MININO- AND PREPARATION OF CLAYS. 63 



suspended in the liquid while the coarser grains set- 

 tle on the bottom of the tank. The water with the 

 suspended clay is then drawn ofp to the settling tanks. 



A modification of this coufsists in the use of a large 

 cylinder closed at both ends and set in a horizontal 

 position; through this cylinder passes an axis with 

 iron arms, the revolution of the latter serving to break 

 up the clay, which is discharged through a hopper at 

 the top. A current of water passes through the cylinder 

 and carries the fine clay particles with it while the 

 coarse ones are left behind in the machine. The speed 

 of the current has to be regulated by experiment, 

 for if too much water is used coarse material 

 Mill be washed out of the cylinder, and 

 conversely, if the current is too slow the clay 

 will not yield a sufficient percentage of 

 washed product. One objection to this apparatus is 

 that it 'has to be stopped from time to time to remove 

 the coarse sand from the machine. 



The method most commonly used at the present- 

 day for washing kaolin, is by troughing and its gen- 

 eral detail is as follows : 



As the kaolin comes from the mine it is generally 

 discharged into a log washer, which consists of a semi- 

 cylindrical trough in which there revolves a horizont- 

 al axis, bearing short arms. The action of these arms 

 breaks up the kaolin more or less thoroughly, depend- 

 ing on its density, and facilitates the subsequent wash- 

 ing. The stream of water directed into the log washer 

 sweeps the kaolin and most of the sand into the wash- 

 ing trough, which is about 15 inches wide and 12 

 inches deep. It may be wider and deeper if the kaolin 

 is very sandy; in fact it should be. The troughing is 

 about 700 feet long, and to utilize the space thorough- 

 ly, it is broken up into sections, 50 feet to each is a 



