86 NEW YORK STATE MUSEUM 



while the fine powder which if allowed to remain long in contact 

 with the heat would- become dead-burned, passes quickly to the rear 

 end of the cylinder tmder the blast of air. The gases and hot air 

 pass out of the cylinder and into the forewarmer where whatever 

 heat remains in them is utilized in heating the crude gypsum on its 

 v/ay to the cylinder. The gases and air, then with a temperature of 

 but 80° F., pass into collecting chambers to recover any dust of 

 plaster in them, and thence out through the stack. 



In Europe gypsum is commonly burned in lump form in arched 

 kilns, which are built of masonry and somewhat resemble the ordi- 

 nary brick kiln of this country. ^ The heat from the central fire pit 

 is conducted through radiating channels, which are constrncted of 

 the larger gypsum blocks, and then finds its way upward in the 

 spaces between the lumps to issue finally through fines in the roof. 



Plaster of paris used in porcelain and china ware manufacture 

 requires careful preparation, as it must form a light, porous mass 

 when set. This grade is made mostly in France and Germany. 

 The calcination is often carried out in brick ovens, the gypsum 

 being stirred frequently during the process. An improved type of 

 oven that is now employed in Germany for making porcelain plaster 

 takes the form of a long room constructed of brick into which the 

 gypsum is carried on cars.^ These have racks holding five or more 

 shelves which are loaded with rock that has been previously crushed 

 to i-indi size or smaller. The room is heated by a furnace below, 

 the gases passing through fines in the walls and not coming into 

 contact with the gypsum. The temperature is maintained uniformly 

 at 140° C. (284° F.). Three charges are burned in a week, and 

 the output of each is about 8 or 9 tons of calcined plaster. 



Resultant product. The product resulting from the operations 

 just described is a finely divided calcined plaster. If a pure gyp- 

 sum has been used it will consist of calcium sulphate plus a small 

 residue of water, the amount depending upon the degree to which 

 the calcination is carried. The ideal composition of plaster of paris 

 is represented by the formula CaSOi- ^ HoO which calls for 93.8 

 per cent of calcium sulphate and 6.2 per cent of water. These per- 

 centages are approached in high grade plaster of paris, which finds 

 special uses, but most wall plasters contain a considerable propor- 

 tion of impurities due to the admixture of clay, lime, magnesia 



etc. with the gypsum. 



» _^..^____ 



1 Grimsley, G. P. Technology of Gypsum. Mineral Industry. 1899 ' 

 7:390. 



2 Wilder, F. A. The Gypsum Industry of Germany. Iowa Geol. vSur. 

 An. Rep't 12. 1902. p. 217. 



