Problems with a small/medium sized glass casting ...

Hello All, I've been casting for the past 4 years (average of 5-6 casts a month of something or the other). I've been using plaster+silica moulds with chicken wire as needed. I've had reasonable results so far EXCEPT for this one exasperating piece ...

This is a piece about 9"x4"x6" - it's a figure head with a fair amount of detail on it. I used the lost-wax technique to make the mould. All my moulds have 2" thick mould walls. I've tried to cast this with the following results ...

1. Hydrocal + silica mould - mould cracked right at the end of the firing
2. Plaster + silica mould - mould did not fill even after 6 hours in the kiln
3. Plaster + silica mould - mould cracked towards the end again.

I've slowly come to the conclusion that perhaps there is something in my house water that is making the plaster behave wierdly and disintegrate _faster_ than usual.

These moulds tend to get reasonably heavy (imagine a block of plaster about 12"x10"x10") - so the whole cast + dry + dewax process is getting on my nerves, and with nothing to show for it in the end.

Has anyone used other mould materials ? I've heard fly-ash being talked about - has anyone used it with good results ?

Thanks, Krishnan Sick of staring @ a broken plaster mold - after a weeks labour!

Fly-ash is composed of vitreous micro-balls. It has a fairly sharp melting characteristic, depending on the ash fusion temperature of the coal in question. It also has pozzolanic properties, so you can get an hydraulic bond with the filler that you use. How it would stand up to your technique, I've no idea.