Here is a thing I did not know: increasing primary air (the air that enters the kiln through the aperture at the bottom of the burner) relative to secondary air (air that enters the kiln through all the other avenues, such as the burner ports themselves) shortens flame length.
At the risk of boring the pants right off my non-potter readers, I found this great article about combustion and flame dynamics by Louis Katz at Texas A&M University-Corpus Christi. Read the article if you are interested -- I highly recommend it to anyone thinking of building a gas kiln. I found a couple of take-aways relevant to my particular situation:
- Katz mentions raising the bag wall as a solution to a cool top. I had arrived at that idea more or less intuitively, and it seems to have borne out if the recent bisque firing is an indication; but it's nice to have the validation.
- He also suggests pulling a brick from just under the arch, to create a sort of second flue, allowing the kiln to behave somewhat as an updraft, pulling the flame higher in the chamber. I never thought of that! Next time I fire, I will have to brick up the door in such a way as to make this possible.
In other news, I purchased a new triple-beam scale from Ebay to replace my old one, which after 15 years of service flung itself from a tabletop, breaking an irreparable part. Got the new one for about 60% of retail, including the shipping. I considered a digital version, but, like the power windows in my car, it seemed like just one more thing that could go wrong and necessitate replacement. Give me a good old mechanical device any day.