THE PHYSICAL PROPERTIES OF CLAY. 85 



are molded and dried with care, the variation in the breaking 

 strength of the individual briquettes should not vary more than 

 15 or 20 per cent., but with some very plastic clays it is extremely 

 difficult to keep the variation within these limits. 



In making a complete test of the New Jersey samples the ten- 

 sile strengths given are usually the average of 10 or 12 briquettes. 

 In a number of cases where it was only desired to get an ap- 

 proximate idea of the strength of a clay, but two or three were 

 broken. The greatest variation usually appeared in clays of high 

 tensile strength, in which case the fracture nearly always 

 occurred in the head, indicating that the briquettes broke before 

 the limit of their strength was reached. 



The tensile strength of clay briquettes is expressed in pounds 

 per square inch, but. since the briquette shrinks in drying, the 

 strength actually obtained in testing will be less than that for a 

 square inch, and the result must be increased in proportion to the 

 amount the briquette has shrunk. 



The following figures give the range in tensile strength shown 

 by clays from the different formations in New Jersey, and it will 

 be seen from an inspection of these figures that the clays of any 

 one formation may show a wide range in their tensile strength. 



Range of tensile strength of clays from different formations in Nezv Jersey 

 expressed in pounds per square inch. 



Minimum. Maximum. 

 Post- Pleistocene and Pleistocene other than 



Cape May. 65 297 



Cape May, 90 291 



Cohansey, 54 293 



Asbury Clay, 107 182 



Alloway Clay, 80 453 



Clay Marl IV, 195 (one sample tested) 



Clay Marl III, 134 (one sample tested) 



Clay Marl II, 126 286 



Clay Marl I, 127 251 



Raritan (extreme limits), 20 251 



If the tensile strength tests of a number of clays from dif- 

 ferent localities are grouped according to the kind of clay, a 

 somewhat similar variation is found. 



