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!