86 CLAYS AND CLAY INDUSTRY. 



Minimum. Maximum. 



Kaolins, 20 60 



Fire clays, (Flint clays) 150 



Brick clays, 50 300 



Pottery clays, 50 250 



The washed ball clays of New Jersey showed the lowest ten- 

 sile strength of all the samples tested, while the clay from the 

 railroad cut north of Alloway was the strongest of the series 

 examined, and had an average tensile strength of 453 pounds 

 per square inch, with a maximum of 506 pounds. 



With such a variation existing in the tensile strength of clays, 

 it becomes a matter of importance to< know the cause of this 

 variation. It is a well known fact that all clays, shrink in drying, 

 and that this shrinkage is accompanied by a drawing together of 

 the particles. Indeed, some clays shrink to such a hard mass as 

 to suggest a close interlocking of the grains, which, it seems to 

 the writer, may be the explanation of the tensile strength shown ; 

 that is to say, those clays in which the interlocking of the par- 

 ticles is the tightest will show the highest tensile strength, and 

 vice versa. If this is true it becomes necessary to determine, if 

 possible, what arrangement or size of particles produces the 

 tightest and strongest structure. 



When any series of clays is tested it is evident that the highly 

 sandy ones have a low tensile strength, 1 and very fine-grained 

 ones are in all cases, as far as the writer's experience goes, also 

 low in their tensile strength. 2 Many samples having a high 

 tensile strength have considerable grit, but still they shrink to a 

 very dense body, indicating a considerable percentage of fine par- 

 ticles. E. Orton, Jr., 3 attempted to determine the effect of the 

 fineness of grain on the tensile strength of clays by taking a very 

 fine-grained clay and mixing different sizes of sands with it, the 

 sand being obtained by grinding and screening vitrified bricks. 

 His conclusions were "(1) that the tensile strength of mixtures 

 of a plastic ball clay with equal quantities of nonplastic sands 



1 See Asbury clay, sample 695 S, Table, Chap.. XVIII. 



2 See Raritan clay, sample 723. Table, Chap. XVIII. 



3 Transactions American Ceramic Society, Vol. II, p. 100, and Vol. Ill, p. 

 198. 



