David I spall the rock to small usable stone and lay it in the tray. Stuff like that agate from Washington and Brazilean agate, Mookite,coral, chert. I turn it up to 500dgs and let it go all nite. When I go to work in the morning I unplug it and it is usually cool and nice and waxy when I get home in the PM. The last oven, I paid $5 for at the Flea market.