Tarnishing is a common surface phenomenon where a metal loses its luster and darkens through a chemical process. Pure gold (24-karat) is chemically inert and does not react with sulfur compounds under normal conditions. However, most gold jewelry is an alloy, and it is the other metals in that mixture that react to cause discoloration. The visible dark film is a form of surface corrosion, specifically sulfidation, which occurs when alloyed metals are exposed to sulfur-containing substances. This chemical change often requires cleaning to restore the jewelry’s original shine.
The Critical Difference Between Pure Gold and Alloys
The distinction between pure gold and gold jewelry is defined by the karat system, which measures the proportion of gold in an alloy. Pure gold (24K) contains 99.9% gold atoms, but its inherent softness makes it impractical for everyday wear. To increase durability, gold is mixed with tougher metals like silver, copper, nickel, or zinc, creating an alloy. For example, 10-karat gold contains only 41.7% gold, making it significantly more susceptible to tarnishing than 14-karat gold (58.3% gold).
Tarnishing occurs because these non-gold metals are significantly more reactive to sulfur compounds. Sulfur compounds, particularly hydrogen sulfide (\(\text{H}_2\text{S}\)) found in the air, aggressively react with the silver and copper components of the alloy. This chemical interaction is known as sulfidation, where the sulfur binds to the alloy metal’s surface. The resulting compound is primarily silver sulfide (\(\text{Ag}_2\text{S}\)) or copper sulfide (\(\text{Cu}_2\text{S}\)), which appears as a dark brown or black film.
Common Sources of Sulfur That Cause Tarnishing
The sulfur compounds responsible for sulfidation are common in everyday life, existing in both gaseous and trace solid forms. Air pollution is a significant environmental source, particularly industrial emissions and vehicle exhaust, which release sulfur dioxide (\(\text{SO}_2\)) and hydrogen sulfide. High humidity and warm temperatures accelerate the tarnishing process by promoting the chemical reaction between these gases and the metal surface.
Many household and personal items also contain small amounts of sulfur that can interact directly with gold alloys. Rubber products, such as elastic bands, latex gloves, and certain shoe soles, contain sulfur used in their manufacturing process. Wool and some types of paper or cardboard can release sulfur compounds that contribute to tarnishing when jewelry is stored improperly.
Trace sulfur compounds are also present in substances that come into direct contact with skin. Natural perspiration contains sulfur compounds which, combined with the body’s oils and salts, can react with the alloy metals. Personal care products like lotions, perfumes, and cosmetics may also accelerate the chemical process when applied near jewelry. Certain foods, such as eggs and onions, also release sulfur-containing gases that speed up tarnishing.
Preventing Tarnishing and Cleaning Affected Gold Jewelry
Minimizing exposure to sulfur sources is the most effective strategy for slowing the tarnishing of gold alloys. Proper storage is a simple preventative measure, ideally using airtight containers or jewelry pouches lined with anti-tarnish fabric. Placing anti-tarnish strips inside the storage container can help by absorbing sulfur compounds present in the air.
It is advisable to remove gold jewelry before engaging in activities that involve direct contact with known culprits. This includes swimming in chlorinated pools or natural hot springs, using household cleaning agents, and applying cosmetics or perfumes. Wiping jewelry with a soft, clean cloth after each wear is a practical step to remove skin oils, sweat, and environmental residues before storing the piece.
For gold jewelry that has already developed a surface tarnish, gentle cleaning methods are recommended for safe restoration. A non-abrasive approach involves soaking the piece in a solution of mild dish soap and warm water, followed by light scrubbing with a soft-bristled brush. Specialized, chemically treated polishing cloths can effectively remove the dark sulfide layer without causing damage. For severe tarnish, consulting a professional jeweler is the best course of action, as they can employ ultrasonic cleaning or other methods to safely restore the metal’s brilliance.