Brass is a metal alloy composed primarily of copper and zinc. Yes, brass does oxidize, but the result is a process called tarnishing or the development of a patina, which is fundamentally different from the destructive “rust” that affects iron. The confusion often arises because the term “oxidation” is used broadly to describe any reaction with oxygen, but the specific outcome for brass is one of protection rather than decay. This surface change is a natural process that can be either polished away or left to enhance the object’s character.
How Brass Reacts to Air and Moisture
Brass tarnishes through a chemical reaction driven by the copper content in the alloy, which readily interacts with components in the air. When exposed to oxygen, a layer of copper oxide forms on the surface of the metal. This initial dark layer is the first stage of the tarnishing process.
The presence of moisture and specific atmospheric pollutants accelerates this reaction significantly. Sulfur compounds, often found in polluted air, react with the copper to create copper sulfides, which appear as a dark film. When moisture and carbon dioxide are present, the surface compounds transform into green or bluish-green copper carbonates, commonly known as verdigris. This resulting layer of tarnish is generally self-limiting. The thin film creates a barrier between the underlying brass and the surrounding environment, effectively sealing the surface and slowing further corrosion.
The Difference Between Tarnish and Iron Rust
The corrosion process on iron, specifically called rust, is chemically and structurally distinct from the tarnishing of brass. Rust is hydrated iron(III) oxide, a reddish-brown substance that forms when iron reacts with oxygen and water. Unlike the dense layer on brass, iron rust is porous, flaky, and non-adherent.
This porous structure means that as the rust forms, it flakes away, continually exposing fresh, underlying iron to the environment. The process is aggressive and self-perpetuating, leading to the eventual structural failure of the iron object. Rust is also a form of volume expansion, causing swelling and cracking.
In contrast, the patina that forms on brass is a dense, tightly-adhering layer of metal compounds. This layer acts as a passivation coating, sealing the surface and protecting the bulk of the metal from further reaction. Brass corrosion is considered self-limiting because the surface layer stabilizes the metal, preventing the deterioration seen in rusting iron.
Practical Steps for Cleaning and Protecting Brass
Cleaning brass tarnish involves removing the oxidized surface layer using gentle abrasion or chemical dissolution. One effective method utilizes the acidity of common household ingredients. A paste made from white vinegar, salt, and flour combines the acid needed to dissolve the tarnish with a mild abrasive to lift the residue.
Similarly, a paste of lemon juice and baking soda uses the citric acid to break down the copper compounds, while the baking soda provides a light scrubbing action. After applying any cleaning mixture, it is crucial to rinse the brass thoroughly with water to neutralize any remaining acid. Immediately dry and buff the surface with a soft, lint-free cloth to prevent new oxidation from beginning due to residual moisture.
Protection Methods
For long-term protection, especially on pieces that are handled often or exposed to the elements, a protective barrier is the most effective measure. Applying a microcrystalline wax creates a thin, non-visible coating that physically blocks air and moisture from contacting the brass surface. The wax can be gently applied, allowed to set, and then buffed to a soft sheen.
Another common method is to apply a clear lacquer or varnish to the clean, dry brass. This provides a durable, hard-shell barrier that completely isolates the metal from oxygen and humidity. While lacquers offer superior protection for many years, they may require professional removal and reapplication if the coating is scratched or starts to chip, as moisture can become trapped underneath and cause concentrated tarnishing.