BRICK AND BUILDING STONE 281 



bricks being equal to about 8 per cent of the volume of the machine'. 

 The cast-iron blocks are of two sizes, the larger being about 2J in. 

 square and 4^ in. long, with rounded edges and weighing at first 

 1\ Ib. The smaller are about 1^-in. cubes, with rounded edges. About 

 225 Ib. of the smaller size and 75 Ib. of the larger size are used; 

 1800 revolutions are required, and must be made at the rate of about 

 30 per minute.* 



During the first 600 revolutions the effect of the rattler action on 

 the brick is to chip off the corners and edges. Thereafter the action 

 is more nearly abrasive. 



232. Absorption test of brick. A brick which absorbs a great 

 amount of water is likely to be weakened and injured by frost. To 

 measure the amount of absorption, a dry brick is taken and a deter- 

 mination of its absorbing capacity made, as in the case of stone 

 (Article 226). 



Ordinary brick will absorb from 10 to 20 per cent of its own 

 weight, and paving brick from 2 to 3 per cent. 



This test is now little used, since a brick that fails in the absorp- 

 tion test is of such poor quality that it will also fail when subjected 

 to the crushing and cross-bending tests. 



* See specifications of the National Brick Manufacturers' Association for rattler test. 



