The best method for cleaning mirrors is much the same as that for window glass, but with one major difference. It’s best to keep as much liquid as possible away from the edges, where it can drip behind the surface and damage or separate the reflective surface from the back of the mirror. Excess liquid can wick up between the glass and the reflective backing, which causes oxidation. That’s how mirrors get those black edges, particularly along the bottom.

 

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Use a fast-drying glass cleaner sprayed onto a lint-free cloth, NOT the mirror, and then wipe it clean. Dry the edges thoroughly with a clean, lint-free cloth.

 

Remove stuck-on dirt or adhesive with a sharp, single-edged razor blade on moistened glass instead of a duller tool such as a putty knife, which can scratch the glass. Scrape in one direction only to avoid trapping abrasive grit between the blade and the glass.

Here’s an inexpensive alternative to commercial glass cleaners: Fill a spray bottle with equal parts water and “wash,” the blue, methyl-alcohol-based windshield washing fluid for automobiles. It cleans well, dries quickly, and rarely leaves streaks. Be sure to label the bottle contents clearly and store safely away from small children. The methyl alcohol found in wash doesn’t have especially dangerous fumes but is toxic when ingested.

Reference:

Don Aslett in The Cleaning Encyclopedia: Your A to Z Illustrated Guide to Cleaning Like the Pros.