GRIMSLEY. | Technology of Gypsum. 87 
PAYEN’S EXPERIMENTS. 
Lavoisier, in his memoir quoted above, states that on heating 
gypsum the water was removed at two different stages, and that 
three-fourths of it was much more easily removed than the re- 
mainder. Payen, in 1830, confirmed this result, and found that 
gypsum commenced to lose water at a temperature of 115°C., 
and that the loss rapidly increased up to 204°C. Payen regarded 
a temperature of 110° to 120°C. to be the best in the manufac-_ 
ture of plaster, but he found by a series of experiments that 
plaster could be made at a lower temperature if burned long 
enough. The lowest temperature found was 80°C., and then a 
long time was required. 
Payen’s” results are included in the following summary : 
1. The set of plaster is due to a crystallization of hydrous 
sulphate of lime. 
2. The temperature the lowest at which plaster can be made 
is 80°C., and the process of manufacture is very easy. 
3. A temperature of 110° to 120°C. is sufficient to deprive 
plaster of all its water and to cook completely. 
4. Plaster in small particles favors the drying. 
5. Calcium sulphate heated to about 250°C. is dehydrated ; 
at 300° to 400° it loses completely the properties of hydration, 
or the power of gaining again the water of crystallization, and 
resembles then hydrated sulphate of lime found in nature. If 
heated higher, it may result in melting the sulphate of lime. 
6. The hardening of plaster by alum is perhaps due to the 
formation of a double sulphate of potash and lime. 
CHATELIER’S EXPERIMENTS. 
Chatelier in 1887 conducted some very elaborate experiments 
on the effects of temperature on gypsum. He powdered the 
gypsum rock and placed it in a paraffin bath, and connected a 
thermometer in the bath with a chronograph. On applying 
heat up to 200° C., or 392° F., there was a constant rise in tem- 
perature, with twoexceptions. The first halt occurred at 128°C., 
or 262° F., and the second at 168° C., or 325° F. The first one 
39. Chimie Industrielle, 1830, quoted by Landrin, Annales de Chimie, 1874. 
