Almost completely finished a large reno project. Plumber replaced two old clunky radiators with new baseboard radiators. First he installed reversed - with air valve on bottom, water intake on top and of course, no heat. He reversed it several days later, bled all the radiators, then disposed of the black sludge from the radiator runoff into the new tub. The tub now has many chips in the enamel. The contractor and plumber claim this could not have been from putting the runoff into the tub. I do not know where else the chips could have come from? Can anyone help? Does the black radiator runoff have sufficiently large particles in it that, when tossed carelessly into the tub, could have chipped the tub surface? Any advice you have is welcomed.