Brass is a metal alloy composed primarily of copper and zinc. When exposed to the atmosphere, brass naturally undergoes oxidation, often called tarnishing. This chemical process involves the metal surface interacting with oxygen and moisture in the air, leading to a dulling of its original sheen. The resulting film is called a patina, which is typically a self-limiting layer that protects the underlying metal from more extensive corrosion.
The Visual Result of Patination
The initial oxidation of brass results in a gradual loss of the metal’s shine rather than an immediate bright color. This process begins with a dulling of the surface, often progressing to a reddish-brown or dark brown hue. Over a long period, and depending heavily on the environment, the metal develops the characteristic blue-green color known as verdigris. The specific shade can range from a light teal to a darker, almost black-green, representing the final stage of atmospheric exposure.
The Chemical Mechanism of Tarnish
The color change occurs because the copper component of the alloy is significantly more reactive to air than the zinc. Initially, copper reacts with oxygen to form a thin layer of copper oxides on the surface. These oxides start as reddish cuprous oxide, which then further oxidizes into black cupric oxide. This combination of oxides is responsible for the metal’s early darkening and dull-brown appearance.
As the process continues, moisture and carbon dioxide from the air react with the copper oxides. This secondary reaction forms basic copper carbonates, the chemical compounds responsible for the vivid blue-green patina, or verdigris. Although the zinc in the alloy also oxidizes to form zinc oxide, this compound is generally white and less visible. Therefore, the copper compounds dominate the visual result of the tarnish, creating a complex mixture of copper and zinc compounds.
Factors Influencing Oxidation Rate and Shade
Several external variables influence the speed and final shade of the patina. High humidity and prolonged exposure to moisture significantly accelerate the rate of oxidation by providing the water necessary for copper carbonate formation. Coastal environments also expose the metal to salt and chlorides, which can lead to faster and more intense patination.
Airborne pollutants, particularly sulfur compounds from industrial emissions, also alter the tarnish. These sulfur gases react with the brass to form black copper sulfides, resulting in a darker, sometimes almost black patina before the green verdigris appears. Furthermore, the alloy’s specific composition matters; brass with a higher percentage of copper will develop a green patina faster than alloys with a higher zinc content.
Prevention and Restoration
The oxidation process can be slowed down significantly through protective measures for those who prefer to maintain the original bright, golden look. Applying a clear lacquer or a coat of protective wax creates a barrier that prevents oxygen and moisture from reaching the metal surface. These coatings must be reapplied periodically, as they wear down over time and can chip, allowing tarnish to begin in localized spots.
If tarnish has already developed, restoring the original luster requires gentle mechanical or chemical cleaning. Simple restoration methods involve using a mild abrasive paste, such as a mixture of flour, salt, and vinegar, which removes the oxidized layer without damaging the underlying metal. After cleaning with any acidic solution or commercial polish, it is important to rinse the brass thoroughly with water and dry it completely to halt any remaining chemical reaction and prevent immediate re-tarnishing.