12 NEW YORK STATE MUSEUM 



there is manifest a decided decrease in the land plaster industry, 

 which seems to indicate a growing uncertainty among agriculturists 

 of the positive value of gypsum as a general fertilizer. The enor- 

 mous development of the trade in calcined plasters, however, may 

 have had something to do with bringing abo-ut this decline by divert- 

 ing the attention of the producers to a broader, if not more re- 

 munerative, outlet. 



The literature on the subject of land plaster is so voluminous 

 that it can hardly be summarized profitably in a few pages. In 

 fact the real action of gypsum on soils appears even now to be 

 little understood. Storer,^ one of the more recent writers, ascribes 

 the effects to a combination of chemical and mechanical processes. 

 According to him ground gypsum acts mechanically in flocculating 

 loose soils and disintegrating stiff clay soils. In a chemical way 

 it releases part of its oxygen to combine with nitrogenious and car- 

 bomaceous substances in the soil and also decomposes silicates like 

 feldspar, from which it sets free potassium sulfate available for 

 assimilation by the plant roots. The last reaction is thought to be 

 the more important. Aside from the possible benefit of the con- 

 tained lime, gypsum appears thus to add no direct fertilizing ele- 

 ment to land. 



Gypsum in portland cement manufacture. According to E. C. 

 Eckel^ gypsum is universally employed as a retarder in portknd 

 cement manufactured by the rotary kiln process, such cement being 

 characterized by a high lime content and rapid set. From 2 to 3 

 per cent of gypsum is added usually to the clinker before grinding, 

 in order to insure thorough incorporation. Besides its function in 

 retarding the set of portland cement, the gypsum seems to exert a 

 strengthening influence, at least in the early stages of setting. The 

 addition is more often made in the forni of raw gypsum than as 

 calcined plaster, though of course the latter is more effective weight 

 for weight. The lower value of raw gypsum, however, more than 

 counterbalances the increased quantity necessary, as compared with 

 calcined plaster. 



Miscellaneous uses. Raw white g}^psum is ground and made 

 into crayons which are said to be superior in some respects tO' chalk 

 crayons. The American Crayon Co. of Sandusky, O. manufactures 

 such crayons along with a variety of similar materials. 



Finely ground white gypsum is sold under the name of terra 

 alba for various purposes. It is a common basis of cheap white 



1 Chemistry of Agriculture. 1887. i :2o6. 



2 Cements, Limes and Plasters. 1905. p. 534 et seq. 



