Tarnish is a surface discoloration that occurs when a metal chemically reacts with substances in its environment. While pure, 24-karat gold does not tarnish, the dark black film seen on gold jewelry is a reaction involving the other metals mixed into the gold. This discoloration on 10k, 14k, and 18k pieces is common because gold must be alloyed with less noble metals to achieve the necessary hardness for jewelry. Understanding this blackening allows for effective cleaning and proactive management.
Why Pure Gold Does Not Tarnish
Gold is classified as a noble metal, meaning it exhibits exceptional resistance to chemical action, corrosion, and oxidation. Pure 24-karat gold does not readily bond with oxygen or sulfur compounds, which are the primary atmospheric elements responsible for tarnish on other metals.
However, pure gold is too soft for practical use in jewelry, making it susceptible to bending and scratching. Jewelers must therefore mix it with other metals, which is reflected in the karat system.
The karat number indicates the proportion of pure gold by weight; for example, 14k gold is 14 parts gold out of 24 total parts, or about 58.3% gold. The higher the karat, the greater the percentage of pure, non-reactive gold, and the more resistant the piece is to tarnishing. Conversely, lower-karat gold, like 10k, contains a higher proportion of reactive alloy metals.
The Chemical Causes of Blackening
The blackening of gold jewelry is caused by the chemical reaction of the alloy metals, primarily silver and copper, with atmospheric or contact agents. These metals are much more reactive than gold and readily participate in a process called sulfidation. Sulfur compounds, present in the air, human sweat, and skin oils, react with the silver and copper content.
When these alloy metals meet sulfur, they form dark compounds such as black silver sulfide and black copper sulfide. This dark layer of corrosion is what we recognize as tarnish. The black residue can also result from “contact tarnish,” where external substances like cosmetics, lotions, or perfumes contain reactive chemicals transferred directly to the jewelry’s surface.
Another significant chemical culprit is chlorine, commonly found in cleaning products and swimming pools. Chlorine can chemically attack the alloy metals, leading to the formation of dark metal chlorides. Repeated exposure to chlorine can also cause stress corrosion cracking, which weakens the metal structure, especially in areas under tension. The acidity and salts present in human sweat further accelerate both the sulfidation and the chloride reactions.
Cleaning and Restoration Methods
Removing tarnish involves using gentle methods that lift the corrosion products without damaging the underlying gold alloy. For mild discoloration, a basic cleaning solution of warm water and a few drops of mild dish soap is often sufficient. The jewelry should be soaked for up to ten minutes to loosen any surface grime and oils.
A soft-bristled toothbrush, such as one designed for children, can then be used to gently brush the surface and crevices of the piece. After scrubbing, the jewelry must be thoroughly rinsed under warm, running water to remove all soap residue. Finally, dry the piece completely with a soft, lint-free cloth.
For heavier tarnish, a safe at-home electrochemical cleaning method can be employed using aluminum foil and baking soda. A glass bowl is lined with aluminum foil, and the tarnished jewelry is placed on top, ensuring it touches the foil. A solution of hot water and several tablespoons of baking soda is poured over the jewelry, creating a chemical reaction called ion exchange. This process effectively transfers the tarnish from the jewelry to the aluminum foil, restoring the metal’s brightness.
Preventing Future Discoloration
Minimizing future blackening requires controlling the jewelry’s exposure to reactive agents. It is advisable to remove all gold jewelry before engaging in activities that involve exposure to harsh chemicals, such as cleaning with bleach or swimming in chlorinated water.
Sweat and body oils accelerate tarnishing, so pieces should be taken off before strenuous exercise. A simple habit is to apply all cosmetics, lotions, perfumes, and hairsprays before putting on gold jewelry. Allowing these products to absorb fully into the skin first reduces contact with the alloy metals.
Routine maintenance, such as gently wiping down the jewelry with a soft cloth after each wear, can remove skin oils and residues before they react. For long-term storage, keeping gold jewelry in a cool, dry place is essential to prevent moisture buildup that facilitates chemical reactions.
Airtight containers or jewelry boxes with anti-tarnish lining or strips can absorb sulfur and moisture vapor from the air. Storing pieces separately also prevents them from scratching one another.