Monday, August 9, 2010

lessons learned

The Kiln Shed after a Summer Storm


I'm starting to get my hands back into the clay. Sales at Windy Ridge have been slow, so similarly (is that a word... probably not) to fellow potter/blogger Brandon Phillips, I've been doing some construction work. But we need more pots for a show that starts September 1st, so I'm back to the pottery for a short stint.

Nothing new to report as today is my first day back at the wheel, so I thought I would post on various things that we learned from our last firing:

Glazes

I was pretty disappointed when I unloaded our "oil spot/tenmoku" glaze from the kiln. This is what I was hoping for:
and this is what we ended up getting:

The glaze, although quite different from our goals, has slowly started to grow on me. It's another lesson on how little we can trust the results out of our quick- firing, quick- cooling test kiln. Obviously the glaze is going through some serious crystallization in the wood kiln. I'll mix up another batch of tests for the next wood firing. Last time I added 20 percent EPK hoping the extra alumina would stop the crystallization. This time I'll try two different approaches: one with a blend of progressively more silica, and one with a blend of progressively more alumina (maybe in the form of EPK, maybe in the form of pure alumina hydrate... still undecided on this one). Both of these materials are useful for retarding the crystallization process, albeit different types of crystals if my understanding is correct.

Clay

Our new clay turned out well--slightly less sheen and flashing than our old clay, but still quite nice. Best part? No cracking! I might try slightly lowering the fire clay from 20% to 10-15% on the next batch, but over all I'm quite happy with the changes. The only cracking we got was on serving bowls such as this one:




I am quite sure this cracking is my fault. Maybe to thin at the corner? This is a new shape for me so I can't say for sure. Any ideas out there?

Kiln

Finally our kiln. The closer spacing on the grates was just what we were hoping for. The only thing I'll change for next time is to raise the stoking door. I had kept it pretty low in the past thinking that would give us better control of the embers if they needed to be moved around.

Don Hammond and I study the ember situation

With the new grate spacing, though, all the low stoking door did was to make it impossible to really build up the ember pile. Stacking the stoking door another 10-12 inches higher should be just right.

One issue I have yet to really wrap my head around is the top of the first chamber. It never gets the same amount of ash and reduction as the lower half of the first chamber:


I assume it's simply from having such a tall first chamber with a relatively short firebox. The simple solution would be to put glazed pots on the top several shelves, although I think I will be stubborn for at least a couple more firing and see if we can't get at least some more reduction from the pots on the top shelf. Again, any suggestions here are more than welcome. Our next firing will be a small load, but in early October we are hoping to fire all three chambers and fire for 72 hours instead of our usual 48-50. If we still can't get anything going on on the top shelf after that.... then we'll switch to putting glazed ware up there.


The Third Chamber Earthenware Tests:

Christy was very pleased with her first round of earthenware tests from the third chamber. It seems as though, until we have time to build her a separate small kiln, the third chamber will work well.


Final lesson:

This one's pretty obvious... we need some new posts:



I'd rather not have to unloaded anything like this again.

Joe

edit:
sorry y'all- blogger's getting all crazy with the text size... not sure why

6 comments:

ang said...

i read the whole post without noticing the change in font size!! now that's weird...:)) glad to see the earthenware tests came out well and i rather like the top half picture or are they not as shiny as they look from here? onwards with the testing...cheers

deanandmartinpottery said...

Hey Joe and Christy,

The first chamber of your kiln looks like there is a big heat discrepancy from top to bottom. I was wondering does the back of the front chamber above your flues have any other draft holes toward the middle and upper part of the wall. This particular thing helps in drafting heat and fly ash through the upper parts of a tall chamber. This is something that has been used in kilns with taller first chambers that I have seen. Hope ya'll are doing well. The firing looks really good. Ya'lll got some great pots out hope they all got good homes quickly. Have a great rest of the summer!!

Joe and Christy said...

Jeffery,
yeah we decided not to sweat the temperature differences in the first chamber this firing as almost all of it was unglazed. In the past though when we have kept a closer watch on the temp, the end product was the same. In other words, the top was hotter due to our clamping down the dampers, but the ash build up and reduction were still similar to what we've seen this firing.
Our first chamber's arch comes down to rest on top of the flue, so adding flue exit up higher isn't really an option... You are probably right though, that would help things out...
As it stands right now as long as we are patient toward the end of the firing we can even the temperature out in the first chamber, by slowing the draft down. I'm just not sure how to even out the reduction levels. Not that I'm looking for perfectly even levels of reduction though out the chamber... I'm always curious though how/why things work the way they do.
Joe

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