its abrasive so rubbing it off might not help much
it can also be pretty corrosive when combined with water depending on the makeup of the ash.
i would suggest using something like a leaf blower or compressed air hose to get a lot off before you try washing.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent