^6 NEW YORK STATE MUSEUM 



Of these, the first two steps are carried out on calcining plaster 

 and their reversal on the setting of plaster. 



Davis classes freshly made plasters into four groups : 



1 Those consisting mainly of the half hydrate, containing 6 to 8 

 per cent water. 



2 Those containing soluble anhydrite and very hygroscopic, with 

 less than 6 per cent HgO. 



3 Plasters containing more than 7.5 per cent HgO and consisting 

 of half hydrate mixed with some gypsum. 



4 " Dead burnt " plasters containing less than 6 per cent water 

 but not hygroscopic and setting slowly ; these contain' ordinary 

 anhydrite. 



Setting of plaster. The property of plaster, or the calcined gyp- 

 sum, to set on mixing with water gives it its chief value. Gypsum cal- 

 cined at temperatures varying perhaps from 100° C. to 500° C. 

 and mixed with water will, after a period of from a few minutes 

 to a day, take up water and become a hard mass. 



The cause of setting has long been an unsettled and debatable 

 theme, though the fundamental principle was laid down by La- 

 voisier in the investigations already noted. In addition to the ex- 

 periments that have been described he carried on one more. Into 

 a large vessel of water he threw some powdered plaster and al- 

 lowed it to sink. He says, " In passing through the liquid, each 

 molecule of plaster took back its water of crystallization amd fell 

 to the bottom of the dish under the form of small brilliant needles, 

 visible only with a high power lens." Examined with a lens they 

 proved to have the regular form of gypsum. He concluded that 

 the setting with water " is nothing more than a simple crystalliza- 

 tion " ; gypsum, deprived of its water, reabsorbs it greedily and 

 again becomes crystalline. Lavoisier thought that his investiga- 

 tions left no doubt as to the cause of the hardening of plaster, and 

 that there remained '' nothing to be desired in explanation of the 

 problem." Though the change is caused primarily by a crystal- 

 lization and the taking up of water, the chemical, crystallographic 

 and physical changes in all their steps are far from clear ;' as stated 

 by Mr Davis,' "' the problem has proved to be one of extraor- 

 dinary difficult)^ and in spite of the investigations made by such 

 well known chemists as Marignac, Le Chatelier and Van't Hoff, 

 an amount of confusion exists with regard to the subject which 

 is almost without parallel in inorganic chemistry." 



Landrin^ made an elaborate investigation into the setting of plas- 

 ter and brought forward the theory that the plaster partially dis- 



' loc. cit. 



* Ann. de Chimie, 1874. p. 434. 



