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So this last firing
Mar. 26th, 2020 05:38 pmOnce again, I was struggling with a kiln that was heating up on top faster than the bottom. I was being proactive, starting my adjustments earlier in the firing to keep on top of the difference. Conventional wisdom is to close the damper on this kiln, presumably to make the exhaust gases take longer in exciting, dumping more heat.
Our damper has three settings marked in sharpie on the kiln shelf: 3 (with open and closed hashes about half an inch apart) the widest open setting; 2, the almost-closed setting for body reduction; and 1, about half-way between the two. No, it doesn't make sense from a linear perspective, but think of it temporally. The first setting, 1, is where the damper stays overnight, through warm-up, and into the initial part of the firing. When cone 08 or 06 drops, the damper goes to 2 for body reduction. After a half hour, you bring it out to 3 for the rest of the firing, tweaking between the open or closed mark depending on whether the kiln's hotter on top or bottom. Always adjusting for kiln atmosphere, flame flickering in the chimney.
At least, that's the theory. As I mentioned, I'd been struggling with evening the temperature, pushing in the damper half an inch, then another. At one point, around 3:30 pm, I had cone 9 completely down on top, cone 8 only halfway on the bottom, and was getting desperate. I didn't want to over-reduce my pots--dark brown is as unattractive as ghost-white as a background for paintings--so I was also carefully watching the gas pressure.
I pushed the damper all the way back to 1--the overnight setting--brought the gas back from 1.8 to 1.6". Again, the same setting I use for overnight pre-heat. And I had a tiny brainstorm.
I'd done something similar last firing, managed to arm-wrestle the cones into submission, but wound up with a lot of oxidation afterward. Maybe, I thought, I should be adjusting the primary air as well, by turning the air shutters on the burners. Wind them back to the overnight (2 turns) setting as well.
So I tried it. And dropped cones 8 and 9 on the bottom in 30 minutes flat. Thirty minutes later, both cone 10s started bending, and continued to move together till they touched down, millimeters apart (still a tiny bit cool on the bottom) at 5:45 pm. 45 minutes and 2 units of gas sooner than usual.
What happened? I think it had to do with flame length.
If you've ever adjusted a bunsen burner (or an oxyacetylene torch), you know that you get a big, mushy flame when you first light it. Then, as you add the air (or oxygen), the flame gets shorter, tighter, hotter. The hottest part of the flame is just below the tip.
In a kiln, changing the damper doesn't affect the length of the burner flame; it just pulls it around in the kiln. Reducing the primary air, though, does. The flame gets longer, and in a down-draft kiln, doubles back on itself, dumping more heat (from the tip, remember) in the bottom of the kiln.
When Steve taught me to fire this kiln, twenty years ago, I learned to control atmosphere and heat from the damper. Primary air was something you set and left alone. I should have known better--when I fired updraft kilns at Viterbo, we had shutters on the blower fans to control air. At Buck's Rock, I could tweak the air shutters and damper on that updraft kiln to put the heat (and reduction) anywhere I wanted. I don't know why it took me so long to realize I was ignoring a potential tool for firing here.
I feel a little like a photographer who's been controlling exposures by changing the shutter speed, accepting the occasional wobbly, blurry shot, never thinking that changing the F-stop could accomplish the same thing.
Our damper has three settings marked in sharpie on the kiln shelf: 3 (with open and closed hashes about half an inch apart) the widest open setting; 2, the almost-closed setting for body reduction; and 1, about half-way between the two. No, it doesn't make sense from a linear perspective, but think of it temporally. The first setting, 1, is where the damper stays overnight, through warm-up, and into the initial part of the firing. When cone 08 or 06 drops, the damper goes to 2 for body reduction. After a half hour, you bring it out to 3 for the rest of the firing, tweaking between the open or closed mark depending on whether the kiln's hotter on top or bottom. Always adjusting for kiln atmosphere, flame flickering in the chimney.
At least, that's the theory. As I mentioned, I'd been struggling with evening the temperature, pushing in the damper half an inch, then another. At one point, around 3:30 pm, I had cone 9 completely down on top, cone 8 only halfway on the bottom, and was getting desperate. I didn't want to over-reduce my pots--dark brown is as unattractive as ghost-white as a background for paintings--so I was also carefully watching the gas pressure.
I pushed the damper all the way back to 1--the overnight setting--brought the gas back from 1.8 to 1.6". Again, the same setting I use for overnight pre-heat. And I had a tiny brainstorm.
I'd done something similar last firing, managed to arm-wrestle the cones into submission, but wound up with a lot of oxidation afterward. Maybe, I thought, I should be adjusting the primary air as well, by turning the air shutters on the burners. Wind them back to the overnight (2 turns) setting as well.
So I tried it. And dropped cones 8 and 9 on the bottom in 30 minutes flat. Thirty minutes later, both cone 10s started bending, and continued to move together till they touched down, millimeters apart (still a tiny bit cool on the bottom) at 5:45 pm. 45 minutes and 2 units of gas sooner than usual.
What happened? I think it had to do with flame length.
If you've ever adjusted a bunsen burner (or an oxyacetylene torch), you know that you get a big, mushy flame when you first light it. Then, as you add the air (or oxygen), the flame gets shorter, tighter, hotter. The hottest part of the flame is just below the tip.
In a kiln, changing the damper doesn't affect the length of the burner flame; it just pulls it around in the kiln. Reducing the primary air, though, does. The flame gets longer, and in a down-draft kiln, doubles back on itself, dumping more heat (from the tip, remember) in the bottom of the kiln.
When Steve taught me to fire this kiln, twenty years ago, I learned to control atmosphere and heat from the damper. Primary air was something you set and left alone. I should have known better--when I fired updraft kilns at Viterbo, we had shutters on the blower fans to control air. At Buck's Rock, I could tweak the air shutters and damper on that updraft kiln to put the heat (and reduction) anywhere I wanted. I don't know why it took me so long to realize I was ignoring a potential tool for firing here.
I feel a little like a photographer who's been controlling exposures by changing the shutter speed, accepting the occasional wobbly, blurry shot, never thinking that changing the F-stop could accomplish the same thing.