Brass is a common alloy prized for its warm, golden appearance, frequently used in decorative hardware, musical instruments, and fixtures. Over time, this attractive luster diminishes as the metal develops a dull, darkened film known as tarnish. This discoloration is the visible result of a natural and unavoidable chemical process acting upon the metal’s surface. Understanding this chemistry, a form of corrosion distinct from iron rust, provides the knowledge necessary to slow the reaction and preserve the alloy’s original shine.
The Makeup of Brass
Brass is a metal alloy, a blend of two or more metallic elements. Its primary components are copper and zinc, with proportions determining the final color and properties of the material. The ratio typically ranges from about 60% to 90% copper, with the remainder being zinc.
The copper component is primarily responsible for the tarnishing reaction. While zinc contributes to the metal’s strength and workability, copper is the more chemically reactive metal when exposed to atmospheric elements. Consequently, brass compositions with a higher copper content tend to show signs of tarnish more rapidly.
The Core Chemical Reaction
Tarnish is a thin layer of corrosion that forms on the brass surface, distinct from the porous iron oxide known as rust. The initial stage is oxidation, where copper atoms react with oxygen in the air to form copper oxide. This layer first appears as a slight dulling of the metal’s sheen.
The dark discoloration is primarily caused by sulfidation. Airborne sulfur compounds, such as hydrogen sulfide, readily react with the copper in the alloy. This reaction produces copper sulfide, which creates the typical dark brown or black tarnish layer. This sulfide layer is stable and forms a protective, self-limiting film that slows further corrosion of the underlying metal.
A green or bluish-green layer, often mistaken for rust, can also form, particularly in humid or coastal environments. This is usually a copper carbonate compound, resulting from copper oxides and sulfides reacting further with carbon dioxide and moisture. This buildup fundamentally changes the surface appearance of the metal.
Environmental Factors That Speed Up Tarnish
Environmental conditions significantly accelerate the chemical reactions that lead to brass tarnish. High humidity and moisture are primary accelerators because water acts as an electrolyte, facilitating the transfer of electrons necessary for oxidation and sulfidation. Brass located in damp environments, such as bathrooms or kitchens, will tarnish much faster than items kept in dry conditions.
Air pollution is another major contributor, particularly in urban or industrial areas with high concentrations of sulfur compounds. These pollutants provide the sulfur needed for the rapid formation of the dark copper sulfide layer, quickly turning the metal black. Exposure to salts, such as in coastal areas, or contact with acidic substances, like those found in acid rain, also hastens the corrosive effect.
Physical handling also plays a role in discoloration. Natural oils, salts, and acids from human skin are transferred onto the metal surface, creating localized areas of high reactivity. This often results in uneven, streaky tarnish patterns where the brass was touched. Even common household cleaners containing ammonia or other strong chemicals can strip away the metal’s protective surface and speed up the degradation process.
Protecting Brass From Tarnish
The most effective strategy for preventing tarnish involves creating a physical barrier between the brass and the reactive elements in the environment. Applying a clear protective coating, such as a lacquer or specialized clear coat, seals the metal surface off from oxygen, moisture, and airborne pollutants. This method is highly effective and can maintain the brass’s original luster for many years, though the coating must be reapplied if it wears or chips.
For unlacquered brass, controlling the surrounding environment is the next best approach to slow the chemical reactions. Storing or displaying items in areas with low humidity and stable temperatures minimizes the moisture available to act as an electrolyte. Minimizing physical contact with unsealed brass objects also reduces the transfer of skin oils and salts that initiate localized tarnish.
Chemical inhibitors can also be used, often in the form of specialized waxes or polishes that contain compounds like benzotriazole. This chemical bonds directly to the copper atoms on the surface, forming a thin, invisible layer that blocks further reaction with sulfur and oxygen. Regular cleaning with a non-abrasive polish removes the existing tarnish mechanically, and a protective coating or inhibitor can then be applied to delay its return.