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offcntrMy last glaze firing was, say, less than successful. Strong swath of oxidation along the left side, and a big swipe of over-reduction on the right. Talking to the other potters at the club, I learned that Jon has had a similar problem with his most recent firing, though specific oxidation/reduction locations varied. Tea just noticed oxidation. So it's probably not just a badly controlled firing on my part.
Excessive oxidation/reduction is a problem, not just because I like my pots to be consistent. While the former makes bleached, white pots that I'm reluctant to sell, the latter makes pots that can be actively dangerous, because the chrome green overglaze doesn't properly fuse into the glaze surface. I can sell white pots as seconds; bubbly ones, I either have to fix or break.
I've got one testing for my next firing--I mixed up gerstley borate and water with a little bit of gum arabic, and painted it over the green to see if it will settle down on refiring. In the meantime, I need to fix the kiln.
I notice that the bag walls are leaning outward, restricting flame from the burners. I point this out to Tea, and he and I reverse the top two courses of brick, so they lean back inward. This should allow for easier transit of flame into the kiln. It seems to help--Tea reports good reduction throughout his next firing.
Encouraged, I do a thorough walk-around, both inside and out, taking notes. Then I start tinkering.
To begin with, the chimney is showing gaps between bricks. This happens when the bricks expand with heat during a firing, then don't move back when they contract. Solution? Bang on them with a hammer and a piece of two-by-four. (The wood can be used both to direct the force against a particular brick or to spread it out so the brick doesn't break.) The chimney now seems noticeably tighter. I also take a look up the chimney to make sure it's not being blocked by anything. Normally, this is done with a mirror, but I've misplaced mine, so instead go digital and take a picture with my cell phone. Flue looks clear.
The door of the kiln is binding on the left side, even after I tap the bricks tight. Closer inspection shows that the door sill has moved inward, also with expansion, leaving gaps in the brick. This is a strong possibility for my culprit, as gaps and leaks provide extra oxygen. I bang the door jambs tighter, too.
Lastly, I take some photos of the inside of the burners. Interestingly, both of the left-side burners show a lot of crud and debris inside the venturi tubes. Don't know if it's enough to obstruct the flame, but I vacuum them out, just to be safe.
Tomorrow I load the kiln and fire it. Monday, when I unload, I hope to have good news.
Excessive oxidation/reduction is a problem, not just because I like my pots to be consistent. While the former makes bleached, white pots that I'm reluctant to sell, the latter makes pots that can be actively dangerous, because the chrome green overglaze doesn't properly fuse into the glaze surface. I can sell white pots as seconds; bubbly ones, I either have to fix or break.
I've got one testing for my next firing--I mixed up gerstley borate and water with a little bit of gum arabic, and painted it over the green to see if it will settle down on refiring. In the meantime, I need to fix the kiln.
I notice that the bag walls are leaning outward, restricting flame from the burners. I point this out to Tea, and he and I reverse the top two courses of brick, so they lean back inward. This should allow for easier transit of flame into the kiln. It seems to help--Tea reports good reduction throughout his next firing.
Encouraged, I do a thorough walk-around, both inside and out, taking notes. Then I start tinkering.
To begin with, the chimney is showing gaps between bricks. This happens when the bricks expand with heat during a firing, then don't move back when they contract. Solution? Bang on them with a hammer and a piece of two-by-four. (The wood can be used both to direct the force against a particular brick or to spread it out so the brick doesn't break.) The chimney now seems noticeably tighter. I also take a look up the chimney to make sure it's not being blocked by anything. Normally, this is done with a mirror, but I've misplaced mine, so instead go digital and take a picture with my cell phone. Flue looks clear.
The door of the kiln is binding on the left side, even after I tap the bricks tight. Closer inspection shows that the door sill has moved inward, also with expansion, leaving gaps in the brick. This is a strong possibility for my culprit, as gaps and leaks provide extra oxygen. I bang the door jambs tighter, too.
Lastly, I take some photos of the inside of the burners. Interestingly, both of the left-side burners show a lot of crud and debris inside the venturi tubes. Don't know if it's enough to obstruct the flame, but I vacuum them out, just to be safe.
Tomorrow I load the kiln and fire it. Monday, when I unload, I hope to have good news.