METHOD OF MANUFACTURING BRICK 



159 



essentially of a semi-cylindrical trough, six to 10 feet long, in 

 which there revolves a shaft, bearing knives set spirally around 

 it, or a worm screw six or more inches wide. The material is put 

 in at one end, and the knives or thread mix it up. At the same 

 time it is worked along to the other end of the trough from 

 which it is discharged into the machine. The pug mill may be 

 closed or open ; the former is better as there is a more uniform 

 pressure on the clay while it is being tempered, and a more 

 thorough mixing results. Water is also added from a faucet at 

 the upper end of the trough until the clay is in the right condi- 

 tion. The angle of the knives with relation to the shaft can be 

 changed so that the clay can be moved along slower or faster as 

 it is desired. The trough of the pug mill is of iron or wood, 

 usually the former. A pug mill, according to its size, will in 10 

 hours temper clay enough for from '25,000 to 60,000 brick. Pug 

 mills take up less room than ring pits and do not require as much 

 power to operate them. They will also, if desired, discharge the 

 clay directly into the molding machine. They are used chiefly 

 with stiff mud machines. 



Molding. — Having prepared the clay by one means or another, 

 according to its character, and somewhat according to the 

 machine to be used, the next step is to mold it into bricks. The 

 old-fashioned method of molding bricks by hand is rapidly dying 

 out, yet every now and then we come across a yard where it is 

 still in vogue. In New York the soft mud process is the most used. 

 There are a number of different types of machines but the funda- 

 mental principal of them all is the same. A soft mud machine 

 consists essentially of an upright box of wood or iron and gener- 

 ally of a rectangular shape. In this is a vertical shaft bearing 

 several knives horizontally. Attached to the bottom of the shaft 

 is a device such as a curved arm, which forces the clay into 

 the press box. The molds are put in at the rear of the machine 

 and fed forward underneath the press box automatically. The 

 empty mold sliding into place shoves out the filled one. A boy 

 sands the molds before placing them in the machine in order to 

 prevent the clay from sticking. The clay is fed to the machine 

 at the upper end of the box. Often there is a pug mill attached 

 to the machine. In all these machines the material gets an addi- 

 tional amount of mixing by the knives on the vertical shaft. In 



