Silver does not rust, but it develops a dark layer on its surface over time. As a precious metal, silver (Ag) is chemically distinct from base metals that undergo true corrosion. Understanding the chemical differences between this surface change and rust clarifies why silver maintains its structural integrity even when discolored. This unique surface reaction is a natural process that can be managed easily.
The Direct Answer: Silver Does Not Rust
Rust is strictly defined as the corrosion of iron (Fe) or iron-containing alloys like steel. This oxidation reaction requires oxygen and water, leading to the formation of hydrous iron(III) oxides, the familiar reddish-brown compounds. Silver is classified as a noble metal, meaning it resists chemical attack and does not readily react with oxygen (O₂) under normal atmospheric conditions. Because silver is not iron and does not undergo this specific reaction, it cannot form iron oxide. The low chemical reactivity of silver allows it to resist oxidation and corrosion, which is why it is valued.
Understanding Silver Tarnish
The dark discoloration seen on silver is not rust, but a compound called silver sulfide (Ag₂S). This surface layer forms through sulfidation, not oxidation, when silver metal contacts sulfur-containing compounds in the environment. These compounds, such as hydrogen sulfide (H₂S) or sulfur dioxide (SO₂), react with the silver to create the silver sulfide layer. Silver sulfide is a stable, dark-gray or black compound that adheres to the surface, causing the metal to appear dull. Unlike rust, which flakes away and deteriorates the iron, the silver sulfide layer does not cause the underlying silver metal to deteriorate.
Common Causes of Tarnish Acceleration
The presence of sulfur compounds in the air is the primary driver of the sulfidation process. Air pollution, containing elevated levels of hydrogen sulfide and sulfur dioxide from industrial emissions, significantly accelerates this reaction. High humidity also increases the rate of tarnishing by providing a medium for the chemical reaction. Many everyday items contain sulfur and can cause rapid tarnish when placed near silver objects. Natural body oils and perspiration may also contribute to tarnish formation on jewelry.
Common Tarnish Sources
- Certain foods, such as eggs, onions, and mustard
- Materials like wool, felt, latex, and rubber products
Proper storage in airtight containers or anti-tarnish bags is a practical preventive measure against sulfur-containing materials.
Simple Methods for Cleaning Tarnished Silver
Tarnished silver can be cleaned through two main approaches: using a mild abrasive or employing an electrochemical method.
Abrasive Cleaning
For light tarnish, a soft cloth treated with a mild silver polishing cream or a gentle silver polishing cloth is effective. These products contain fine abrasives that carefully remove the silver sulfide layer without scratching the metal. Unlike abrasive polishes, which slightly rub away a microscopic layer of the metal, the electrochemical method does not remove any silver itself.
Electrochemical Cleaning
A non-abrasive method utilizes an electrochemical reaction to reverse the sulfidation and restore the silver. This involves placing the tarnished silver in a heat-resistant dish lined with aluminum foil, ensuring the silver touches the foil. Hot water, baking soda (sodium bicarbonate), and salt (sodium chloride) are added to the dish. The baking soda acts as an electrolyte, attracting the sulfur atoms from the silver sulfide to the aluminum metal, which transfers the sulfur and turns the silver sulfide back into silver metal. After soaking for several minutes, the silver should be removed, rinsed thoroughly with water, and dried with a soft cloth.