GYPSUM DEPOSITS OF NEW YORK 8 1 



izer Co. of St Louis. The reduction is accomplished by means of 

 hammers carried on a rapidly revolving horizontal axis and work- 

 ing against a corrugated breaker plate. The machine is said to 

 take rock that will pass through a 2-inch ring and crush from 12 

 to 15 tons an hour through a 30-mesh screen. 



The Stedman disintegrator, which is characterized by a series of 

 concentric cages with steel crossbars, the adjacent cages revolving 

 in opposite directions and crushing the rock by impact, is employed 

 in some of the western plants. The roller mills in use for grind- 

 ing flour is also said to be serviceable for gypsum. 



T-here seems to be no standard of fineness for plasters, such as 

 obtains in cement manufacture. The size of the particles, however, 

 is not without influence upon the setting qualities, though within 

 the moderate limits of variation in ordinary- practice the degree 

 of fineness does not appear to be very important. A series of 

 sieve tests on marketable plaster from the middle western districts 

 has been published by the Iowa Geological Survey,^ the results of 

 which show that an average of 70 per cent of the ground plaster 

 will pass through a sieve with 74 mesihes to the linear inch, about 

 60 per cent through a lOO-mesh sieve and 44 per cent through a 

 200-mesh sieve. 



Calcination. The chemical features of tlie calcination process 

 are described elsewhere in detail. Though the process is simple 

 in theory, as well as in its mechanical requirements, it demands a 

 degree of experience and skill to insure a uniformly satisfactory 

 product. 



The common kettle method of calcination as used in this coun- 

 try is an adaptation of the earHer practice by which plaster of paris 

 was made on a small scale in a cauldron kettle over an open fire. 

 The modern kettles are cylinders of boiler steel, nearly square in 

 vertical section, set upright on a brick foundation. Their diameters 

 range from about 8 to 10 feet. The sides are constructed of sheet 

 iron ^ to ^ inch thick, while the lx)ttoms which must withstand 

 extremes of temperature are usually cast from the best grade of 

 scrap iron, and their thickness varies from ^ inch at the edges 

 to 4 inches in the centers. The bottoms are arched upward rising 

 about a foot at the crown. Some kettles are made with sectional 

 bottoms, so that in the case of breakage it is only necessary to 

 replace the broken part instead of installing a new bottom. The 

 cover is of sheet iron and has a trap through which the charge is 

 introduced. 



'An. Rep't 12. 1902. p. 162. 



