412 STORY B. LADD 



plaster. Whether a retarder weakens the cement plaster or 

 not is a disputed question among architects and plasterers, 

 although it is conceded that the addition of too much foreign 

 material reduces the strength of the mass of interlacing crys- 

 tals. As a general rule the prolongation of the period of crys- 

 tallization or set is determined by the quantity of retarder 

 added to or incorporated with the plaster. Calcined plaster, 

 which sets slowly, whether retarded or not, is known as ce- 

 ment plaster. 



It is a well known fact that when a salt crystal is dropped 

 into a supersaturated solution of a salt, as for instance sodium 

 sulphate, the whole mass immediately crystallizes. Crystals, 

 or even solid particles of foreign material, will hasten the crys- 

 talhzation. Hence, to make a quick-setting plaster, as for 

 dental use, some other crystallizing salt, as alum or borax, is 

 added, in small quantity, to the calcined gypsum. The result 

 is that, after solution, the added salt has a tendenc}^ to crys- 

 taUize, and thus starts the process in the dehydrated gypsum. 



When calcined gypsum is steeped in a solution containing 

 from 8 to 10 per cent of alum, and then dried and again burned 

 with a dull red heat of uniform and constant temperature 

 (Greenwood's process), a plaster is produced which after setting 

 is very much harder than the ordinary plaster. Casts made of 

 such plaster solidify gradually, but finally acquire a hardness 

 similar to that of alabaster or marble, and present a translucent 

 appearance resembling these substances. According to Lan- 

 drin, this change in the plaster is not to be accounted for by 

 the formation of double sulphates of lime and alkali, nor by 

 the crystals of calcium sulphate being imbedded in alumina; 

 but rather by the calcium carbonate in the plaster stone being 

 converted by the alum into calcium sulphate. A liquid con- 

 taining from 8 to 10 per cent of sulphuric acid acts in a similar 

 manner. 



Keene's hard cement is produced by impregnating plaster 

 of Paris with a solution of 1 part of borax and 1 part of cream 

 of tartar in 18 parts of water, drying and burning at a low red 

 heat for six hours. Borax alone serves the purpose. The 

 plaster hardens more slowly as the solution is more concen- 

 trated. 



