Real silver does not rust, which is a common misconception often resulting from the dark discoloration that appears on the metal’s surface. Rust is a term that specifically describes the corrosion of iron, a process where iron reacts with oxygen and water to form iron oxide, a reddish-brown substance. Silver, a different type of metal, undergoes a distinct chemical reaction that results in a black or grayish film, properly called tarnish.
Clarifying the Terminology: Rust versus Tarnish
Rust and tarnish are both forms of corrosion, but they involve different chemical processes and compounds. Rust is the irreversible degradation of iron or iron alloys, like steel, through oxidation. The final product is hydrated iron(III) oxide, which flakes off, continually exposing fresh metal to the elements and compromising the object’s structural integrity.
Tarnish, by contrast, is a surface-level phenomenon that forms on metals such as silver, copper, and brass. This reaction is primarily a form of sulfidation rather than oxidation. The resulting layer is silver sulfide (\(\text{Ag}_2\text{S}\)), which appears as a dark film. This dark layer does not compromise the underlying metal structure and often acts as a protective barrier against further reaction, unlike rust.
Silver’s Minimal Reaction with Pure Water
Silver is classified as a noble metal, meaning it is highly resistant to chemical attack and corrosion from pure water. If a silver object were submerged in clean, distilled \(\text{H}_2\text{O}\) with only dissolved oxygen, virtually no degradation would occur. The issue arises when water is not entirely pure, which is the case for tap water, pool water, or seawater. Impurities like chlorine and various salts found in non-distilled water can significantly accelerate the tarnishing process. While the water itself is not the direct cause of the black discoloration, its presence facilitates the chemical reactions involving other compounds that lead to tarnish.
The True Cause of Silver Degradation: Sulfur Compounds
The primary chemical agent responsible for silver tarnish is sulfur, which is abundantly present in the environment as various compounds. Silver reacts most readily with hydrogen sulfide (\(\text{H}_2\text{S}\)), a gas found in trace amounts in the air from sources like pollution and the natural breakdown of organic matter. This reaction forms the thin layer of silver sulfide on the surface.
The presence of moisture and higher temperatures accelerates the rate of this sulfidation reaction. Sulfur compounds can also come from surprising household sources, including certain foods like eggs and onions, as well as materials like wool felt, latex gloves, and rubber bands. Even the oils and sweat from human skin contain sulfur compounds that react with silver jewelry. The reaction happens even when silver is stored, as long as it is exposed to air containing these trace sulfur gases.
Practical Steps for Preventing Tarnish
Preventing tarnish involves minimizing a silver item’s exposure to the sulfur compounds and moisture that accelerate the reaction. Proper storage is one of the most effective strategies for long-term preservation.
Storage and Maintenance Tips
- Store silver in airtight containers or specialized anti-tarnish bags to limit its exposure to atmospheric hydrogen sulfide.
- Placing anti-tarnish strips or packets of silica gel inside storage containers can absorb airborne sulfur and excess moisture, creating a drier, less reactive micro-environment.
- Avoid storing silver near materials known to emit sulfur, such as rubber or newspaper, which uses sulfur-containing inks.
- Regular, gentle cleaning with a soft polishing cloth specifically made for silver can remove the very early stages of tarnish.
- These early stages often appear as a subtle yellowing, which should be removed before they develop into dark, visible discoloration.