Gold alloys can and often do tarnish over time. This discoloration is a chemical reaction that affects the surface of the metal, causing it to darken or dull. Pure gold is a noble metal that is chemically inert and does not react with oxygen or sulfur compounds in the air. However, pure gold is too soft for practical jewelry use, so it is alloyed with other metals to increase its strength and durability. It is these non-gold components in the alloy that are susceptible to tarnishing, which is primarily a form of sulfidation or oxidation.
The Contribution of Non-Gold Metals
The fundamental reason gold alloys tarnish lies with the reactive non-gold metals mixed into the composition, most commonly copper and silver. Gold itself is highly stable, possessing a chemical structure that resists bonding with other elements like oxygen or sulfur. The alloying metals, however, are not noble and readily react with compounds found in the environment.
When gold jewelry is exposed to air, moisture, or chemicals, the copper and silver on the surface oxidize or react with sulfur-containing gases. This reaction forms a thin layer of copper sulfide or silver sulfide on the surface, seen as a dark, dull film that obscures the natural luster of the gold. Copper sulfide often appears as a reddish-brown discoloration; silver sulfide creates a darker, sometimes black, film.
How Gold Purity Affects Tarnish
The gold karat system serves as a direct indicator of an alloy’s tarnish resistance because it represents the percentage of pure gold. Karats are measured on a scale of 24: 24-karat gold is 100% pure, 18-karat gold is 75% pure, and 10-karat gold is only 41.7% pure. Since only the non-gold metals tarnish, alloys with a lower karat value contain a significantly higher proportion of reactive metals, making them much more susceptible to discoloration.
For instance, 10-karat gold tarnishes faster and more noticeably than 18-karat gold because its composition is over 58% non-gold metals. The type of alloy also plays a major role, as evidenced by rose gold, which achieves its color from a high concentration of copper. This increased copper content, while producing a distinct pink hue, dramatically increases the alloy’s reactivity. White gold, typically alloyed with nickel, palladium, or silver, is often plated with rhodium, a noble metal that offers an additional protective layer against tarnish.
External Factors That Accelerate Tarnish
While the alloy composition determines the potential for tarnish, external factors act as catalysts that speed up the chemical reaction. Household cleaning products are a major accelerant, particularly those containing chlorine or bleach, which can weaken the alloy and cause surface pitting or staining. Exposure to sulfur compounds in the environment also hastens the sulfidation process.
Sulfur-containing agents are commonly found in air pollution, rubber products, and certain cosmetic items like lotions, perfumes, and hairsprays. Human body chemistry also contributes, as sweat contains salts and acids that react with the copper and silver in the alloy. High humidity and moisture exposure further promote the tarnishing process by creating a favorable environment for the chemical reactions to occur.
Cleaning and Preventing Tarnish
Cleaning light tarnish from gold alloys is easiest using a gentle approach with household materials. Submerge the jewelry in warm water mixed with a few drops of mild, non-detergent dish soap, then use a soft-bristled brush to gently scrub the surface and remove the tarnish film or buildup. After cleaning, rinse the piece thoroughly with clean water and pat it completely dry with a soft, lint-free cloth.
Preventative care is the most effective strategy for maintaining the alloy’s shine. Always remove gold jewelry before using harsh chemicals, cleaning, swimming in chlorinated pools or saltwater, or engaging in activities that cause heavy perspiration. Proper storage is also beneficial, as keeping pieces in a dry, airtight container or a jewelry box lined with anti-tarnish fabric helps limit exposure to humidity and airborne sulfur. Wiping the jewelry down with a soft cloth after each wear removes skin oils and product residue, slowing the rate of tarnish formation.