We did this in college foundry work for investment casting.
We mixed talc in with the plaster to make it easier to
calcine (drive off the water). If I recall properly, each
molecule of gypsum tends to retain two molecules of water so
by reducing the gypsum (plaster) content you can reduce the
bonded water significantly. I don't remember the proportions
we used. Experiment a bit. You just need enough plaster to
hold it all together.  Preheat the mold before pouring.

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-
> bounces@mit.edu] On Behalf Of veegee
> Sent: Monday, May 13, 2013 10:08 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Making a new spur gear
>=20
> On 2013-05-13 8:57 PM, IVP wrote:
> > I'm guessing you'd need to let it cure properly so the
hot
> metal
> > doesn't heat any remaining free water and crack the mold
>=20
> This is a well-known procedure. I'm not inventing any new
> methods. After
> the plaster hardens, it is to be baked in an oven for a
few
> hours to get
> out as much water as possible. Then it's safe to pour
liquid
> aluminum
> into it. But I'll be heating the mold with the blow torch
> before doing
> anything with the aluminum to make sure it's free of water
> as an extra
> precaution.
>=20
> I'll be melting outside, on a solid concrete slab, with
the
> mold inside
> a hole of a cinder block.
>=20
> If it feels unsafe, I'll dig a hole in the ground, line it
with
> gravel
> and use coal instead of a blow torch so I don't have to be
> near the
> aluminum while it's melting into the mold. Cover with a
> concrete slab
> with a hole to breathe and let it sit until the coal burns
out.
> Doesn't
> get any safer than that.
>=20
> Aluminum melts at low temperatures, and this method has
> been used to
> cast alloys of much higher temperature.
>=20
> If it really becomes an issue melting the aluminum safely,
I'll
> just use
> zinc and melt it under a candle or something outside, and
> stay far away
> so as not to breathe any fumes (zinc fumes are toxic).
>=20
> But the volume of metal I'm dealing with is so incredibly
> small that I
> very highly doubt that I'll run into any issue safely
casting it.
>=20
> I just made the plaster mold and it turned out much better
> than I
> expected, even after creating too thick of a mix. The
> precision of the
> plaster getting into all the notches of the gear is
incredible.
>=20
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