Tarnishing is a dulling or discoloration of metal surfaces, caused by a chemical reaction that forms a thin layer of corrosion. While this process affects many metals, gold generally resists tarnishing. Pure gold largely remains unaffected, but understanding why some gold items do tarnish requires examining their composition and environmental interactions. The difference lies in whether the gold is pure or alloyed with other metals.
The Nature of Pure Gold
Pure gold, specifically 24-karat gold, is nearly 99.9% pure. This high purity is the primary reason it does not tarnish. Gold is classified as a “noble metal,” indicating its low chemical reactivity.
Unlike many other metals, pure gold does not readily react with oxygen, sulfur, or most other common environmental substances. This chemical inertness means it resists forming compounds on its surface, which leads to tarnishing. Consequently, 24-karat gold maintains its shine and appearance over long periods.
How Gold Alloys Tarnish
Gold jewelry is rarely made from pure gold because 24-karat gold is very soft and susceptible to scratching and deformation. To enhance its hardness, durability, and sometimes alter its color, gold is alloyed with other metals like copper, silver, zinc, and nickel.
The tarnishing on gold jewelry occurs not because of the gold itself, but due to these other metals in the alloy. These reactive metals, particularly copper and silver, can chemically react with environmental elements. Such reactions often involve sulfur compounds, oxygen, and moisture, forming a dark, dull layer on the surface.
For instance, silver in a gold alloy can react with hydrogen sulfide to form silver sulfide, appearing as dark spots. Copper can also oxidize, resulting in discoloration. This chemical change on the surface of the alloying metals is what is perceived as gold tarnishing.
Understanding Gold Purity and Tarnishing
The purity of gold in jewelry is measured in karats. The proportion of pure gold to alloying metals directly influences the likelihood and severity of tarnishing. Lower karat gold, such as 10K or 14K, contains a higher percentage of more reactive alloying metals compared to 18K or 22K gold.
For instance, 10K gold is about 41.7% pure gold, while 14K gold is approximately 58.3% pure gold. With a greater proportion of copper, silver, or nickel, lower karat gold is more susceptible to tarnishing. Different alloy compositions also affect tarnishing.
Yellow gold, often alloyed with silver and copper, is generally resistant. White gold may contain nickel or palladium and can be rhodium-plated to enhance its white appearance and tarnish resistance. While white gold itself does not typically tarnish, the rhodium plating can wear off, revealing the underlying gold alloy which may have a yellowish tint or contain reactive metals.
Minimizing Tarnishing and Care
While pure gold does not tarnish, gold alloys can, making proper care beneficial. Environmental factors and substances commonly encountered can accelerate the tarnishing process. These include sweat, cosmetics, perfumes, lotions, cleaning products, chlorine, and air pollution, especially sulfur compounds.
To reduce tarnishing:
Store gold jewelry in a dry, cool place, away from moisture and air exposure.
Store individual pieces in soft pouches or jewelry boxes with separate compartments to prevent scratches and limit exposure to elements.
Clean regularly with mild soap and warm water to remove dirt, oils, and residues that can contribute to tarnish.
Remove jewelry before swimming, showering, applying cosmetics, or using harsh chemicals to protect the alloying metals from reactive substances.