GYPSUM DEPOSITS OF NEW YORK 



79 



CALCINING TEMPERATURE 



SET 



BEGINNING 



AFTER 



SET 



COMPLETE 



AFTER 



120°C. 

 2 5©°C. 

 450°.. 

 500° . . 

 600°. . 

 650°. . 

 700°. . 

 720°. . 

 1185°.. 



Minutes 



16 minutes 

 6 minutes 

 over 5^ hours 

 24 hours 

 24 hours 

 no set 

 no set 

 no set 

 no set 



This indicates that the most efficient temperature for calcining is 

 at about 250° C (482° R). 



TECHNOLOGY OF GYPSUM PLASTERS 



Plaster of pans and wall plasters 



The manufacture of the different calcined plasters is based on 

 similar methods, though there is considerable variance in the details 

 of practice and equipment of the plants. In every case the crude 

 gyipsum from the mines or quarries must undergo the two' opera- 

 tions of crushing and calcination. 



The plasters made in New York and also practically all those 

 manufactured in this country belong to the half hydrate class, i. e. 

 their basis is plaster of paris. Their varied qualities depend mainly 

 upon the proportion of impurities present in the original rock and 

 upon the addition of artificial materials to hasten or retard ^ the 

 setting process. The anhydrous plasters which include the so called 

 " cements " and the German flooring plasters form a distinct group 

 that can best be considered under a separate head. 



Crushing. The crushing of the material may be performed 

 either before or after calcination. The general practice in this 

 CQiuntry is to make a partial reduction at least before burning, 

 though abroad the crude rock is often calcined in arched kilns in 

 a manner similar to the burning of limestone. With the kettle 

 process, which is widely used in American plants, the rock is re- 

 duced to a fine powder before calcination. The introduction of 

 rotary cylinders for calcining among the newer plants involves a 

 change of the crushing process whereby the rock is subjected to 



