NEW JERSEY BRICKMAKING INDUSTRY. 251 



broken or crushed, so as to give a reliable average, and the 

 results are given in condensed form, in the table on page 256. 

 Each manufacturer has been furnished with a copy of the tests 

 made on his own brick, although in this report no names are 

 published in connection with these tests, even though, in the 

 majority of cases, the results are very creditable. 



'Explanation of Tests. 



Crushing test. — This test was made in order to- determine „ the 

 number of pounds pressure per square inch that a brick will 

 stand before it crushes under a load. This strength will vary 

 with the density of the brick, degree of hardness to which it has 

 been burned, character of the raw materials and freedom from 

 flaws. In most bricks it rarely falls below 2,000 pounds per 

 square inch, and may reach .15,000 or more pounds, although in 

 actual use (that is when set in the wall), the bricks are seldom 

 compelled to' stand the weight that they are capable of resisting. 



The test to determine a brick's crushing strength is commonly 

 made in a specially constructed machine. Half bricks are usually 

 tested, because a whole brick has so large a surface area that it 

 might resist a greater pressure than could be applied by the 

 machine. Before crushing, the two 1 opposite surfaces of the 

 brick (in this case the top and bottom) must either be ground 

 perfectly smooth and parallel, or else they must be built up to 

 this condition by the application of a layer of plaster of paris. 

 The reason for this is that in the testing machine the brick is set 

 between two steel surfaces, and unless its surface fits perfectly 

 against these, the pressure will not be evenly distributed. 



Before crushing, the area of the brick's surface is measured, 

 and, the total load necessary to crush the brick, divided by this 

 area, gives the crushing strength in pounds per square inch. It 

 will be seen from the table that the average crushing strength of 

 the 174 New Jersey bricks which were tested was 5,104 pounds 

 per square inch, ranging from an average of 661 pounds for 

 those of one manufacturer up to 13,873 pounds for those 

 of another, while the minimum and maximum for individual 



