Understanding Gold’s Stability
Pure gold, known scientifically as Au, is classified as a noble metal. It exhibits exceptional chemical inertness, resisting reactions with most elements and compounds. Gold does not readily oxidize or tarnish when exposed to air, moisture, or common environmental factors. Its atomic structure makes it inherently stable, preventing it from forming new compounds that could alter its appearance.
However, the gold used in jewelry is rarely pure gold. Jewelers create gold alloys by mixing pure gold with other metals, such as copper, silver, zinc, or nickel, to enhance its durability, alter its color, and reduce its cost. The karat system indicates the proportion of pure gold in an alloy; for instance, 14-karat gold contains 14 parts gold and 10 parts other metals out of 24. It is these non-gold metals in the alloy that are susceptible to chemical reactions and are primarily responsible for any discoloration observed in gold jewelry.
Why Gold Jewelry Discolors
The discoloration of gold jewelry, often appearing as a dark or black residue, primarily results from chemical reactions involving the non-gold metals within the alloy. A common cause is sulfur compounds in the environment. These compounds, found in polluted air, some rubber products, and even certain foods like onions and eggs, react with metals like copper and silver in the alloy to form dark metal sulfides. These sulfides appear as a black film on the jewelry’s surface.
Chlorine, found in swimming pools, hot tubs, and household cleaning products, also contributes to discoloration. Chlorine reacts aggressively with alloy metals, particularly nickel and copper, forming metal chlorides. These compounds appear as black or green stains and can even corrode the jewelry’s surface. Additionally, everyday substances such as cosmetics, perfumes, lotions, and sweat can contain chemicals that react with the alloy metals. These reactions can deposit a dark residue or accelerate the tarnishing process.
Restoring and Protecting Gold
Restoring discolored gold jewelry involves gentle cleaning to remove tarnish or residue. A method is to soak the jewelry in a solution of warm water mixed with a few drops of mild dish soap for several minutes. Gently scrub with a soft-bristled brush, such as an old toothbrush, to dislodge any film. Rinse the jewelry thoroughly under clean running water and dry it completely with a soft, lint-free cloth.
To prevent future discoloration, take several protective measures. Remove gold jewelry before exposure to harsh chemicals, such as cleaning, swimming in chlorinated water, or applying lotions and perfumes. Proper storage is also important; storing jewelry in a cool, dry place, ideally in individual cloth pouches, minimizes exposure to air and humidity. Regularly cleaning gold jewelry, even when it does not appear discolored, helps remove skin oils and environmental contaminants before they can cause reactions.