Plaster molds in 1250-1350 F range

[Lauri Levanto]

As far as I know
plaster calcinates ( loses the chrystal water)
between 42 C and 240 C (= 107-420 F) range.
This is how dry plaster is made reactive.

Something else happens to plaster around 1300 F.
After that it does not harden again if water is added.
Does anybody know what really happens there.

Something is gassed out, you get huge bubbles if
you slum into a palster mold in that temp.
Is this a one-time process, counteracted by
prefiring the plaster mixture molds before slumping?

-lauri

[rosanna gusler]

whatever it is i bet it is the same thing that happens when you try to fuse/slump on whiting (calcium carb i think) at those temps. don't you wish you had paid more attention in basic chem. ?????? i do. rosanna

[Brad Walker]

Something else happens to plaster around 1300 F.
After that it does not harden again if water is added.
Does anybody know what really happens there.

Something is gassed out, you get huge bubbles if
you slum into a palster mold in that temp.
Is this a one-time process, counteracted by
prefiring the plaster mixture molds before slumping?

All plasters break down when they reach a certain temperature. For gypsum plaster, this temperature is between 1300 and 1500F -- different plasters break down at different temperatures, but most types are in this general range.

When a plaster breaks down, it basically loses its strength and its ability to act as a binder and hold the mold together. Once a plaster breaks down, there's really no way to get the strength back (but you can mix fired plaster in a new investment, just make sure you use 10% or less of the old stuff with the new).

I don't really understand your comment about getting huge bubbles if you slump into plaster, since I have done this many times without a problem. What temperature are you slumping? Into pure plaster or plaster mixed with a refractory? How dry is the mold when you fire? And what firing schedule?

[Don Burt]

whatever it is i bet it is the same thing that happens when you try to fuse/slump on whiting (calcium carb i think) at those temps. don't you wish you had paid more attention in basic chem. ?????? i do. rosanna

I use powdered whiting on my kiln shelves for work up to 1385°. I don't recall ever having done a large flat piece that might be prone to trapping air, but I've never had any issues with small items. Bert said elsewhere that 1400° is a problem with whiting.