MANUFACTURE OF PLASTERS. - - 43 



the contents cf the kettle settle. Further heating, however, brings on 

 boiling again, at about 290 F. (=143 C.), part of the water of combin- 

 ation being now driven off. The point at which the process is complete, 

 340-396 F., is known to the expert calciner by the manner in which 

 the material boils and by its general appearance, and at the proper 

 moment the calciner allows the charge to blow out through a small 

 gate at the bottom and in the side of the kettle, controlled by a lever. 

 In plaster-of-Paris plants thermometers are commonly used to govern 

 the temperature exactly, but in gypsite plants, whose raw material is 

 not so uniform in composition, the proper point varies slightly and is 

 usually best known by an experienced calciner. 



The escaping steam is let off by means of a stack let into the sheet- 

 iron cover of the kettle parallel with the smoke-stack, and this stack 

 contains near its base a separator similar to the steam-separators, for 

 the purpose of retaining the plaster-dust. It has been found by raising 

 the iron cover about 18 inches and putting on proper sides, that it 

 furnishes a chamber above the boiling material and greatly assists 

 the escape of the steam from it. 



From the kettle the hot material runs like water into a fire-proof 

 pit. The kettles are usually run in couples so that one pit will do for 

 two kettles ; and one chute will do for two kettles in filling, as the kettles 

 are r un at slightly different periods. Each kettle contains a charge of about 

 five tons of manufactured material, and requires from two to three hours 

 to calcine properly. After cooling slightly, the manufactured material is 

 elevated into a revolving screen, which separates all small particles and 

 foreign matter and renders the product uniform. The screenings which 

 usually amount to from ^ to 1 per cent only are sent back to buhrstone and 

 reground. It is usual to have a series of screw-conveyors and elevators 

 both in front of and behind the screen, so as to mix the material thor- 

 oughly. Owing to the temperature of the material, all conveyors, 

 elevators, and interior linings must be of metal, and the screen is made 

 of wire cloth. From the screen the material is conveyed to the storage- 

 bins (which are usually arranged to hold 100 or 200 tons), of which there 

 are several, so as to separate, if desired, the runs of different days. The 

 material is usually allowed to fall from a screw at the top of the building, 

 first, that it may spread out and let the different portions mix thor- 

 oughly, and, secondly, that it may cool in passing through the air. 



Temperature and water determinations made by Slosson and Moudy 

 during an actual run of the Laramie (Wyo.) plaster-plant are given 

 in the following table. The kettle used carried a charge of about five 

 tons, and the run was completed in about three hours. As shown by 



