FLOORING-PLASTERS AND HARD-FINISH PLASTERS, 75 



may be slightly harder than the third sample, which is entirely unaltered. 

 Under the microscope the alabaster gypsum after twelve days showed 

 several gypsum crystals, while the third sample showed only scattering 

 ones. 



"The essential result of the investigation, therefore, is, that in the 

 heating of gypsum after total dehydration, which occurs at about 190, 

 the capacity to bind water is at first retained, and is only gradually 

 lost, either by more intense or by longer heating. The retention of 

 the crystalline form, which is probably due to burning without previous 

 division into small bits, checks this so-called dead-burning, and is there- 

 fore of technical importance. We found no evidence to support the 

 statement that, after dead-burning, a new binding capacity appears 

 at a high temperature, in which case even the natural anhydrite would 

 be suitable for burning floor-gypsum." 



Methods of manufacture. Flooring-gypsum is, therefore, a pure 

 plaster, entirely free from water. It is manufactured by burning pure 

 gypsum, broken into lumps but not finely crushed, in a vertical kiln. 

 The fuel, usually coal, is burned on a grate set at one side of the kiln, 

 and the hot gases pass directly through the mass of gypsum, though 

 neither fuel nor ashes come into direct contact with it. The tempera- 

 ture reached is, according to Wilder, about 500 C. The gypsum must 

 not be exposed to this temperature for more than four hours, for a 

 longer heating would deprive it entirely of its setting properties, as 

 noted by Van't Hoff in the paper presented above. 



Uses of flooring-gypsum.* As its name denotes, flooring-gypsum 

 (Estrichgips) is extensively used in Germany for floors, giving a very 

 hard and durable surface. As the material attains this hardness only 

 when it is protected from moisture during setting, care must be taken 

 to give it a suitable foundation. If the material dries unevenly or 

 very rapidly cracks will appear on its surface. In this case the floor 

 should be covered with water until the surface is soft and the cracks 

 closed, after which it is allowed to dry again. After standing about 

 twelve hours and becoming fairly hard the floor is pounded with wooden 

 mallets and smoothed with trowels. 



Pure flooring-plaster gives the best results for hardness, but for 

 economy it may be used in a mixture of two parts plaster to one part 

 sand, ashes, etc. A cubic meter of hardened flooring-gypsum weighs 

 about 2000 Ibs., equivalent to a weight of about 57 Ibs. per cubic foot. 



Flooring-plasters are manufactured on a fairly large scale in Germany, 

 but have not been made or utilized in England or the United States. 



* The data on the uses of flooring-gypsum are largely taken from Wilder's 

 paper, cited previously. 



