Tarnish is the dark, dull layer that appears on silver objects over time, causing them to lose their characteristic bright luster. While many people refer to this process as silver “oxidizing” due to its visual similarity to the rusting of iron, the term is scientifically misleading. The darkening film results from a chemical reaction with airborne compounds, not a direct combination with oxygen. Understanding the true chemistry clarifies why silver changes color and how its brilliance can be maintained.
The Noble Nature of Silver
Silver is classified as a noble metal, known for its resistance to corrosion and chemical attack. This stability means silver does not react readily with oxygen (\(\text{O}_2\)) or water (\(\text{H}_2\text{O}\)) under normal atmospheric conditions. Unlike iron, which rusts easily by forming iron oxide (\(\text{Fe}_2\text{O}_3\)), silver resists forming silver oxide (\(\text{Ag}_2\text{O}\)) because that compound is less stable than the pure metal. This low reactivity confirms that visual tarnish is not the same process as simple oxidation seen in other metals.
The Chemical Process of Tarnish Formation
The actual chemical process responsible for tarnish is called sulfidation, not oxidation. Tarnish is a thin layer of silver sulfide (\(\text{Ag}_2\text{S}\)) that forms when silver atoms react with trace amounts of sulfur-containing gases in the environment. The primary reactant is typically hydrogen sulfide (\(\text{H}_2\text{S}\)), a gas present in the atmosphere even at low concentrations.
The resulting silver sulfide forms a thin, dense, black coating on the metal’s surface. This compound is intensely dark because its crystalline structure absorbs light across the visible spectrum, making the silver appear dull and darkened. The sulfidation process is favored because silver sulfide is an extremely stable compound, meaning that once it forms, it is very difficult to break down. Humidity and moisture in the air act as a catalyst, speeding up the chemical reaction rate between the silver and the sulfur compounds.
Common Sources of Sulfur That Accelerate Tarnish
The sulfur compounds needed for the sulfidation reaction are common in everyday life, explaining why silver tarnishes even when stored indoors. Air pollution, primarily from the combustion of fossil fuels, releases sulfur dioxide (\(\text{SO}_2\)) and hydrogen sulfide (\(\text{H}_2\text{S}\)) into the atmosphere. These atmospheric gases cause silver to tarnish slowly over time.
Many common household items contain sulfur and accelerate the tarnishing process through direct contact or proximity. These include natural fabrics like wool, materials containing latex or rubber, and items such as newspaper or cardboard.
Foods and Body Chemistry
Many foods, such as eggs, onions, and garlic, release sulfur compounds that can quickly blacken silver cutlery. Human body chemistry also contributes, as natural skin oils and sweat contain trace sulfur compounds. This is why silver jewelry worn regularly tarnishes faster.
Methods for Slowing and Reversing Tarnish
Slowing the rate of tarnish involves reducing the silver’s exposure to airborne sulfur compounds. Prevention begins with proper storage, such as placing silver in airtight containers or using anti-tarnish bags and cloths designed to absorb sulfur gases. Storing silver items in areas with low humidity, away from known sulfur sources, can significantly delay the onset of tarnish.
Once tarnish has formed, it can be removed either mechanically or chemically.
Mechanical Removal
Mechanical removal involves polishing with a cloth and a mild abrasive cleaner, which physically rubs off the layer of silver sulfide. This method is effective but removes a microscopic amount of the silver metal each time.
Chemical Reduction
The non-abrasive method uses a chemical reduction process, often employing a hot bath containing baking soda and aluminum foil. In this electrochemical reaction, the aluminum transfers the sulfur from the silver sulfide back to the aluminum. This reverses the sulfidation and converts the silver sulfide back into pure silver metal.