Silver tarnishes, causing its surface to become dull and discolored over time. This process is often mistaken for rust, but it is a distinct chemical reaction that results in a dark, usually grayish-black, film. Pure silver is too soft for practical use, so most silver items, like sterling silver, are alloyed with other metals to increase durability. The presence of these other metals, typically copper, makes the alloy significantly more susceptible to this discoloration.
The Chemical Reaction Behind Tarnish
Silver’s discoloration is not oxidation, or rusting, which occurs when iron reacts with oxygen and water. Instead, silver undergoes sulfidation, a reaction with sulfur compounds found in the air. The primary culprit is hydrogen sulfide gas (H2S), a compound naturally present from industrial pollution and the decomposition of organic matter. When silver atoms encounter hydrogen sulfide, they react to form silver sulfide (Ag2S). This black compound adheres tightly to the metal’s surface, creating the tarnished appearance. This thin layer does not damage the underlying metal structure and is self-limiting, acting as a protective barrier to shield the silver beneath from further reaction.
Slowing Down the Tarnishing Process
Since tarnish is caused by airborne sulfur compounds, the most effective preventative measure is limiting the silver’s exposure to the surrounding environment. Storing silver in an airtight container, such as a sealed polyethylene bag or a tightly closed jewelry box, significantly slows sulfidation. Adding an anti-tarnish strip or cloth provides an extra layer of protection by absorbing sulfur and other pollutants from the air inside the sealed container.
Controlling the Environment
Controlling the storage environment also helps, as high humidity and temperature fluctuations accelerate the tarnishing process. Placing a desiccant packet, like silica gel or a piece of chalk, inside the container can help absorb excess moisture.
Avoiding Contaminants
It is also beneficial to keep silver away from common household items that emit sulfur compounds. These include certain fabrics like wool and felt, rubber products like rubber bands and latex gloves, and foods such as eggs, onions, and table salt. Items should be cleaned immediately after use if they contact these contaminants. Skin oils, cosmetics, and perfumes also promote the reaction, making it important to clean jewelry after wearing it and before storing it.
Effective Methods for Removing Tarnish
Once silver sulfide has formed, there are two primary methods for restoring the metal’s shine. The first involves mechanical removal using abrasive polishes or specialized polishing cloths. This method physically rubs away the layer of silver sulfide, but it also removes a microscopic amount of the underlying silver metal with each cleaning.
Chemical Reduction
A second, non-abrasive method uses a chemical reduction process that reverses the sulfidation reaction. This technique is often performed at home using aluminum foil, baking soda, and hot water. When the tarnished silver is placed in a hot solution of water and baking soda, in direct contact with the aluminum foil, an electrolytic reaction occurs. The aluminum is more reactive than the silver, causing it to “steal” the sulfur atoms from the silver sulfide, converting the tarnish back into metallic silver. The tarnish transfers from the silver object to the aluminum foil, which becomes coated in a dull, grayish film. This cleaning method is gentler because it only changes the chemical composition of the tarnish layer rather than physically scratching the metal’s surface.