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cone 10 kiln disaster - help?

updated sat 19 dec 09


Cathi Newlin on wed 16 dec 09

I'm having pretty frustrating results firing my kiln since moving it and
me back to Ca. I'm hoping for some insight from the pottery gurus out
there on what I'm doing wrong.

I fire ^ 10, typically reduction, which is to say that I can get a
reading of 2300f on my pyrometer and get a fully bent cone 11 using
Orton cones.
My kiln is an old electric one that I have converted to propane.
Inside dimensions are 27" high and 23" across.
Its a bit dilapidated, I'll admit, but (to quote Hedwig), its what I
have to work with. I have 3 brand new Coralite shelves that are 15"
round (well, actually I have 1 full round and 4 1/2 rounds.

I have a 2 burner system that has a pilot valve (good for candling) and
a valve that controls both burners simultaneously. The 100 gal propane
tank goes through a pressure regulator before feeding the burners as the
specs on the burner are not to exceed 4 psi (the regulator goes to 20 psi).

I have been trying to be more precise in my firing schedule, relying
largely on John Britt's "Complete Guide to High Fire Glazes" and using
the R2 and R3 firing schedules.

This last firing, which was so frustratingly disastrous, was R3 and I
was able to keep it within about an hour of what was graphed out in the
book. I did not have a cone pack in the kiln because I just plain forgot
to put it in. I shut the kiln off at 2250f and did soak it a little at
the top.

I probably reduced a bit heavier than I should have, and did have some
trouble with the slow cool, but I don't really think those are relative
to what I found when I opened the kiln next morning.

I am currently using 2 clay bodies - Quyle Kilns' Sierra Gold (high
iron, ^6-10 clay body local to me) and Laguna's 1/2 and 1/2, which is
1/2 white stoneware and 1/2 porcelain.
I'm not happy with the Laguna body - its very lumpy and difficult to
throw, but I have 125 lbs of it, so I am largely handbuilding with it
till its gone.

This last firing was mostly to test some new glazes, both alone and with
my 3 very stable glazes.
So most of the stuff was not really important, except for the top shelf.

On the bottom shelf, I loaded some small bowls and syrup pitchers, a
donut-shaped bird feeder and some test tiles. It was about 12" off the
kiln floor. Most of the ware was the Quyle clay, and all was thrown.

The second shelf, about 7" above that, had some tea bowls, test tiles
and small bowls.
All thrown items, most with extruded handles, and an even mix of the
Quyle clay body and the Laguna body. Some of these pieces were glazed
with an ash glaze and those pieces were stilted up with pot shards.

Third shelf, which was about 7" below the roof of the kiln and the stack
was loaded with 5 slab-built soap dishes and a small bowl made with the
Laguna body and glazed with a simple clear glaze.

When I opened the kiln, every one of the soap dishes was beyond just
cracked. They looked like they had been sort of torn apart. The small
bowl also had a crack from the lip down about 1/2 way to the foot.

Some of the pieces hung over the shelf at their edges, but others did
not. I did have a couple crack (not like this) during bisque, so I
checked each one for cracks an defects before I glazed them.

On the middle shelf, there was no breakage and the glazes seemed to
mature just fine, though a couple of the insides of bowls seemed a
little under fired and the Willie Helix glaze was a sick brown color on
everything I had applied it on.

On the bottom shelf, everything seemed to fire just fine and the glazes
seemed to mature well. Again, no breakage.

I feel like I'm just not getting something.
I did a short apprenticeship 5 years ago with a commercial potter, but
beyond that and a few long-ago college classes, I'm largely self-taught.
I don't really know any of the potters in my area, though I did join an
area potters' guild recently.

I'm pretty confident in my ability to produce decent work up through the
bisque firing, but then it just all seems like a crap shoot. I would
like to keep going in the direction I am, and become proficient in
high-fire techniques. I know my kiln is a sorry sight, but for now, it
must suffice.

Sure would appreciate any thoughts anyone has on what went wrong and how
I can correct these issues and someday pull something from the kiln I
don't want to break right away.

Pics of my kiln, the damage etc are here:

Cathi Newlin, Angels Camp, Ca
California Boxers in Need:

Dean on thu 17 dec 09

Aloha, I've got an Aim updraft about the same size as yours. I did a couple=
of modifications that you might consider..
Add 1" of fiber around the kiln with a pad of fiber for the roof. Handle wi=
th gloves. Your kiln will fire hours faster and you'll save lots of gas.
Put a damper of some kind on the exit flue for better firing control and re=
duction control.(didn't see one in your picture.)
I have 6" of space under my bottom shelf to act as a firebox. Helps keep th=
e pots away from the hot spots and makes a more uniform atmosphere. I added=
two rows of brick to the height of the kiln so I wouldn't lose stacking sp=
Good luck,