660 NEW YOKK STATE MUSEUM 



used, but each pit in this case holds enough material for the daily- 

 use of two machines. 



Pug mill. This machine, like the ring pit just described, is used 

 for thoroughly mixing the clay, or clay and sand as the case may 

 be, before introducing it into the machine. It consists essentially 

 o'f a semi-cylindric trough, 6 to 10 feet long, in which revolves 

 a shaft, bearing knives set spirally around it, or a worm screw 6 

 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 till the clay is in the 

 right condition. 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 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 60,00'0i 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. 



In some works a double form of pug mill is used. This has two 

 axles bearing knives, instead of one. They revolve in opposite di- 

 rections, (pi. 106^"^) 



Screens 



When clay is molded in the dry condition, or when shale is used 

 instead of soft, plastic clay, it is important that the material be first 

 ground to the proper degree of fineness. 



As the material comes from the dry pan or other apparatus used 

 to pulverize it, it is carried to screens, which allow the sufficiently 

 fine material to pass through while those particles which are too 

 coarse go back to the crushing machine. 



Three general types of machine are used, inclined, rotary, and 



