131 

 Mixing. 



33. All proportions should be stated by weight; the 

 quantity of water to be used should be stated as a per- 

 centage of the dry material. 



34. The metric system is recommended because of the 

 convenient relation of the gram and the cubic centimeter. 



35. The temperature of the room and the mixing water 

 should be as near '21 C. (70 F.) as it is practicable 

 to maintain it. 



36. 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. 



Fie. 4. 



37. 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. 



38. Method. The material is weighed and placed on 

 the mixing table, and a crater formed in the center, into 

 which the proper percentage 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 t*n additional 1^6 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 pro- 

 tected by gloves, preferably of rubber. 



