If you do find something that works, let us know. I have a lovely firepit with a boulder surround. Following some over-exhuberant fires by the younger crowd (bigger is always better I guess...) I'm not too keen on the resultant scorch that runs all the way up the insides and even across the tops of many of the rocks. I expected the bottom inside to blacken, but would like to clean off the tops if possible.
On the other hand, we regularly incorporate burnt logs into our landscape, and mosses and lichens colonize it quickly. This one provides winter interest, and then gets almost totally covered during the growing season with perennials. It also shows more burnt surface during the summer, as some of the mosses dry up in the sun.
