No determination appears to have been published of the
specific gravity of plaster prepared by complete baking at a low
temperature. The theory is, however, confirmed by the results obtained
by workers on the subject of mineralogical synthesis, who have shown
that the material which has been produced at high temperatures has the
specific gravity and other physical properties of the mineral
anhydrite.
It was formerly supposed that plaster prepared by baking at a
temperature above 300 degrees loses completely its power of setting.
Later observations, however, as those of Landrin, negative this view.
Between 300 degrees and 400 degrees Landrin obtained plasters setting
almost instantaneously when mixed with a small amount of water. When
the temperature employed approached 400 degrees, the set plaster was
softer, but the setting still took place quickly. These observations
appear to show that the change to anhydrite is a very gradual process
at temperatures below a red heat.
Reference has been made to the differences in (1) time of setting of
plaster and (2) in hardness of the resulting material. Both of these
properties are affected by the mode of baking.
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