Brass is an alloy composed of copper and zinc, and like many metals, its surface reacts with its environment over time. Moisture significantly accelerates the natural process of tarnish, meaning brass does tarnish in water. Tarnish is a form of corrosion that appears as a dulling or discoloration on the metal’s surface, unlike rust, which is a specific form of iron oxide that brass does not produce. Water acts as a powerful catalyst, speeding up the chemical reactions that cause this surface change.
The Chemical Process of Tarnish
Tarnish is a chemical reaction where metal atoms on the brass surface bond with non-metal elements from the surrounding air and moisture. Brass is susceptible because its components, copper and zinc, are reactive with oxygen and sulfur compounds. This surface discoloration is not a sign of internal degradation, but rather a thin, protective layer forming on the exterior.
The reaction often begins with copper reacting with oxygen to form copper oxide, a black film that darkens the metal. This copper oxide can then react with airborne substances like carbon dioxide and moisture to create copper carbonate, which appears as a greenish layer, similar to the patina seen on old copper structures. Sulfur compounds, often present as pollutants, can also react with the brass to form copper sulfide, contributing to the dark, dull appearance. The zinc within the alloy forms a white, powdery zinc oxide, which adds to the overall dullness of the tarnished surface.
How Water Accelerates Oxidation
Water acts as an electrolyte that increases the speed of chemical reactions. When water is present, it creates a conductive medium on the brass surface, allowing ions to move more freely and facilitating the electron transfer necessary for oxidation. While high humidity accelerates tarnish, direct exposure to liquid water is a more powerful factor.
The composition of the water itself is crucial, as it often carries corrosive agents that attack the brass. Saltwater contains high levels of chloride ions that are aggressive and can lead to pitting corrosion. Hard water leaves behind mineral deposits like calcium and magnesium, which trap moisture against the surface and create localized corrosion sites. Highly acidic or alkaline water (low or high pH) increases corrosion rates by dissolving the protective oxide layer. This exposure can trigger dezincification, a process where zinc selectively leaches out of the alloy, leaving behind a porous, weakened, copper-rich structure.
Preventing Water-Induced Tarnish
The most effective strategy for preventing water-induced tarnish is creating a physical barrier between the brass surface and the environment. Applying a clear protective coating, such as a lacquer or a durable wax, is the standard method for items used in high-moisture areas. Lacquers offer a long-term, hard-shell barrier that seals the metal from air and moisture, making them ideal for outdoor fixtures or plumbing.
For decorative indoor items, a high-quality paste wax or a thin layer of mineral oil can be buffed onto the surface to provide a temporary layer of protection. This protective film must be regularly reapplied, as it wears away with handling and cleaning. Simple, attentive care is also effective, involving immediately and thoroughly drying any brass item that contacts water, removing the electrolytic medium before a reaction can begin. Storing brass in a low-humidity environment, perhaps with a desiccant, also helps slow the rate of natural tarnishing.
Restoring Tarnished Brass
Once tarnish has formed, restoration requires removing the thin layer of oxidized metal without damaging the underlying brass. For light tarnish, a gentle cleaning with warm water and mild dish soap followed by immediate drying can restore luster. When the tarnish is heavy, a mild abrasive or a chemical cleaner is necessary to dissolve the oxidized film.
Many common household ingredients contain the mild acid required to break down the tarnish, such as a paste made from lemon juice, salt, and flour, or a mixture of vinegar and salt. These acidic cleaners must be applied, allowed to sit briefly, and then rinsed off completely to stop the chemical reaction. Commercial brass polishes use a combination of a chemical agent and a fine abrasive to lift the tarnish and leave a bright surface. Use a soft cloth and avoid aggressive scrubbing, which can create microscopic scratches and accelerate future tarnishing.