Remove the sculpting clay from its container and flatten it into a large circle. Press the circle flat against the surface of the fire box to lift away loose soot. Repeat this step until most of the soot has been removed.
Brush away remaining soot using a hand brush. Sweep the soot up into a dust pan and dispose of it.
Fill a bowl with full strength white vinegar. Saturate the bristles of your scrub brush with the vinegar, then begin to scrub away any remaining soot stains using a rapid circular motion. Scrub the entire internal surface of the fire box until all of the soot has been removed.
Rinse the surfaces of the fire box with fresh water, then pat dry with a towel. Clean the inside of your fire box monthly to avoid thick soot buildup.