90 CLAYS AND CLAY INDUSTRY. 



higher than that of either of the clays alone, or vice versa. 

 While no> experiments were made with the object of proving 

 this point, some results were obtained in the course of the physical 

 work on the New Jersey clays that have an important bearing, 

 as follows : 



Of several clays tested from the Asbury clay, near Asbury 

 Park, one was a slightly gritty, black clay, with an average 

 tensile strength of 182 pounds per square inch. The other was 

 a plastic loam, whose average tensile strength was 137 pounds 

 per square inch. A mixture of the two in equal proportions, 

 however, had an average tensile strength of 258 pounds per 

 square inch. Another clay from a different formation (Lab. 

 No. 695) had an average tensile strength of 108 pounds per 

 square inch, while a mixture of equal parts of this clay and sand 

 showed a tensile strength of but 65 pounds per square inch. In 

 the latter case the decrease in strength was due to' the excess 

 of sand. 



The tensile strength of a clay is used by some as a means of 

 measuring its plasticity, but there are probably fewer advocates 

 of this idea than there were a few years ago. It is true that 

 many very plastic clays have a high tensile strength, and that 

 many very lean ones have a low tensile strength, but we cannot 

 say more than this. Many clays, as, for example, some of the 

 Clay Marls, have a lean, gritty feeling when wet, and yet show 

 a high tensile strength when air dried. Again, many of the 

 Raritan clays feel very sticky and plastic, and yet their tensile 

 strength may be under 100 pounds per square inch. Some clays 

 of high bonding power will, when tested alone, develop little 

 tensile strength, but this is because the high plasticity of the 

 clay causes it to shrink, warp and crack so much in drying that 

 it is impossible to* get a briquette free from flaws. When mixed 

 with sand or grog, its shrinkage and cracking are greatly reduced, 

 and the proper texture is developed for high tensile strength. 

 Some of the No. 1 fire clays of the Woodbridge district, which 

 are very fine grained, crack so much when dried alone that it 

 is impossible to get a flawless briquette for testing. 



