Possible Causes:
1. Desilvering (Mirror Rot)
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This happens when moisture gets behind the mirror, damaging the reflective silver backing.
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It usually shows up as cloudy or shadowy patches that don’t wipe away.
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Common in bathrooms due to humidity and moisture creeping in from the edges.
2. Etching from Harsh Chemicals
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Even though it’s not streaky, repeated use of strong cleaners like ammonia or bleach can damage the mirror coating, leaving a dull or faded patch that looks like a smudge but doesn’t come off.
3. Manufacturing Flaw or Glass Imperfection
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Sometimes mirrors can have built-in flaws, where the silver layer wasn’t properly applied or cured.
How to Confirm:
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Light test: Shine a flashlight from an angle — if the mark is still visible and looks like it’s inside or under the glass surface, it’s most likely desilvering.
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Touch test: Run your finger gently across it — if it’s completely smooth, it’s not residue or dirt.
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Water test: Lightly mist water over the area — if the mark becomes more visible or disappears briefly, it’s likely on the underside.
What You Can Do:
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Unfortunately, if it’s desilvering or etching, there’s no way to fix it — you’ll need to replace the mirror or frame it to hide the edges if the damage is there.
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If you’re unsure, you could also try polishing it gently with a glass polish, but if it doesn’t change, the issue is internal.