The blackening that occurs on silver coins is not rust, but rather a process known as tarnishing. Tarnish forms when silver is exposed to sulfur compounds in the surrounding air, resulting in a natural chemical reaction. This discoloration is a thin, dark layer of silver sulfide, which is distinct from the oxidation that affects other metals. The presence of silver sulfide alters the coin’s surface reflectivity, causing it to appear dull and eventually black.
The Chemical Reaction Behind Silver Tarnish
The dark coating on a silver coin is the result of a chemical bond between silver (Ag) and airborne sulfur compounds, most commonly hydrogen sulfide (H2S). Trace amounts of this gas are present in the atmosphere, often released from industrial processes or the decomposition of organic matter. The reaction creates silver sulfide (Ag2S).
The resulting layer of silver sulfide is extremely thin, yet highly visible due to its dark color and the way it scatters light. This reaction occurs even on pure silver, though the rate of tarnish varies depending on the metal’s purity. Most silver coins are not pure silver, but rather an alloy like sterling silver, which contains 7.5% copper for added strength.
The inclusion of copper accelerates the tarnishing process compared to fine silver (99.9% silver). Copper is more reactive than silver and readily combines with moisture and sulfur compounds, contributing to the discoloration. Consequently, coins made from sterling silver or other lower-grade alloys blacken much faster.
Environmental Factors That Speed Up Blackening
The rate at which a silver coin turns black depends heavily on the storage environment. High relative humidity is a major contributor, as moisture on the coin’s surface helps dissolve sulfur gases, making them more reactive with the metal. Coins stored in damp basements or coastal regions tarnish faster than those kept in dry, stable environments.
Air pollution is another significant factor, especially in areas with high levels of industrial emissions or vehicle exhaust, which introduce more sulfur compounds. Many common household items also contain sulfur and cause rapid tarnish if they contact silver. Human contact contributes to the problem, as natural oils, sweat, and acidity from skin accelerate the reaction.
Sources of Sulfur
Common household items that contain sulfur include:
- Certain types of rubber bands
- Wool felt
- Latex paint
- Foods like eggs and onions
Safe Methods for Reversing Tarnish
When reversing tarnish on a valuable silver coin, the method must balance cleanliness with the risk of damaging the surface or numismatic value. Commercial dips or polishes use chemical agents or fine abrasives to remove the silver sulfide layer. However, abrasive polishes can leave minute scratches, and aggressive chemical dipping can strip away desirable natural toning, potentially decreasing the coin’s worth.
A safer, non-abrasive method uses an electrochemical reduction process that converts the silver sulfide back into metallic silver. This technique requires a bowl lined with aluminum foil, hot water, baking soda, and salt. When the coin touches the aluminum in the hot, electrolytic solution, the aluminum reacts with the silver sulfide, pulling the sulfur from the silver and restoring the metal’s original surface. The baking soda removes the thin layer of aluminum hydroxide that forms, ensuring the reaction proceeds effectively.
How to Slow Down Future Tarnish
Slowing tarnish involves environmental control and proper storage to minimize the coin’s exposure to sulfur and moisture. Storing silver coins in airtight containers is a primary defense, limiting contact with airborne sulfur compounds and general pollution. Use inert storage materials, such as certified coin holders or PVC-free plastic flips, since certain materials can off-gas chemicals that accelerate tarnishing.
Controlling humidity is effective, often achieved by placing desiccants, such as silica gel packets, inside the storage container to absorb excess moisture. Anti-tarnish strips or cloths are also recommended, as they absorb sulfur gases within the enclosed space, acting as a chemical shield. Finally, minimize direct handling; use cotton gloves when touching coins to prevent transferring skin oils and salts that initiate the process.