This comes up from time to time and the reason why I would strongly advise against it is Pyrolysis, where the molecules are changed by burning in a sense. Because of this action on the chemical and its possible adulterants, there is no way to know exactly what is in your sample. There could be tars that will damage your veins and generally speaking the presence of burnt organics creates a change in pH, adding ash to water creates an acidic environment.
Because of all of these I would strongly advise against it and say there is no way to clean it because you dont know what you even have, from a chemical stand point.