MANUFACTURE OF PLASTERS. 41 



from the top, sometimes level with the top, to facilitate shoveling the 

 raw material into the kettle, and the kettle with its supports is in the 

 basement, with storage room for fuel conveniently arranged in front 

 of the kettle. Ports are made through the side of the base ring, and 

 the heat from the furnace is deflected by bridges around the surface 

 of the kettle, so that the heat may cover every part of the kettle, pass 

 through the flues, and finally make exit through a regular stack. 



At the top the kettle is covered with a sheet-iron cap having a mov- 

 able door, through which the raw material is introduced, usually by 

 a chute fed by an elevator. 



The shipping-weight of an 8-foot kettle is about 15,000 Ibs., and 

 of a 10-foot kettle about 18,500 Ibs. Their list prices are about $1200 

 and $1600, respectively. 



The kettles are usually arranged in line and worked in pairs, with 

 one feeding chute and one pit for calcined material for each pair. It 

 is necessary that the material in the kettle be kept constantly agitated, 

 and for this purpose a line shaft carrying a 1-inch vertical pinion-wheel 

 runs over the kettles and a 4-inch vertical shaft with a 5-inch hori- 

 zontal crown-wheel runs from this to the bottom of each kettle, being 

 supported by a saddle placed between the flues. At the bottom of 

 the vertical shaft a curved cross is attached, to which are affixed movable 

 teeth with paddles, run at 15 revolutions per minute, which are so 

 adjusted as to throw the material from the periphery to the center. 

 From 10 to 25 H. P. are required to run one agitator. Should the agita- 

 tion stop, or the teeth become broken, the material settles down on 

 the bottom, and, owing to the intense heat, the bottom is almost in- 

 stantly melted through. The material, which when hot is very fluid, 

 runs through like water and quenches the fire. Stoppage of the agita- 

 tion can usually be detected by the calciner, who stands above and is 

 supposed to watch the process of calcination constantly. 



In burning plaster of Paris the temperature does not exceed 340 F., 

 but when gypsite (gypsum earth) is used a higher temperature is re- 

 quired, averaging close to 396 F., probably owing to the foreign matters 

 included in the gypsite. 



In starting a kettle, the heat is gradually applied, and the crude ma- 

 terial is gradually fed in and constantly agitated. This process is slow and 

 requires some length of time, owing to the vast amount of mechanically 

 held water which must be evaporated when the wet gypsum earths 



I, re used. Material is gradually added until the kettle is full, and as the 

 emperature rises the contents boil violently, much like water, at 220- 

 130 F. (105-110 C.). When the mechanically held water is evaporated 



