Perhaps you're just trying to remove the price tag which has been carelessly affixed to the front of your new mirror, or maybe you're cleaning up after a repair. Either way, if you have glue on your mirror, you want it gone without a trace—no scratches or residue. Depending on the size of the glue deposit and the type of glue, you can usually achieve your goal with a razor blade. If not, you may need a solvent or a lubricant. Soapy water often gets the job done, but some problem adhesives, such as super glue and epoxy, call for stronger solvents. You probably have what you need in your paint cabinet.
Start with a Razor Blade
Whether or not the situation ultimately calls for lubricant or a solvent, some scraping will be involved, so you might as well start with that. It may be all you need to do. Scraping with a razor blade works best when you put the blade in a holder—preferably one with a long handle. If you're removing hardened adhesive, work the edge of the blade under the glue and pry gently. The entire deposit may come off at once, or you may be able to chip it off in pieces. When removing soft adhesive, such as sticker or masking tape adhesive, the blade will usually remove most of it, leaving a thin film that you can wash off with soap and water.
Use a Lubricant
Lubricants loosen the bond between the glue and glass and make scraping easier. Soap and water is the easiest to use and most available. Fill a spray bottle with water, add a few drops of dish soap, shake and spray. Give the mixture five or ten minutes to seep between the glue and the glass before you scrape. Latex paint remover is also an effective lubricant, and it will probably also dissolve some of the glue. You may also have success with spray penetrating lubricants, which are designed to loosen stuck and rusted metal.
Call in the Solvents
Some types of glue—particularly acrylocyanates (super glue), epoxies and silicone adhesives—need extra help to persuade them to loosen their grip. Try acetone. If you don't have any in your paint closet, look for some acetone-based nail polish remover in the cosmetic drawer. Acetone is recommended for removing super glue, and it also softens polyurethane and epoxy adhesives. If you're trying to remove silicone caulk, soak it with denatured alcohol, mineral spirits or vinegar. None of these solvents will actually dissolve silicone, but they will soften it and make it easier to scrape.
Acetone and alcohol evaporate quickly, so to give them time to work, soak a rag or paper towel and hold it over the glue for a few minutes before scraping.
The big gun: When all else fails, reach for lacquer thinner. Products differ, but most lacquer thinners contain a cocktail of strong solvents, including toluene, xylene, methyl ethyl ketone and acetone. Provide plenty of ventilation when using this product, and use it sparingly. Lacquer thinner is strong stuff, so follow label directions and warnings carefully. Moisten a rag and hold the rag on the glue for a few minutes, then scrape.
After you've removed the bulk of the glue, you can take care of the residue with soap and water. Add a capful of vinegar per quart of solution to increase the cutting power. Wipe the area from where you removed the glue down with this solution, using a soft, non-abrasive cloth or a paper towel. While you're at it, it's a good idea to clean the entire mirror to get rid of streaks that may become visible only when the lighting changes.