74 G ITEW YORK STATE MUSEUM 



in order to prevent drying cracks, are placed, after the lapse of 

 24 hours, or at least only after they have set, with their smooth 

 surfaces on a metal plate and exposed, for at least one hour, to 

 a temperature of from 110° C to 120 (230 to 248 F) until no 

 more water escapes. For this purpose the drying closets in use 

 in chemical laboratories may be utilized. If, after this treatment, 

 the cakes show no edge cracks, the cement is to be considered in 

 general of constant volume. If cracks do appear, the cement is 

 not to be condemned, but the results of the decisive test with the 

 €akes hardening on glass plates under water must be waited for. 

 It must, however, be noticed that the heat test does not admit 

 •of a final conclusion as to the constancy of volume of those ce- 

 ments which contain more than 3^ of calcium sulfate (gypsum) 

 or other sulfur combinations. 



For making the final test, the cake made for the purpose of 

 determining the time of setting, for slow setting cements, is 

 placed under water after the lapse of 24 hours, but, at all events, 

 not until after it is set. For quick setting cements this can be 

 done after a shorter period. The cakes, especially those of slow 

 setting cement, must be protected against drafts and sunshine 

 until their final setting. This is best accomplished by keeping 

 them in a covered box lined with zinc, or under wet cloths. In 

 this manner the formation of heat cracks is avoided, which are 

 generally formed in the center of the cake, and may be taken by 

 an inexperienced person for cracks formed by blowing. 



In order to obtain concordant results in the tests, sand of uni- 

 form size of grain and uniform quality must be used. This 

 standard sand is obtained by washing and drying the purest quartz 

 sand obtainable, sifting the same through a sieve with 60 meshes 

 per square cm (387 per sq. inch), thereby separating the coarsest 

 particles, and by removing from the sand so obtained, by means 

 of a sieve of 120 meshes per square cm, the finest particles. The 

 diameter for the wires of the sieve shall be .38 mm, and .32 mm 

 respectively. Since not all quartz sand even und»r the same 

 method of treatment, gives the same resulting strengths in the 

 mortars, one must know whether the standard sand at one's dis- 

 posal gives concordant results with the standard sand furnished 

 by the German society of cement manufacturers and also used 

 at the royal testing station at Berlin (Charlottenburg). 



For each test, in order to obtain correct average results, at 

 least six test pieces are to be made. Tensile test pieces can be 

 made either by hand or by machinery. 



