66 GENERAL DISCUSSION OF CLAYS. 



self cleaning. Such screens are barrel shaped, and the 

 water, with the kaolin in suspension, is discharged 

 into the interior and passes outward through the screen 

 cloth. As the screen revolves, the dirt caught is car- 

 ried upwards and finally drops; but instead of falling 

 down upon the other side of the screen, it falls upon a 

 board, which diverts it out upon the ground. 



The settling tanks, into which the kaolin and the 

 water are discharged, may be and often are about 

 eight feet wide by four feet deep, and fifty or more 

 feet long. As soon as one is filled the water is diverted 

 into another. 



The larger a tank, the longer will it take to fill it, 

 and allow the kaolin to settle, and delays due to this 

 cause them to be expensive, especially when the market 

 takes the output of washed kaolin as soon as it is ready. 



Small tanks have the advantage of permitting the 

 slip to dry more quickly, especially when the layer of 

 clay is not very thick, and furthermore a small pit 

 also takes less time to fill and empty, but one dis- 

 advantage urged against a number of small tanks is 

 that a thorougly average product is not obtained ow- 

 ing to the thin layer of settlings and the small amount 

 iii each. In addition to this a series of small tanks 

 requires considerable room. 



The advantages claimed for large tanks are that the 

 clay can be discharged into any one for a considrable 

 period, and, if the clay deposit varies in character, the 

 different grades get into one tank and a better average 

 is thereby obtained. 



If the kaolin settles too slowly, alum is sometimes 

 added to the water to hasten the deposition. When 

 the kaolin is settled, most of the clear water is drawn 

 off, and the cream like mass of kaolin and water in the 



