BEIOK YARDS 233 



at a charge, taking two hours for the operation. Such a charge 

 makes from 180 to 190 gallons of ware. The ground clay is 

 rolled into masses and "wet blanketed" to keep until used. At 

 some factories steam tracer mills are used for doing the grind- 

 ing. They can grind about 1,200 pounds in 60 to 100 minutes, 

 but eliminate blebs or substances in the clay and particles of 

 foreign matter. The balls of clay are successively divided by a 

 wire into two parts, these halves being each time struck together 

 in such a manner that the two pieces unite different faces. The 

 clay is next turned to the required shape. Crocks and fruit 

 jars are the simplest kinds of product made and after that come 

 jugs, milk-pans and churns. At small works the ware is often 

 laid on flat boards in the sun to dry. At the larger works drying 

 is usually done in closets or rooms heated by a series of steam 

 pipes. 



The dry ware now has to be slipped, that is to say, it is covered 

 with a wash consisting of a very fine ground clay suspended in 

 water. By this means the ware becomes covered with a film of 

 clay. These slip clays vitrify easily to a black glaze which covers 

 the ware. The slip commonly used is obtained from the Hudson 

 river clays at Albany and a partial analysis of it is as follows : 



Sesquioxide of iron 1 . 43 



Potash * 3.17 



Soda 74 



5.34 



There are also present large percentages of carbonates of lime 

 and magnesia, which add to the fusibility of the clay. For other 

 analyses see sewerpipe. 



A difficulty experienced in the use of slip glazes is the tendency 

 to blister. By many potters this is considered to be due to 

 hasty burning, and it is thought by them that it may be overcome 

 by heating the ware at a low red heat for several hours before 

 raising the temperature. After this a rapid raising of the heat is 

 harmless. The kilns in which the ware is burned vary consider- 

 30 



