laster of
Paris class. The mode of preparation is to calcine the gypsum at
temperatures which depend on the class of cement to be produced. If
plaster of Paris is to be made, calcination is carried out at about 204
Deg. C. (=400 Deg. F.); at this temperature, gypsum, CaS04.2H20, loses
three-quarters of its combined water and becomes 2CaSO4.H20. If a cement
of the Keene's cement class is to be prepared the temperature used is
higher, e.g. 500 Deg. C. (=932 Deg. F.), and the whole of the combined
water of the gypsum is expelled, the anhydrous sulphate CaSO4 being
obtained.

Plaster of Paris; Keene's cement.

To produce plaster of Paris European practice consists in baking the
mineral in ovens, and in America in heating it in kettles. Both
processes are inferior in economy to calcination in rotatory kilns, a
process which may be regarded as the method of the present and the
immediate future. Keene's cement and its congeners are made in fixed
kilns so constructed that only the gaseous products of combustion come
into contact with the gypsum to be burnt, in order to avoid
contamination with the ash of the fuel.

The setting of plaster of Paris depends on the fact that when
2CaSO4.H2O is treated with water it dissolves, forming a
supersaturated solution of CaSO4.2H2O. The excess held temporarily in
solution is then deposited in crystals of CaSO4.2H2O. In the light of
this knowledge the mode of setting of plaster of Paris becomes clear.
The plaster is mixed with a quantity of water sufficient to make it
into a smooth paste; this quantity of water is quite insufficient to
dissolve the whole of it, but it dissolves a small part, and gives a
supersaturated solution of CaSO4.2H2O. In a few minutes the surplus
hydrated calcium sulphate is deposited from the solution, and the
water is capable again of dissolving 2CaSO4.H2O, which in turn is
fully hydrated and deposited as CaSO4.2H2O. The process goes on until
a relatively small quantity of water has by instalments dissolved and
hydrated the 2CaSO4.H2O, and has deposited CaSO4.2H2O in felted
crystals forming a solid mass well cemented together. The setting is
rapid, occupying only a few minutes, and is accompanied by a
considerable expansion of the mass. There is reason to suppose that
the change described takes place in two stages, the gypsum first
forming orthorhombic crystals and then crystallizing in the
monosymmet