298 CLAYS AND CLAY INDUSTRY. 



iron pan in which there revolves a frame bearing 2 narrow iron 

 wheels, 30 to 36 inches in diameter. As this frame revolves, the 

 wheels by means of a gearing, travel from the centre to the cir- 

 cumference of the pan and then back. The clay and water are 

 placed in the pan and the action of the wheels grinds and cuts it 

 up, the tempering taking from one to two hours. The action 

 of such a machine is quite thorough, but considerable power is 

 required to operate it. 



Pug mills. — The principle of these is similar to' those used in 

 brick manufacture (p. 225), but they differ in being upright or 

 vertical. The clay and water are added at the top and slowly 

 mixed, being at the same time forced down to the opening at the 

 bottom of the box. 



Tables. — Kneading tables are used at some factories for work- 

 ing the clay by machine instead of wedging it by hand. Although 

 much used abroad, their introduction into this country has been 

 rather restricted. The machine consists of a circular table about 

 6 feet in diameter, the upper surface of which slopes outward. 

 On this are 2 conical rolls, 20 to 30 inches in diameter and about 

 8 inches wide. These rolls have corrugated rims, and are at- 

 tached to opposite ends of a horizontal axis, having a slight verti- 

 cal play. The clay is laid on the table and as the rolls travel 

 around on it, the clay is spread out into a broad band. A second 

 axle carries 2 other pairs of rolls of the same shape but smaller 

 size, which travel around in a horizontal plane. These rolls press 

 the band of clay together again. In this way the clay is subjected 

 to alternating vertical and lateral pressure and all air spaces are 

 thus closed. The rolls make 10 to 12 revolutions per minute, 

 and the machine kneads 2 to- 3 charges of 350 pounds per hour. 



Molding. 



After the clay has been properly tempered, the next step in the 

 process of manufacture is molding. As indicated above this is 

 done in four different ways, the clay having first been thoroughly 

 kneaded, usually by hand, in order to insure its complete homo- 

 geneity and freedom from all air bubbles. 



