Daniel on tue 19 feb 08
Hi Ron, All,
We had a case the other week of a piece that fragmented explosively
in the kiln. It was some high iron stoneware - probably recycle of
questionable parentage - that was glazed principally on the inside. It
was a small enclosed form with a small opening in the bottom - a
pommegrante I think. Anyhow, the first firing was not a great success so
some additional glaze accent was added on the outside and then it was
refired. Still the outside was not completely glazed. It blew up.
Looking at it I think it must have blown up on cooling firing fragments
everywhere. The glaze was a semi matte and I think it must have blown
before some of the glosses got really frozen according to what I was
told but I was not able to inspect them much. I'm still not sure if it
was the quartz or the cristobalite transition that took it out. I tend
to think the latter but without seeing other pieces its a little hard to
tell. Anyhow, the only reasonable explanation I could come up with was
glaze/body fit mismatch exacerbated but refiring a high iron body in
reduction to cone 10. This would also point to cristobalite rather than
quartz.
I've never seen this dramatic an example before and perhaps my
diagnosis is suspect but its the best I could make of the evidence so
far. Any other thoughts or things I could/should check ?
Thx
D
> Hi Kathi,
>
> It happens a lot and it does no good for handmade pottery sales.
>
>
Earl Brunner on tue 19 feb 08
I've had refires of high fired work explode rather violently in the early stages of the second glaze fire. I attributed it to moisture getting into the clay and not being able to get out quickly enough on the second glaze firing. Nasty to clean up and fragments in a lot of other pots, glazed on fragments, since the explosion wasn't discovered until the firing was over.
Earl Brunner
Las Vegas, NV
----- Original Message ----
From: Daniel
To: CLAYART@LSV.CERAMICS.ORG
Sent: Tuesday, February 19, 2008 9:09:01 PM
Subject: Explosive dunt - a case in point
Hi Ron, All,
We had a case the other week of a piece that fragmented explosively
in the kiln. It was some high iron stoneware - probably recycle of
questionable parentage - that was glazed principally on the inside. It
was a small enclosed form with a small opening in the bottom - a
pommegrante I think. Anyhow, the first firing was not a great success so
some additional glaze accent was added on the outside and then it was
refired. Still the outside was not completely glazed. It blew up.
Looking at it I think it must have blown up on cooling firing fragments
everywhere. The glaze was a semi matte and I think it must have blown
before some of the glosses got really frozen according to what I was
told but I was not able to inspect them much. I'm still not sure if it
was the quartz or the cristobalite transition that took it out. I tend
to think the latter but without seeing other pieces its a little hard to
tell. Anyhow, the only reasonable explanation I could come up with was
glaze/body fit mismatch exacerbated but refiring a high iron body in
reduction to cone 10. This would also point to cristobalite rather than
quartz.
I've never seen this dramatic an example before and perhaps my
diagnosis is suspect but its the best I could make of the evidence so
far. Any other thoughts or things I could/should check ?
Thx
D
> Hi Kathi,
>
> It happens a lot and it does no good for handmade pottery sales.
>
>
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Tom at Hutchtel.net on wed 20 feb 08
Earl,
We've had the same experience a couple of times. At first I attributed it
to cristobalite formed in the firs firing, but now I'm of the mind that it's
just the speed with which we fired. We followed our usual sequence (gas
fired, Minnesota flattop, 2 Ward 500,000 btu burners.) Start one burner to
warm a bit, peeps open, 2 (high and low on each side and 2 on the door. The
last time I took temp measurements high and low, front and back. It amazed
me how fast the top of the kiln came up to over 500 degrees, so I slowed
down and had to work to keep it from going too fast and to try to get the
heat down.
It would easily have gone over 1000 degF up top while the bottom was still
at 2-300. So now I'm convinced that it's an expansion explosion and when we
have refires in the kiln, you just have to slow it down. And yes the
explosion is violent. ruined 1/3 the pots in the kiln from stuff blasted all
over. Oh yes, it's always been pieces up high that blow.
Also, it's on the way up as evidenced by the way the dust and shards are all
on horizontal or shallow bowls and are fused into the glazes on those
pieces.
Tom Wirt
Hutchinson, MN
twirt@hutchtel.net
www.claycoyote.com
> I've had refires of high fired work explode rather violently in the early
> stages of the second glaze fire. I attributed it to moisture getting into
> the clay and not being able to get out quickly enough on the second glaze
> firing. Nasty to clean up and fragments in a lot of other pots, glazed
> on fragments, since the explosion wasn't discovered until the firing was
> over.
>
> Earl Brunner
> Las Vegas, NV
>
>
Ron Roy on sat 23 feb 08
Hi Daniel,
A couple of questions - how long between firings? Any chance for it to get
wet - is the clay vitrified? One way to tell is how much the clay gets
reoxidized - into the ware. If only a thin skin of clay is brown and the
rest is black or grey then the clay is well vitrified. If the colour goes
in farther that means not as vitrified.
You can tell if it happened on the way up or down easy - if on the way up
the glaze will have melted and the pieces will be stuck to the shelves
where the glaze touches - and the edges of glaze will not be sharp. If it
fractured on the way down the glaze edges at the fractures will be razor
sharp.
Best regards - RR
>Hi Ron, All,
>
> We had a case the other week of a piece that fragmented explosively
>in the kiln. It was some high iron stoneware - probably recycle of
>questionable parentage - that was glazed principally on the inside. It
>was a small enclosed form with a small opening in the bottom - a
>pommegrante I think. Anyhow, the first firing was not a great success so
>some additional glaze accent was added on the outside and then it was
>refired. Still the outside was not completely glazed. It blew up.
>Looking at it I think it must have blown up on cooling firing fragments
>everywhere. The glaze was a semi matte and I think it must have blown
>before some of the glosses got really frozen according to what I was
>told but I was not able to inspect them much. I'm still not sure if it
>was the quartz or the cristobalite transition that took it out. I tend
>to think the latter but without seeing other pieces its a little hard to
>tell. Anyhow, the only reasonable explanation I could come up with was
>glaze/body fit mismatch exacerbated but refiring a high iron body in
>reduction to cone 10. This would also point to cristobalite rather than
>quartz.
>
> I've never seen this dramatic an example before and perhaps my
>diagnosis is suspect but its the best I could make of the evidence so
>far. Any other thoughts or things I could/should check ?
>
>Thx
>D
>> Hi Kathi,
>>
>> It happens a lot and it does no good for handmade pottery sales.
>>
>>
>
>______________________________________________________________________________
>Clayart members may send postings to: clayart@lsv.ceramics.org
>
>You may look at the archives for the list, post messages, change your
>subscription settings or unsubscribe/leave the list here:
>http://www.acers.org/cic/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots2@visi.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Lee on sun 24 feb 08
I had a reglazed platter explode in a refiring. Moisture must have
seeped into the clay and couldn't escape fast enough.
--
Lee in Mashiko, Tochigi Japan
http://mashikopots.blogspot.com/
"Let the beauty we love be what we do." - Rumi
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