856 NEW YOKE STATE MUSEUM 



9 Corundum 

 10 Diamond 



If the hardness of the brick is such that it can be scratched by 

 quartz but not by orthoclase, its hardness is no. 7 of the scale. 

 . Vitriiied products should show a hardness of 6-Y. 



Determination of deleterious impurities 



The stone or brick is to be subjected to a damp atmosphere for 

 a period of time. If it contains any lumps of carbonate of lime 

 or pieces of pyrite, these will soon sl^ow themselves by causing 

 the brick to flake off. The moist atmosphere can be produced by 

 placing the brick under a bell glass containing a bowl of water. 

 The method suggested by an international committee appointed 

 to decide on a standard test, was that a portion of the brick should 

 be put in a Papin digester containing vapor under one fourth 

 atmospheric pressure for three hours. 



It is advisable in all cases to subject the raw material to an 

 examination to see if any harmful impurities are present. 



Determination of soluble salts 



These are determined by breaking off chips of the brick and 

 grinding these to 100 mesh fineness. 25 grams of this powder 

 are boiled for one hour in 250 cc of water. The water is then 

 filtered, and from this filtrate by evaporation the amount of dis- 

 solved salts is determined. Salts of vanadium show themselves 

 by the presence of a green tint on the surface of the wet brick 

 after it has been set aside in a place protected from dust. 



Resistance to weathering 



One method of testing this is to subject the bricks, wliich have 

 been immersed in water, to a freezing temperature, which can 

 be easily done by covering them with a mixture of ice and 

 salt. The frozen bricks are then subjected to water having a 

 temperature of 20° C. This process of freezing and thawing is 

 repeated 20 or 25 times. The particles which break off in the 



