Those clay baths are refreshing and good for your skin, but the black residue left in the tub is not from mercury. My brother mixed some clay and water in the tub, did not get in it, let it sit for 20 minutes and then drained the tub. Guess what? There was a black residue left in the tub! I am really annoyed with that website and there advertisements. That website really encourages you to believe that the black in the tub is from metals from your body. What a rip-off!