Gold jewelry often loses its luster or develops dark spots, leading to the assumption that the gold itself has changed color. Pure gold (24 Karat, chemical symbol Au) is one of the least chemically reactive elements; it does not tarnish, rust, or corrode under normal conditions. The perceived color change is not due to the gold reacting, but rather the other components mixed with it or a physical change to the surface. Nearly all gold jewelry is an alloy, and the chemical instability of these non-gold metals is the true source of discoloration.
The Stability of Elemental Gold and the Role of Alloys
Gold’s purity is measured using the karat system (K), where 24K represents 100% pure gold. Since pure gold is exceptionally soft and prone to scratching, it is alloyed with other metals like copper, silver, palladium, and zinc to increase its hardness and durability. For example, 14K gold contains 58.3% gold (14 parts out of 24), with the remaining 41.7% consisting of these other metals.
The specific metals chosen determine the final color of the jewelry. Yellow gold is typically alloyed with copper and silver. Rose gold gains its pink hue from a higher concentration of copper. White gold is created by mixing gold with white metals, such as palladium or nickel, to create a paler appearance. The lower the karat number, the higher the percentage of these alloying metals, which are generally more reactive than gold.
Chemical Reactions Causing Surface Discoloration
The most frequent cause of visible color change is tarnishing, a chemical reaction between the non-gold alloy metals and external compounds. This reaction is primarily observed in lower-karat gold pieces because they contain a greater proportion of reactive metals. The gold remains stable, but the alloy metals on the surface form new, dark or discolored compounds.
Environmental Factors
Sulfur compounds, found in air pollution, cosmetics, and natural rubber, are a main culprit in tarnishing. These compounds react with the silver content in the alloy to form silver sulfide, a black compound that creates a dark film on the jewelry’s surface. Exposure to chlorine, often found in swimming pools or household cleaners, can cause corrosion, pitting, and permanent damage to the alloy structure, particularly in low-karat gold.
Personal Chemistry
Body chemistry also contributes to discoloration through the acids and salts present in perspiration. When sweat contacts the copper alloy, it can form copper salts that are sometimes green or blue, leading to the temporary “green finger” effect. Lotions, perfumes, and other personal care products can also leave a residue or contain chemicals that accelerate the oxidation of the alloy metals.
Physical Wear and the Loss of Plating
A different type of color change occurs when the material on the jewelry’s surface is physically removed rather than chemically altered. This is most noticeable in rhodium-plated white gold and gold-plated items.
Rhodium-Plated White Gold
White gold alloys naturally possess a faint yellowish or grayish tint due to their gold content. They are coated with a thin layer of rhodium, a silvery-white metal, to achieve a brilliant white appearance. Over time, friction from daily wear causes the rhodium layer to wear away, exposing the warmer, yellowish color of the white gold alloy beneath. Rings often show this loss of plating earliest, typically requiring professional re-plating every one to two years to restore the finish.
Gold-Plated Items
Gold-plated or vermeil jewelry consists of a thin layer of gold bonded onto a base metal, often brass or copper. Physical abrasion, scratches, or prolonged rubbing can completely remove this thin surface layer of gold. Once the gold layer is breached, the underlying base metal is exposed, resulting in a color change that may appear silver, reddish, or grey depending on the core material.
Maintaining Color and Restoring Luster
Preventing color change involves limiting the jewelry’s exposure to chemical aggressors and abrasive forces. It is advisable to remove gold jewelry before using household cleaners, swimming in chlorinated water, or applying lotions and perfumes. Storing pieces separately in soft pouches or a lined jewelry box minimizes physical friction and tangling, which prevents premature wear of platings.
Cleaning Tarnish
For general dullness or light tarnish caused by chemical reaction, a simple cleaning method is effective. Soak the jewelry in a solution of mild dish soap and warm water, followed by a gentle scrub with a soft-bristled brush. This helps lift surface grime and tarnish. The piece should then be rinsed thoroughly and dried completely with a soft, lint-free cloth.
Professional Restoration
When discoloration is due to the physical loss of a surface layer, such as with white gold or plated items, simple cleaning will not restore the color. Professional intervention is necessary to restore the original appearance. Jewelers can re-apply the rhodium plating to white gold or chemically re-plate thin gold layers onto plated items, renewing the piece’s surface color and brilliance.