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.
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