Gold (chemical symbol Au) is a dense, yellow precious metal valued for its luster and resistance to corrosion. In its purest form, known as 24-karat gold, the metal is naturally soft and highly malleable. This pliability means pure gold is easily scratched or deformed, making it unsuitable for objects requiring structural integrity and daily wear. To retain gold’s beauty and value while adding practical strength, it is combined with other metals to form an alloy.
Why Gold Requires Alloying
The primary reason for mixing gold with other metals is to significantly increase its mechanical strength and durability. Pure gold is so soft that it registers only 2 to 2.5 on the Mohs scale of hardness, which means it can be easily marred or lose its shape, especially in fine jewelry settings. Introducing base metals alters the atomic structure of gold, enhancing its resistance to abrasion and bending.
Alloying also makes the final product more cost-effective for consumers. Since pure gold is expensive, diluting the content with less costly metals lowers the overall material price. The addition of alloying agents also changes the melting point, which is advantageous during manufacturing and casting processes.
The Karat System of Measurement
The Karat system (K) is the standardized method for measuring the proportion of pure gold within an alloy. This measurement is based on 24 parts, where 24 Karat is considered pure gold (99.9% or more Au). The Karat number indicates how many of those 24 parts are pure gold, with the remainder being the alloying metals.
For instance, 18 Karat gold is composed of 18 parts gold and 6 parts of other metals, translating to 75% pure gold. A lower purity, like 14 Karat gold, contains 14 parts gold and 10 parts alloy, making it 58.3% pure gold. The lowest commonly accepted standard, 10 Karat gold, is 41.7% pure gold.
Primary Metals Mixed with Gold
A variety of metals are mixed with gold, each contributing specific properties to the final alloy.
Silver and Copper
Silver is a common alloying metal that helps maintain malleability while lightening the color slightly. It is frequently included in yellow and green gold alloys to balance the hue and improve workability. Copper is added to increase the alloy’s strength and contribute a reddish tint. Copper significantly increases the hardness of the final product, making it often used for durable items such as rings and clasps.
Nickel and Palladium
Nickel was historically used as a primary component in white gold alloys, providing a pale, white color and substantial hardness. However, due to nickel’s potential to cause skin allergies, its use is restricted in certain regions and is often replaced with other metals. Palladium, a platinum group metal, serves as a premium alternative for creating white gold. It offers a naturally whiter color than nickel-based alloys and is hypoallergenic.
Zinc
Zinc is often used in small amounts as a deoxidizer during the casting process. It helps produce a smoother, cleaner metal surface.
Creating Different Gold Colors
The visual appearance of gold alloys is determined entirely by the specific mix and ratio of the non-gold metals.
Yellow Gold
Yellow gold, the traditional color, is created by alloying gold with a combination of silver and copper. The silver prevents the copper from imparting too much of a red tone. A standard 18K yellow gold alloy contains 75% gold, with the remaining 25% often split between roughly 16% silver and 9% copper.
Rose Gold
Rose or pink gold owes its distinctive warm color to a higher concentration of copper in the alloy mixture. Increasing the copper content while reducing or eliminating the silver pushes the color from a soft pink to a rich, reddish hue. For example, 18K rose gold is 75% gold, but the remaining 25% is almost entirely copper.
White Gold
White gold is produced by combining gold with white metals like nickel or palladium to bleach the natural yellow color. Because the resulting alloy often retains a slight yellowish cast, a final layer of rhodium plating is commonly applied to achieve the bright, pure white finish seen in commercial jewelry.
Green Gold
Green gold, a less common alloy, is primarily made by mixing gold with a high proportion of silver. This is sometimes done with a small amount of copper or cadmium, resulting in a pale, greenish-yellow metal.