Corinne Null on wed 10 apr 96
Help! Opened the cone 10 gas fired updraft kiln yesterday to find my first
explosion. A teapot that had previously been glaze fired, and had a few
dabs of new glaze on it, had blown its top and spout. It was on the top
shelf in the center of the kiln. Fragments were found even on the bottom
shelf ware. Many unhappy students :(
In discussing this with other potters, I have heard several similar stories,
but no one knows why this happens. It sure makes me wary of ever refiring
anything again! Any clues as to why this happens? Is there any way to do
it safely?
We fire very slowly. Pilots for about 7 hours, then about 17 hours to 2300
degrees or so - this is for our convenience.
With the wealth of experience among the collective here, I imagine someone
must have this figured out. Thanks.
Corinne Null
Bedford, NH
cnull@mv.mv.com
tom huber on wed 10 apr 96
there is a small amount of moisture in the pot .Because it is glazed the
moisture cannot escape as it normally would during fireing. it expands,
nowhere to go. the pot explodes I know I lost a kiln load .
At 09:29 AM 4/10/96 EDT, you wrote:
>----------------------------Original message----------------------------
>Help! Opened the cone 10 gas fired updraft kiln yesterday to find my first
>explosion. A teapot that had previously been glaze fired, and had a few
>dabs of new glaze on it, had blown its top and spout. It was on the top
>shelf in the center of the kiln. Fragments were found even on the bottom
>shelf ware. Many unhappy students :(
>
>In discussing this with other potters, I have heard several similar stories,
>but no one knows why this happens. It sure makes me wary of ever refiring
>anything again! Any clues as to why this happens? Is there any way to do
>it safely?
>
>We fire very slowly. Pilots for about 7 hours, then about 17 hours to 2300
>degrees or so - this is for our convenience.
>
>With the wealth of experience among the collective here, I imagine someone
>must have this figured out. Thanks.
>Corinne Null
>Bedford, NH
>
>cnull@mv.mv.com
>
Tracy Dotson on wed 10 apr 96
looks like we all need for this to happen at least once refireing pots in
salt. Mine was a coffee mug that I decided to refire (you know that growth
that comes from 3 weeks of setting on the shelf) . I rinsed out most of the
gunge, dried it off with a towl, put it in that space on the bottom shelf
that I didnt want to leave empty. Yes !!! small projectiles through out the
entire stacking (mostly pots I ask my friends for to fill the Kiln). It
looks like moisture had penetrated into the vitrifed body letting steam do
its thing. I now try to preheat a piece before refireing in the salt kiln
(usually on top of a bisque kiln). Needless to say , I don't clean up old
mugs that way any more. Good luck Tracy Dotson Penland NC
BobWicks@aol.com on thu 11 apr 96
In a message dated 96-04-10 09:31:53 EDT, you write:
> A teapot that had previously been glaze fired, and had a few
>dabs of new glaze on it, had blown its top and spout. It was on the top
>shelf in the center of the kiln. Fragments were found even on the bottom
>shelf ware. Many unhappy students :(
Hi Corinne:
I guess most of us have had similar experiences like you describe. In many
cases where there is steam generated where it is sealed within the clay body
you will generally have an explosion. Often times wet glazed projects can be
the cause. There are other things such as plaster chips in the body. When
this heats up it swells and pops (calcium pop I believe is the term used). I
usually hold each project to my cheek before loading: if its cold I know it's
wet and needs to be dryer. I frequently microwave questionable projects for
about 10 minutes. This works consistently well. I am assuming that you
bisquet everything before glaze firing??Right? You might try bisquet firing
to a higher cone, say cone 8 in your case.
I'm sure others will have comments on your dilemma and I wish you good luck.
Don't give up. Pottery is a cause effect phenomena and you will always find
a cause.
Bob
Gerry Barbe on thu 11 apr 96
On Thu, 11 Apr 1996 BobWicks@aol.com wrote: . I am assuming that you >
bisquet everything before glaze firing??Right? You might try bisquet
firing > to a higher cone, say cone 8 in your case. > > > Bob
Hi there,
A cone 8 bisque seems high to me. My glaze fire is cone 6. Is
something new happening out there? Do you high fire folk bisgue higher
than we cone 6 folk glaze?
This was the teapot's second glaze firing. If the lid was stuck
on the teapot, i.e., not removed after the first firing. And if the
spout holes filled with glaze after this first firing or during the
second firing, then any trapped air inside the pot would make it
explode. It seems to me that however it happened, air got trapped
inside the teapot. I would be interested to know if the spout holes glazed
over.
Anyway, my experience has been that my pots always crack in a
second cone 6 fire (they survive lower temp second fires). Perhaps it's
the clay body.
Regards, Marilyn >*>* in Ailsa Craig where spring has sprung
Gerry Barbe - gbarbe@julian.uwo.ca
Dept. OB/GYN
University of Western Ontario
London, Ontario, CANADA
______________________________________________________________________________
tony nankervis on thu 11 apr 96
Corinne
Over the last 10 years I have been multiple firing all of my work in a
long woodfired kiln (5 day firings) sometimes up to 5 or 6 times to get the
depth and quality of surface I require.
Only in one firing have I had the problem you mention. Several large
platters had been stored where they could get wet during rain. I believe
what happened was that despite being vitrified, some water was able to
penetrate deep into the clay body during the storage period. Because the
structure of the vitrified body is so tight, a normal firing cycle isn't
anywhere near long enough to allow the steam from that heated water time to
get out without exploding its way through the piece. In fact after
exploding three pieces in the firing I spoke of, I unpacked the kiln and
removed the debris, repacked and candled the kiln to 250 degreed C over a
20 hour period and then blew another one up!!. The water stays in there
good!
My best guess is that the teapot that you mention might have been used a
few times before it was refired and that somehow liquid has managed to seep
into the minuscule pores which exist even in vitrified ware.
Hope this is of some help - Cheers
Tony Nankervis
Ceramics Dept
Visual Arts
Southern Cross University
Lismore 2480 NSW Australia
LBlos72758@aol.com on thu 11 apr 96
In probably over 300 items being repeatedly refired, I have only encountered
one item that cracked on the third firing. It was a sink in the bottom of my
electric kiln. I don't know the real reason for this breakage - I did add
glaze with each firing until the poor thing had about 6 coats on it. The
jury is still out on this verdict.
Linda
Ithaca, NY
art_selsor@vino.emcmt.edu on fri 12 apr 96
We fire to ^6 at MSU-B and I have once in a great while seen things
explode on the refiring of a glaze. I think ih has to do with location
in the kiln and thermal shock to a dense (already vitrified) clay body.
I discourage students to refire glazed pieces, but if they insist, I
try to put them in a buffered area of the kiln where they don't get
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