I unloaded the kiln today. Near perfect firing. I fired on Saturday when the wind finally calmed down. It was very bad firing from the beginning, similar to the last firing. I decided to move around the oxyprobe to see what going on.
I could not get good reduction reading with the oxyprobe next chimney while it read heavy reduction at spy holes. I could not reduce the chimney side of the kiln by changing damper or blower setting at all. I re-read Nils Lou’s Art of firing while the kiln was acting strange. The book explain the importance of the tuberance. Then I thought, blower does not do anything. Then what…. Gas pressure!
I was hesitance to raise the gas pressure, first 3.5 PSI, which I have never done before, it did not change the reading that much, but when raise to 4 PSI, it changed the atmosphere of the kiln dramatically. I started seeing the reading evened out at three locations at 600-620 as I opened the damper accordingly. I have never had this much gas pressure available before the move (I knew this time have up to 7PSI), but never think to use that much since the kiln was firing fine with below 3.3 PSI for 15 years. I was the same building across the hall from the current studio 15 years ago for two years.
All the things I have tried last three years, but the gas pressure. I actually lowered the pressure I was using because the glaze defect I thought was due to the rapid raise or to hot as it happened where the flame hit directly. Now I think, I got the heavy reduction near the door, and which was causing the glaze to react strangely, not heat. I do not have enough pull at the chimney, so it reduced heavily before the exit flue. I needed the push by the gas pressure. I thought opposite, I had too much updraft (14′ of chimney) and getting the oxidation. I was wrong. I am looking forward the next firing. I think I have solve the problem.