8o ' NEW YORK STATE MUSEUM 



a preliminary reduction to uniform size and after calcination is 

 given a second treatment for pulverization. 



The first step in reduction is performed in a coarse crusher, by 

 which the rock of size convenient for handling is broken to lumps 

 of about I -inch diameter. The crushers commonly used are of the 

 jaw or gyratory types, the preference in New York plants being 

 given to the former. One formx of jaw crusher or " nipper " spe- 

 cially devised for gypsum plants is shown in plate i8. The mov- 

 able jaw, as well as the end plate, sometimes has a corrugated sur- 

 face which prevents the soft material from clogging the outlet. 

 The machine shown in the illustration weighs 13,000 pounds and 

 will crush each hour from 15 to 30 tons of rock. 



Fro^m the coarse crusher the gypsum passes into the " cracker." 

 This machine works like a coffee mill, having a corrugated shell 

 of inverted conical shape within which revolves a corrugated spin- 

 dle [pi. 19]. The machines have a capacity of from 3 to 12 tons 

 an hour and crush to about pea size. 



After this treatment, the gypsum is ready for charging into 

 rotary cylinders if these are used for calcination. For the kettle 

 process, however, it is next run through a fine grinder of which 

 there are several forms well adapted for the purpose. In the 

 mills first erected the grinding was universally done by buhrstones, 

 and this practice continues to be quite common, though it has been 

 superseded in most of the modern plants by more improved meth- 

 O'ds. The stones are set the same as in flour mills and may be of 

 French or domestic make. The small expense of such an oiifit is 

 its chief recommendation and is offset by the necessity of redress- 

 ing the stones from time to time, an operation that requires a 

 high degree of skill. 



An improvement on the horizontal millstones for grinding gyp^ 

 sum is the use of a vertical mill which can be run at a higher 

 speed. This type is common abroad. The Sturtevant Mill Ca 

 of Boston manufactures a vertical mill of special construction that 

 has been installed in several plants. The stones are built up of 

 emery blocks set in a metal shell aroimd a central disk o^f buhrstone. 

 The emery blocks are held secure by metal filled in while molten. 

 A 36-'inch Sturtevant mill is shown in section in plate 20. The 

 mill is supplied with an automatic feeder from which the gypsum 

 is carried by a worm conveyor and forced between the stones. 



Another machine in use for pulverizing gypsum is illustrated in 

 plate 21. It is made by the Williams Patent Crusher & Pulver- 



