It has long been a familiar fact that gypsum yields on baking a
material which possesses the power of setting with water to a firm
mass, this setting being accomplished much more quickly than is the
case with mortar.
The explanation of the setting of plaster was first given by
Lavoisier, who pointed out that gypsum is an hydrated salt, and that
the set plaster is in fact gypsum reformed, the change brought about
by baking being merely loss of water of crystallization. The beds of
gypsum of most importance both formerly and at the present time in the
plaster manufacture occur in the neighborhood of Paris in the lower
tertiary formation. Different beds differ (1) in respect of character
and quantity of admixed materials and (2) in the structure of the
gypsum itself. With regard to the first point, some deposits contain a
notable proportion of carbonate of lime, a fact which under certain
circumstances may considerably influence the character of the plaster.
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. 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
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.
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 the large percentage of water they usually contain. Thus,
four samples of ordinary plaster analyzed by Landrin have an average
of 90.17 per cent. of CaSO4 and 7.5 per cent. of water, while two
samples of best plaster contained 89.8 per cent. of CaSO4 and 7.93 per
cent. of water. These numbers do not add up to 100, the difference
being due to silica and other impurities of the original gypsum,
amounting altogether to about 3 per cent.
It might be suggested that the reason why these plasters set more
slowly than completely dehydrated plaster is owing simply to the fact
that they contain, apparently, some unaltered gypsum, which serves to
_dilute_ the action. Were this so, a similar result, as far as time of
setting is concerned, should be obtained with a plaster containing a
corresponding quantity of dead-burnt material. This, however, is not
found to be the case. The time of setting appears, then, to be
connected in some special and peculiar manner with the retention of
water by the burnt plaster.
The following explanation of this connection is offered, an
explanation only tentative at present, owing to want of experimental
data.
The following substances are known:
Gypsum, and set plaster, CaSO4 + 2 H2O, containing 20.93
per cent. of water.
Plaster completely burned at moderate temperature, CaSO4,
probably amorphous.
Anhydrite and dead-burned plaster, CaSO4, crystalline.
Selenitic deposit from boilers, 2 CaSO4 + H2O, or CaSO4 +
1/2 H2O, containing 6.2 per cent. of water.
The circumstance that the hot calcium sulphate can crystallize with 1/4
its normal amount of water indicates that for this proportion of water
it has a greater attraction than for the other 3/4. Having a similar
bearing is the fact that when burned at lower temperatures, gypsum
only loses the last portions of water with extreme slowness.
Now, if it be the case that anhydrous calcium sulphate has a greater
attraction for the first half molecule of water, then the operation of
hydration will proceed very rapidly at first, more slowly afterward.
Many such cases are known, e.g., that of copper sulphate. Conversely,
if only 3/4 of the water of hydration be expelled during the baking of
gypsum, the material obtained should hydrate itself more slowly. For
our present purpose it will be convenient to recalculate the numbers
given by Landrin (_vide supra_) so as to make the calcium sulphate and
water add up to 100. This treatment of the numbers gives a mean result
for the six analyses of 7.68 per cent. of water, the amounts not
varying by more than 1 per cent.
It will be seen that the dehydration has never passed the composition
corresponding to 2 CaSO4 + H2O; indeed, the material approximates
more nearly to the composition 3 CaSO4 + H2O. It appears probable,
therefore, that in the successful preparation of plaster the whole, or
nearly the whole, of the gypsum is changed, but that this change does
not result in the production of CaSO4, or of a mixture of CaSO4 and
CaSO4 + 2 H2O, but of a lower hydrate of calcium sulphate.
In the case of the analyses, given by Landrin, of fine plaster for
potteries, the percentages of water (8.14 and 8.08) correspond closely
to that of a hydrate, 3 CaSO4 + 2 H2O, which would contain 8.1 per
cent. of water.
Some surprise may have been excited by the fact that the well known
method of revivifying hydrated calcium sulphate has recently formed
the subject of a patent (Eng. pat., No. 15,406).
The method described in the specification consists in reducing the
materials (waste moulds, etc.) to small lumps, and baking between the
temperatures of 95° and 300°. It is mentioned that the whole of the
water must not be expelled. This is no doubt correct, but it must be
effected by regulating the _time_ of baking, since by prolonging the
operation all the water of crystallization can be expelled far below
300°. To secure even baking the mass is kept stirred by mechanical
stirrers, a necessary precaution, since the operation is to be carried
out in an ordinary kiln. The process is stopped when a portion of the
plaster is found to set in the required time, a method of regulation
which will probably be found to work well in practice.--_Chem. Trade
Jour._
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