COMPOSITION, PROPERTIES, AND TESTS OF PLASTERS. 67 



TABLE 20. 

 EFFECT OF ACCELERATORS ON RATE OF SET. 



References on properties and tests of plasters. The following brief 

 list covers the principal papers on this subject. 



Bailey, E. H. S. On the chemistry of gypsum, plaster of Paris, and cement 

 plaster. Vol. 5, Reports Kansas Geological Survey, pp. 134-170. 



Marston, A. Preliminary tests of stucco and plaster made by the Civil En- 

 gineering Department of Iowa State College. Vol. 12, Reports Iowa 

 Geological Survey, pp. 224-235. 1902. 



Slosson, E. E., and Moudy, R. B. The Laramie cement plaster. 10th Ana, 

 Rep. Wyoming, College Agriculture and Mechanics, 1900. 



Anon. Tests for plaster of Paris. Stone, vol. 25, pp. 331-334. 1903. 



Hardening gypsum and plaster. The following methods of harden- 

 ing an ordinary plaster have been recently recommended : * 



(1) Two to four per cent of finely ground marshmallow-root are 

 intimately mixed with powdered plaster and the mixture kneaded 

 to a dough with 40 per cent of water. The resulting mass resembles 

 a stiff clay, hardens in about an hour, and finally becomes hard enough 

 to cut, file, or bore. A harder and tougher mass may be obtained by 

 increasing the quantity of marshmallow-root to 8 per cent. Gum, 

 dextrin, or glue may be substituted for the marshmallow-root if more 

 convenient. If the objects are to be exposed to high temperatures 

 shellac may be used. 



(2) Six parts of gypsum are mixed with one part of freshly slaked 

 lime and the mixture is soaked with a concentrated solution of mag- 

 nesium sulphate. In preparing this mixture, too much gypsum must 

 not be poured into the water, and the mixture must be stirred quickly 

 so that lumps do not form. The smaller the quantity of water used 

 the thicker and firmer is the cement. The porosity caused by the 

 gradual loss of water can be obviated by soaking the objects in a solu- 

 tion of ozocerite or wax in oil of turpentine, varnish, or hot tar, or by 

 coating them with shellac. 



* Journ. Soc. Chem. Industry, vol. 21, p. 347. 



