It can take about 50 years in a damp location for a mirror to achieve that aged, spotty look that easily compliments many different decors. Modern mirrors are backed by a sheet of mirrored coating, which is in turn backed by a coat of paint to protect it from damage. By removing the protective layer of paint and damaging the mirrored surface with acid, a crafty decorator can make her own "antique" mirror in a matter of hours.
Choose a well-ventilated area for your workspace and cover anything that you do not want to be exposed to chemicals or paint.
Put on goggles, gloves and a ventilation mask.
Remove the paper backing on the mirror, if there is one, to expose the coat of paint covering the mirrored coating.
Brush a thick layer of paint stripper onto the areas you wish to antique. Let the stripper sit for five minutes, then wipe it off with a dry cotton rag. Repeat as necessary until the silver backing is revealed. Lightly scrape the loosened paint with a putty knife if necessary. Be careful not to scrape the mirror's surface.
Wipe off any remaining traces of the paint stripper.
Pour muriatic acid into a spray bottle. Pour slowly and carefully and do not spill a drop. Use a funnel if necessary. Tightly close the acid and store it away from your workspace.
Apply the acid to the back of the mirror by misting it on the areas you have stripped of paint. You can also apply the acid with an eyedropper to achieve larger spots. Let the acid react for approximately one minute. Keep an eye on it while it is reacting; some applications may take longer than others to achieve the desired results.
Blot excess acid with paper towels, then lightly wipe the area with a damp cotton rag and allow it to dry completely.
Apply a thick coat of black paint to the antiqued areas. Follow this with a coat of black spray paint applied to the entire back of the mirror. Reattach the paper you removed in step one or reapply a new sheet of craft paper.
Attach wire and hang the mirror.
Don't pour the acid before you need it, and only pour a small amount at a time.