Bronze is copper alloy. In modern times, bronze is an alloy of copper and any metal except zinc. It is generally more expensive than brass and more corrosion resistant. Bronze forms a patina (green color) which is protective to the metal and is often seen on artwork. Reproduced it is called Verde. Bronze will deteriorate rapidly if exposed to moisture and chlorides or sulfides.
Solid bronze often is lacquered (at the factory) to protect the finish. Lacquered bronze only needs dusting and an occasional wiping with a damp cloth. Have the lacquer replaced if it cracks or peels.
Keep bronze pieces as clean as possible. Accumulations of dust and dirt can eat into the metal surface. Dust regularly using a soft cloth. Do not rub too vigorously, especially on any protruding parts. If a bronze piece has been neglected for a long time and is covered with grime, thoroughly clean it with a soft brush. Remove all dust from crevices and notches and then lightly rub the entire surface with a soft flannel cloth. For a more thorough cleaning, carefully wash with a solution of 1 tablespoon of salt and 3 1/2 quarts of water. Rinse well. Polish with copper polish followed by glass wax.
If you want a high polish, dip a cloth into liquid wax and apply to the piece. When dry, buff lightly to a high gloss. This wax treatment also may be given to bronze pieces that are kept outdoors. Weathered bronze usually darkens; however, this is natural and does not harm the piece.
"Bronze disease" is one of the most serious hazards of bronze. This disease, caused when chlorides and oxygen combine in a damp environment, also attacks brass and pewter. The disease takes the form of a sudden outbreak of small patches of corrosion and is distinguished by rough, light green spots. "Bronze disease" usually can be stopped by washing the piece in repeated changes of boiling hot, distilled water. You may have to soak the object for a week or more in distilled water. If this treatment does not work, consult a museum expert about using a strong solution of sodium sesqui-carbonate or have your piece treated by a professional.
General Purpose Bronze Cleaner: Salt, Vinegar, and Flour. Dissolve 1 teaspoon salt in 1 cup white vinegar. Add enough flour to make a paste. Apply paste to bronze and let sit for 15 minutes to 1 hour. Rinse with clean, warm water, and polish dry.
Courtesy of MSU Extension