Need help getting rid of dark areas on shower wall tile
The bottom few rows of wall tile in our shower have these dark areas, which I assumed was mildew, but perhaps not. The house was built in 1931, and I believe this is ceramic tile. I've attached two pics, one farther away and one closeup.
The dark areas were there many years before I came along. The dark areas are not cracked, per se, but it's almost as if just the outer surface of the tile is "crackling," and the lines where it is crackling are dark colored. Is that what is referred to as crazing? (And yes, there is also an actual large crack, but I'm not referring to that).
I've tried spraying on Tilex Mildew Root Penetrator and Remover (contains bleach), letting it sit, and manually scrubbing it, and also using an electric rotating brush. But it doesn't make any difference.
Is there anything that can be done to remove the dark areas other than replacing the tile? If there is some sort of chemical that would work, I want to be sure it won't damage the tile (i.e. make the outer surface crack off, or something). Thanks.