152 KEW YORK STATE MUSEUM 



10,000 to 12,000 pounds per square incli are probably sufficient. It 

 should be homogeneous throughout. It should be dense and not 

 absorb over two or at the most three per cent, of water. 



In the manufacture of paving brick, the clay must be thoroughly 

 drepared before being molded. Some clays can be ground, 

 screened and pugged as soon as taken from the bank, while others 

 have to be weathered or soaked before crushing. The machine 

 used for molding depends on the clay. Some clays make a first- 

 class paving brick by the dry press process, while others give the 

 best quality with a soft mud machine. In any case the green 

 brick should be as dense as possible. After molding, the bricks 

 are dried in tunnels. The drying should not be hurried. Burning 

 is usually done in down draft kilns. The kiln should not be too high 

 in order to avoid the bricks in the lower part of it being crushed 

 out of shape by the weight of those above, at the time the fires 

 are hottest. In burning, the fires are raised till temperature of 

 vitrification is reached, and they are held at this temperature for 

 from 24 to 48 hours. Cooling is done very slowly, thus annealing 

 the brick. The term "vitrification " is a misnomer. To vitrify a 

 brick would be to convert it into a glass in which state it would 

 be brittle and useless. What takes place is that the bricks are 

 raised to a temperature sufficient to flux the potash, lime and 

 iron with the silica and give a dense brick, and it is in order to 

 thoroughly accomplish this, that the brick is kept for 24 or more 

 hours at the point of fluxing or " vitrification." 



Testing paving brick 



1. Absorption. To determine the amount of water which a 

 paver will absorb, it is soaked in water for 20 hours and weighed 

 before and after. The increase in weight is the amount of water 

 absorbed. 



2. Abrasion. The bricks are weighed and then put in a rattler 

 together with foundry shot and the rattler revolved for several 

 hours at 52 revolutions per minute. The bricks are again 

 weighed, the loss being due to abrasion. 



Another method of making this test is to grind the brick on a 

 horizontal stone, 14 feet in diameter and making 28 revolutions 

 a minute. This is kept up for eight hours, the brick of course 

 being weighed before and after. 



