Understanding Gold Tarnish
Tarnish is a common surface discoloration that appears on many metals, including gold items. This phenomenon alters the aesthetic appeal of a metal surface, making it appear dull or darkened rather than bright and lustrous. It is a chemical process affecting the appearance of gold jewelry and other gold objects.
Tarnish on gold is a form of mild corrosion that develops on the metal’s surface due to chemical reactions with airborne substances. Pure 24-karat gold, which is nearly 100% gold, is highly unreactive and does not typically tarnish. Gold jewelry and other gold items can tarnish because they are not made of pure gold. Instead, they are alloys, containing a mixture of gold and other metals. These other metals, such as copper, silver, and nickel, are more reactive than gold and are primarily responsible for the discoloration.
The Role of Gold Purity
The purity of gold, indicated by its karatage, directly influences its susceptibility to tarnishing. Lower karat gold, such as 10k or 14k, contains a higher proportion of non-gold metals compared to 18k or 22k gold. These less noble metals, including copper, silver, and zinc, readily react with compounds in the environment. For example, 10k gold is approximately 41.7% gold and 58.3% other metals, making it more prone to tarnishing than 18k gold, which is 75% gold.
Different gold alloys also exhibit varying tendencies to tarnish based on their specific composition. Rose gold, for instance, contains a higher percentage of copper, which can oxidize and contribute to a reddish or brownish tarnish over time. White gold often includes nickel, palladium, or silver, and these metals can also react with external agents. The tarnishing reaction primarily involves these alloying metals interacting with sulfur, oxygen, or other compounds present in the air, forming metal sulfides or oxides that manifest as discoloration on the surface.
Environmental and Personal Contributors
Various external factors contribute to the acceleration of tarnish formation on gold alloys. High humidity levels in the environment can facilitate chemical reactions on the metal surface. Air pollution, particularly the presence of sulfur compounds like hydrogen sulfide, is a significant environmental contributor. Common household chemicals, such as those found in cleaning products, bleach, or even sulfur-containing foods like eggs, can also trigger or accelerate tarnishing when they come into contact with gold jewelry.
Personal factors also play a role in how quickly gold items might tarnish. Skin oils and sweat contain compounds that can react with the alloying metals in gold, leading to discoloration. Cosmetic products, perfumes, and lotions, when applied while wearing gold jewelry, can leave residues that initiate or speed up the tarnishing process. These substances act as catalysts or direct reactants, promoting the chemical changes.
What Tarnished Gold Looks Like
Tarnished gold typically exhibits a noticeable change in its visual appearance, moving away from its original bright, reflective luster. The discoloration can range from a subtle dullness to a distinct, dark film on the surface. Common colors of tarnish include black, brown, green, or even reddish hues. The specific color often depends on the type of alloying metals present in the gold and the compounds with which they have reacted.
For example, a high copper content in the gold alloy might lead to greenish or reddish-brown tarnishing. If silver is a prominent alloying metal, the tarnish may appear black or dark gray.