THE PHYSICAL PROPERTIES OF CLAY. 105 



The cone numbers used in the different branches of the clay 

 working industry in New Jersey are as follows : 



Common brick, 08-01 



Hard-burned, common brick. 1-2 



Buff front brick, 6-8 



Hollow blocks and fireproofing, 03- 1 



Terra cotta, 5- 7 



Conduits, 7- 8 



"White earthenware. 8-9 



Fire bricks. 9-12 



Porcelain 1 I -13 



Red earthenware. 08-05 



Stoneware. 6-8 



While the temperature of fusion of each cone number is given 

 in the preceding table, it must not be understood that these cones 

 are for measuring temperature, but rather for measuring pyro- 

 chemical effects. Thus if certain changes are produced in a clay 

 at the fusing point of cone 5, the same changes can be reproduced 

 at the fusion point of this cone, although the actual temperature 

 of fusion may van- somewhat, due to variations in the condition 

 of the kiln atmosphere. As a matter of fact, however, repeated 

 tests with a thermoelectric pyrometer demonstrate that the cones 

 commonly fuse close to the theoretic temperatures. 



Manufacturers occasionally claim that the cones are unreliable 

 and not satisfactory, forgetting that their misuse may often be 

 the true reason for irregularities in their behavior. It is un- 

 necessary, perhaps, to state that certain reasonable precautions 

 should be taken in using these test pieces. The cones are com- 

 monly fastened to a brick with a piece of wet clay, and should 

 be set in a vertical position. After being placed in a position 

 where they can be easily seen through a peephole, the latter should 

 not be opened widely during the burning lest a cold draft strike 

 the cones, and a skin form on its surface and interfere with its 

 bending. Moreover, one set of cones cannot be used to regulate 

 an entire kiln, but several sets should be placed in different 

 portions of the same. One advantage possessed by a cone over 

 trial pieces is that the cone can be watched through a small 

 peephole, while a larger opening must be made to draw out the 



