34 BULLETIN 704, U. S. DEPARTMENT OF AGRICULTURE. 
at least 20 hours. After 24 hours in moist air, the briquets are immersed 
in clean water in storage tanks of non-corroding material. The air and water . 
should be maintained as nearly as practicable at a temperature of 21° C. 
(70° F.). The moist closet may consist of a soapstone, slat, or concrete box, 
or a wooden box lined with metal. If a wooden box is used, the interior 
should be covered with felt or broad wicking kept wet. The bottom of the 
moist closet should be covered with water. The interior of the closet should 
be provided with nonabsorbant shelves on which to place the test pieces, the 
shelves being so arranged that they may be withdrawn readily. Tests may 
be made with any standard machine. The briquets are tested as soon as they 
are removed from the water. The bearing surfaces of the clips and briquets 
must be carefully centered and the load applied continuously at the rate of: 
600 pounds per minute. Testing machines should be frequently calibrated in 
order to determine their accuracy. Briquets that are manifestly faulty, or 
which give strengths differing more than 15 per cent from the average value 
of all test pieces made from the same sample and broken at the same period, 
should not be considered in determining the tensile strength. The average 
tensile strength in pounds per square inch of not less than three briquets is 
compared with the average tensile strength of not less than three standard mor- 
tar briquets made at the same time and by the same operator wtih the same 
cement and Ottawa sand.. 
Rattier Test for Paving Brick. (A. S. T. M. Standard Test C7-15.)— 
Foliowing is a brief description of the rattler test. For a detailed description 
reference should be made to the book of A. 8. T. M. Standards, 1916. 
The standard rattler test is used for the purpose of determining the resistance 
to wear of vitrified paving brick. The machine should conform in every re- 
spect to the standard requirements covering the construction of the rattler. 
The abrasive charge consists of 10 cast-iron spheres, weighing when new 73 
pounds each, and a sufficient number of small spheres, weighing when new 
0.95 pounds each, to bring the collective total weight of the charge to as 
nearly 300 pounds as possible. Both small and large spheres should be 
standardized from time to time, as called for in the detail specification. 'The 
brick charge consists of 10 brick, none of which would be rejected on visual 
inspection. They should be thoroughly dried previous to testing. The test 
consists of subjecting the brick to 1800 revolutions of the rattler at the rate of 
not less than 294 nor more than 304 revolutions per minute. At the conclusion 
of the test, the brick are reweighed, rejecting any piece weighing less than 1 
pound. The percentage of wear is calculated from the following formula: 
Original weight—final weight 
Original weight 
Per cent wear= <100. 
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