Brass oxidation is a chemical reaction that occurs when brass comes in contact with air. Brass is an alloy of copper, and contains about 70 percent copper and 30 percent zinc, and is a bright yellow-gold in appearance. When brass is kept clean and dry, the surface remains stable, and a protective patina of corrosion can develop that protects the metal underneath. Brass that is stored in heavily polluted environments, exposed to salt water, or mishandled can develop more serious corrosion that can damage the integrity of the metal.
Brass is prized for its ornamental characteristics, and is a common alloy in museum pieces and historical artifacts. However, the risk of corrosion makes proper care essential for antique brass. Clara Deck, Senior Conservator at The Henry Ford in downtown Detroit, writes that the first line of defense is to provide a good storage environment.
A safe, natural patina can develop as brass oxidizes, preventing further damage from exposure to the air. This patina can appear reddish, black to brown, or green to blue and will often form a thick crust. A patina can be purposefully applied.
Harmful corrosion can leave pitting on the surface, commonly called "brass disease." This process differs from a patina in that the oxidized coating won't easily flake off, and will continue corroding the surface of the brass indefinitely. This process occurs in areas of high humidity, and can be accelerated by airborne pollutants or dust and grime left on the surface of the brass.
Brass can be protected from oxidation using a patina or a varnish. Some common varnishes for copper alloys include nitrocellulose, acrylic, epoxy coatings, silicone coatings, alkyd coatings, urethane coatings, vinyl, and cellulose acetate butyrate. If a coating is broken, dark streaks will appear where oxidation occurs.