Silver is classified as a noble metal, meaning it exhibits an inherent resistance to chemical attack and oxidation in moist air. The dark film that forms on silver objects is not a deep, destructive corrosion but a superficial chemical change known as tarnish. Tarnish is chemically distinct from true corrosion. This process involves multiple environmental factors, with water playing a specific role in accelerating the reaction.
Corrosion Is Not Tarnish
True corrosion, such as the rusting of iron, is a process where the metal reacts with oxygen and water to form a metal oxide that flakes away, progressively weakening the material. Silver resists this type of oxidation under normal atmospheric conditions. It does not readily react with oxygen or water alone because silver oxide (\(\text{Ag}_2\text{O}\)) is an unstable compound that easily decomposes back into silver metal.
The familiar black or grey film on silver is not an oxide but silver sulfide (\(\text{Ag}_2\text{S}\)), which forms through a process called sulfidation. This reaction occurs when silver metal (\(\text{Ag}\)) comes into contact with sulfur-containing compounds, primarily hydrogen sulfide (\(\text{H}_2\text{S}\)) gas in the atmosphere. The resulting silver sulfide layer is extremely thin and adheres closely to the metal’s surface, which is why tarnish is considered a self-limiting surface phenomenon. Unlike rust, tarnish does not eat away at the underlying silver, but it does dull the metal’s highly reflective appearance.
The chemical transformation is represented by the formula \(2\text{Ag} + \text{H}_2\text{S} + \text{O}_2 \rightarrow \text{Ag}_2\text{S} + \text{H}_2\text{O}\). This equation shows that atmospheric oxygen is also involved in the reaction, but the presence of the sulfur compound is the necessary factor. The high stability of silver sulfide strongly favors the reaction, even when the concentration of hydrogen sulfide in the air is extremely low. The formation of this stable compound explains why silver is susceptible to tarnishing despite its nobility.
How Water Acts as a Catalyst
While pure water is harmless to silver, the water encountered in daily life is far from pure and plays a significant role in the tarnishing process. Water acts as a carrier for the necessary reactants and serves as an electrolyte to facilitate the chemical change. The moisture in the air, or a water film on the silver surface, allows sulfur compounds to dissolve and make ionic contact with the metal more easily than they would in a dry environment.
The water itself contains dissolved gases, most notably the sulfur-containing gases picked up from the atmosphere. Additionally, water carries various mineral impurities and dissolved ions that accelerate the reaction rate. Tap water contains small amounts of sulfur compounds, and sources like swimming pools or seawater often contain elevated chloride ions. These chlorides can react with silver to form silver chloride, contributing to the overall surface degradation.
Water’s ability to act as an electrolyte allows for efficient electron transfer, speeding up the rate at which tarnish forms. Consequently, silver exposed to damp air or immersed in water containing contaminants darkens much faster than silver stored in a dry environment. The presence of water is not the cause, but it is the indispensable medium for the sulfidation reaction to proceed noticeably.
Environmental Accelerants of Degradation
Several external environmental factors significantly increase the rate at which silver tarnishes. High humidity is a major factor because it ensures a continuous thin layer of moisture is present on the silver surface. This moisture film enables the airborne sulfur compounds to dissolve and react quickly with the metal.
Temperature also plays a role, as chemical reactions proceed faster when heat is applied. Silver tarnishes more rapidly in warm, humid conditions than in cool, dry ones because the increased energy accelerates the molecular interactions.
The level of air pollution is a significant external factor, as air in industrial or urban areas often contains higher concentrations of sulfur-containing gases like sulfur dioxide and hydrogen sulfide. These pollutants provide the primary reactant needed for the sulfidation of silver. Exposure to materials that off-gas sulfur, such as rubber, wool, or paint fumes, also functions as a localized environmental accelerant.
Prevention and Restoration Methods
Managing the environment is the most effective method for preventing silver tarnish, focusing on reducing exposure to moisture and sulfur compounds. Proper storage involves placing silver items in airtight containers or specialized anti-tarnish bags that physically block the entry of reactive gases. Anti-tarnish strips, which are chemically treated to absorb sulfur from the air, offer additional protection.
Immediately dry silver items that have been exposed to water, especially harsh sources like chlorinated pool water or saltwater, using a soft, clean cloth. This action removes the electrolyte layer and any dissolved impurities before they can react with the metal. Regular, gentle polishing with a treated cloth also removes trace amounts of tarnish before it becomes visibly dark.
For restoration, tarnish can be removed either mechanically or chemically. Polishing cloths use a fine abrasive to physically rub off the silver sulfide layer, removing a minute amount of silver. A non-abrasive method utilizes an electrochemical reaction, typically involving a bath of warm water, baking soda, and a piece of aluminum foil. In this bath, the aluminum reacts with the silver sulfide, converting the tarnish back into silver metal and transferring the sulfur to the foil.