Silver is a valuable metal prized for its bright luster, yet it is highly susceptible to a common form of surface degradation that causes it to darken. Many people confuse this process with the rusting that affects iron, especially when water appears to be present, leading to the question of whether silver can truly rust. Understanding the difference between these two chemical reactions is the first step in properly caring for silver pieces.
The Direct Answer: Silver Tarnishes, It Does Not Rust
Silver does not rust because the term “rust” is chemically defined and applies exclusively to the corrosion of iron and its alloys, such as steel. Rust is the reddish-brown substance known as iron oxide, which forms when iron reacts with oxygen and moisture. The corrosion process that affects silver is instead called tarnishing.
Tarnish on silver typically manifests as a dull, gray, or black film on the metal’s surface, which contrasts sharply with the flaky, reddish-brown color of iron rust. Unlike rust, which often continues to eat away at the metal structure, tarnish is a thin layer of corrosion that primarily affects the silver’s appearance.
The Chemical Process Behind Silver Tarnish
The chemical reaction that causes silver to tarnish involves sulfur compounds rather than the oxygen required for iron to rust. Silver reacts most readily with trace amounts of sulfur-containing gases in the air, primarily hydrogen sulfide, to form a black compound called silver sulfide (\(\text{Ag}_2\text{S}\)). Hydrogen sulfide is found everywhere, released from natural sources like volcanoes and oceans, as well as from common household items like rubber bands and certain foods.
The full reaction requires silver, hydrogen sulfide gas, and oxygen to produce silver sulfide and water. Moisture does not directly cause the tarnish, but it plays a secondary role by significantly accelerating the reaction. High humidity acts as an electrolyte, creating a favorable environment that allows the sulfur compounds to more easily interact with the silver surface. The resulting silver sulfide layer dulls the metal’s highly reflective surface, causing discoloration.
Practical Steps for Prevention and Removal
Since tarnish is caused by sulfur and accelerated by moisture, prevention methods focus on reducing exposure to these two elements. Storing silver in a cool, dry place with low humidity is a simple way to slow down the tarnishing process. Pieces can be wrapped in anti-tarnish cloth or acid-free tissue paper and kept in airtight containers to create a barrier against atmospheric sulfur compounds. Anti-tarnish strips or packets of silica gel can also be placed inside storage containers to absorb gases and moisture.
Tarnish can be removed using chemical and electrolytic methods, which are often preferred over abrasive silver polishes that can wear away the metal over time. A popular household technique is the electrolytic cleaning method, which uses a bath of hot water, baking soda, and aluminum foil. The tarnished silver is placed in contact with the aluminum foil within this solution, and a chemical reaction occurs where the aluminum trades places with the silver, converting the black silver sulfide back into silver metal. The baking soda serves to remove a protective oxide layer from the aluminum, ensuring the reaction proceeds. After the tarnish transfers to the foil, the silver should be rinsed and dried thoroughly with a soft cloth to prevent spotting.