228 CLAYS AND CLAY INDUSTRY. 



required. It does not produce a product with smooth faces and 

 sharp edges, but this defect can be overcome by re-pressing- the 

 product. 



Stiff-mud process. — In this method of molding (PL XXVI, 

 Fig. 2), the clay is tempered with less water and consequently 

 is much stiffen The principle of the process consists in taking 

 the clay thus prepared and forcing it through a die in the form 

 of a rectangular bar, which is then cut up into' bricks. The 

 machine now most used is known as the "auger" type, the 

 "plunger" type having nearly disappeared from use. Its gen- 

 eral form is that of a cylinder closed at one end, but at the 

 other end tapering off into a rectangular die, whose cross section 

 is the same as either the end or the largest side of a brick. With- 

 in this cylinder, which is set in a horizontal position, there is a 

 shaft, carrying blades similar to those of a pug mill, but at the 

 end of the shaft nearest the die there is a tapering screw. The 

 internal shape of .the die is variable, depending on the make of 

 the machine. It is heated by steam or lubricated by oil on its 

 inner side, in order to facilitate the flow of the clay through it. 



The tempered clay is charged into the cylinder at the end 

 farthest from the die, is mixed up by the revolving blades, and at 

 the same time it is moved forward until seized by the screw and 

 pushed through the die. Since this involves considerable power, 

 it results in a marked compression of the clay. With such con- 

 ditions, there will naturally be more or less friction between the 

 sides of the bar and the interior of the die, causing the centre of 

 the stream of clay to 1 move faster than the outer portion. Much 

 attention has, therefore, been given to the construction of the 

 die, in order to overcome this and facilitate the flow of the clay 

 as much as possible. In case the amount of friction between 

 die surface and clay is greater than the cohesion in the plastic 

 mass, the bar of clay is likely to tear on the edges, producing 

 serrations like the teeth of a saw. The effect of the screw at 

 the end of the shaft, together with the differential velocities with- 

 in the stream of clay, also produces a laminated structure in the 

 brick, which is often greatest in highly plastic clays, but is some- 

 times marked in clays of only moderate plasticity when machines 



