10 NEW YORK STATE MUSEUM 



effect; dilute hydrochloric acid is the common solvent for labora- 

 tory purposes. 



Gypsum parts readily with its water of crystallization. By par- 

 tial dehydration it is converted into the half hydrate (CaS04. 

 >^HoO), which is plaster of paris. In ordinary practice of plaster 

 manufacture where the dehydration is performed in closed recep- 

 tacles, the temperature is not allowed to exceed about 4C>o°F. If 

 the temperature is maintained above that point for. any length of 

 time, complete dehydration results and the product then has the 

 composition of the mineral anhydrite (CaSO^). This mineral is 

 insoluble and useless for plaster, but the artificial product, if cer- 

 tain conditions are observed in burning, is capable of uniting with 

 water and enters into special grades of plaster. The dehydration 

 of gypsum can be accomplished at temperatures much lower than 

 those in manufacturing practice under the influence of hygroscopic 

 materials like concentrated acids or currents of dry air. 



Varieties of gypsum. Crystallized g3^psum is commonly known 

 as selenite. It is colorless and transparent in pure condition. It 

 finds limited application in optics and has been used in the past as 

 a substitute for glass for some purposes. 



Satin spar is the name given to the fibrous variety, which is really 

 an aggregate of parallel or radiating acicular crystals. It ranges 

 from white to colorless, has a pearly luster and is often found in 

 veins which intersect the larger bodies of massive gypsum and its 

 inclosing rocks. 



Rock or massive gypsum is the usual gypsum of commerce. It 

 is generally made up of an intricate intergrowth of small crystals. 

 The white or dehcately tinted variety of even texture is the ala- 

 baster used for sculptures and objects of art. More often the rock 

 gypsum has a dark color such as gray, drab, brown or nearly 

 black, according to the character and amount of impurities present. 

 These include organic matter, lime and magnesia carbonates, clay, 

 iron oxids and silica. The actual proportion of gypsum in rock 

 from different localities ranges within Avide limits, from as low as 

 50 or 60 per cent up to 97 or 98 per cent in exceptionally pure 

 types. 



Still another variety of gypsum, known as gypsite or g3'psum 

 earth, is common in somiC parts. It is an incoherent surface de- 

 posit consisting of small gypsum crysta'ls mixed with quartz, lime 

 carbonate and organic matter. It occurs in the Middle Western 

 States, where it is used to some extent for plaster manufacture, and 

 in Europe. ■ 



