Paul B on mon 10 apr 06
i have always soaked at top temp for at least a while and then dropped it
slowly, holding again for 15-30 mins at a lower temp. But the last few
firings i have held in the cone 10-11 range for over an hour, then dropped
to the cone 8-9 range and held for over two hours (much of it closer to
cone 8). From what i had read, the idea is that soaking too close to top
temp for an extended period of time allows the reactions taking place
within the body as it matures to continue, with gasses being released
througout this time. The results, for me, were sporadic bubbles and
pinholes on the surface of the glaze - something that i have dealt with on
and off for close to a year. Sometimes a lot, sometimes very few; sometimes
in one part of the kiln but not in another, sometiems throughout; sometimes
only on certain glazes, others not; plates and platters never a problem,
but mugs, bowls, vases more susceptible to this problem.
Anyway, the long soak in the cone 8-9 range seems to have solved the
problem (along with some adjustments in the clay body and the way the
glazes are applied) because the last few firings have not shown a single
bubble, blister or pinhole.
Does anyone else do this on a regular basis? Can anyone confirm if the
theory i have is correct (as to why it helps) or maybe i have just been
lucky lately? I will give it 3-4 more firings before i can say for sure
this is what solved the problem, but i would be interested in hearing from
anyone else with a similar firing schedule.
thanks,
Paul
Ivor and Olive Lewis on tue 11 apr 06
Dear Paul B,
Not certain what your Theory assumes. Is it that alteration of the clay =
makes the difference, or that changing the way you apply your glaze is =
responsible or that extending time ate temperature is the solution to =
the problem ?
The only way to be sure is to test each factor independently.
But, I would suggest that elimination of pinholing would be more certain =
if e temperature were held high for a longer period. This gives more =
time for refractory constituents that control viscosity and surface =
tension to dissolve into the melt. These factors are important since =
lower viscosity values allow bubbles to discharge and pinholes to heal =
over.
Check your Glaze Seger Formula against the information given in Frank =
Hamer's Dictionary (Viscosity). There is a table indicating which oxides =
enhance and which diminish viscosity in a glaze.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
=20
karen gringhuis on tue 11 apr 06
Paul -
Although I do soak at top temp when I attempt to even
up the tiers of my electric kiln, as well as down fire
for about the 1st 2-300 degrees of drop - I don't
really do your firing plan.
BUT -- perhaps your pinholing problems come from
bisque firing too fast. Once I reach red heat, I let
the kiln temp rise only about 100 degrees per hour so
that outgassing has lots of time to occur.
Just a thought.
Karen Gringhuis
KG Pottery
Box 607 Alfred NY 14802
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Eleanora Eden on wed 12 apr 06
Paul,
For my low fire glaze firings I reach temp then back off 100F/hr 2 hours and then
soak for 30-40 minutes. This smooths out any bubbling that was happening. So
I think your theory is sound and is independent of temperature range.
Eleanora
>i have always soaked at top temp for at least a while and then dropped it
>slowly, holding again for 15-30 mins at a lower temp. But the last few
>firings i have held in the cone 10-11 range for over an hour, then dropped
>to the cone 8-9 range and held for over two hours (much of it closer to
>cone 8). From what i had read, the idea is that soaking too close to top
>temp for an extended period of time allows the reactions taking place
>within the body as it matures to continue, with gasses being released
>througout this time. The results, for me, were sporadic bubbles and
>pinholes on the surface of the glaze - something that i have dealt with on
>and off for close to a year. Sometimes a lot, sometimes very few; sometimes
>in one part of the kiln but not in another, sometiems throughout; sometimes
>only on certain glazes, others not; plates and platters never a problem,
>but mugs, bowls, vases more susceptible to this problem.
>Anyway, the long soak in the cone 8-9 range seems to have solved the
>problem (along with some adjustments in the clay body and the way the
>glazes are applied) because the last few firings have not shown a single
>bubble, blister or pinhole.
>Does anyone else do this on a regular basis? Can anyone confirm if the
>theory i have is correct (as to why it helps) or maybe i have just been
>lucky lately? I will give it 3-4 more firings before i can say for sure
>this is what solved the problem, but i would be interested in hearing from
>anyone else with a similar firing schedule.
>thanks,
>Paul
>
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