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Originally Posted by rockm
[...] I thought the details were worth explaining, not only to show such home-made kilns are meant for a purpose other than what Zen was asking about, but also to emphasize the possible dangers of building your own.
In light of Zen's idea of using coal, metal pipe, duct tape and a hair dryer to build a furnace and bellows, I thought the warning was warranted.
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Here is a nice webpage on simple kilns. A firepit or simple kiln cannot reach maturation temperature for stoneware clay, if indeed stoneware type clay is what the stream holds. Stoneware matures at well over 2,000 degrees F. Just heat leakage from a 2,000 F firepit would melt the duct tape and blow drier if it was near enough to be effective as a bellows, I would think. The effect of a blowdrier on coals would require the bellows to be very close to the coals. Try hooking a blow drier up to a vacuum cleaner hose and see how well it blows through five feet of hose... not well. Then pity the poor children who used to work the bellows which upped the temps in the old beehive kilns during the Industrial Revolution. They had to be very close to the kiln to be effective, it must have been a terrible, dirty, and fatal job.
My kiln doesn't get that hot (I fire to the 1900 F range) but it is glowing orange when at its peak, which takes 6 hours for the electric kiln to reach... I can't imagine how long it would take to get a coal fire to reach such temps. Remember, the coal's heat would have to be far in excess of the temp range you would want inside the kiln chamber, as the chamber and ware always lags behind the temperature of the actual heat source. The loss of heat drops rapidly in proportion to the distance from the heat source.
Interesting, as always. Have fun at the clay place!
Joanie