Using a cast iron mold for fused glass

Jewelry Discussion
1

I have just started doing some fused glass experimenting in a small
kiln (6 x 6 x 3) --Can a small cast iron, steel or aluminum shape be
used as a mold? They would not melt at the 1500 or so temperature of
the full fused glass – but the expansion and contraction of the
metal may cause an issue. Does anyone know? Also —do I need to
coat the metal with kiln wash?

2 
I have just started doing some fused glass experimenting in a
small kiln (6 x 6 x 3) --Can a small cast iron, steel or aluminum
shape be used as a mold? They would not melt at the 1500 or so
temperature of the full fused glass -- but the expansion and
contraction of the metal may cause an issue. Does anyone know? Also
---do I need to coat the metal with kiln wash?

You can get answers to questions about glass fusing and casting at
the warmglass board at www.warmglass.com. I was a moderator on that
board at one point in time but have no present affiliation.

As for your questions…Aluminum won’t hold up to the heat of
casting which requires about 1500F or higher.

Molds of any material must be coated with kiln wash or the glass will
stick. Firing glass inside of metal is problematic because of the
expansion/contraction issues. It may get stuck or it may break. I
have successfully slumped glass inside of metal molds, but they were
simple bowl molds and were only slumped, not taken to casting temps.

Typically, cast glass is fired in a refractory material, such as
jewelery casting investment or casting investments that are
specifically designed for casting glass. I have personally used
jewelry casting investment with success when casting glass. Using a
one-time mold materials like investment will allow for undercuts,
which can’t be done with a reusable mold.

Good luck!
Geri Comstock

3 
do I need to coat the metal with kiln wash?

Yes, you can use iron or steel molds -not aluminium as it melts at
about the same temperature that fused glass starts at. You will have
to avoid molds with sharp corners and undercuts and they will have
to be coated in a suitable release agent- kiln wash will do or a mix
of alumina and kaolinite powder. You can buy proprietary mixes which
are in my experience better. An alternative is kiln paper.

To prevent the glass sticking to the prepared mold you need to heat
it up fairly quickly and hold at you max temp for a very short time.
How long will depend on the mold size and hence glass volume and also
frit/glass piece size. The alternative is to go to a higher
temperature and then crash cool the melting glass back down to 510
dec C and then hold at this annealing temp for at least an hour. When
you start experimenting keep a note of what you are using and what
results you get. It will prove invaluable in the future even if you
are recording disasters at first.

Best wishes,
Nick Royall