STATE GEOLOGIST. 310) 
of mortar that can be mixed tends to produce greater uniformity in the 
results. 
51. The molds should be wiped with an oily cloth before using. 
52. All proportions should be stated by weight; the quantity of water 
to be used should be stated as a percentage of the dry material. 
53. The metric system is recommended because of the convenient 
relation of the gram and the cubic centimeter. 
Aone 
Nae ee 
Ye -—- == 
SS Te ee Ne eee 
Fig. 80. Standard form of brickette for tensile strength. 
54. The temperature of the room and the mixing water should be as 
edu ines Oun(7On ule) asmiteisn practicable to umaintainyit 
55. he sand and cement should be thoroughly mixed dry. The 
mixing should be done on some non-absorbing surface, preferably plate 
glass. If the mixing must be done on an absorbing surface it should be 
thoroughly dampened prior to use. 
56. The quantity of material to be mixed at one time depends on 
the number of test pieces to be made; about 1,000 gr. (35.28 oz.) makes 
a convenient quantity to mix, especially by hand methods. 
57. Lhe committee, after investigation of the various mechanical 
mixing machines, has decided not to recommend any machine that has 
thus far been devised, for the following reasons: (1) The tendency 
of most cement is to “ball up” in the machine, thereby preventing the 
working of it into a homogeneous paste; (2) There are no means of 
ascertaining when the mixing is complete without stopping the machine, 
and (3) the difficulty of keeping the machine clean. 
58. Method.—The material is weighed and placed on the mixing 
table, and a crater formed in the center, into which the proper percentage ~ 
