76 CEMENTS, LIMES, AND PLASTERS. 



Under the next group (Hard-finishing Cements), however, will be found 

 descriptions of a number of products which have been manufactured 

 in these latter countries and which are very closely related to the dead- 

 burned plasters. The only difference in technology between the two 

 groups is that while the flooring-plasters are prepared from pure gypsum 

 the hard-finishing cements are prepared from gypsum to which alum 

 or some similar material has been added. 



Hard-finish Plasters. 



The materials grouped under this name include those plasters which 

 owe their hardness and slow set not only to being burned at high tem- 

 peratures, but to the fact that they have been treated with alum or 

 other chemicals during manufacture. As thus defined, the hard-finish 

 plasters include the various materials known commercially as "Keene's 

 cement", " Parian cement", " Mack's cement", etc. 



Keene's cement. The most prominent representative of the group 

 of hard-finishing cements is that known to the trade as Keene's cement. 

 Originally manufactured under English patents which have now ex- 

 pired, the term "Keene's" is applied by various manufacturers to their 

 product, in the same manner as the term "Portland" has become gener- 

 alized. Large quantities of Keene's cement are annually imported, 

 while its manufacture in the United States has been of late years suc- 

 cessively begun. 



Keene's cement is sharply distinguished from the other members 

 of the group of hydrate cements (or "plasters"), not only by the proper- 

 ties of the product, but by its method of manufacture. In its prepara- 

 tion a very pure gypsum is calcined at a red heat, the resulting dehy- 

 drated lime sulphate is immersed in a bath of alum solution and, after 

 drying, is again burned at a high temperature. After this second burn- 

 ing the product is finely ground and is then ready for the market. This 

 sketch of the process is a general outline of the methods used, and 

 in the essentials is followed in all plants, though slightly modified at 

 different plants according to the experience gained by each manufacturer. 



The gypsum used should be as pure as possible, and especially it 

 should be free from such impurities as might tend to discolor the product, 

 which should be a pure white. Nova Scotia gypsum has been tried 

 and, for some reason, found to be unsatisfactory. Even the Virginia 

 gypsum, which on analysis shows but a trace of iron oxide, is not en- 

 tirely satisfactory; for on heating to the temperature necessary for 

 the manufactuie of Keene's cement, minute red streaks appear in the 



