Pure gold, known as 24-karat gold, maintains its vibrant metallic yellow color almost indefinitely. The karat system measures purity, with 24K representing 99.9% pure gold. Since pure gold is too soft for everyday wear, nearly all gold jewelry is an alloy, mixed with other metals to increase strength and durability. This alloy content, not the gold itself, is responsible for almost all instances of discoloration, tarnishing, or color change.
The Elemental Stability of Pure Gold
Gold (Au) is classified as a noble metal, meaning it is highly resistant to chemical attack. This low reactivity is due to its stable electron configuration, which prevents it from easily forming chemical bonds with other elements like oxygen or water vapor.
Pure 24-karat gold will not oxidize, which is the chemical process of forming a metal oxide layer, commonly known as rust or tarnish. This inherent chemical stability means that elemental gold remains bright and untarnished, maintaining its color for millennia.
The Role of Alloys in Gold Discoloration
Gold used in jewelry is mixed with metals like copper, silver, zinc, and nickel to create a durable alloy. These non-gold additives are the primary source of discoloration. For instance, 14-karat gold is only 58.3% gold, with the remaining 41.7% composed of these more reactive alloy metals.
The non-gold metals readily react with sulfur compounds in the air, creating a thin, dark layer of metal sulfide on the surface. This process, known as tarnishing, causes the jewelry to appear dull or blackened over time. Lower-karat pieces (10K or 14K) are more susceptible to tarnishing than higher-karat options (18K or 22K) because they contain a greater percentage of reactive metals.
The alloys also determine the gold’s color; for example, rose gold has high copper content, and white gold includes nickel or palladium. It is these specific metals that react and cause the color shift.
Common External Causes of Surface Change
The surface color of gold jewelry can change due to direct chemical exposure. Chlorine, especially in swimming pools or household bleach, poses a threat to gold alloys. Chlorine aggressively attacks alloy metals like copper and silver, leading to discoloration and weakening the metal’s structure. Prolonged exposure to high concentrations of chlorine can even cause a 14K white gold ring to become brittle and break down.
Contact with mercury is another specific cause of discoloration. Mercury quickly forms a soft, dull gray-white compound called an amalgam when it touches gold, immediately changing the appearance. Furthermore, common substances like lotions, makeup, hairspray, and sweat can accumulate on the surface, creating a sticky film. This residue traps dirt and oils, making the gold appear dark or duller, though this is a build-up rather than true chemical tarnish.
Gold-plated items, which have a thin layer of gold over a base metal, are particularly vulnerable. As the thin gold layer wears away through friction, the underlying base metal is exposed. This exposed base metal tarnishes quickly, causing a noticeable change in color and shine.
Cleaning and Protecting Gold
Preventing discoloration requires careful wear and maintenance to minimize chemical reactions. Jewelry should always be removed before engaging in activities involving chemicals, such as cleaning with bleach, swimming in chlorinated water, or taking a hot tub soak. It is also advisable to put on gold jewelry after applying cosmetics, lotions, and perfumes to prevent residue build-up.
For general maintenance, a simple home cleaning method involves soaking the jewelry in a solution of warm water and a few drops of mild dish soap for 10 to 15 minutes. A soft-bristled toothbrush can then be used to gently clean the crevices and remove accumulated dirt and oils. After rinsing, dry the piece with a soft, lint-free cloth.
For discoloration caused by mercury exposure or deep tarnish that resists home cleaning, a professional jeweler is necessary. They can safely remove the amalgam or polish away the corrosion without damaging the piece.