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What temperature does plaster of Paris calcine at?

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What temperature does plaster of Paris calcine at?

Post  vtquestion on Mon Aug 04, 2014 8:54 am

I have found tons of conflicting information about lost wax investment casting on the Internet. So to try to understand what happens when a plaster of Paris mold is heated in a kiln to drive off bound water, I would first like to know what the specific temperature that this reaction occurs at:

CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O

Wikipedia says "about 150C" -- but is there a more specific temperature, derived from the formula for the reaction itself?

(Wikipedia does list a derived specific temperature for similarly calcined limestone CaC03 under the subject "calcination". Is this possible for CaSO4 as well?)

Second, under the plaster of Paris section Wikipedia states that there are two occurrences of water being driven off and that the the second happens around 200C and produces an "anhydrite." but doesn't elaborate further. Does this mean there are no water molecules attached above 200C? Or is anhydrite a general term meaning the same 0.5H20 is still attached?

Thank you for your help! I'd like to really understand the chemistry basics here. That way I can interpret the loads of conflicting information on the casting forums.


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