410 STORY B. LADD 



plaster, and soils with no potash compounds are not benefited 

 in this respect by it. 



The phenomena attendant upon the baking and hardening 

 of plaster of Paris were first studied by Lavoisier. In the 

 Comptes Rendus of February 17, 1765, appears the following: 

 ''After having removed the water of hydration from gypsum 

 by heat, if it be presented to it again (this is commonly known 

 as the mixing or tempering of plaster), it takes it back with 

 avidity, and suddenly assumes a state of irregular crystaUiza- 

 tion; the small crystals which form become confused with each 

 other, the result being a very hard mass." Lavoisier discov- 

 ered that when plaster was baked at too high a temperature it 

 lost its pecuHar property of setting, and that when it was 

 heated the water of crystaUization was removed at two dif- 

 ferent stages, three fourths of it being much more easily remov- 

 ed than the remainder. Payen, in 1830, found that gypsum 

 began to lose its water of crystallization at 115° C, and that 

 the loss increased rapidly as the temperature rose. He con- 

 cluded that a temperature of from 110° to 120° C. was the best 

 for calcination, but that plaster of Paris could be made at a 

 lower temperature, even as low as 80° C, if the burning were 

 continued long enough. He found that gypsum was dehydrat- 

 ed if heated to about 250° C, and from 300° to 400° C. it lost 

 completely the properties of hydration. 



In the manufacture of plaster of Paris the gypsimi rock is 

 crushed and ground into a flour in a buhr mill, a roller mill, or a 

 disintegrator. The ground gypsum is then passed into storage 

 bins. In some mills the flour gypsum is conveyed to the bins 

 by an air blast. 



The calcining kettles are made of boiler steel and hold 

 about 8 tons; the flame is carried around the sides by flues, so 

 as to heat all parts. In charging, the ground gypsum is run 

 slowly into the kettle, which is kept at a temperature of 212° F. 

 or over; about an hour and a half is consumed in filling to a 

 depth of 5 feet. The material is kept in motion by a mechan- 

 ical stirrer, it being of the greatest importance that the partly 

 calcined plaster shall not remain in contact with the heated 

 iron. The slow filling tends to keep the heat constant or nearly 

 so during the charging, and as the boiling increases the plaster 



