"Wes" wrote in message
...
"Ed Huntress" wrote:

It's good to make a point of the fact that you have to know what you're
doing to use plaster molds for bronze temperatures, or you're in for an
explosion.

I'm thinking plaster is like gypsum and under high heat will give up water
vapor. So the
mould would have to be baked to drive it off.

My experience with bullet casting is any water combined with hot metal is
catastrophic. I
was using Marvelux for flux, it has an affinity for water and I didn't
preheat the spoon I
used to flux with before putting it in the pot. BANG! OUCH!

Wes

Right. And when you get up to bronze-casting temperatures, the problem is
*much* more dangerous.

I have a big collection of literature from US Gypsum on casting plasters,
which I haven't consulted for, oh, maybe 20 years g, and it's buried deep
in my research material somewhere, but I think there's enough information
around the Web to put the story together. Using plaster (which is gypsum) to
cast aluminum is one thing -- although you still have to calcine it for
that. For bronze, you have to cook it a lot more, at a higher temperature,
and you need to have heat-resistant additives in it. Vermiculite (expanded
mica) is a traditional one. I don't recall how much you're supposed to use.

Since the OP is talking about an open mold, the one thing you don't have to
worry about here is venting. That's another big issue when you use plaster
for casting molds.

Personally, I'd first try sand with a dusting of graphite powder. If you
dust it just right, it fills in the texture of the sand and gives you a
really smooth finish. You can sprinkle it in from an old stocking or a piece
of pantyhose. Don't let your wife see it...

--
Ed Huntress