Tarnish describes a dulling or discoloration on a metal’s surface. While gold is widely recognized for its resistance to such changes, gold jewelry can appear tarnished under specific circumstances. This is not typically due to the gold itself, but rather the other metals mixed with gold to create a more durable material. Understanding the characteristics of gold alloys, chemical processes, and environmental interactions clarifies why this occurs.
The Unique Properties of Gold
Pure gold (24-karat) possesses exceptional chemical inertness, resisting reaction with oxygen or most other elements. This is why pure gold does not corrode or tarnish. However, pure gold is very soft and malleable, making it unsuitable for most jewelry applications. To enhance its strength and durability, pure gold is combined with other metals, forming gold alloys.
Alloying metals frequently include copper, silver, nickel, palladium, or zinc. The karat rating indicates the proportion of pure gold; for instance, 18K gold contains 75% pure gold, while 14K gold contains 58.3%. These non-gold metals within the alloy are susceptible to reactions that lead to tarnishing. The higher the karat number, the greater the gold content and the less susceptible the alloy is to tarnishing.
The Chemical Reactions Behind Tarnishing
Tarnishing of gold alloys primarily involves chemical reactions between non-gold metals and sulfur-containing compounds in the environment. Metals like silver and copper, common components, readily react with these sulfur compounds. This reaction forms metal sulfides, such as silver sulfide or copper sulfide, which appear as a dark, dull film on the jewelry’s surface. This layer of metal sulfides is responsible for the black or brown discoloration.
While sulfur reactions are the predominant cause, oxidation (a reaction with oxygen) can also contribute to discoloration. Oxidation causes metals to lose electrons when exposed to oxygen, leading to changes in appearance. Pure gold itself has very low reactivity and is unlikely to oxidize. The tarnishing process is superficial, affecting only the outermost layer of the metal alloy.
Everyday Factors That Cause Tarnishing
Various environmental and contact factors contribute to the tarnishing of gold alloys.
Environmental Factors
Airborne pollutants, such as hydrogen sulfide and sulfur dioxide, are common sources of sulfur compounds. High humidity levels can accelerate these chemical reactions.
Chemical Exposure
Household cleaning products, particularly those containing ammonia or chlorine (like bleach), can significantly accelerate tarnishing and cause damage. Cosmetics, perfumes, and hairsprays also contain chemicals that react with alloy metals.
Body Chemistry
Even natural skin oils and sweat, which contain sulfur and acids, can interact with the metals in gold jewelry, leading to tarnishing over time.
Preventing Gold Tarnishing
Preventing gold alloy tarnishing involves minimizing exposure to reactive agents.
Proper Storage
Jewelry should be kept in a dry, cool place, away from direct sunlight and humidity. Anti-tarnish bags, cloths, or pouches can limit air exposure and prevent scratches. Adding silica gel packets to jewelry boxes can absorb excess moisture.
Activity Avoidance
Remove gold jewelry before activities involving exposure to chemicals or moisture. This includes showering, swimming (especially in chlorinated pools), cleaning with household products, exercising, or applying cosmetics and perfumes.
Regular Care
Gently wiping jewelry with a soft cloth after each wear helps remove skin oils and residues. Regular, gentle cleaning with mild soap and warm water helps maintain the jewelry’s luster.