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NEW YORK STATE MUSEUM 



characterized by slow setting when mixed with water and by a 

 hardness superior to that of the half hydrate class. They are used 

 more specially as material for floors and for hard finishing of walls, 

 and corresponding to these uses two general varieties may be 

 recognized — Estrichgips or flooring plaster which was first intro- 

 duced in Germany, and the so called cements, of which Keene's 

 cement is a common example. 



The manufacture of flooring plaster is still centered largely in 

 Germany. Its technology has been described briefly by Wilder.^ 

 The nature of the material is still not well understood, though an 

 investigatioii by Van't Hoif and G. Just^ has thrown some light on 

 the subject. 



According to Wilder, Estrich gypsum is prepared by calcination 

 of rock gypsum at a temperature of about 500° C. The rock is 

 not crushed, but taken directly from the quarry to the kiln. The 

 kiln resembles that used in lime burning. The gypsum blocks are 

 thrown in at the top and pass over an inclined grate which lies 

 over a fireplace. They slowly work their way over the grate, 

 through a constricted space, and finally, when calcined, fall into a 

 cooling chamber. No attempt is made toward a close control of 

 temperature. 



Estrich gypsum has come into general use in Germany as a fl.oor- 

 ing material for office buildings, factories etc., where it takes the 

 place of Portland cement. It admits of coloring and polishing, so 

 as to yield a good imitation of marble or other attractive stone. 



Hard finish plasters or gypsum cements are made from anhy- 

 drous plaster by treatment with some chemical. The best known 

 representative of these plasters is Keene's cement, Avhich was first 

 manufactured in England. The burning process is performed in a 

 vertical kiln, somewhat similar to that just described, where the 

 rock reaches a red heat. The dehydrated material is then treated 

 with a 10 per cent alum solution, after which it is again burned at 

 high temperature and ground for use. The action of the alum is 

 perhaps to assist the solution of the dead-burned gypsum. The 

 plaster sets slowly and when quite stiff can be softened again with 

 water, without impairing its hardening power. The high tempera- 

 ture at which it is burned tends to oxidize any iron present so that 

 a perfectly white product can be made only from rock gypsum that 

 is practically free from such impurity. 



^ Op. cit. p. 219. 



2 Kgl. Preuss. Akad. Wissensch. 1903. 1:249. A translation of this 

 paper appears in E. C. Eckel's Cements, Limes and Plasters. 



