The discoloration often observed on gold jewelry is commonly mistaken for the oxidation of the gold itself, but the reality is more chemically complex. Pure gold is one of the most stable elements, but the gold items people wear daily are mixed with other metals, which are the true source of any color change. The visual changes, ranging from simple dullness to dark spots, signal a chemical reaction involving these companion metals and substances in the environment. Understanding this distinction helps in knowing how to prevent and correct the cosmetic issue many jewelry owners face.
Why Pure Gold Does Not Oxidize
Gold, represented by the chemical symbol Au, is classified as a noble metal, prized for its exceptional resistance to chemical reactions. The atomic structure of gold is inherently stable, meaning its outer electrons are strongly bound and not easily shared with other elements like oxygen or sulfur. This characteristic makes elemental gold nearly inert, preventing it from forming oxides or corroding under normal environmental conditions. A piece of pure gold, known as 24 Karat (24k) gold, will not tarnish, rust, or change color, even when exposed to air and water for centuries. The stability of pure gold means that the discoloration seen on jewelry is never the result of the gold atoms themselves oxidizing.
The True Cause of Gold Jewelry Discoloration
The discoloration seen on gold jewelry is a reaction involving the metal alloys mixed with the gold to increase its strength and durability. Pure 24k gold is too soft for everyday wear, so it is combined with metals like copper, silver, and zinc to create alloys such as 18k, 14k, or 10k gold. The lower the karat level, the higher the proportion of these more reactive non-gold metals, and the more susceptible the piece is to tarnishing.
Sulfidation and Perspiration
One primary mechanism for this color change is sulfidation, where the silver and copper in the alloy react with sulfur compounds found in air pollution, rubber, and certain foods. This reaction forms a thin layer of silver sulfide or copper sulfide on the surface, which is visible as tarnish. Perspiration also plays a significant role, as the body’s natural sweat and oils contain sulfur and chloride compounds that accelerate the chemical process.
Chlorine Exposure
Another potent cause of discoloration is exposure to chlorine, particularly in swimming pools or household cleaning products. Chlorine is highly reactive and can attack the alloyed metals, causing rapid corrosion that manifests as a color change. This chemical assault is especially damaging to lower-karat pieces.
Identifying Tarnish Versus Surface Residue
When people ask what “oxidized gold” looks like, they are describing the visual result of these alloy reactions. True chemical tarnish typically presents as a black or dark brown film, often appearing as streaks or dark spots on the metal surface. In lower karat gold, particularly if it has a high copper content and has been exposed to acidic environments or chlorine, the discoloration can sometimes appear with a greenish or bluish tint.
This chemical tarnish is distinct from the cloudy appearance caused by common surface residue, which is not a chemical reaction of the metal itself. Residue from makeup, lotions, perfumes, skin oils, or soap scum accumulates on the jewelry, causing a general dullness or a film that obscures the metal’s natural luster. Unlike tarnish, which is a chemical alteration of the alloy’s surface, this residue is simply a buildup of foreign material that can be easily wiped away. The location of the discoloration can also be a diagnostic clue; tarnish is often concentrated in crevices and areas of high friction, while surface residue tends to create a uniform, hazy coating. The extent of the color change is directly related to the alloy composition, with 10k gold showing a greater tendency toward dark spots than 18k gold.
Cleaning and Prevention Methods
Cleaning chemically tarnished gold involves carefully removing the thin layer of metal sulfide or oxide without damaging the piece. A simple and safe method is to soak the jewelry in a solution of warm water mixed with a few drops of mild dish detergent. After soaking for 10 to 15 minutes, the piece can be gently scrubbed with a soft-bristled toothbrush to dislodge buildup, followed by a thorough rinse with clean water and patting dry with a soft, lint-free cloth. For more stubborn tarnish, a professional polishing cloth designed for jewelry can buff the surface and restore the shine. Milder approaches are always recommended, and harsh chemicals like bleach or ammonia should be avoided, as they can damage the alloy and structural integrity.
Preventing discoloration relies on minimizing the jewelry’s exposure to reactive compounds and moisture.
- Always remove gold items before swimming, especially in chlorinated water.
- Remove jewelry before using household cleaners, which often contain harsh chemicals.
- Apply cosmetics, lotions, and perfumes before putting on jewelry to ensure ingredients have dried.
- Store gold pieces in a cool, dry place, ideally in individual soft pouches or lined jewelry boxes, to prevent scratching and reduce exposure to air and moisture.