Carole Fox on thu 29 jun 06
In a recent glaze firing in my electric kiln, I had major problems with
bloating. Since I've never had problems with bloating before, I checked
the archives found many useful posts on the topic. I've pretty much
concluded that the bloating, which only occurred in the dark brown clay
body that I use, was due to overfiring by one cone (^7 when ^6 was the
intent). I had pieces made from two other lighter colored bodies in the
load, and none of those bloated. All of the bodies are ^6 bodies that I
have used many times before, everything had been slow bisqued to ^04, and
all pieces had been glazed with the same combination of 2 glazes.
My question is this:
Does shape of the piece affect the likelihood of bloating? All of the
bloated pieces were low, open bowls. I had several other pieces in the
load made from the same dark clay body that did not bloat - these were all
more vertical pieces - mugs, cruets, and small pitchers. Both bloated and
unbloated pieces were distributed throughout the kiln.
Any thoughts?
Carole Fox
Dayton, OH
William & Susan Schran User on thu 29 jun 06
On 6/29/06 9:13 AM, "Carole Fox" wrote:
> Does shape of the piece affect the likelihood of bloating? All of the
> bloated pieces were low, open bowls. I had several other pieces in the
> load made from the same dark clay body that did not bloat - these were all
> more vertical pieces - mugs, cruets, and small pitchers. Both bloated and
> unbloated pieces were distributed throughout the kiln.
Carole,
Do you recall the distribution of the pieces in the bisque firing?
Were the pieces that bloated all on the bottom shelves of the bisque firing?
My thinking is the work that bloated may not have actually gotten to ^04 in
the bisque firing.
Were the pieces that bloated thicker or thinner than those that didn't
bloat?
Did the pieces that bloated all have the same glaze on them?
Thinking maybe one glaze is sintering/melting before the others and sealing
the clay surface.
-- William "Bill" Schran
Fredericksburg, Virginia
wschran@cox.net
wschran@nvcc.edu
Carole Fox on thu 29 jun 06
On Thu, 29 Jun 2006 12:48:50 -0400, William & Susan Schran User
wrote:
>Were the pieces that bloated all on the bottom shelves of the bisque
firing?
>My thinking is the work that bloated may not have actually gotten to ^04
Actually, none of the pieces that bloated were on the bottom shelf during
the bisque firing. Also, I had witness cones on all the shelves, and all
fired to ^04 with the tip touching the shelf. I did have all the bowls
stacked rim to rim in pairs during the bisque. I have never had a
problem with this before, but I do wonder if stacking the bowls this way
alters the heat absorption by those pieces. I have more pieces from that
bisque load that haven't been glazed yet. It will be interesting to see
what happens when they are fired to ^6, as the bloated stuff should have
been.
>Were the pieces that bloated thicker or thinner than those that didn't
>bloat?
All pieces in the load were of pretty much the same thickness - there were
a only couple that were slightly thicker, and yes, they did bloat but so
did several others.
>Did the pieces that bloated all have the same glaze on them?
>Thinking maybe one glaze is sintering/melting before the others and
sealing the clay surface.
All the pieces were glazed the same - an iron red glaze slightly
overlapped with a tan glaze, both poured. I fired a bunch of test buttons
with these glazes at varying thicknesses on the same body earlier this
year at ^6 and all was well.
I know for sure that at ^7 the dark clay body was fired higher than the
clay manufacturer recommends, and this was my error. I just didn't watch
the shelf cones closely enough at the end of the firing. So I'm not
surprised there was a problem. I'm just wondering whether it was pure
coincidence that all the bloating occurred in flat pieces while none of
the more vertical pieces show signs of the problem.
thanks for your thoughts!
Carole Fox
Dayton, OH
Snail Scott on fri 30 jun 06
At 05:32 PM 6/29/2006 -0400, you wrote:
>...I did have all the bowls
>stacked rim to rim in pairs during the bisque. I have never had a
>problem with this before, but I do wonder if stacking the bowls this way
>alters the heat absorption by those pieces...
Not the heat absorption, so much, but: I have
seen stacked or enclosed bisque which appeared
to have gotten some slight reduction during
the bisque firing, due to incomplete burnoff
of the organics. A longer bisque firing would
no doubt eliminate this, but the restricted
space inside stacked forms seems to make it
tougher to get a really clean bisque.
Most of the time, it doesn't matter much, but
very high-iron bodies are more susceptible to
the bad effects of inadequate burnout. It may
be that your usual bisque, even on the stacked
forms, was just adequate to meet the needs of
your normal ^6 firing, but barely. They were
just on the safe side of a fine line, before.
When overfired to ^7, the _very_ slightly less-
clean bisquing of those stacked forms was enough
to make a difference.
The triple risk factors for bloating are
inadequate burnout during bisque, high-iron
clay, and overfiring. Until this latest firing,
you only had two of those. The overfiring,
though only very slight, was just the last straw.
If it didn't happen before during your ^6
firings, it probably won't happen again, but
if you want to give yourself a safety margin
against the possibility of future overfiring,
try adding a soak to the end of your bisque,
or put little spacing wads between the rims of
your stacked bowls.
-Snail
Ron Roy on fri 30 jun 06
Hi Carol,
Probably what happened - when the bowls are stacked rim to rim - any
organics burning off will be trapped inside - the iron in the body will be
reduced and that will overflux the clay - I'm sure that is the reason.
If you had covered the taller pieces it would probably have happened to
them as well.
Keep in mind that raw clays used to mix bodies will have different amounts
of organics in each batch - we need to design our firings and stacking with
this in mind - especially when we are firing iron rich clays.
Perhaps - if you used some clay on the rims - to keep the bowls a bit apart
- the organics would have a chance to burn off properly.
RR
>Actually, none of the pieces that bloated were on the bottom shelf during
>the bisque firing. Also, I had witness cones on all the shelves, and all
>fired to ^04 with the tip touching the shelf. I did have all the bowls
>stacked rim to rim in pairs during the bisque. I have never had a
>problem with this before, but I do wonder if stacking the bowls this way
>alters the heat absorption by those pieces. I have more pieces from that
>bisque load that haven't been glazed yet. It will be interesting to see
>what happens when they are fired to ^6, as the bloated stuff should have
>been.
>
>>Were the pieces that bloated thicker or thinner than those that didn't
>>bloat?
>
>All pieces in the load were of pretty much the same thickness - there were
>a only couple that were slightly thicker, and yes, they did bloat but so
>did several others.
>
>>Did the pieces that bloated all have the same glaze on them?
>>Thinking maybe one glaze is sintering/melting before the others and
>sealing the clay surface.
>
>All the pieces were glazed the same - an iron red glaze slightly
>overlapped with a tan glaze, both poured. I fired a bunch of test buttons
>with these glazes at varying thicknesses on the same body earlier this
>year at ^6 and all was well.
>
>I know for sure that at ^7 the dark clay body was fired higher than the
>clay manufacturer recommends, and this was my error. I just didn't watch
>the shelf cones closely enough at the end of the firing. So I'm not
>surprised there was a problem. I'm just wondering whether it was pure
>coincidence that all the bloating occurred in flat pieces while none of
>the more vertical pieces show signs of the problem.
>
>thanks for your thoughts!
>
>Carole Fox
>Dayton, OH
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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