98 GEOLOGY OF OHIO. 



pipe or brick factories. This machine will be described in detail later 

 on. In regard to its use in this connection it would seem a sensible 

 change. As to the cost, a wet pan is as cheap or nearly as cheap as the 

 tracer and it has the advantage of requiring far less power in proportion 

 to the work it is capable of doing. In a wet pan the clay is forced by the 

 scrapers to pass under the wheels continuously.' In ten minutes in the 

 wet pans, the clay is ground more than in an hour in the tracer, and there 

 is far less likelihood of any lumps remaining unreduced. The chief ob- 

 jection urged to it is that the tempering is so rapid that the clay requires 

 frequent wetting or it becomes too dry and that in emptying a charge 

 while the pan is in revolution the temper of the first portion removed will 

 be softer than the last portion. The tracer has to be stopped to empty it 

 and the product is therefore all alike. The capacity of one wet pan 

 would be sufficient to grind clay for the largest factories now in opera- 

 tion, which are using three or four tracers for the same purpose. 



The washing process has been introduced largely into the stoneware 

 industry in the last ten years. Prior to that the only attempt at washing 

 clay was by boiling it in iron pans. The washing plants now in use are 

 in all respects similar to those used in all branches of the pottery industry 

 and need be described only once to answer for all. 



The theory of the process is to beat the clays into a thin "slip;' or fluid 

 pulp with water, and then after sifting out all coarse particles, remove the 

 water and soluble impurities of the clay by filtration. 



The machinery is substantially the same everywhere. The clay is 

 reduced to a slip in the blunger, a vat generally of wood but sometimes of 

 iron. An upright shaft driven from overhead by gears, revolves in the 

 center of the tub and three or four stiff blades arranged like a propeller 

 are fixed near the bottom. As the clajr is shoveled into this tub of water 

 it is kept constantly in motion and reduced to a pulp. Sometimes the 

 blunger is a double one and has two shafts and two sets of arms, which 

 are set so that their circles of revolution overlap. The shafts revolving 

 in contrary directions give an extremely vigorous action to the machine, 

 • and it is wonderful how hard and rocky a cla}' can be reduced to a fluid 

 pulp in these machines. 



The screens are the next machine employed. They are generally 

 made of fine brass wire cloth stretched on rectangular frames which are 

 rapidly shaken back and forth by a high speed crank or eccentric. The 

 slip is delivered from the blunger on to the screen at its highest point 

 which slopes away at a small pitch toward either side. The liquid and 

 fine clay run through, are collected and conducted to an agitator. The 

 coarsest particles, including gravel, coarse sand, chips, etc., are retained 

 on the wire and by the constant vibration are carried down and off the 

 screen into troughs to receive the refuse. The fineness of the mesh for 

 stoneware is about one fortieth of an inch; it is not considered best to use 

 too fine a mesh in this business. 



