ARTIFICIAL STONE, OR CONCRETE 



the filled air-spaces, however, for even the finest shot 

 has none of the adhesive qualities of the particles 

 of cement. 



MIXING THE MATERIALS 



Since thorough mixing is so essential in making 

 blocks, or in building with concrete on a large scale, 

 steam-driven mixers are used which produce an enor- 

 mous quantity of concrete of uniform consistency, 

 although hand-mixing is still in general use in any but 

 the largest operations. Machine-mixed concrete is 

 usually of greater strength than that made by hand, 

 and is likely to be more uniform in color, as the amount 

 of water used in each batch can be better regulated. 

 The exact shade of the finished block, when the pro- 

 portions of the solid substances are the same, depends 

 to a great extent upon the amount of water used. 



Mixing by hand is usually done on a board platform. 

 The sand to be used is spread upon the platform and 

 the dry cement spread over this, the substances being 

 mixed thoroughly by turning with a shovel before 

 being moistened. Water is then sprayed upon the 

 mixture, which is stirred constantly. When thor- 

 oughly moistened, the gravel or broken stone is added, 

 the mass turned repeatedly, until ready for the block 

 molds. 



Mechanical mixers are made in a variety of shapes 

 and on many different principles. Some of the larger 

 ones in use on extensive building operations are con- 

 tinuous producers, a steady stream of concrete emerg- 

 ing from one end of the machine, while the cement, 



