So I put some rocks in my fireplace tonight for about 10 minutes. I dropped them into a big pot of water. The water went from cold, to very near boiling. I wouldn't trust it to be hot enough to kill anything, plus the water was full of soot afterward. I was thinking that if I ever had to use this method, I wouldn't have a metal pan at all, and would be using out a hollowed log, which may hold heat better than metal. Also, I doubt I'd have as much water as I was trying to boil tonight.

Any tips for doing this? I know not to try and heat wet rocks, as they can shatter/explode. I'm curious about how to keep the ashes out of the water, or if you are supposed to pour the water through a t-shirt after boiling or something. Also, is 10-15 minutes too short of time to heat the rocks?