PART I. PLASTERS. 



CHAPTER I. 

 COMPOSITION, DISTRIBUTION, AND EXCAVATION OF GYPSUM. 



THE mineral called gypsum is the raw material which serves as the 

 basis for the manufacture of plaster of Paris, " cement plaster", and 

 the various related types of plasters. In the present chapter the com- 

 position, properties, varieties, mode of occurrence, origin, and distribu- 

 tion of gypsum will be described in the order named, after which the 

 methods and costs of quarrying and mining gypsum will be discussed. 



Chemical composition. The mineral gypsum, when absolutely pure, 

 is a hydrous sulphate of lime, made up of one molecule of lime sulphate 

 combined with two molecules of water. The chemical formula of 

 gypsum is therefore CaS04 + 2H 2 O. This, when reduced to percentages 

 of weight, corresponds to the following: 



Gypsum (CaSO 4 + 2H 2 O) = (CaS ' ) 



The 79.1% of lime sulphate can, in turn, be considered as being made 

 up of 32.6% of lime (CaO), plus 46.5% of sulphur trioxide (SO 3 ). 

 Reduced to its ultimate components, the composition of pure gypsum 

 may therefore be represented as follows: 



f Lime (CaO). . 32.6% 



Gypsum (CaSO 4 + 2H 2 O) = \ Sulphur trioxide (SO 3 ) ...... 46.5 



[Water (H 2 0) .............. 20.9 



100.0 



Deposits of gypsum large enough to be worked for plaster are, how- 

 ever, rarely even approximately as pure as this. Gypsum as excavated 

 for a plaster-plant will usually carry varying and often high percentages 

 of such impurities as clay, limestone, magnesian limestone, iron oxide, 



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