248 CLAYS AND CLAY INDUSTRY. 



except in the Hackensack district, where the heat probably does 

 not exceed the melting point of cone 08. Measurements made in 

 some down-draft kilns showed that a temperature sufficient to 

 fuse cones 1-2 was reached. In the burning of pressed brick a 

 much higher temperature is reached, the buff brick manufactured 

 in New Jersey being burned at cones 7 to 9. Measurements made 

 with cones in some down-draft kilns showed a difference of as 

 much as two cone numbers between the temperature of the top and 

 bottom. Such a difference is sufficient to make the brick in the 

 top considerably harder than those in the bottom. 



SHRINKAGE MEASUREMENTS OE NEW JERSEY BRICKS. 



At many of the brickyards measurements of all three dimen- 

 sions were made of the freshly molded and burned bricks, and 

 from these both the linear and cubical air and fire shrinkages were 

 calculated, the former being measured on the greatest length of 

 the brick. 



If a brick is burned by itself the shrinkage will probably be 

 equal in all directions, but if the same brick is placed under con- 

 siderable weight, it seems probable that the percentage of shrink- 

 age will be greater in a vertical direction than a horizontal one. 

 This becomes quite apparent when the tables of shrinkage given 

 below are examined, for in these it will be noticed that the linear 

 fire shrinkage in some cases is zero, while the cubical fire shrinkage 

 is several per cent. For purposes of comparison, therefore, it 

 would seem more desirable to< use the latter rather than the former. 

 The results obtained are given in the following tables, the first one 

 representing the stiff-mud bricks, and the second the soft-mud 

 ones. 1 



1 In calculating the air and fire shrinkage, the freshly molded brick is used as 

 basis of length. Figures show the per cent, of shrinkage. Those preceded 

 by a dash ( — ) show expansion. 



