Tarnish is defined as a dulling or darkening on a metal’s surface, resulting from a chemical reaction with surrounding elements like oxygen or sulfur. The simple answer is that pure gold, in its 24-karat form, does not chemically tarnish. Discoloration is a frequent experience, however, because most gold items are not pure gold but are alloys mixed with highly reactive metals. This distinction between the pure element and the manufactured alloy determines a piece’s long-term resistance to discoloration.
The Chemical Stability of Pure Gold
The element gold, with the chemical symbol Au, is classified as a noble metal, known for its high resistance to chemical attack. Gold’s atomic structure makes it one of the least reactive metals on the periodic table. This inherent stability means it does not readily bond with oxygen or moisture found in the air, the process that causes rust in metals like iron. Pure gold remains chemically inert even when exposed to most common acids or atmospheric sulfur compounds. This resistance to oxidation and corrosion establishes the scientific baseline for its enduring luster.
How Alloys Affect Tarnish Resistance
Pure gold is soft and easily scratched, making it impractical for everyday jewelry. For this reason, gold is mixed with other metals to create an alloy that provides strength and durability. The purity of the gold alloy is measured in karats (K); lower numbers like 18K, 14K, or 10K indicate a higher proportion of non-gold metals. It is these non-gold metals that are the true source of discoloration.
The most common alloy metals are copper and silver, both of which react easily with their environment. Silver reacts readily with sulfur compounds to form silver sulfide, a dark layer that appears as tarnish. Copper readily oxidizes when exposed to air and moisture, leading to a darkening or even a greenish hue. Consequently, a lower karat gold, such as 10K, is significantly more susceptible to tarnishing than 18K gold.
Identifying Causes of Discoloration
Discoloration on gold alloys is accelerated by a variety of external environmental factors that trigger a reaction with the copper and silver content. Exposure to chlorine is one of the most aggressive threats, as the chemical can react with the alloy metals to form metal chlorides, which appear as black spots or a generally dull surface. This makes wearing gold jewelry in chlorinated pools or while using bleach-based cleaning products a primary cause of quickened tarnish.
Sulfur compounds, naturally present in the air, cause gradual darkening, but they are highly concentrated in substances like rubber, certain foods, and industrial pollutants. Chemical residues from common personal care products also contribute to surface dullness. Lotions, cosmetics, perfumes, and hairspray leave a thin film on the metal that traps moisture and sweat, creating a micro-environment that accelerates the alloy metals’ reaction. Furthermore, an individual’s body chemistry, particularly highly acidic sweat, can increase the rate at which the non-gold metals corrode.
Restoring the Shine
Fortunately, because tarnish on gold alloys is a surface-level phenomenon, the original shine can usually be restored through simple, safe cleaning methods. A mild solution of warm water and a few drops of gentle dish soap is effective for removing residues and light tarnish. The piece should be soaked for a few minutes to loosen any buildup, then gently scrubbed with a soft-bristled brush, such as a soft toothbrush, to dislodge the dull film.
After cleaning, the jewelry must be thoroughly rinsed under clean water to remove all soap residue, as any remaining film can attract dirt and speed up future discoloration. The final step is to dry the piece completely with a soft, lint-free cloth or let it air dry before wearing or storing. For heavy discoloration or buildup in intricate settings, professional cleaning with specialized equipment, like ultrasonic cleaners, may be necessary to safely restore the metal’s bright finish.