put in in pieces of about
21/2 inches in thickness. After the baking several lumps are broken up
and examined to see that there are no shining crystalline particles,
which would indicate that some of the gypsum had remained unchanged.
Before use the plaster is ground very fine. This point is of
considerable practical importance. The consistency attained should be
such that the material may be rubbed between the finger and thumb
without any feeling of grittiness. Should there be particles of a size
to be characterized as "grit," these will after use appear at the
surface of the mould, with the result that the mould will have to be
abandoned long before it is really worn out, i.e., before the details
have lost their sharpness.

It is manifestly of considerable practical importance to understand
the conditions which determine the time of the setting up of plaster.
According to Payen, the rapidity of setting, provided the plaster has
dehydrated at a temperature sufficiently low, depends entirely on the
structure of gypsum employed. Thus, according to him, the fibrous
kinds gives a plaster setting almost instantaneously. The water, he
says, penetrates the material freely, setting takes places almost
simultaneously throughout the mass. The hydration of each particle is
accompanied by an expansion, and under the conditions specified, this
expansion being unresisted takes place to the maximum extent, with the
result of leaving cavities between the crystals, and producing a set
plaster of less coherence and density. On the other hand, where
granular crystalline gypsum has been used, setting begins at the
surface of each group of crystals before the water has penetrated to
the interior; the hydration is in consequence more gradual, and
resistance being offered to the expansion of the inner parts, a harder
and denser material is obtained. That this expansion contains an
element of truth is indicated by the practice of employing the
granular crystalline variety for the preparation of moulding plaster.
The explanation appears, however, to be inadequate in several
respects, especially in view of the fact that plasters for moulding
are reduced to a fine state of division before use. It seems as if
this treatment must, in great part at any rate, break up the
crystalline aggregates.

In order to discover a more satisfactory explanation, let us examine
the results of the chemical analysis of plasters used in commerce. One
is struck by