CLAY WORKING INDUSTRIES. 191 



fifty pounds of iron. When the table, which is covered with sharp san d 

 and water, is rotated, the under side of the brick will be worn away to an 

 extent strictly proportional to its hardness. This test is valuable in 

 measuring the hardness only ; it gives us no true indication of what may 

 be expected from the same material under the combined blow and rolling 

 friction of actual wear. 



The crushing strength is often obtained as a test of the value of the 

 material. At one time it was considered the best that could be applied. 

 The facts wrought out by a comparatively short series of tests in the crush- 

 ing strength go to show, however, that this factor is of very little real 

 value in determining the quality of the paving material. 



Progressive tests in the same kind of material at different stages of 

 hardness generally show that the highest crushing strength is attained 

 sometime before the best vitrification, and that the best crushing strength 

 is at a pDint where the absorption is much too high. Also, no condition of 

 actual wear in any way approximates the stress of the testing machine, 

 and as has been stated before, there is no benefit from applying tests which 

 only give the relative endurance of materials under conditions they will 

 never be called on to meet. 



In classifying the various Ohio bricks on the results of the test, the 

 absorption and ratling tests alone were considered; the crushing tests 

 were made with great care and are recorded with the other tests, but 

 enough was seen in making these tests to decide that crushing strength is 

 not a quality worthy of notice in getting the comparative excellence of 

 brick paving materials. 



A test was made in one of the western cities not long ago, by laying 

 a circular track and paving it with bricks of the various kinds to be tested. 

 They were put down with all the care that a city street would receive as 

 to bedding and cement, etc. A heavy, broad tread, wheel, carrying a 

 heavy load of iron was now pivoted from the center of the circle and run 

 round the track for hours at a time; after the proper duration of the test 

 the bricks were takemup and their loss determined. ' This test, while it 

 is too cumbersome to be of much public benefit, approximates more 

 closely to an exact determination of the relative value of the material 

 than anything but actual trial can do. 



The probabilities are that an absorption test and a rattling test, in 

 which the conditions of the test are made absolutely uniform everywhere, 

 will soon become recognized as the best modes of making the preliminary 

 tests of paving material. 



The manufacturers of the state were requested by the Survey to fur- 

 nish each a sample of five (5) of their paving brick for a test, and with 

 the exception of a few factories, the samples were received and tested, 

 The tables of the actual figures obtained in each kind of test will he 

 found in an appendix to this chapter, where those interested can trace the 

 exact behavior of their material; the summary of the results, however, is 

 included in the following table: 



