

MANUFACTURE OF PLASTERS. 



37 



After being reduced as above described, the gypsum is calcined. 

 Usually it is necessary to regrind some of the product which comes from 

 the kettles; and this may be accomplished in any of the fine grinders 

 above noted. 



When the rotary process is used, it is customary not to pulverize the 

 material until after calcining. As calcined plaster is much easier to 

 grind than .crude gypsum, a considerable saving in power and repairs 

 is effected by this difference in practice. 



FIG. 8. Stedman disintegrator, showing cage construction. 



Calcining in ovens. In the manufacture of the higher grades of 

 plaster of Paris it is necessary that the material should be calcined 

 with extreme uniformity and at exactly the proper temperature. This 

 uniformity in burning is attainable in ovens, though the process is 

 necessarily expensive in fuel and labor. For these reasons the oven 

 process has not been used in the United States, though it still persists 

 in Europe for certain grades of plasters. 



Calcining in kettles. The favorite process in the United States, 

 particularly in the plaster-plants of the Middle West, is that in which 

 the calcination is effected in kettles. As noted later in discussing 

 continuous calcining processes (pp. 46-50) the kettle process is slow, 

 low in output, and expensive in fuel. For these reasons it will probably 

 disappear as the continuous rotary calciner becomes perfected; but 

 at present it is still used in the majority of American plaster-plants. 

 The statements above should not be construed as a too sweeping con- 



