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Page 22
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
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. The hardest material is
frequently obtained from the quick-setting plasters, but for certain
purposes this rapidity in setting is of great practical inconvenience.
Thus the moulder in pottery work must have leisure to fill in every
detail of a design often complicated and intricate before the material
with which he is working becomes intractable. Thus for many of the
more refined purposes to which plaster is applied, extreme hardness in
the set plaster is of less vital importance than a convenient period
of setting. On the other hand, plasters which set very slowly give as
a rule too soft a material, as well as being inconvenient in use.
Plasters which hit off the happy medium are alone suitable for the
work of the potter. The finer varieties of plaster prepared especially
for use in potteries are obtained by a treatment which differs in many
respects from that described above for the commoner kinds. In the
first place, the direct contact of fuel or even flame is avoided,
since this reduces some of the sulphate to sulphide of calcium, the
presence of which is in many respects objectionable. Secondly, it is
necessary that there should be a better control over the temperature,
since, as has been seen, if the heating be carried too far the
plaster, if not partially dead burnt, will set too quickly for the
particular purpose to which it is to be put.
The arrangement employed in France is known as the _four a boulanger_,
or baker's furnace. The temperature attained in the furnace itself
never exceeds low redness. The material preferred is the softer kind
of the granular variety of gypsum. This is 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.
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