610 CEMENTS, LIMES, AND PLASTERS. 



50. Gang moulds, which permit moulding a number of briquettes 

 at one time, are preferred by many to single moulds; since the greater 

 quantity of mortar that can be mixed tends to produce greater uni- 

 formity in the results. 



51 o The moulds should be wiped with an oily cloth before using. 



MIXING. 



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. 



54. The temperature of the room and the mixing water should be 

 as near 21 C. (70 F.) as it is practicable to maintain it. 



55. The 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 1000 gr. (35.28 oz.) 

 makes a convenient quantity to mix, especially by hand methods. 



57. The 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 with- 

 out stopping the machine, and (3) the difficulty of keeping the machine 



58. Method. The material is weighed and placed on the mixing 

 table, and a crater formed in the center, into which the proper per- 

 centage of clean water is poured; the material on the outer edge is 

 turned into the crater by the aid of a trowel. As soon as the water has 

 been absorbed, which should not require more than one minute, the 

 operation is completed by vigorously kneading with the hands for 

 an additional 1 minutes, the process being similar to that used in 

 kneading dough. A sand-glass affords a convenient guide for the 

 time of kneading. During the operation of mixing, the hands should 

 be protected by gloves, preferably of rubber. 



