14 NEW YORK STATE MUSEUM 



ture of plaster boards or blocks for the construction of walls and 

 floors. The boards or sheets are built up in alternating layers of 

 plaster and some supporting material such as paper, excelsior, 

 fibrous talc, etc. They are nailed directly to the studdings and 

 joists of buildings and are then covered with a fresh coat of 

 plaster. Further details of their manufacture are given on page 



45. 



The employment of gypsum wall plasters in the place of lime 

 plasters has developed rapidly of late years and now represents 

 the most important single application of gypsum, at least in this 

 courutry. Wall plasters consist of plaster of paris and some fiber- 

 like hair or wood with the addition of a retarder. Their advan- 

 tages over lime plasters are many, including more rapid set, greater 

 spreading power, less shrinkage on drying, and ability to unite 

 with coloring agents so as to produce any desired tint. On the 

 otlier hand they are somewhat more expensive than lime and in- 

 ferior to it in deadening sounds. A special preparation of plaster, 

 glue and pigments is sold under the name of alabastine for the 

 tinting of walls. | 



The manufacture of anhydrous plasters, of which Keene's .ce- 

 ment and flooring plasters ('' Estrichgips" of the Germans) are 

 examples, is not carried on to any extent in this country. They 

 are characterized by slow setting and superior hardness. Keene's 

 cement, which is representative of a number of materials sold 

 under special brands for hard finishing of walls, is made by cal- 

 cining gypsum at red heat, after which the dehydrated plaster is 

 immersed in a solution of alum and again ignited at high tempera- 

 ture. " Estrich " gv^psum is the soluble form of artificial anhy- 

 drite prepared by calcination of g}psum at a temperature of about 

 500° C. for a period of not more than four hoiu-s. Further details 

 concerning these plasters are given in another chapter. 



Gypsum mortar can be made by using plaster ground to' about 

 the size of building sand and mixing witli five to eight parts of 

 water. ^ Tests prepared with German plaster show a crushing 

 strength which for a number of different mixtures averages ii.i 

 kilograms per square centimeter, higher than the results obtained 

 with lime mortar and exceeded only by those for cement mortar. 



In mixing plaster for wall plaster or mortar it is pointed out 

 that the plaster should be added to the water, the lumps quickly 

 broken and the mass stirred as little as possible.^ The plaster 



» Scientific American Sup. 1907. 64:18. 

 2 Scientific American Sup. 1907. 63:26207. 



