From: Chris Plicht 
To:  atm{at}shore.net
Reply-To: Chris Plicht 


Hi Bob and all,

plaster in general is Calcium sulfate, CaSO4. In it's hydrated form two
molecules of water (H2O)  are embedded with the CaSO4 in the crystal that
forms, so the chemical formula is CaSO4 * 2 H2O. This water is driven out
with heat, leaving the plaster as we buy it. Dental plaster sure has some
additives, but the basics are the same.

The molecular weights are 136,14 for CaSO4 and 36,031 for 2 molecules of
water. One can use those numbers as weight units (grams or ounces or
whatever)  to calculate the theoretical mass of water needed, e.g. 136
ounces of plaster need 36 ounces of water. This compares pretty good with
the information given on the package of the plaster (+/- 10% or so). The
weight ratio is about 4 to 1, so one can calculate easily other amounts of
water for a given mass of plaster.

This is the theory. However, it may be a good practice to use a bit more
water than the this. Now I have to go and fix that distorted pitch tool
that was sent to me with an almost finished mirror.

Chris

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