Things You'll Need
1 cup ammonia
Blue stains can begin to develop in your bathtub's drain if you have copper or brass pipes or plumbing fittings. This is because the brass or copper oxidizes and small amounts of it are left on the bathtub's surface. Luckily, you can often remove this stain the same way you would remove any other mineral deposit on your bathtub. Clean the blue stain as soon as you notice it to prevent the blue color from soaking into the tub where it may be harder to remove.
Mix 1 cup ammonia with 1 cup water.
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Dip a cloth into the ammonia mixture and place the cloth over the blue stain. Let the cloth sit on the stain for at least 20 minutes. This will give the ammonia time to loosen the stain.
Remove the cloth.
Dip a toothbrush into the ammonia mixture and brush away the rest of the blue stain.
Rinse the area with clear water.