Wood burns in two distinctly different stages. The first, and most obvious, is the burning of gasses produced when wood is heated. Wood begins to gasify at about 500°F. The second is the burning of the charcoal. This happens, for the most part, after the materials that form the gasses have been driven out of the wood. The coals in your ash pit serve to provide some heat to the kiln and to gasify the freshly stoked wood, mostly through radiant heat energy. As the gasses burn in a wood kiln, they typically produce very long flames. These flames can be easily over 30 feet long. Charcoal produces very hot, but very short, flames. The flame from charcoal is normally less than a few inches long.
All of these issues are relevant to building and firing any wood-burning kiln.One of the demonstrations I take my students through when we begin talking about kiln design is to bring an oxygen-acetylene torch into the classroom. If the torch is ignited with only acetylene (fuel), it produces a very long, very dirty flame.
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One can quickly pass his or her hand through this flame without any real danger, but it will be covered with black soot. As oxygen is added, the flame shortens and becomes significantly hotter. As the flame shortens with the changing oxygen-fuel ratio, smaller flame tips appear in the center of the flame. This is the place where the flame is the hottest.
The more defined the tips are, the hotter the flame. You want this part of the flame in the firing chamber of a kiln, not in the firebox or the flue. If you have a small kiln and a fuel that develops a long flame, you need to either redesign your kiln to use the length of the flame, or simply shorten the flame to bring the hottest part of the fire back into the chamber where the pots are stacked.
As with the acetylene example, the easiest way to shorten the flame and make it hotter is to add oxygen. If you have electricity at the kiln site, adding a blower is one of the easiest and most controllable ways of adding oxygen. A small squirrel-cage fan that will deliver about 100 cubic feet of air per minute will supply all the air you will need to fire a small kiln. You can fabricate a bolt-on connector to attach the pipe to the blower, or duct tape a piece of automotive tail pipe to the blower.
You should realize that the end of the metal pipe will be subjected to a great deal of heat and will have to be replaced after a number of firings. Place the pipe in the ash pit of your firebox and adjust the air-input damper to the desired air flow. You will finnd that the flame around the blow pipe will be very intense. This system will allow you to fire your kiln with a much smaller firebox than would normally be needed in a natural-draft kiln.
The smaller firebox will require more frequent stoking, simply because it will not hold as much fuel as a larger box. Increasing the flue height would be the last choice in a small kiln. If you do this, you must be certain that you have air intake ports and a flue cross section large enough to allow easy circulation of hot gasses.
A damper will be essential for control. This will be less responsive than a forced-air system and will vary more due to atmospheric conditions, because it depends on lowered pressure to bring air into the kiln. So, more air shortens the flame and more air increases the temperature of the coal bed to help gasify your fuel more quickly.
**First published in 2008