Turkey roasters like the one pictured work great. I pour in about 1" of playground sand and arrange the spalls to heat treat, filling with sand as I get about half way and continuing until I fill the whole thing and dome as much as will fit with the lid on top. One thing to remember is to initially heat it to to only about 150-175 degrees for 24 hours. You might get by with less pre-heating, but remember water boils at 212 degrees and if there is any moisture in your stone, guess what happens when it turns to steam inside your rock? You can test with a probe thermometer but check all over.
Another thing to remember about roasters is that the sides and bottom are much hotter than the middle because that's where the elements are located. Just stick a thermometer in different spots once you get it heating and you'll see. I have hit almost 700 degrees on the sides of mine, but it's one of those antique ones that gets hotter. So I usually only turn the knob on mine to about 400 for most cherts that need 450 to 500.
It's all a big experiment until you know your rock and roaster and their quirks. I don't bother to ramp mine down with sand in it because it will hold the temperature and slowly drop over about 24 hours, which is a plenty slow downward ramp. When I am heating I usually ramp up more carefully 200 for 4-6 hrs, then up to 300, wait an hour then up to 400 (and the same rate for higher temps). I hold the temp 24 to 48 hours leaning towards the 48 side and depending on the rock. When done, turn it off and let it set until it is cool through and through, certainly no hotter than 100 or so at the most if you are in a hurry.
Wal mart sells these around most holidays for about $25. Use the heck out of it and if it burns up - take it back! Sometimes you can find the old ones at the flea markets, but believe me, 400 gets hotter than 400.
