CLAY WORKING INDUSTRIES. 153 



In the bricks made by the wet processes, the preliminary cohesion is 

 accomplished by water, and burning merely perpetuates the bonds 

 already formed. In the dry press brick, the kind of vitrification required 

 to establish a bond between the particles would be scarcely less than 

 fusion. 



Hence it is to the stiff mud processes that we must look for the 

 paving bricks of the future. 



_ The machines in use for the manufacture of brick by the stiff mud 

 process can be classified as, 1. Plunger machines ; 2. Auger machines ; 

 3. Combination of thest principles 



Plunger machines are those in which the tempered clay is introduced 

 in a cylinder, or press box, and forced out through the die by pressure of 

 a piston operating from in rear of the clay. 



This is the simplest and most natural method of forming clay or any 

 plastic material into a required shape or size. 



The character of the bar of clay produced by being thus thrust out 

 of a die or constricted opening by pressure from in rear, is a very im- 

 portant field for study. To begin with, there are certain qualities which 

 it is thought probable are inherent in any bar of clay which is formed by 

 expulsion from a constricted opening. A study of the product obtained 

 from a large number of different machines, working on different clays 

 indicate this very strongly. 



When plastic tempered clay is put under pressure before a constricted 

 opening it issues forth and its flow from this opening appears to operate 

 under the same laws that govern the flow of fluids. That part of the 

 clay will be in most rapid motion, which is furthest from any solid fric- 

 tional surface, just as water in the center of a stream flows faster than its 

 edges. Hence the center of a bar of clay is sure to move faster than the 

 outside. These conditions become less marked in producing a large bar 

 than in a medium sized one. A bar having a section 6x11 inches has but 

 little tendency to drag on the. outside and run ahead in the center, be- 

 cause the inside area bears so large a proportion to its surface. There are 

 about two inches square area to every lineal inch of surface in such a bar 

 In a bar 3x4 there is less than one inch square area for each inch of 

 surface. Hence the center will t e more likely to run ahead in the small 



bar. 



But if we reduce the size of the bar to 1 x 4 the tendency to run ahead 

 in the center again nearly disappears, for while plastic clay under pressure 

 acts like a fluid, it is a very stiff fluid, and in sections where the surface 

 is so large in proportion to the area the retarding effect is nearly equal 

 all over the bar, and it comes out at nearly equal speed in all parts of its 



section. 



The sizes of aperture then at which the clay develops the greatest 

 tendency to run at unequal speeds, lies in the limits of those sizes that 



