The Shop > Metal Stuff
Plaster of Paris, Investment, etc.
awemawson:
I know I think where Steve is coming from - there are many woolly descriptions giving 'largely correct' facts, and he want's the actual facts in the sense of an accurate scientific approach that can later be built on.
BTW to play devils advocate I can boil water at FAR lower temperatures than you quote above (*) :lol:
(* cf lousy cup of tea on the top of Everest)
vtsteam:
heh, Andrew, at STP and not yer muddy stuff!
I just realized I'm asking this question in the wrong place. I need to find a chemistry answers forum and just ask about the formula reaction. Back with that soon -- if I get an answer.... :coffee:
vtsteam:
And here's the answer, attached.
And it shows the to changes and why there is a range, and where that range begins and ends and the proportions along the way.
And anhydrous definitely means no water.
awemawson:
If only you'd asked - I'd have told you THAT ...... :lol:
vtsteam:
So here's what we can say for sure:
Plaster of Paris (gypsum) when heated goes through 2 separate chemical reactions that release bound water. These are:
CaSO4 .2H2O --> CaSO4 .1/2 H2O
and
CaSO4 .1/2 H2O + 1 1/2 H2O --> CaSO4 + H2O
The first begins as the gypsum is heated at 100C (212F) and continues until it reaches 150C (302F).
The second begins as the gypsum passes 150C, and ends when it reaches 185C (365F)
After reaching 185C it has been converted to anhydrite and all water has been released.
Now there are almost certainly other reasons for the claims of heating to 600F or 600C even, but they have nothing to do with the plaster investment conversion process.
What they probably have to do with is baking out the wax.
Which is the next topic I'm interested in.
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