802 NEW YORK STATE MUSEUM 



kaolin fall on the screen, and pass tlirongli, otherwise they run off 

 and are lost. A slight improvement is, to have two or three 

 screens overlapping one another, so that whatever does not get 

 through the first will fall on the second. If the vegetable matter 

 and sticks are allowed to accumulate, they clog the screen, and 

 prevent the kaolin from running through; consequently stationary 

 screens must be closely watched. 



The revolving screens are fai' better; for they are self-cleansing. 

 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 carried upward and finally drops; but, instead of falling down 

 on the other side of the screen, it falls on a board, which diverts it 

 ■but to the ground. 



The settling tanks, into which the kaolin and the water are dis- 

 charged, may be and often are about 8 feet wide by 4 feet deep 

 and 50 or more feet long. As soon as one is filled the water is 

 diverted into another. The larger a tank the longer it will take 

 to fill it, and allow the kaolin to settle. Clays obtained in this 

 manner are expensive, particularly when the market takes the out- 

 put of washed kaolin as soon as it is ready. Small tanks have the 

 advantage of permitting the slip to dry more quickly, specially 

 when the layer of clay is not very thick; furthermore a small pit 

 takes less time to fill and empty. But one disadvantage urged 

 against a number of small tanks is that a thoroughly average prod- 

 uct is not obtained, owing to the thinness of the layer of settlings 

 and the small amount in each. In addition a series of small tanks 

 require considerable room. The advantages asserted in the case 

 of large tanks are that the clay can be discharged into any one for 

 a considerable 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. "WTien the kaolin has settled, 



