Otoe, Roaster Ovens work just fine for most of the rock. The exception is for the rocks that require high heat like Novaculite. There are some drawbacks that you need to be aware of. You hit it right when you were talking about temperatures. Talk about that in a second. First what you need to do is find the right roaster oven. Most of the ovens you will find on todays market have heat settings that go up to 450 Deg. I was lucky to find one in like new never used condition that cranks up to 500 Deg. Warning.
Don't ever crank one of these things up on your kitchen counter or mom
will be getting a new counter in short time. What you need to do is without the insert fill the oven up with clean sand and crank up the heat to about 150 overnight to make sure the sand is dry, mix it up a little to make sure. At a temperature of 150 make yourself a graph of at least six to eight quarters of the oven. With a good thermomiter check the heat range of each grid and mark it down. I did this in 25 Deg. steps all the way up to 500. You will have drastic heat differences at each setting so be carefull. Once you know how your oven works you will be able to follow the heating recipies of each rock in the area that will give you the most consistant heat. Sounds like alot of work but after the initial work it works quite well and simple. I find this best works with rock such as Texas flit that sometimes works a lot better with a mild heat and great with your aggets and coral. I also heat burlington and pioria with great results. This is the poor mans kiln but it works fine. Good luck. For more info check out (Roasting Rocks, The Art and Science of Heat Treating, with D.C. Waldorf. I found this helpfull. The DVD comes with a Rock Recipe Book. Good luck Sleddman.
