[vtquestion]

Hello all, I appreciate the opportunity to ask this question here.

Problem: 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 from a chemist's perspective what the specific temperature that this reaction occurs at:

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

I have seen variously, 140-180C, 600F, 600C, 650C, etc, It's all over the map.

Wikipedia says "about 150C" -- but is is that even right and 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 in 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 conflicting information on the casting forums.

[Borek]

Best thing to do would be to find a thermogravimetric curve - that clearly shows at what temperatures changes occur.

Something like curves here: http://www.tainstruments.com/library_download.aspx?file=TS29.PDF

Not that these things don't happen at a single temp - it is not a typical phase change like melting. More like they start when the temperature is X and they end when the temperature is Y (and both can a function of how fast the temperature goes up).

Anhydrite means "no hydration water at all".

[vtquestion]

Ahh, thank you Borek, that is perfect. Now I understand what how much, and why!

[Enthalpy]

http://www.alloyavenue.com/vb/archive/index.php/t-461.html?s=d56fdee1e8e3c74e0cf0c765dd367491
check other model hobbyist websites, they cast metal propellers.