Paul Borian on thu 17 dec 09
i seriously doubt the cracks had to do with the firing schedule, unless you
fired it exremely fast or the cracked pots got hit directly by the burners
- but even these two scenarios don't necessarily cause cracks. You could,
however, try a slower firing at lower gas pressure and see if that helps at
all - and of course always put the cones in there. i suggest that you make
the cone packs as soon as you start glazing and put them on the lid of the
kiln so you don't forget them again.
I am sure there will by plenty of other suggestions here but my impression
is that this problem has to do with the clay body and how the pots are
made, dried, bisque fired, etc., rather than the glaze firing.
paul
---------- Forwarded message ----------
From: Cathi Newlin
Date: Dec 16, 2009 5:39pm
Subject: Cone 10 Kiln Disaster - help?
To: Clayart@lsv.ceramics.org
CC:
> 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:
> http://squarepegarts.blogspot.com/
> --
> Cathi Newlin, Angels Camp, Ca
> cathi@box49.com
> box49@caltel.com
> cathi@SquarePegArts.com
> -------------------------------
> California Boxers in Need:
> http://CaliforniaBoxer.org
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