ric system. Gypsum thus crystallized is in its normal
monosymmetric form, more stable under ordinary conditions than the
orthorhombic form. Correlatively in its process of dehydration to form
plaster of Paris, monosymmetric gypsum is converted into the
orthorhombic form before it begins to be dehydrated.

The principles which govern the preparation and setting of the other
class of calcium sulphate cements, that is, cements of the Keene
class, are not fully understood, but there is a fair amount of
knowledge on the subject, both empirical and scientific. The essential
difference between the setting of Keene's cement and that of plaster
of Paris is that the former takes place much more slowly, occupying
hours instead of minutes, and the considerable heating and expansion
which characterize the setting of plaster of Paris are much less
marked.

It is the practice in Great Britain to burn pure gypsum at a low
temperature so as to convert it into the hydrate 2CaSO4.H2O, to soak
the lumps in a solution of alum or of aluminium sulphate, and to
recalcine them at about 500 Deg. C. On grinding they give Keene's
cement. Instead of alum various other salts, e.g. borax, may be used.
The quantity of these materials is so small that analyses of Keene's
cement show it to be almost pure anhydrous calcium sulphate, and make
it difficult to explain what, if any, influence these minute amounts
of alum and the like can exert on the setting of the cement. It seems
probable that the effect of the salts is inconsiderable, and that the
governing condition is the temperature at which the cement has been
burnt. The setting of Keene's cement takes place by the same sort of
process which has been described for the setting of plaster of Paris,
the chief differences being that the substance dissolved is anhydrous
calcium sulphate and that the operation takes a longer time.

All cements having calcium sulphate as their base are suitable only
for indoor work because of the solubility of this substance. They form
excellent decorative plasters on account of their clean white colour
and the sharpness of castings made from them, this latter quantity
being due to their expansion when setting.

See D.B. Butler, _Portland Cement_ (London, 1905); E.C. Eckel,
_Cements, Limes and Plasters_ (New York, 1905); G.R. Redgrave and
Charles Spackman, _Calcareous Cements_ (London, 1905); F.H. Lew