A common misconception is that gold is entirely impervious to chemical change, suggesting a pure gold coin should remain flawless forever. Tarnish is a chemical surface reaction that dulls a metal’s shine, but gold fundamentally resists this process. Gold is classified as a noble metal, meaning it should not react with its environment. If pure gold is so chemically resilient, why do some gold coins eventually appear dull, spotted, or darkened? The answer lies not in the gold, but in the other metals mixed with it to create a durable coin.
Understanding Gold’s Resistance to Corrosion
Gold’s exceptional stability is rooted in its atomic structure, placing it in a class of elements known as noble metals. This classification indicates that the metal resists chemical action, particularly oxidation and corrosion, under ordinary conditions. The gold atom holds its outermost electrons tightly, making them unavailable to form bonds with other elements.
Tarnish occurs when a metal’s surface reacts with sulfur compounds or oxygen in the environment. While iron rusts through oxidation and silver tarnishes through sulfidation, pure gold (Au) does not readily participate in these reactions. Its low chemical reactivity means it will not form stable oxides or sulfides when exposed to air, moisture, or most common acids. This inherent resistance is why 24-karat gold, which is nearly 100% pure, maintains its brilliant luster indefinitely.
This establishes that any discoloration found on a gold coin cannot be the result of the gold atoms themselves tarnishing. Only extremely harsh chemical agents, such as the corrosive acid mixture aqua regia, can dissolve gold. Therefore, the presence of tarnish must be attributed to something other than the gold content.
The True Cause of Discoloration in Gold Coins
The vast majority of gold coins, including high-purity bullion, are alloys containing other metals to increase their strength. Pure gold is highly malleable and too soft for circulation or maintaining sharp numismatic detail. Therefore, it is combined with metals like copper and silver. These alloying components are significantly more reactive than gold and are the actual culprits behind any discoloration.
The most common form of discoloration is caused by the oxidation of copper content within the alloy, manifesting as small, dark spots or a reddish haze. Copper is highly susceptible to reacting with oxygen and moisture in the air, creating a copper oxide layer on the coin’s surface. This is particularly noticeable in coins with lower gold content, where a higher percentage of copper is used for greater durability.
Another primary source of surface change is sulfidation, where reactive metals, especially silver, interact with sulfur compounds in the atmosphere. Exposure to high humidity or contact with materials like cardboard, rubber bands, or certain plastics can accelerate these chemical reactions. For instance, storing a coin in a soft plastic flip made of Polyvinyl Chloride (PVC) can lead to a sticky, green residue due to the plasticizer reacting with the alloy metals.
Protecting and Cleaning Your Gold Investment
The preservation of a gold coin relies heavily on minimizing its exposure to environmental elements and reactive substances that cause alloying metals to discolor. The best practice involves proper storage, meaning keeping coins in airtight containers made of inert materials. Hard plastic capsules, acrylic holders, or polyester film (Mylar) are preferred because they do not off-gas chemicals that react with the metal surface.
Handling should be avoided whenever possible, as oils and salts from human skin can transfer to the coin and accelerate corrosive processes. When handling is necessary, lint-free cotton gloves should always be worn, and the coin should be held only by its edges. Storing gold coins in a climate-controlled area with low humidity will further reduce the likelihood of discoloration.
Cleaning gold coins is generally discouraged by experts, as aggressive action can scratch the surface and diminish the coin’s numismatic value. For coins with light surface dust or minor, non-corrosive residue, a quick rinse in warm, distilled water with a small amount of mild, non-detergent soap may be used. The coin must be dried completely using a gentle air-dry method or a soft, inert cloth to prevent water spots. Harsh chemicals, abrasive materials, and excessive rubbing must never be used, as these actions cause irreversible damage to the coin’s finish.