26 GENERAL DISCUSSION OF CLAYS. 



TENSILE STRENGTH. 



The tensile strength or the binding power of a clay 

 often stands in relation to its plasticity, but not al- 

 ways. It exerts an important effect in connection 

 with the cracking of the ware in drying. The com- 

 mon method of determining it is to form the plastic 

 clay into briquettes of the same shape as those used in 

 the testing of cement. When air-dried they are tested 

 in the regular cement testing machine, and their ten- 

 sile strength per square inch is determined. Before 

 breaking, the cross section of the briquette must be 

 carefully measured,, as the clay shrinks in drying and 

 the tensile strength per square inch has to be calcu- 

 lated from this sectional area. , 



The tensile strength of air-dried clays is extremely 

 variable. In kaolins it is from 5 to 10 pounds per 

 square inch ; in brick clays 60 to 75 pounds per square 

 inch and even 100 pounds; in pottery clays from 150 

 to 175 pounds. 



Some very plastic clays show as much as 200 and 

 300 pounds per square inch, and a tensile strength of 

 even 400 pounds has been recorded. 



The strongest Alabama clay were the highly plastic 

 one from Chalk Bluff, which had a maximum tensile 

 strength of 384 pounds per square inch, while the 

 Choctaw County one showed only 5 pounds per square 

 inch. 



The Alabama clays were all ground and passed 

 through a thirty mesh sieve before testing. 



Very fine grained clays seem to be lacking in tensile 

 strength as they are in plasticity. 



SHRINKAGE. 



All clays undergo a shrinkage in drying and an ad- 

 ditional shrinkage in burning, the first is known as 

 air — , the second as fire-shrinkage. Some clays shrink 

 most in drying, others most in burning, and conse- 

 quently the amount is variable and depends on the 

 amount of water absorbed, on the amount of lime in 



