CHAPTER II. 



CHEMISTRY OF GYPSUM-BURNING. MANUFACTURE OF PLASTERS 



BEFORE taking up the actual methods and details of plaster-manu- 

 facture, it will be of advantage to discuss briefly the chemical and physi- 

 cal principles on which the industry is based. 



Chemistry of gypsum-burning. Pure crude gypsum is a hydrous 

 sulphate of lime, with a chemical formula CaS0 4 +2H 2 0. This corre- 

 sponds to the composition: 



CaSO 4 +2H 2 



(Lime(CaO) 32.6% 



Lime sulphate (CaSO 4 ) \ Sulphur trioxide 



I (S0 3 ) 46.5 



Water (H 2 O) =20.9 



79.1% 



100.0 



If pure crude gypsum he heated to a temperature of more than 

 212 F. and less than 400 F., a certain definite portion of the water 

 of combination will be driven off, and the gypsum thus partially de- 

 hydrated will be plaster of Paris. Plaster of Paris has the formula 

 , corresponding to the composition: 



CaSO 4 +H 2 O 



f Lime sulphate (CaSO 4 ). 

 \ Water (H 2 O) 



93.8% 

 6.2 



Three fourths of the original water of combination have therefore 

 been driven off in the course of the process. Dehydration to this extent 

 can, as above noted, be accomplished at any temperature between 

 212 F. and 400 F. In actual practice, however, it is found most 

 economical of fuel and time to carry on the process at the highest allow- 

 able temperatures; and 330 to 395 F. may be regarded as the usual 

 limiting temperatures for plaster-manufacture. 



About 400 F. is a critical temperature, for if gypsum be heated at 

 temperatures much above this, it loses all of its water of combination, 

 becoming an entirely anhydrous sulphate of lime, and useless as a 

 normal plaster. Under certain conditions, however, gypsum burned 

 at temperatures above 400 F. gains valuable properties. Such highly 



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