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Author: Subject: Simple and inexpensible DIY mini-charcoal "furnace"
12AX7
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[*] posted on 26-10-2007 at 18:16


Calcium silicate vis. portland cement is a really crappy refractory. It dehydrates on heating (it sets due to a hydration reaction) and turns to powder at high temperatures, before sintering into a glassy, sagging mess at higher temperatures. You want to use lots of clay, and lots more of something as refractory but without all the shrinkage. A rudimentary mixture is 1 part clay, 2 parts sand. It really should be fired into bricks (up at cone 10-15 or hotter if possible), but for backyard purposes, it suffices to pack it into a steel drum and abuse it from there.

Perlite and vermiculite are cheap insulating fillers, but they aren't very refractory.

Antwain: the less sodium, calcium, iron, etc. in the refractory, the longer it will last. You don't want glaze, you want dry crusty ceramic walls. They'll last the longest and stay the cleanest.

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Antwain
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[*] posted on 26-10-2007 at 18:55


Ok. Well my friends sister is a potter (if that is what people who make stuff with clay are called) so I could probably commission her to make me something. The place she goes to also has a proper kiln too, so that seems like a good idea.
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12AX7
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[*] posted on 26-10-2007 at 20:29


Ah, excellent.

Make sure to use a custom mix -- your average pug of stoneware, porcelain, etc. contains feldspar and other fluxes, which cause a vitreous phase on firing. It'll work anyway, but it won't work as hot.

Tim




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ciscosdad
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[*] posted on 31-10-2007 at 16:47
Plaster


Yes plaster is good until it reaches the 1100 deg C decomposition mark, but needs to be dried out beforehand at much lower temps (100 to 200 c or wait around in warm weather for a few weeks). I don't think you can use it as a crucible. Furnace/forge walls only.
It is not too strong at the high temps but I have had absolutely no sign of problems with the floor of my forge, and that has had a fair few hours work since I built it.
Bear in mind it is always protected with a couple of centimeters of ash, and no direct air blast. I have not seen any separation of the plaster from the steel of the base, although I'm sure there will be cracks.

Just allow for plaster's limitations. Perhaps some high temp insulation (expensive) to line the inside and use the plaster for outer layers where the temp gradient has got things down to 1000 or less.
For anything around 700 to 900 (brass, Al, zinc, Mg and a great raft of other metals work in this range. You can also distil Oleum/SO3 from FeSO4 at 700 or so. Looks like a good application right there.
12AX7 has a lot of experience in this metal casting area. His advice is worth reading.

If you want extreme temps you will have to take extreme measures and pay for the necessaries or get innovative. Less than 1000 deg C is easily achieved, and this range does a lot of what we backyarders want.
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