Tarnishing is a common chemical process that affects many metals, causing a loss of their metallic luster and a dulling of the surface. This reaction creates a thin, discolored film on items like jewelry, silverware, and decorative objects. Unlike rust, which is a deep, flaking form of iron corrosion, tarnish is a superficial coating that changes the metal’s color without compromising its structural integrity.
The Chemical Reaction That Causes Tarnish
Tarnish results from a chemical reaction called sulfidation, which differs from the oxidation that causes rust. The metal reacts with trace amounts of sulfur-containing gases in the air, most notably hydrogen sulfide ($\text{H}_2\text{S}$). This gas, often present from industrial pollution or decomposing organic matter, bonds with the metal atoms on the surface. The resulting compound is a metal sulfide, responsible for the characteristic dark, often black or grayish, discoloration.
For silver, the reaction forms a layer of silver sulfide ($\text{Ag}_2\text{S}$), a black compound that affects only the outermost atomic layers. The chemical transformation is represented by the equation: $\text{2Ag(s)} + \text{H}_2\text{S(g)} \to \text{Ag}_2\text{S(s)} + \text{H}_2\text{(g)}$. High relative humidity and moisture accelerate this reaction, as water acts as a medium for corrosive agents to interact with the surface. Because the sulfide layer is thin, it is considered a surface effect rather than deep, destructive corrosion.
Common Household Metals That Tarnish
The most recognizable metal to tarnish is silver, including sterling silver, due to its high reactivity with atmospheric sulfur compounds. The dark film of silver sulfide is particularly noticeable against the metal’s bright white color. Copper is also highly susceptible, though its tarnish often involves both sulfidation and oxidation, leading to a greenish or bluish-green layer known as verdigris (copper carbonate or oxide).
Brass and bronze, both copper alloys, also show vulnerability to environmental factors. Brass, a mix of copper and zinc, develops a dull oxide layer. Bronze, an alloy of copper and typically tin, tarnishes much like pure copper. These metals are prone to tarnish because their atomic structures readily bond with sulfur and oxygen, initiating the surface reaction quickly.
Safe Methods for Tarnish Removal
One common method for tarnish removal is mechanical polishing, which uses a polishing cloth impregnated with a fine abrasive to physically rub away the metal sulfide layer. This process is effective for mild tarnish and safe for most solid metal items, but it removes a microscopic amount of the underlying metal along with the tarnish. For heavily tarnished items, a more efficient, non-abrasive approach is the chemical reduction method, often performed using household supplies like aluminum foil, hot water, baking soda, and salt in a non-metal bowl.
The aluminum foil acts as an anode, and the hot water and baking soda solution act as an electrolyte, creating an electrochemical cell. When the tarnished silver touches the aluminum, the reactive aluminum “steals” the sulfur from the silver sulfide. This converts the tarnish back into silver metal while forming aluminum sulfide on the foil. This process reverses the tarnish chemically without removing any silver. However, the hot water and solution may damage porous gemstones or antique finishes, and this technique should be avoided for silver-plated items, as the base metal may react unexpectedly.
How to Slow Down the Tarnish Process
Preventing tarnish relies on limiting the metal’s exposure to the atmospheric sulfur that drives the chemical reaction. Storing items in a low-humidity environment is effective, as moisture accelerates the sulfidation process. For long-term storage, placing items in airtight plastic bags or containers minimizes contact with the air.
Specialized anti-tarnish strips or cloths are also available; these materials are treated with a chemical that absorbs sulfur gases from the air, creating a protective microenvironment. It is also beneficial to avoid storing metal items near common household materials that contain sulfur, which accelerates tarnishing. These materials include:
- Rubber bands
- Latex gloves
- Wool
- Certain foods like eggs, which release hydrogen sulfide