GRIMSLEY. | Technology of Gypsum. 89 
uniformly 1.56 grammes which corresponds with one and one- 
half equivalents of water so that the compound should contain 
about 6.2 per cent. of water as shown by the formula given. 
Ordinary plaster of Paris usually contains about seven per cent. 
of water, so it is a definite hydrate with the formula (CaSQ,)., 
lel). 
The second halt, as has been given, took place at 163° C., or 
325 KF. From this temperature to 221° C., or 430° F., no change 
was noted in the plaster; but beyond this temperature, the 
plaster when mixed with water did not absorb it readily and 
only set afteralong time. If the heat reaches 343°C., or 650° F., 
the plaster acts like an anhydrite, and it is said to be dead burned 
and will not set on addition of water. If the gypsum is heated 
further, the substance melts, forming a crystalline mass on 
cooling which cannot be decomposed by heat except in the pres- 
ence of organic matter, when it changes into CaS. If this sub- 
stance is acted upon by carbon dioxide gas (CO,) and water, 
sulphuretted hydrogen gas will be formed. 
The temperature of burning is thus seen to be all important, 
.andjcalls for skill and experience on the part of the calciner. 
If overburned, the plaster sets very slowly. If underburned, 
the’new hydrate is not formed, and the plaster will not set. 
The reason why the commercial plasters vary in quality un- 
doubtedly depends to a very great extent on this care of calcin- 
ing and the companies are very careful to select the best men 
they can find to watch the kettles which are used in the plaster 
manufacture. Some of the calciners in the Kansas field have 
spent many years in learning their trade, and the skill they 
employ in determining the right point to draw the plaster is re- 
markable and wonderful to one who watches them. ‘The prin- 
ciples outlined above, though perhaps wholly unknown to them 
from a chemical standpoint, are familiar from a practical point 
of view. They all have their secret tests and clues which are 
jealously guarded. 
