1 bucket warm water 1/2 cup washing soda (look in the laundry aisle of grocery store)
Wash surface, wearing gloves; then rinse.
For really tough jobs, make a thick washing soda paste by mixing it with water, and then spread the paste on the soot. Keep the paste damp by spraying every hour or so with a spray bottle. Leave the paste on overnight, and then rinse. Make certain to only use this formula on inert surfaces such as glass and stone, as this amount of washing soda can peel off paint, wax, etc.