Why is there a black powder in my tub?

After every shower or bath there is a very fine black substance, almost like a powder in the bottom of my bathtub.


Any ideas why?Why is there a black powder in my tub?
Perhaps it is the onset of a mold problem.


Black mold will seem like dust.


Make sure your bathroom has adequate ventilation.


Could also be a high mineral content in your water.


Or possibly old galvanized piping.


Or lastly, an old water heater.


Squeegee the tub dry after you use it and spray some bleach-water on it to see if that helps..