What causes Black Spots?
The dark discoloration often seen along the edges or bottom of a mirror is known as mirror rot, black back or desilvering. This damage is caused by the oxidation of the mirror’s reflective coating—typically silver or aluminum—on the back of the glass. Oxidation commonly occurs when moisture or harsh chemicals, such as ammonia or citric acid (found in many cleaning products), come into contact with the mirror’s backing.
Proper Mirror Installation
To prevent mirror rot from the start, it’s important to install mirrors correctly. When setting a mirror in place, always use small neoprene or polycarbonate setting blocks—typically about 1/8” thick. These blocks provide a crucial air gap beneath the mirror, allowing moisture and cleaning residues to evaporate rather than pool behind the glass.
Although setting blocks are usually clear, they can be visually concealed using a j-channel. J-channels come in various anodized finishes, offering a neat, stylish trim that complements the mirror while hiding the blocks from view.
Cleaning Existing Mirrors
If your mirror is already installed without setting blocks, don’t worry—you can still take steps to preserve its appearance and longevity by using the right mirror maintenance products and techniques.
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Use ammonia-free foaming glass cleaner. I recommend Sprayway Glass Cleaner. Its foaming formula reduces the risk of runoff that can damage the backing.
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Where to buy: Sprayway is available at Fast Glass Mirrors & More (770-428-0819) and many Ace Hardware locations.
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If using ammonia-based cleaners: Always spray the cleaner onto a cloth, never directly onto the mirror, to prevent liquid from seeping behind the glass.
Quick Tip for Tough Spots
For stubborn marks on the mirror surface, try using 0000-grade (ultra-fine) steel wool. Be sure the steel wool is:
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New (never previously used)
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Free of rust or residue
This ultra-fine grade is safe for mirror maintenance and won’t scratch the glass when used gently.
Looking for a New Mirror?
If you’re considering replacing an old mirror or starting fresh with a custom design, we’ve got you covered. Visit our Custom Mirrors page to explore options tailored to your space, style, and needs.