8 4 



CLAYS AND CLAY INDUSTRY. 



The tensile strength is measured by molding the tempered 

 clay into briquettes, of the form and dimensions shown in 

 Fig. 26, and, when they are thoroughly air dried, pulling 

 them apart in a suitable testing machine. The cross section 

 of the briquettes when molded is 1 square inch, and, after 

 being formed, they are allowed to dry first in the air and then 

 in a hot-air bath at a temperature of ioo° C. (212 F.). When 

 thus, thoroughly dried the briquette is placed in a machine, in 



Fig. 26. 



The outline and dimensions of a briquette made for testing the tensile strength of clay. 



which its two ends are held in a pair of brass clips, and is sub- 

 jected to an increasing tension until it breaks in two: Theoreti- 

 cally the briquette should break at its smallest cross section, with 

 a smooth, straight fracture, and when this does not occur it is 

 due either to a flaw in the briquette or because the clips tend to 

 cut into the clay. In such event the briquette breaks across one 

 end, and to prevent this it is necessary to put some soft material, 

 such as asbestos, pasteboard or rubber between the inner surface 

 of the clip jaws and the sides of the briquette. If the briquettes 



