Controlling the heat w/ a hot plate can be difficult. It is also not safe to put a ceramic bowl directly on a hot plate--even a Pyrex one. It's just like putting a bowl on a burner. Every spa I've ever worked in uses a turkey roaster. It's more shallow than a crockpot, you can get at the stones more easily (and fit more stones in a turkey roaster) and the inner lining comes out easily so that you can wash and disinfect it more easily than you can a crock pot. At the spa I currently work in, we use a crock pot to heat our towels for facials, etc. Both work great in their respective jobs. If for some reason, you find you need to go the hot plate route, heat water in a metal pan, and either put your rocks into that or put the bowl on top of the heated water, remembering that if you should drop a stone, especially a big one, the bowl will more than likely break. The turkey roaster truly is the best option. It's also very helpful to have rubber gloves, tongs, and a stack of towels right by your work station. The towels serve to blot the rocks so your client will not get burned by hot water dripping off the rocks. They also serve to keep the rocks warm after you'v'e taken them out of the roaster. 140 degrees F is the MAX your rocks should be heated to. Keep a meat thermometer right by your turkey baster, as the temperature gauge is not always calibrated correctly.