REMARKS ON BRICKS 153 



3. Crushing tests. The brick to be tested is put on edge 

 between the two plates of a crushing machine. The amount of 

 pressure is noted at which the brick cracks, and also that at 

 which it crushes. Before testing, the two sides of the brick on 

 which the pressure is exerted should be ground perfectly smooth 

 and be made exactly parallel. To further insure an even sur- 

 face some material, such as paper or cardboard or plaster of paris, 

 is put between the surface of the brick and plate of the 

 machine. The following experiments made by Prof. I. O. Baker* 

 show that for the same brick the results obtained vary with, 

 the method of preparing the surface. The bricks tested by him 

 were prepared in the following manner : 



1. Grinding as nearly flat as possible on convex side of emery 

 stone and crushing between self-adjusting, parallel cast iron 

 plates. 



2. Removing the irregularities of surface and crushing between 

 blotting paper. 



3. Removing the irregularities of surface and crushing between 

 straw boards. 



4. Removing irregularities, coating with plaster of paris and 

 placing under slight pressure until set (12-24: hrs.), and then 

 crushing. 



5. Coating with plaster of paris which was afterward ground 

 down on a sand paper disk, to the surface of the brick so as to 

 leave a minimum thickness with a perfectly fiat surface and then 

 crushing. 



After a number of experiments no great difference was found 

 between the first three, but difficulties connected with the last 

 two rendered them worthless. With a uniform grade of brick 

 the first three methods gave 7000 to 9000 pounds as crushing 

 strength of cubes. Some samples of the same lot of brick were 

 prepared on rubbing bed at marble works, and the strength of 

 these carefully prepared cubes ranged from 16,0i/0 to 21,000 

 pounds per square inch, showing that a very small difference in 

 flatness of surface makes a great difference in the apparent 

 strength. 



* Clay worker, June, 1692. 



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