The most direct answer to whether 24-karat (24k) gold tarnishes is no, due to its unique chemical structure. By definition, 24k gold is pure gold, meaning it is 99.9% gold with no other metals mixed in. Tarnish is a form of corrosion, a chemical reaction that occurs when a metal reacts with elements like oxygen or sulfur. Pure gold is highly resistant to this degradation, which is why it has been valued for millennia.
Why Pure Gold Does Not Tarnish
Tarnishing, which causes a metal to darken or develop a dull film, is primarily the result of oxidation or sulfidation. This chemical process involves a metal losing electrons to form a new compound, such as a metal oxide or metal sulfide, on its surface. Gold is classified as a noble metal, meaning it is highly resistant to chemical reactions under ordinary conditions.
The element gold (Au) possesses an extremely stable atomic structure that prevents it from readily reacting with oxygen, moisture, or sulfur compounds. Its valence electrons, which are involved in forming chemical bonds, are held tightly to the nucleus. This low availability of electrons makes gold chemically inert, meaning it will not easily form the surface compounds that constitute tarnish.
This stability is partly due to relativistic effects, a phenomenon significant in heavy elements like gold. These effects cause the outermost electron orbital to contract, drawing the electrons closer to the nucleus. The tighter binding of these electrons makes them less available to participate in the oxidation reaction that causes tarnish.
The Role of Alloys in Lower Karat Gold
The confusion about gold tarnishing arises because most gold jewelry is not 24k purity. Karat is a measure of gold content, and lower karats (like 18k, 14k, and 10k) are alloys—mixtures of gold and other, less noble metals. These alloys are created to increase the hardness and durability of the gold, since pure 24k gold is relatively soft.
The metals most commonly mixed with gold include copper, silver, zinc, and nickel. It is these additive metals that are responsible for any tarnishing seen on gold jewelry. Copper and silver are particularly prone to sulfidation, reacting with trace amounts of hydrogen sulfide gas in the air to form dark silver sulfide or copper oxide compounds.
The lower the karat number, the higher the percentage of reactive alloy metal present, which correlates to the jewelry’s tendency to tarnish. For example, 10k gold is only about 41.7% pure gold, compared to 18k gold which is 75% pure. Consequently, a 10k piece is significantly more susceptible to tarnishing than an 18k piece due to its larger proportion of copper and silver.
Explaining Discoloration on 24k Jewelry
While pure 24k gold does not undergo the chemical reaction of tarnishing, it can become dull or dirty in appearance. Any perceived discoloration on pure gold is not tarnish, but rather a layer of surface contamination or buildup. This film is caused by external substances that adhere to the gold rather than reacting with it chemically.
Common culprits include residues from cosmetics, perfumes, skin lotions, and natural oils present in sweat. Over time, these substances accumulate on the surface, creating a visible, dull film that diminishes the metal’s natural luster. Unlike true tarnish, which is a chemical change to the metal, this surface film can be easily removed through gentle cleaning.
Another source of discoloration is contact with harsh chemicals, particularly chlorine found in cleaning products or swimming pools. Although gold is highly stable, chlorine can attack the metal structure, especially at high concentrations, leading to damage distinct from oxidation. This external damage emphasizes that even pure gold needs occasional care to retain its brilliance.