QUARRY MATERIALS OF NEW YORK 43 



of a drill. . The common practice in laboratories for the testing of 

 roadstones is to subject a specimen of definite dimensions to the 

 abrading action of a grinding disc. The loss of weight after the 

 disc has revolved a certain number of times is a measure of the 

 hardness. In the laboratories of the State Department of Highways 

 at Albany, the test is carried out on a core of rock, i inch in 

 diameter and 3 to 4 inches long, obtained with a diamond drill. 

 The ends of the core are faced off and then the latter is weighed. 

 One end is placed against a Dorry grinding machine, so as to bear 

 with constant pressure upon the disc upon which quartz sand of 

 standard quality and size is fed. The disc is revolved 500 revolu- 

 tions at the rate of 2000 revolutions an hour, when the core is taken 

 out, reversed end for end, and ground for another 500 revolutions. 

 The loss in weight in grams is noted. One-third of this loss sub- 

 tracted from 20 is the relative hardness. A hardness below 14 

 is considered soft, 'between 14 and 17 medium and above 17 high. 

 Toughness may be defined as the resistance to rupture from im- 

 pact by a falling body. It differs from hardness in that it depends 

 mainly upon the texture of the material, more especially the manner 

 in which the components are interlocked. Fibrous aggregates like 

 those of talc, serpentine and gypsum, though possessing little hard- 

 ness, are very resistant to rupture, as shown by the difficulty in 

 pulverizing such materials in a ball mill. Tests for toughness are 

 commonly carried out on roadstones, but have less value for build- 

 ing materials. The method of testing toughness as adopted in the 

 New York State Department of Highways is as follows : 



The toughness test is made by taking two core pieces 

 one inch in diameter which have been obtained with the 

 diamond drill, as was done for the hardness test. The 

 ends of these core pieces are accurately and carefully 

 smoothed off so as to form cylinders 1 inch in height. 

 They are then placed on a firm, level bearing in an impact 

 machine, securely clamped and subjected to blows through 

 a one-kilogram weight. The first blow of the hammer 

 is from a height of 1 centimeter. Each succeeding blow 

 is from a height 1 centimeter greater than the preceding 

 one. The number of blows, which equals the drop ex- 

 pressed in centimeters of the last blow required to break 

 the core, is considered as the toughness of the stone. The 

 toughness of the stone is represented by the average of 

 the two core pieces broken. A toughness below 13 is 

 considered low, between 13 and 19 medium and above 

 19 high. 



