It has long been a familiar fact that gypsum yields on baking a
material which possesses the power of setting with water to a firm
mass, this setting being accomplished much more quickly than is the
case with mortar.

The explanation of the setting of plaster was first given by
Lavoisier, who pointed out that gypsum is an hydrated salt, and that
the set plaster is in fact gypsum reformed, the change brought about
by baking being merely loss of water of crystallization. The beds of
gypsum of most importance both formerly and at the present time in the
plaster manufacture occur in the neighborhood of Paris in the lower
tertiary formation. Different beds differ (1) in respect of character
and quantity of admixed materials and (2) in the structure of the
gypsum itself. With regard to the first point, some deposits contain a
notable proportion of carbonate of lime, a fact which under certain
circumstances may considerably influence the character of the plaster.
In the matter of structure two principal varieties occur (1) granular
and (2) fibrous. Further, hardness of the granular kind varies
considerably. These differences of structure in the original material
appear to exercise an influence on the properties of the plaster. Thus
according to Payen the plaster formed from the granular variety sets
more gradually than that derived from the fibrous, and forms a denser
mass. The softer kinds of the granular gypsum are those principally
used in the production of plaster for the moulds of potteries.

In the old fashioned process which is still employed for making the
common kinds of plaster, the material is exposed to the direct action
of flame. Large lumps are placed in the lower part of the furnace,
above them smaller lumps, and, after the heating has been carried on