CLAYS OF KEW TOEK 545 



Anotlier method requires tliat the clay be formed into a bar of 

 known cross-section. When dried, the bar is held in a horizontal 

 position by supports under the two ends, and the weight noted 

 which is needed to cross-break it when the pressure is applied to 

 the central portion of the bar. 



An objection to: this method is that very plastic clays are apt 

 to develop structural peculiarities, which cause their tensile strength 

 to appear much lower than it really is. In such cases, it has been 

 suggested that the clay be pulverized and "mixed with an equal 

 quantity of fine sand. If when made into briquets this mixture 

 shows a higher tensile strength than the clay alone, it is an indi- 

 cation that the low tenacity of the original clay was due to flaws. 



The tensile strength of air-dried clays is highly variable. The 

 following figures may be taken as representing the average for 

 different clays, in pounds per square inch. 



Pounds 



Kaolins 5- 15 



Brick clays 60- 75 or even 100 



Pottery clays , 150-1Y5 



Some very plastic clays 200-300 



Shrinkage 



All clays shrink in drying, and again in burning. The first is 

 known as air, the second as fire shrinkage. Some clays shrink 

 more in drying, others in burning, consequently the amount ia 

 variable. 



Air shrinkage depends partly on the amount of water absorbed, 

 and partly on the grain or texture of the clay. 



Air shrinkage. As soon as evaporation of water takes place 

 from a clay, it begins to shrink, and within certain limits, the 

 greater the amount of water absorbed, the greater the air shrink- 

 age. Plastic clays show this property in a marked deg-ree. The 

 shrinkage continues till all the clay particles are in contact with 



