684 



Report of the State Geologist. 



pipe being tempered in a few minutes. The charge is generally removed by- 

 means of a long-handled shovel pivoted on an upright arm. Wet-pans have a 

 greater capacity and are more efficient than pug-mills, but consume more 

 power. 



Pug-mills consist of a horizontal or inclined trough in which there 

 revolves a shaft bearing knives or a worm screw. The material with water 

 is charged at one end and, by means of the screw thread or knife blades, is 

 mixed thoroughly and at the same time passed to the other end of the trough, 

 where it is discharged. To insure thorough tempering, pug-mills should not 

 be less than eight or ten feet long. They are extensively used hy paving- 

 brick manufacturers, and so arranged that their contents are discharged 

 directly into the molding machine. 



Molding. Paving brick may be manufactured by one of three processes, 

 viz., the stiff-mud, soft-mud or dry-clay process. 



Stiff -in ad 'process. This is the most used by paving-brick manufacturers. 

 In this process the clay is discharged from the machine through a rectangular 

 steel die, whose cross-section may be 9x4 or 2x4 inches, depending on whether 

 the brick is to be end-cut or side-cut. As the bar of clay issues from the 

 machine it is received on the cutting table and- cut up into bricks, either by a 

 series of parallel wires fastened to a moveable frame, or by means of a revolv- 

 ing wheel, also bearing a series of wires. The capacity of the machine may 

 be materially increased by having a double or triple die, so that more than 

 one bar of clay issues at the same time, but a single die will give an 

 auger machine a capacity of 50,000 brick in ten hours. The clay should 

 be thoroughly pugged before being charged into the machine. Auger 

 machines combine economy and large capacity, and their use has not only 

 become widespread but necessary to enable the manufacturer to compete 

 successfully with his rivals. 



Auger machines are adapted to a wide range of clays, except very lean 

 or very plastic ones. The former seldom have enough cohesion to hold 

 together in passing through a die, and the bar of clay cracks and tears. Very 

 plastic clays develop a series of concentric laminations in the brick which are 

 a serious detriment to its strength. The laminations in two bricks made from 

 the same shale may be very differently developed, if molded in machines of 

 different make Much attention is, therefore, paid to improving the construc- 

 tioi) of the dies and other portions of the machine as small changes may often 

 cause considerable difference in the structure of the brick. 



