Copper, a reddish-orange metal, is widely used in various applications, from plumbing to decorative items. When exposed to water, copper does tarnish, and water often plays a significant role in this process. While copper itself does not directly react with water, its presence facilitates the chemical reactions that lead to tarnishing. This discoloration is a surface phenomenon, impacting the metal’s appearance.
Understanding Copper Tarnish
Copper tarnish is a form of corrosion resulting from chemical reactions between the metal and elements in its environment. This process is primarily oxidation, where copper reacts with oxygen. Initially, this reaction forms reddish-brown copper(I) oxide (Cu₂O) on the surface, which can then further react to create black copper(II) oxide (CuO). These oxides are thin surface layers that alter the copper’s original bright, metallic luster.
Beyond oxygen, other compounds contribute to tarnish formation. Sulfur compounds, such as hydrogen sulfide and sulfur dioxide, can react with copper to produce dark, dull copper sulfides (Cu₂S). Additionally, copper can react with carbon dioxide and hydroxide ions, leading to the formation of a greenish layer known as patina, often seen on aged copper structures like roofing. This patina is a stable form of copper carbonate that can, over time, protect the underlying metal from further corrosion.
How Water Influences Tarnish
Water acts as a medium and catalyst for the tarnishing process. The primary reactant involved is dissolved oxygen within the water. Even distilled water contains dissolved oxygen, which can slowly oxidize copper, leading to brown discoloration over time. The presence of moisture significantly accelerates these chemical reactions.
Different types of water can influence the rate and nature of tarnishing. Tap water, containing dissolved minerals and chlorine, can accelerate the process; chlorides from evaporated tap water can contribute to “water stains” and faster corrosion. Acidic water (pH below 7.0) or alkaline water (pH above 8.5) can also increase the rate of corrosion and tarnish formation. Additionally, other dissolved substances in water can contribute to tarnish.
Managing Copper Tarnish
To maintain copper’s original shine and prevent tarnish, several practical approaches can be employed. Keeping copper items dry is a straightforward and effective method, as moisture accelerates the tarnishing process. Wiping copper surfaces thoroughly after use, especially in areas with pooling water like sinks, helps to prevent mineral deposits and tarnish spots. Controlling humidity in storage environments can also slow down tarnish formation.
Applying protective coatings provides a barrier against environmental elements. Lacquers, waxes, or specialized metal protectants, such as carnauba wax or commercial copper protectants, can shield the copper surface from oxygen and moisture. For items not used for food, a thin layer of mineral oil can also be applied immediately after cleaning to prevent re-tarnishing by sealing the copper from air.
When tarnish does appear, various household methods can effectively remove it. Acidic solutions are commonly used, such as a paste made from lemon juice and salt, where the citric acid reacts with the tarnish and salt acts as a mild abrasive. A mixture of white vinegar and salt is another effective cleaning solution; for stubborn tarnish, items can be boiled in a solution of water, vinegar, and salt. Baking soda combined with lemon juice or vinegar also creates a paste that can remove tarnish. After applying any cleaning mixture, it is important to rinse the copper thoroughly with water and dry it completely to prevent new tarnish from forming. Commercial copper cleaners are also available for more severe tarnish, but gentle application is always recommended to avoid scratching the surface.