by Marc Ward
Clay Times Sept. 1996
I often have folks call me to ask questions about their kiln and why it stalls. After I ask about the size of the kiln, I'll ask how many BTU's their burners are rated for.....silence....then, "Oh, they were some old burners someone gave me". I'll ask the same question again.....silence.....then, "well, I'm not really sure." Or another one along the same line is, "These are the burners I had on my old kiln and when I built a bigger kiln, I just used the same ones." Knowing how many BTU's you need to put into the kiln is as important as knowing how much zinc oxide to put into a glaze. Unless you're just goofing around and not trying to get repeatable glaze effects, you would never just throw a handful of this and a handful of that into a glaze. You weigh it out, often to the tenth of a gram. Why are so many potters meticulous about their clay and glazes and so lax about firing knowledge?
Different types of kilns need different BTU amounts. There are three factors that determine BTU input for the normal pottery kiln; kiln construction material, desired final temperature, and firing time. Some of the BTU input figures that follow, have a range of several thousand BTU's. The higher figure will give a fast firing of 6-8 hours, while the lower figure will give a longer firing of 14 hours plus.
All of these figures are BTU's per cubic foot(BTU/CF). Since you're firing the whole kiln and not part of it, this is the total interior volume of the kiln and not just the stacking space. (Next issue I'll give you the formulas for determining the cubic feet of all types of kilns). Multiply the numbers below by your total cubic feet. This will tell you how many BTU's you'll need for your kiln to get to a particular temperature. Divide your total by the number of burners and you'll know how many BTU's you'll need per burner.
If you have a hard brick kiln with 9" walls, you'll need: 12,000 - 17,000 BTU/CF for cone 06, 14,000 - 18,500 BTU/CF for cone 6, and 16,000 - 20,000 BTU/CF for cone 10.
If you have a soft brick kiln with 9" walls, you'll need: 6,000 - 10,000 BTU/CF for cone 06, 8,000 - 13,000 BTU/CF for cone 6, and 10,000 - 16,000 BTU/CF for cone 10.
If your kiln is 6" of ceramic fiber, you'll need: 4,000 - 6,000 BTU/CF for cone 06, 6,000 - 9,000 for cone 6, and 7,000 - 11,000 BTU/CF for cone 10.
Raku kilns are built for speed, so the above numbers for cone 06 firing may not be acceptable (most people don't want to wait hours and hours for the first load to be ready). The numbers below will give you a firing of 20-40 minutes. Remember, these numbers are BTU's per cubic foot. Hard brick Raku kilns; 70,000 BTU/CF. Soft brick Raku kilns; 35,000 BTU/CF. Ceramic fiber Raku kilns; 25,000 BTU/CF.
As all these numbers indicate, there are vast differences in BTU input. This is why it's so important to have an idea of how many BTU's you're putting in the kiln. If you're unsure, the only way to determine it is to call the burner manufacturer. If that's not possible, find out the orifice size using a drill bit as a gauge. When you've determined that, find out the regulated or delivered pressure of the gas and someone familiar with burners can tell you how many BTU's your burner is producing.