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Page 23
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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