Tarnish is a common surface discoloration appearing as a darkening film on silver objects. This dulling effect is not rust, but rather a chemical change specific to silver and its alloys. Sterling silver is highly susceptible to this process due to its specific metallic makeup. Understanding the chemical reactions involved reveals why this metal changes color and how its original luster can be restored.
Composition of Sterling Silver
Sterling silver is defined as a metal alloy consisting of 92.5% pure silver by weight and 7.5% of other metals. This composition is often represented by the millesimal fineness number 925. The inclusion of other metals, traditionally copper, provides the necessary hardness and durability for practical use, as pure silver is inherently soft. However, this copper content is the primary reason sterling silver tarnishes more readily than fine silver (99.9% pure). While silver is resistant to oxidation, the copper atoms within the alloy are highly reactive with atmospheric elements, accelerating the sulfidation reaction that defines tarnish.
The Silver Sulfide Reaction
Tarnishing is fundamentally a chemical process known as sulfidation, where silver atoms react with sulfur-containing compounds in the atmosphere. The most common source of sulfur is trace amounts of hydrogen sulfide (\(\text{H}_2\text{S}\)) gas found naturally in the air. The resulting compound formed is silver sulfide (\(\text{Ag}_2\text{S}\)), a black solid. This layer begins as faint yellow, progresses to brown, and eventually becomes the deep black film recognized as tarnish. The reaction is: \(4\text{Ag} + 2\text{H}_2\text{S} + \text{O}_2 \rightarrow 2\text{Ag}_2\text{S} + 2\text{H}_2\text{O}\).
The copper content in sterling silver further deepens the discoloration. Copper atoms react with sulfur compounds, forming copper sulfide (\(\text{Cu}_2\text{S}\)), which integrates into the tarnish layer. Silver sulfide is non-volatile and adheres firmly to the surface, creating a thin, opaque coating that obscures the metallic shine.
Environmental Catalysts for Tarnish
Several external factors act as catalysts, significantly accelerating the sulfidation reaction. High humidity is a major contributor because water acts as an electrolyte, promoting chemical interaction between the metal surface and sulfur compounds. The presence of moisture allows reactants to dissolve and interact more efficiently. Beyond atmospheric gases, many common household items directly release sulfur compounds that attack the silver.
Sources of sulfur that accelerate tarnishing include:
- Certain foods, such as eggs, onions, and mustard.
- Wool, felt, latex, and rubber products like rubber bands, which emit sulfur-containing vapors.
- Air pollutants, particularly sulfur dioxide (\(\text{SO}_2\)) in industrial or urban areas.
- Substances like perfumes, hairsprays, and some cleaning agents.
Chemical Reversal of Tarnish
Cleaning sterling silver involves methods designed either to remove the silver sulfide layer or to chemically reverse its formation. One common approach utilizes polishes containing mild abrasives or chemical solvents, which physically rub away or chemically dissolve the black silver sulfide film. While effective, this physical removal results in a microscopic loss of silver atoms with every cleaning, potentially thinning the silver surface over time.
A more sophisticated method relies on an electrochemical reduction reaction to reverse the tarnish without removing metal. This process involves submerging the tarnished silver in a hot solution of water and an electrolyte, typically baking soda, alongside aluminum foil. The aluminum acts as a sacrificial metal because it has a greater affinity for sulfur than silver does. A small electrical current is generated, causing the sulfur atoms to transfer from the silver sulfide (\(\text{Ag}_2\text{S}\)) to the aluminum, forming aluminum sulfide. This reaction converts the silver sulfide back into pure metallic silver, restoring the shine without loss of the original metal.