668 NEW YOKK STATE MUSEUM 



tlie frost. Owing to their greater density, dry press brick have t(D 

 be burned more slowly than those made by other methods. 



The stiff mnd process is adaptable mainly, if the best results are 

 desired, to clays of moderate or good plasticity, which will dry in 

 a reasonable time. As the clay in flowing through the die requires 

 much tenacity to escape tear, very siliceous clays are not desirable, 

 and on the other hand very plastic ones tend to develop laminations 

 in the brick. The capacity of the stiff mud machines is very great 

 and their use is increasing, though it is already extensive. 



Repressing of bricks 



Paving brick and front brick are sometimes repressed, the object 

 being to give sharper edges and angles in the case of the latter, and 

 in both cases to produce a brick of more regular size and greater 

 density. 



The repressing is done in a machine known as the repress, 

 operated either by hand or steam power, (pi. 36.) In the hand 

 power machine only one brick is repressed at a time, and one man 

 and a boy can generally repress about 2000 a day. In a 

 steam power machine two bricks are repressed at a time, and the 

 capacity is about 25,000 a day of 10 hours. In each case the 

 pressure is applied vertically, and the dies and other parts of the 

 machine have to be oiled frequently to keep the clay from sticking. 



Eepressing reduces the volume of the brick somewhat, thus in 

 one case a brick before being repressed in a steam power machine 

 measured 8f x 4| x 3i inches, and after it Sii x 4f x 2|-. 



Drying 



The methods employed have already been enumerated in the 

 table given on page 655. 



With few exceptions artificial drying is used only in connection 

 with the stiff mud and dry press process. The drying of bricks 

 should never be hurried, as bricks dried too quickly are apt to 

 crack; but some clays can be dried much more rapidly than others, 

 and so the drying capacity of the plant does not need to be as great 

 as in the case of clays that dry slowly. 



