'Transactions of the Bombay Geographical Society, from September 1850 to June 1852. Edited by the Secretary. Volume X.' [‎228] (361/458)

228
NOTES.
Having been much occupied in plaister-casting, in the early part of 1848, m an
pieces of broken plaister were lying about as the rains approached. To my
prise I found, that as these began to be washed away, they became covered with
crystals, many of which extended some way into the surface and protruded as
much as a quarter of an inch beyond it. They could not, in any case, have been
the result of deposition from solution, as they made their appearance quite as often
on convex surfaces as in cavities, the rain fell with such violence as to prevent
any great accumulation of water ; and the moisture which might have temporarily
accumulated in irregularities, during the fall, passed away the moment it faired.
In observing this I collected a multitude of fragments and placed them in an iron
pan with water, the stucco on all occasions rising above its surface. I was disappointed
to find that no further effects resulted from this than those found to prevail
after the rain had for a week begun. The outer surfaces of all the castings that
had been exposed were considerably hardened and covered with crystals all over.
Some of these castings I had afterwards occasion to pack up, and on those which had
been carefully wrapped in paper and kept dry, I found portions beginning to
harden, or which were, completely hardened into fine semi-transparent crystalline
gypsum, which occupied exactly the same space as before crystallization,>thera
being no crack or trace whatever of shrinkage around it. In the course of casting
I had repeatedly overburnt the gypsum ; it was indeed so much more easily
ground when heated to redness than when merely raised to a temperature of 350°
that the Natives at first overburnt it to save trouble* Though it ground and
sifted beautifully, it either set partially or altogether refused to harden; remaining
wet and clammy, and afterwards breaking to pieces and mouldering in the hand.
In this case the sulphuric acid had either been driven off or partially decomposed,
The stucco when mixed with water gave out numberless bubbles of sulphureted
hydrogen gas, exhibiting the usual foetid smell of gun washings. Even when decom
position had been only partial, the amount of over-heating being very inconsider
able the extrication of gas bubbles» in the process of setting, was often very
troublesome ; altogether detroying the surface of the cast. I found a partial
remedy for this in working the mixture of pounded stucco and water be
twixt the hands as along as possible j that is for a minute or so, till it begau
to thicken; the gas bubbles were thus broken before the stucco was applied
after which it begau to set immediately. When the stucco was damp or under-
burnt I placed it m an iron pot and exposed it to a heat of 300° when number
less vapour bubbles disengaged themselves, and its setting property was
developed to the fullest extent. In the experiment above referred to 1 watched
With the utmost care the effects of rain on the over and underburnt specimens,
but could observe nothing remarkable in either case. The former, as soon as
they became soaked with water, fell to pieces ; they had before merely cohered
mechanically. Some twenty or thirty years ago, a ship load of gypsum app** r3
to have been brought to Bombay, from the Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. . As no use could be