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. of the total, the plaster is
suitable for many applications.

It was early observed that set plaster could be revivified by a second
baking, but attempts in this direction were not uniformly successful,
it being found that the dehydrated substance in some cases refused to
set with water. It behaved in fact similarly to the natural anhydrous
calcium sulphate which is unaffected by water. These failures were
found to be due to the employment of too high a temperature, and such
plaster was termed _dead burnt_. Although this fact was ascertained
long ago, yet ignorance of what had already been done has probably
been the cause of many disappointments in attempts at revivification
which have been made from time to time by persons unacquainted with
the history of the subject.

The view generally adopted with regard to the theory of these
processes is that plaster consists of anhydrous calcium sulphate,
CaSO4, in a condition probably amorphous, different from that of
natural crystallized CaSO4, known to mineralogists under the name
of anhydrite. By the influence of a high temperature it appears
probable that a molecular change is gradually induced with production
of a crystalline structure, and probably an increase of specific
gravity, resulting in the artificial reproduction of the mineral
anhydrite. No determination appears to have been published of the
specific gravity of plaster prepared by complete baking at a low