Brad Collis on thu 10 oct 96
Greetings all, from someone with a puzzling problem.
I have just made a stunning (if I may say so) dinner set in a beautiful rich
blue glaze. It was fired to cone 9 oxidation and all was well.
My intention then was to apply a decorative motif in gold lustre. This I did
and proceeded to refire the wares to 660deg C for the lustre firing.
On opening the kiln, all the bowls and other assorted dishes were fine, but
every single dinner plate (bar one) had cracked in half, the crack following
a circular curve and was the same with every plate. My eyebrows are still
following my receding hairline.
I'd be very grateful for some thoughts as I'm about to bisque fire the new
set of plates (which, to expedite the proceedings I've made using a slipcast
mould) and dread a repeat disaster.
The first set was bisqued to 1000deg C then glaze-fired to cone 9. Should I
have given them a higher bisque? Could I have fired them too fast to 660deg
-- although I really would have only averaged 100deg/hr? Should I have
slowed the cooling, which I can do using the computerised controller?
Any other thoughts?
Many thanks in advance
Brad Collis
Brad Collis
Melbourne Australia
brad@parkhouse.com.au
Ron Roy on fri 11 oct 96
Brad,
That kind of crack is probably related to a mismatch of clay and glaze -
possibly cristobalite in the body. That kind of dunting is more likely to
happen on bigger flat pieces. I would guess it happened on the way up in
the gold firing - outside of plate heats faster than inside, expands and
separates from the inside.
I can be more precise if I know more about the type of body you are using,
the glaze recipe and type of firing - also was the crack open or closed.
If I must guess about how to avoid the cracking in the next firing - go
very slow on the way up in the luster firing - especially between 100 and
300C. You might also protect the edges of the plates from direct heat with
bick or kiln furniture.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Brad Collis on sun 13 oct 96
Roy,
Thanks for your reply. The cracks were closed, and only became apparent when
I picked up the plates and they fell in half.
I will slow the firing right down next time and see what happens. I was
possibly a bit gung-ho, feeling that the 660deg firing was no great stress
for wares already taken to cone 9. I did slow hold the firing at 573 deg for
about 15 minutes, but I may well have been a bit quick in the 100 to 300
bracket.
Regards
Brad Collis
Brad Collis
Melbourne Australia
collis@parkhouse.com.au
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