Wednesday, December 16, 2009

Having Firing Issues - can you help?

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.

This is my kiln.
Its obviously 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 all looked like these!

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.



I will say that the bird feeder was a success, though I will admit it was sheer dumb luck rather than any insight on kiln loading and firing on my part.

Thanks for reading...
CMCK

1 comment:

Paul Vernier said...

Hi, It sounds like you are having several problems. First, put a sticky on the kiln so to always have a cone pack. Other wise, who knows. But, the soap dishes that cracked may not be the resilt of the firing. I was thinking that if you moved from a wetter, more humid climate, they might be drying cracks. I use to fire a Olympic kiln very much like your conversion. They have a tendency to be very uneven without cone packs and a dual pyrometer to control them with. If you had your pyrometer at the bottom, it may also be that the top was a bit out of control. I would not dwell on this worse that expected firing and move on to the next. Be sensitive to the humidity and your slab work. Good luck.

Paul Vernier - Santa Cruz, CA

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