Silver tarnish is the dark discoloration that appears on silver metal, resulting from an unavoidable chemical process. It begins as a faint yellowish haze, progressing through shades of brown, eventually developing into a dark gray or black surface layer. This change is a form of corrosion unique to silver, unlike rust which involves iron and oxygen. Unlike gold or platinum, silver possesses a particular chemical affinity for sulfur compounds found in the air and in many common materials.
The Formation of Silver Sulfide
Tarnish is caused by the formation of silver sulfide (\(\text{Ag}_2\text{S}\)), the compound responsible for the dark coloration. Elemental silver (\(\text{Ag}\)) reacts primarily with airborne sulfur compounds, most notably hydrogen sulfide (\(\text{H}_2\text{S}\)), a gas released by various sources. This reaction is a sulfidation process where silver atoms bond with sulfur atoms from the surrounding environment, rather than a simple oxidation.
The chemical transformation involves the reaction of silver with hydrogen sulfide and oxygen in the air. This process slowly creates a thin, uniform coating of the black silver sulfide compound on the metal’s surface. The initial yellow tint represents the earliest, thinnest layer, which darkens as the layer thickens over time.
While sulfur is the chemical agent, moisture and high heat accelerate the reaction rate. Humidity acts as a catalyst, allowing sulfur compounds to dissolve and interact more easily with the silver surface. A silver piece exposed to sulfur-rich, humid air will tarnish faster than one kept in a dry environment. The resulting silver sulfide layer is inert and causes no structural damage, but it dulls the metal’s reflective surface.
Household and Environmental Sources of Tarnish Agents
Sulfur compounds required for tarnish formation are present in the atmosphere, making the process inevitable for exposed silver. A major source is air pollution, where industrial emissions and the combustion of fossil fuels release sulfur dioxide (\(\text{SO}_2\)) and hydrogen sulfide gases. Even at low concentrations, these atmospheric pollutants trigger the tarnishing reaction.
Inside the home, numerous everyday items contain sulfur and act as tarnish accelerators. Common household materials like vulcanized rubber, found in rubber bands and latex gloves, off-gas sulfur compounds. Certain fabrics, particularly wool and felt, can also release sulfur, posing a risk when silver is stored near them.
Some common foods contribute to a sulfur-rich environment:
- Eggs
- Onions
- Mustard
- Mayonnaise
Human skin oil and perspiration contain sulfur-containing amino acids and chlorides, which is why frequently worn silver jewelry tarnishes rapidly where it contacts the skin. Storing silver in cardboard boxes or near certain adhesives can also be detrimental, as these materials may contain residual sulfur compounds.
Strategies for Prevention and Storage
Controlling the environment is the best approach to slowing the tarnishing process. Silver should be stored away from areas with high humidity and temperature fluctuations, which accelerate the chemical reaction. Keeping silver in a dedicated, cool, and dry location minimizes the catalytic effect of moisture.
For long-term storage, placing silver items in airtight containers or specialized anti-tarnish bags is recommended. These containers create a barrier against airborne sulfur compounds. The bags and cloths are typically made from fabric treated with chemical compounds designed to absorb sulfides.
A more active preventative measure involves using anti-tarnish strips or tabs within the storage container. These products often contain activated carbon or specialized copper-based polymers that chemically neutralize or adsorb sulfur gases before they reach the silver. A single strip can protect the contents of a cubic foot of air space for several months, providing a layer of chemical defense.
Safe Methods for Tarnish Removal
Once silver sulfide has formed, it must be removed to restore the metal’s shine, using one of two safe methodologies. The first is abrasive polishing, which uses commercial pastes or creams containing fine particles, such as diatomaceous earth or aluminum oxide. Rubbing the silver physically abrades the dark silver sulfide layer off the surface. However, this process also removes a minute amount of the underlying silver metal with each cleaning, which can eventually wear down the piece, particularly silver-plated items.
A second, non-abrasive method uses a chemical reduction bath, which reverses the tarnishing reaction without removing the base metal. This technique involves placing the tarnished silver in contact with aluminum foil in a container filled with hot water, baking soda, and sometimes salt. The aluminum has a stronger chemical attraction to sulfur than silver, initiating an electrochemical reaction.
In this bath, the aluminum sacrifices itself by taking the sulfur from the silver sulfide (\(\text{Ag}_2\text{S}\)) and turning it into aluminum sulfide, which may appear as a residue on the foil. This process reduces the silver sulfide back into pure silver metal (\(\text{Ag}\)). This method is unsuitable for pieces with embedded gemstones or hollow components, as the hot solution can loosen settings or leave corrosive residue in crevices. Commercial chemical dips, often based on acidified thiourea, should be avoided due to their corrosive nature and potential to damage the silver’s surface, leading to rapid re-tarnishing.