CHAPTER IV. 

 FLOORING-PLASTERS AND HARD-FINISH PLASTERS 



THE two groups of plasters to be considered in this chapter agree 

 (1) in being prepared by burning gypsum at a higher temperature 

 than is employed in the manufacture of plaster of Paris and " cement " 

 plasters, and (2) in being products which, for plasters, set rather slowly 

 but finally take on great hardness. Because of these last properties, 

 the flooring-plasters and hard-finish plasters are available for certain 

 uses to which ordinary plasters are ill adapted. So much for the resem- 

 blances between the two groups. Their points of difference are, that 

 the flooring-plasters are prepared by simple burning at high tempera- 

 tures, while the hard-finish plasters are produced by a aouble burning, 

 with the additional use of chemicals. 



Neither product is made to any extent in the United States, though 

 a considerable quantity of hard-finish plasters are imported every year. 

 The data obtainable as to processes of manufacture are scanty, and 

 the descriptions published are often contradictory, so that it has been 

 difficult to prepare a satisfactory account of these products. It is 

 believed, however, that the descriptions given below contain no errors 

 of importance. 



Flooring-plasters. 



The flooring-plasters (" Estrichgips " of German reports) include 

 those plasters made by calcination of a relatively pure gypsum at tem- 

 peratures of 400 F. or higher. 



In the literature of gypsum and plaster it is often stated that gypsum, 

 burned at temperatures exceeding 400 F., yields a completely dehy- 

 drated product an artificial anhydrite which is entirely valueless as 

 a structural material, because it has completely lost its property of re- 

 combining with water. This statement is, however, erroneous, for plas- 

 ters burned at such temperatures are regularly made and used. They 

 set with extreme slowness, however, and require very fine grinding. 



Composition of flooring-gypsum. Until very recently no satisfac- 

 tory discussion of this phenomenon had been attempted, and the few 

 published accounts of the manufacturing processes employed were con- 



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