Yes, silver coins do tarnish. Tarnish is a form of discoloration that appears on the surface of silver items, making them look dull, yellow, brown, or eventually black. This natural chemical process begins the moment a coin is exposed to the environment. It is a superficial reaction that creates a thin layer on the metal, distinct from rust that compromises the structural integrity of iron-based metals.
The Chemical Process of Tarnish Formation
Tarnishing occurs when silver atoms react with sulfur compounds present in the air, a process known as sulfidation. The main culprit is often hydrogen sulfide gas (\(\text{H}_2\text{S}\)), a common airborne pollutant, which reacts with the silver (\(\text{Ag}\)) on the coin’s surface. This interaction forms a thin film of silver sulfide (\(\text{Ag}_2\text{S}\)), the substance responsible for the discoloration. The process is self-limiting because the silver sulfide layer acts as a barrier, protecting the underlying metal from further reaction, unlike the continuous corrosion seen with rust.
The purity of the silver significantly affects the speed of this reaction. Pure silver, such as 0.999 fine bullion, resists tarnish better than lower purity alloys. Most silver coins are alloys, often mixed with copper for durability, such as sterling silver (92.5% silver) or coin silver (90% silver). The copper content in these alloys is more reactive to sulfur and oxygen, contributing to the tarnishing process and sometimes producing a greenish tint in addition to the black silver sulfide.
Environmental Factors That Accelerate Tarnish
The rate at which a silver coin tarnishes is heavily influenced by its environment. High humidity and moisture levels accelerate the chemical reaction by helping to dissolve sulfur gases, making them more available to react with the silver surface. A damp environment can quickly turn light yellow tarnish into a dark, thick layer.
The presence of sulfur-containing materials in the storage area is a major factor. Common household items like wool, felt, latex paint, certain types of paper, and especially rubber bands can emit sulfur compounds that rapidly tarnish silver. Airborne pollutants, particularly industrial and vehicle emissions containing sulfur dioxide, also contribute to a faster tarnishing rate. Contact with acidic or salty residues, such as oils and perspiration left by fingerprints, can initiate or accelerate localized tarnish.
Safe Storage and Handling for Prevention
Preventing tarnish begins with proper handling to avoid introducing contaminants to the coin’s surface. Always handle silver coins by their edges using soft cotton gloves or specialized non-latex gloves. This prevents the transfer of skin oils, salts, and acids that accelerate the sulfidation process.
Storing coins in a cool, dark, and dry environment is the first line of defense against tarnish. Airtight storage is the standard for preservation, as it limits the coin’s exposure to sulfur compounds and moisture. Individual coins should be kept in hard plastic capsules, known as “air-tites,” or in inert plastic flips.
Recommended Storage Materials
- Mylar
- Polyethylene
- Polypropylene
It is important to avoid older storage materials, such as soft PVC (polyvinyl chloride) flips, which can break down and leach chemicals that cause irreversible damage and discoloration. Adding anti-tarnish strips, which absorb airborne sulfur compounds, or silica gel packets, which control humidity, inside the storage container provides an extra layer of protection.
Cleaning Tarnished Coins: Methods and Warnings
The general rule among coin collectors is to avoid cleaning silver coins, especially those with numismatic value, as cleaning can permanently reduce their market worth. The natural tarnish, often called “toning” or “patina,” is sometimes desirable and represents the coin’s authenticity and history. Cleaning can leave microscopic scratches or alter the surface, which a professional grading service will detect, resulting in a lower grade and a significant loss in value.
For non-collectible bullion or heavily tarnished silver where aesthetic is the only concern, a gentle soak in mild, phosphate-free soap and distilled water is the safest method. Specialized commercial silver dips can remove tarnish quickly, but they work by chemically dissolving a microscopic layer of the coin’s surface and should be used with caution and for very brief periods. Methods involving abrasive substances, such as creating a paste with baking soda, should be avoided entirely for any collectible coin, as the tiny crystals will scratch the surface. Even the popular aluminum foil and baking soda method, which uses an electrochemical reaction to reverse the tarnish, should be approached with extreme care, as it can be overly aggressive and strip away desirable toning.