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My friend Kim Blatt showed me this trick and my friend Tom Chandanais got me the old refrigerator.
Drill 1/2" holes 6 inches on center in the top and bottom (and between freezer/fridge section)
Add a 40W light bulb and you should be able to maintain the temperature around 100 deg F. Mine generally stays between 105-108F in the summer and 90-100F in fall/winter with a 40 or 60 watt bulb (it's in my garage). In the picture above you'll also see a Vornado fan I use ($14 on Amazon), digital thermometer (not visible) and light bulb socket mounted on the interior wall . . .
After rough turning, I seal the piece with green wood sealer, then let it dry in the kiln for at least a month (depending on how green it was) . . . most Aussie Goldfields burls (the majority of what I import) are pretty low on the moisture content scale already because of the region they grow in (120-130F is common in the Aussie summer), so the real intent of using the homemade kiln with most of these burls is to relieve the internal stresses in the rough turned burl (and prevent future movement).