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There are two types of kilns, brick and glaze. You can build a brick type kiln, that will fire clay to about the same temperature as bricks or flower pots, quite cheaply. Those you can heat with simple BBQ burners. If you insulate it well with fire-brick, it will work dependably for many years.
For a glaze kiln, you need either electrical heaters, oil burner, or gas torch heating, and you need proper refractory bricks. Anything else will either not get hot enough, or it will self-destruct.
Ideally you have both types. You need a brick kiln to slowly and gently fire the dried clay, before you paint any glaze onto it. The fuel costs are way less, and you are less likely to cause cracking.
The glaze kiln needs to get hot enough to melt glass. You can build a frame out of steel and stack white or beige refractory bricks for the insulation. Temperature control is a bit tricky with glaze kilns, because the slightest draft or wind will drastically offset any temperature indicator, that is safely on the outside. One way to get true inside temperatures is with melting cones holding thin cast iron rods apart, and checking electrical connectivity on the outside. They will be near white hot and the upper one will sag, when the supporting cone melts. It will then touch the lower rod and complete the circuit. You use that method to calibrate and write inside numbers onto an oven thermometer attached to the outside.
Brick kilns are easy, but it might be a good idea to buy your first glaze kiln and gain experience with it.
By DearWebby from Black Diamond, Alberta