"Oil-rubbed bronze" is a general term used to describe bronze that is aged or given the simulation of age and has a patina of various shades of deep, rich browns. The chocolaty appeal of oil-rubbed bronze makes it a favored metal for a variety of household features, including light fixtures, cabinet handles, doorknobs, water faucets, towel bars and light switch cover plates. Unfortunately, things like harsh chemical cleaners, abrasive scrub pads and hard water can mar a bronze surface. Use methods of hard-water stain removal that will not compromise the patina or scratch the metal.
Combine 1 part sodium hexametaphosphate powder with 10 parts of distilled water. Sodium hexametaphosphate is used in common commercial water softening and limescale removal products. Stir until the powder is dissolved.
Remove any easily removable hard-water-stained, oil-rubbed bronze fixtures, such as light switch cover plates. Submerge fixtures in the solution.
Wrap items that are not easily removed in towels that have been soaked in the solution.
Leave the solution to saturate and soften the stains for several minutes then scrub the chalky residue away with a soft-bristle scrub brush or a toothbrush.
Rinse the scrubbed item in distilled water and pat it dry with a towel.
Apply a wax coat to protect the bronze item from future water stains. Pick up a gob of microcrystalline wax on a lint-free cloth and smear it over the bronze surface. Allow the wax to dry then buff away the excess with a dry, lint-free cloth. Add one to three additional coats.
Wipe down and dry oil-rubbed bronze fixtures prone to moisture, such as faucets, when they become wet to prevent stains from developing.