GYPSUM DEPOSITS OF NEW YORK 83 



operation, though thermometers are used in some plants as a fur- 

 ther check. The wide range of temperatures at which the burning 

 is completed may be ascribed largely to the variation in the purity 

 of the gypsum. According to Paul Wilkinson^ the temperatures 

 used in the manufacture of plaster of paris from the Kansas rock, 

 which averages very high in gypsum, do not exceed 340° F., while 

 they run about 396° F. as a maximum in the calcination of the 

 impure earthy gypsum. 



If the calcination is finished at too low a temperature the change 

 to half hydrate will be incomplete : the plaster in that case will be 

 deficient in strength. On the ether hand, if the temperature is car- 

 ried too high, there is danger of converting a part of the whole of 

 the charge into anhydrite. Soluble anhydrite results when the over- 

 burning is continued for a short time only and insoluble anhydrite 

 when it is of longer duration and at still higher temperature. The 

 presence of soluble anhydrite in plaster freshly burned is perhaps 

 not -uncommon, though the main ingredient is, of course, the half 

 hydrate. According to Davis any soluble anhydrite in the product 

 will take up moisture from the air to form half hydrate, so that its 

 presence in small amount may have no detrimental effect. 



The time required for the calcination of a charge ordinarily is 

 from two and one quarter, to three hours, depending on individual 

 practice. The fuel consumption with bituminous coal averages from 

 200 to 300 pounds for each ton of plaster. After cooling in the 

 pits the product is elevated to a revolving screen, which removes 

 any coarse material for regrinding, and is then transferred to the 

 storage bins. 



The kettle process has been criticized frequently as uneconomical, 

 and this is undoubtedly a serious drawback. Its simplicity and 

 the fact that plaster makers have grown accustomed to visual 

 methods of controlling the burning operation seem to be the main 

 reasons for its continued favor. As compared with the rotary kiln 

 the kettle consumes for each ton of plaster made, more fuel in cal- 

 cination and more power in agitating the charge, while it is less 

 efficient by reason of its interrupted operation. 



The Cummer rotary kiln is the only continuous calcincr in use 

 in this country. It is made by F. D. Cummer & Son Co. of Cleve- 

 land. The apparatus as installed for operation is shown in plate 23. 



The g>'psum rock is not pulverized as in the kettle process l)ut 

 is crushed to pass through a ;4-iiich I'iiig ^"d delivered to the stor- 

 age bin over the feed spout of the kiln. This consists of a steel 



^m. Inst. Min. Eng. 1897. 27:516. 



