BAILEY. | Chemistry of Plasters. 161 
Chemistry of the Manufacture of Plaster, 
In making the plaster of Paris from gypsum, although the 
process is simply that of. heating to drive off water, there are 
several points of importance to be considered in what seems a 
simple operation. It is stated that’ ‘‘ When gvpsum is heated 
to about 120° to 130° it loses water, and is converted into burnt 
gypsum or plaster of Paris. This well known substance when 
mixed with water combines with it, evolving heat,gand subse- 
quently solidifies, taking the form of the vessel in which it is 
contained. It was formerly supposed that plaster consisted of 
anhydrous calcium sulphate, and that the setting was brought 
about by the direct combination of this with two molecules of 
water reforming gypsum, but the researches of Le Chatelier” 
have shown that the properly burnt plaster still contains about 
seven-eighths per cent. of water, and consists of the hydrate 
2CaSO,+ H,0. When this is mixed with water, part of this hy- 
drate, which is much more soluble than either the anhydrous 
salt or the hydrate with two molecules of water, dissolves in 
the water to form a saturated solution; it very soon, however, 
becomes converted into the hydrate CaSO,+-2H.O, which, being 
less soluble, crystallizes out, thus allowing the water to dissolve 
more of the hemihydrate ; the same process is then repeated un- 
til the whole of the hemihydrate is converted into gypsum. The 
latter separates into long thin prisms, which interlace to form 
the solid cake which remains. 
‘‘Tf the gypsum be heated too strongly in burning, it only 
takes up water very slowly, and is said to be dead-burnt; this 
is due to the whole of the water having been driven off, leaving 
anhydrous calcium sulphate. Such plaster when exposed to 
moist air takes up six-sevenths per cent. of water, and if mixed 
with water sets slowly, but with a normal hardness. 
‘‘An artificial calcium sulphate is now prepared by precipi- 
tating a solution of calcium chloride with dilute sulphuric acid, 
and sold under the name of pearl hardening or ‘annaline.’ It 
is used by paper-makers as a filling for writing paper.’’ 
91. A Treatise on Chemistry, Roscoe and Schorlemmer, new edition (1898), Vol. II, p. 488. 
92. Ann. des Mines, 1887, 345. @ 
