658 NEW YOKK STATE MUSEUM 



particularly to the iDroduction of sucli materials as exhibit coarse 



article and fine grains, such as grogs for instance. 



The material to be ground in ball mills must be air-dried and 



only in those of the intermittent type can damp or wet material 



be introduced. This is necessary, for instance, in the case of 



glazes. If the material to be ground must be kept from contaci; 



with iron, the interior of the cylinder is lined with porcelain, and 



instead of iron balls porcelain or flint ones are used. The capacity 



of ball mills is highly variable, depending on the fineness of the 



product desired, the hardness of the material to be ground, and also 



on the size of the mill, therefore the hourly production will vary 



in the case of grog between 1500 and 3000 pounds. In this case, 



for every 2000 pounds ground in an hour, three to 10 horse power 



is required. 



Wet methods of preparation 



The clay can commonly be tempered directly as it comes from 

 the bank instead of being pulverized, which is always necessary 

 in the case of shales. 



The wet methods employed are: 



SoaJc pits. These are the most primitive contrivances at present 

 used for the preparation of clays. There is a rectangular pit about 

 5 feet deep and 6 feet square. The Long Island pits are usually 

 rectangular in shape. Into this the clay and sand are dumped, 

 water poured on and the mass allowed to soak over night, so as 

 thoroughly to soften it. The following morning the softened ma- 

 terial is shoveled into the machine. Two men — pit shovelers — 

 do this, and it is highly important that they be men of intelligence 

 and attend to their work, seeing that the right proportions of clay 

 and sand are shoveled into the machine, rrom one third to one 

 quarter is the amount of sand added. The operation of mixing the 

 clay and sand is called tempering; the addition of sand is in most 

 cases not necessary, as the majority of clays have sufiicient of it 

 naturally. The object of the addition of sand is to counteract the 

 effect of the alumina, by preventing a too great and uneven shrink- 



