I've been struggling for a while with the firing atmosphere in my glaze kiln. My glazes need to fire in a reducing atmosphere for best results: creamy color, iron speckles, best oxide and stain color reaction.
The problem was getting the atmosphere consistent. There was always an oxidizing spot, usually low and toward the front left, near the door jamb. I assumed an air leak through the door, around the frame, or even through gaps in the floor, but no amount of tightening brick or packing/wadding with ceramic fiber made any difference.
Then one day a couple of months back, while unblocking the burner ports prior to unloading a particularly unsuccessful firing, it occurred to me that the gas pipe along the wall supporting the burners was not actually parallel to the wall of the kiln. The tip of the front burner was actually a good two inches further from the burner port than the back one. Light bulb time!
As I've explained before, you get a reducing atmosphere in the kiln by controlling the ratio of air to fuel in the kiln. Too much air, oxidizing; too little air, reducing. But there's two different sources of air in the kiln, and two ways to control it.Primary
air is pulled through the burners themselves, by the low pressure caused by fast-moving gas coming out of the aperture into the mixer. (This is the venturi
effect, so we call them venturi burners.) A disk threaded onto the back of the burner limits the amount of air drawn into the kiln. Twisting the shutter open allows more air, closed, less.Secondary
air comes in through the burner ports, mostly, but also through the peep holes, cracks in the door, gaps in the floor, what have you. The draft from hot gases exiting up the chimney pulls secondary air into the kiln, where it combines with unburned fuel from the burners. The amount of secondary air is controlled by the damper, a kiln shelf that slides in and out of a slot in the chimney. Close the damper, the kiln reduces. Open it up and the atmosphere becomes neutral or even oxidizing, as the excess fuel is burned more thoroughly.
The problem is, while it's possible to control primary air at the individual burners, secondary air has only one control for the whole kiln. Normally, this isn't a problem, as the gaps and openings are evenly distributed around the kiln, but here I had a burner with a lot more space around it for secondary air. And
it happened to be in the corner of the kiln where oxidation was a persistent problem.
But how to fix it? The gas pipe is old, joints solid with rust at this point, so moving one end closer to the kiln would be a major project. I tried just wedging a piece of 2-by-4 between the burner and wall to push it closer, but it didn't move appreciably. I could try to compensate by cutting back the primary air, or even by running extra gas through that burner, but how much
is difficult to estimate.
So the only thing left is to move the kiln wall closer.
No, not really. What I did is take some thin sections of soft brick, each about an inch thick, and build a collar around the burner port, closing the gap between burner tip and brick.
My first firing after the changes was astonishing. The lower corner was reduced perfectly. There were a few random oxidation spots, both about chest high. One was in the middle of the back wall, the other at the left door jamb, but they were very small, affecting about half a pot each.
The second firing, which I unloaded last night, was perfect
. No oxidation anywhere in the kiln, and no over-reduction (which causes excessively brown pots, and sometimes bubbling in the black and green stains). And the cones? Even, top and bottom, to within millimeters
I've shown the fix to Jon and Beth, two of the other potters who fire this kiln, and explained my repair. I'm not sure how long it will be before I load high-value items (teapots, special orders) into what used to be the death zone. Maybe one more firing? I need to convince myself that this thing is in fact repeatable.