QUARRY MATERIALS OF NEW YORK 45 



of 2000 times an hour. The stone is then taken out, washed, dried 

 and the weight of material less than one-sixteenth of an inch in size 

 computed. The per cent of loss of the original weight is expressed 

 by the French coefficient which is obtained by dividing 40 by the 

 per cent of wear. Thus a stone which loses .4 per cent in weight 

 during the test would show a coefficient of wear of 10. A coefficient 

 of wear below 8 is considered low, between 8 and 13 medium, be- 

 tween 13 and 20 high and over 20 very high. 



Resistance to fire. The resistance of stone to intense heat may 

 be considered one of the important qualities in building stones that 

 should be given consideration by the architect and builder, but 

 which is very often neglected. Fires in cities work great damage 

 upon stone structures. The test of extreme heat followed by sudden 

 chilling from the play of water upon the surface is one that very 

 few stones will pass through with strength and appearance unim- 

 paired. There is, however, considerable variation among different 

 building stones in respect to fire resistance, as may be observed in 

 their condition after a large conflagration like that of Baltimore or 

 San Francisco. Some buildings are completely ruined, so far as 

 the possibility of making any use of the stone work for reconstruc- 

 tion ; others are only damaged as to their exposed parts like the 

 cornices and window openings ; and some appear to be practically 

 uninjured. 



Intense heat causes both physical and chemical changes in stone. 

 The most apparent effect is the spalling and cracking incident to 

 unequal expansion between the outer and inner parts of the blocks. 

 Stone has a very low capacity for transmitting heat; consequently, 

 the interior may be still comparatively cool while the surface is 

 intensely hot. This difference in temperature sets up a stress that 

 disrupts the stone or causes the outer part to flake off in successive 

 layers. The same process takes place in nature where changes of 

 temperature are extreme ; in the arid regions like the Great Basin, 

 the warmth of the sun after a cool night causes the scaling of bare 

 rock surfaces, but of course at a comparatively slow rate. 



The disruption of rocks of complex mineral composition, such 

 as granite, is probably traceable to some extent to the loosening of 

 the bond between the ingredients through intergranular strain. 

 Quartz, feldspar and mica each has its own rate of expansion which 

 must produce a certain amount of differential thrust under rapid 

 temperature changes. Further, most granites hold occluded liquids 

 and gases in closed cavities which were imprisoned during the con- 

 solidation of the mass from its state of liquid fusion. These are 



