CLAY WORKING INDUSTRIES. 149 



The pug mill, as a tempering device is the oldest machine employed 

 The principle on which it is founded is the Archimedes screw. A re- 

 volving bar is equipped with knives, or cutting bars, or propellers, set so 

 as to form an interupted screw thread. These knives or "paddles" stir 

 the clay up and pass it slowly forward in the direction the screw is run- 

 ning. Pug mills are now made horizontal for use in large brick works. 

 The older pug mills used in small brick yards and potters pug-mills are 

 still vertical. The trough in which the screw works is either of iron- 

 bound wooden staves, or sheet iron, and the shaft is usually made of 

 lorged iron and the paddles are either cast solid to slip on this shaft one 

 after another, or are made in two pieces, a cast stub which slips on the 

 shaft as before and a wrought iron or steel blade which bolts fast to the 

 stub and can be removed or repaired with no trouble. The gears which 

 drive the mill are either single or double according to the work which is 

 put on the mill. 



The clay is run into the mill in a continuous stream at one end and 

 is immediately wet with nearly all the water which it requires, further 

 addition of water is effected, if necessary, when the clay has progressed 

 part way down the pug mill. The discharge of the pug mill may be 

 either free or under pressure. Some of the recent forms of pug mills 

 are regular brick machines, forcing the clay out through a constricted 

 opening and cutting it off in small shavings by a revolving cutter outside 

 of the delivery. One mill forces the clay out through a dozen or more 

 holes, one and one half or two inches in diameter and cuts the bar up in 

 sections from one to two inches long. 



The closed delivery pug mills are more efficient in getting uniform 

 plasticity and uniform structure of the tempered clay, than the open 

 top mills. In the latter there is no pressure brought on the clay, it is 

 continually and loosely stirred until it is tipped out at the discharge open- 

 ing in a loosely granular condition. 



The advantages of the closed pug mills over the open ones are mani- 

 fest in the work they do, but the increase of power consumed is very 

 great. The character of the tempering done on a rocky clay not pos- 

 sessed of much natural plasticity is poor and the power consumed is 

 nearly as great as that used in a wet pan, 



In the open mill, much less power is used and the work done is still 

 poorer in quality. 



The advantages which can be claimed for the pug mill are, 



1st. Cheapness of first cost and small expense in repairs. 



2d. The action is continuous and if clay and water be within the 

 control of the temperer, more uniformity is possible than can be 

 attained in the wet pan. Under ordinary conditions, however, this 

 advantage is lost, for not one in ten of the plants using pug mills have 

 any storage bin for securing a regular flow of clay, but are made to 

 depend on the supply of clay that falls from the screen, which in turn 



