning. 



Fire shrinkage. This generally commences when the combined 

 water begins to pass off, or at about 1200° E. It varies in differ- 

 ent iClays and may reach any point betweeii 2^ and 15^. 



The shrinkage in burning may be just as variable as that in dry- 

 ing; it does not depend on the same causes but is influenced by the 

 temperature to which the clay is exposed, percentage of combined ' 

 water and organic matter. 



