Brass is a metal alloy composed primarily of copper and zinc, often with copper making up about two-thirds of the composition. Prized for its golden appearance, durability, and high workability, brass is frequently used in applications requiring low friction and corrosion resistance, such as musical instruments, plumbing fittings, and decorative hardware. Like many metal alloys, brass naturally interacts with its environment, leading to tarnishing, which alters its surface color. This article explores the specific colors brass takes on as it ages and provides guidance on managing this transformation.
The Specific Colors of Brass Tarnish
Tarnishing begins when the brass surface loses its bright, reflective luster and develops a dull film. This initial dullness progresses into a range of dark, earthy tones, typically starting with a brownish discoloration. As the tarnish layer thickens, these colors deepen into dark brown or even an almost black film, obscuring the metal’s original golden hue.
The most distinct color change occurs when brass is exposed to moisture, carbon dioxide, and pollutants, especially in outdoor or high-humidity environments. Under these conditions, a greenish-blue layer forms, known scientifically as verdigris or chemical patina. This blue-green layer results from the copper components reacting to form copper carbonate. While patina is often desirable on architectural elements, tarnish generally refers to the initial, uneven cloudiness or dark streaks that dull the surface.
The Chemical Reaction Causing Tarnish
The discoloration of brass is a surface chemical reaction between the metal and compounds in the air. Tarnish is commonly caused by oxidation and sulfurization, both accelerated by moisture. Oxidation involves the copper and zinc components reacting with atmospheric oxygen to form metal oxides. While zinc often oxidizes first, forming a dull zinc oxide layer, copper drives the most noticeable color changes.
The copper within the alloy reacts to form copper oxide, which initially appears dark brown or black. When airborne pollutants, specifically sulfur compounds, are present, the reaction is called sulfurization. This process creates copper sulfide compounds, which contribute significantly to the dark, blackish film seen on tarnished brass. High humidity, acidic substances, and oils from human skin can dramatically speed up these chemical reactions.
Slowing Down and Preventing Tarnish
Protecting the surface from environmental factors is the most effective method for slowing tarnish. Applying a protective barrier, such as a clear lacquer, seals the brass and prevents direct contact with oxygen and moisture. Lacquer is effective for outdoor items or those not frequently handled, though it will eventually wear off and require reapplication.
For indoor items or those subject to frequent touch, a high-quality paste wax or carnauba wax provides a softer, temporary shield. This wax layer must be reapplied periodically, as it wears down over time, especially with handling. Reducing humidity and avoiding storage near sources of sulfur, such as rubber or certain fabrics, can also mitigate the speed of tarnishing.
Safe Removal and Restoration Methods
When tarnish has formed, restoration begins by determining if the item is lacquered, as the coating must be removed if the tarnish is underneath. For unlacquered brass, light tarnish can often be removed using a soft cloth and a commercial brass polish. For deeper tarnish, mild chemical agents are effective at dissolving the metal oxides.
A homemade paste of white vinegar, salt, and flour, or a mix of lemon juice and baking soda, uses a gentle acid to break down the tarnish. When using any acidic cleaner, it is important to test it on an inconspicuous spot first. Afterward, rinse the item thoroughly with water. Neutralizing any remaining acid with a mild baking soda solution is recommended before drying and buffing the surface.