Soap = fatty acid + lye. How can anyone not know this? Run water through wood ash, boil a bit to concentrate, then mix with any accessible oil. Use eye protection at all times for those trying his at home.
I don't think it would ever have occurred to me to try to turn ash into something I could clean myself with (given that jumping into coal is a quick way to get dirty). Should I find myself in the middle ages, I hope to remember your comment...
