On Mon, 19 May 2008 14:03:56 +0100, stuart noble wrote:

Unlike plaster, I don't think gypsum sets when mixed with water. Kind of
important for this application I would have thought :-)

Well building plaster is (more or less) just gypsum that has been heated
to about 150C, this drives off the water from the calcium sulfate
dihydrate to form calcium sulfate hemihydrate.

2 CaSO4 4H2O 2 CaSO4 H2O + 3 H2O (released as steam).

http://en.wikipedia.org/wiki/Gypsum
http://en.wikipedia.org/wiki/Plaster

So the gypsum is already "set". B-)

--
Cheers
Dave.