Gold purity is measured using the karat system, where 24 karat (24K) represents pure gold. 14K gold, the most common purity level for jewelry, is an alloy consisting of 58.3% gold and 41.7% other metals. These metals are added to increase hardness and durability, as pure gold is extremely soft and would be easily scratched or bent if used alone. The question of whether 14K gold turns black when burned centers not on the gold itself, but on the chemical properties of those alloy metals.
The Immediate Reaction: Why 14K Gold Turns Black
When 14K gold is exposed to high heat, such as from a jeweler’s torch, the surface of the metal does indeed turn black. This discoloration is not the gold component burning, but rather a process called oxidation. Pure gold, symbolized as Au, is a noble metal and is highly resistant to reacting with oxygen, even under extreme heat.
The black layer that forms is known in the jewelry industry as “fire scale” or “firescale.” This fire scale is a thin layer of metal oxides that results from the heat accelerating the reaction between the non-gold metals in the alloy and the oxygen in the surrounding air. The heat provides the energy necessary for the oxygen atoms to chemically bond with the alloy components.
This chemical reaction is confined to the surface layers of the metal and does not penetrate deeply into the gold structure. While the black color is visually stark, it is a superficial chemical change, not a sign of the gold itself being destroyed.
The Role of Alloys in Discoloration
The blackening of 14K gold is directly attributable to the non-gold metals intentionally mixed into the alloy. The remaining 41.7% is typically composed of metals like copper, silver, and sometimes zinc or nickel. Copper is the primary culprit in the formation of the black fire scale.
Copper readily reacts with oxygen when heated, forming a compound called cupric oxide (CuO). This cupric oxide compound is dark brown or black in color, which coats the surface of the jewelry. Silver can also contribute to oxidation, but the black color is predominantly due to the copper content.
The greater percentage of alloys in 14K gold compared to higher-karat gold, such as 18K (75% gold), makes it more susceptible to this reaction. The specific recipe of the alloy dictates the final color of the gold, with yellow and rose gold having higher copper concentrations, thus making them more prone to fire scale formation.
How Jewelers Remove the Black Residue
Fortunately, the black fire scale is not permanent damage and can be effectively removed by jewelers using established methods. The most common technique is known as “pickling,” which involves immersing the heated piece into a warm acid solution.
Modern jewelers typically use a solution based on sodium bisulfate, often referred to as a safety pickle. This mild acid bath works by chemically dissolving the surface layer of metal oxides—the black fire scale—without harming the gold itself. The solution is often heated to around 180°F to accelerate the chemical reaction and more quickly remove the discoloration.
In cases where the oxidation is minor, or for a final finish, abrasive methods may be used. This involves mechanical polishing using specialized compounds and brushes to physically rub away the thin layer of oxide. Both chemical pickling and mechanical polishing effectively restore the jewelry to its original appearance because the blackening is only a superficial layer.