Black edges appear when the reflective metal behind a mirror begins to corrode. The damage often starts around the perimeter, where moisture and cleaning products can reach the backing more easily, and gradually becomes visible through the front of the glass.
This condition is sometimes called mirror desilvering. It is damage behind the glass, not dirt sitting on the surface.
How Do Black Edges Form?
A standard mirror has a reflective metallic layer applied to the back of the glass. Protective coatings are added over that layer, but the perimeter and any damaged areas remain more exposed.
When moisture, oxygen, or unsuitable chemicals reach the metal, they can break down the reflective coating. The affected area stops reflecting normally and appears black, brown, gray, or cloudy from the front.
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