82 NEW YORK STATE MUSEUM 



The kettle is inclosed nearly tO' the top by a brick wall with an 

 open space between for the circulation of heat. The fire chamber 

 below is a little narrower than the kettle and rises from 4 to 7 

 feet above the grate bars. The heated gases pass through ports 

 into the open space at the base, then into flues which are placed 

 hoTizontally in the kettle itself and out through a stack. The flues 

 are built in sets of two or four. In a kettle of two flues they are 

 placed parallel about 8 inches above the crown. The arrangement 

 in a kettle of four flues is shown in plate 22 taken from a photo- 

 graph furnished by Butterworth 81 I.owe, Grand Rapids, Mich. The 

 kettle illustrated measures 10 feet, 4 inches across by 8 feet, 5 

 inches high and will calcine 10 tons of ground gypsum into plaster 

 of paris at a single charge. The weight of the mfetal is about 10 

 tons. 



The kettles are generally installed in line and worked in pairs 

 with a feeding chute and a pit for the calcined product between 

 each pair. In burning it is necessary to keep the gypsum in con- 

 stant agitation, for otherwise the hot mass would soon destroy the 

 kettle bottom. The agitation is accomplislied by nieans of a ver- 

 tical shaft to which paddles are attached and which is turned at 

 the rate of 15 revolutions a minute by means of a crown wheel 

 connecting with a pinion on the mill shafting. From 10 to 25 

 horsepower is required to maintain the agitation. 



The arrangement of an installation in a kettle plant is shown in 

 figure 8, which is reproduced from a drawing furniished by Butter- 

 worth & Lowe. ^ 



In operation, the kettle is charged with ground gypsum through 

 the trap in the cover and is filled in about an hour. Heat is grad- 

 ually applied during the process, and when the temperature reaches 

 220° or a little above, the mass begins to boil vigorously from the 

 escape of the mechanically held moisture. After this is evaporated 

 there is a noticeable settling, and the steam almost ceases for a time. 

 With increasing heat a second ebullition beginis between 280° and 

 290° F., causing the mass to rise to the top of the kettle. The 

 steam now is due to water of crystallization Avhich continues to, 

 come off as the heat is raised. When the boiling begins to slacken, 

 the mass settles again and is ready for removal into the fire brick 

 bins for cooling. The finishing temperature ranges between 350° 

 and 400°, as there is no fixed point which marks the completion of 

 the process. The experienced calciner relies chiefly upon the physi- 

 cal appearance of the plaster, the amount of steam given off and 

 the creaking of the machinery during the settling as guides in the 



