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
for some time, finely divided material is filled in at the top. The
outer portion of the larger lumps is always overburnt, and in the
upper part of the furnace the presence of shining crystalline
particles generally indicates the fact that some gypsum has remained
unchanged. Provided that the amount of unburnt and overburnt material
does not exceed about 30 per cent.
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