232 CLAYS AND CLAY INDUSTRY. 



The capacity of a dry-press machine is about the same as that 

 of a soft-mud one, provided six bricks are molded at a time. Two 

 and four-mold machines are, however, alsoi made. The initial 

 cost of the machinery is considerable, although this may be more 

 than offset by the saving in dryers. 



R^-PRESSING. 



Many soft-mud and stiff-mud brick that are to* be used for 

 fronts are improved in appearance and often in density by re- 

 pressing. This smoothens the surface and straightens and 

 sharpens the edges, of the product, as well as sometimes increas- 

 ing the strength. 1 



The re-press consists essentially of a steel mold box, having 

 both bottom and top movable. The green brick are placed in 

 this mold box, and pressure applied to it by the vertical motion 

 of the top or bottom oi the mold, the effect of this being to 

 re-form the brick to a slight degree. Re-pressing machines are 

 operated either by hand or steam 1 power. In the hand power 

 machines, the bottom of the mold is moved upwards by means 

 of a lever and applies the pressure. With such a machine, one 

 man and two boys can re-press 2,500 to 3,000 per day. In steam 

 power re -presses, both the top and bottom of the mold box move. 

 They are commonly constructed with two compartments, and 

 their capacity is about 25,000 per day of ten hours. In both types 

 of re-presses the dies have to be liberally oiled. Soft-mud brick 

 are allowed to dry for a few hours before re-pressing, but stiff- 

 mud brick can be re-pressed as soon as molded. 



The change in volume that occurs in a brick in re-pressing can 

 be seen from the following measurements of a paving brick. 



Before re-pressing, 8| by 4§ by 3! inches, . . . 119! cubic inches. 

 After re-pressing, S{^ b}' 4| by 2| inches, . . . 109^ cubic inches. 



1 See Table of Brick Tests, Chap. XL 



