Pure gold is an element, designated by the chemical symbol Au, that cannot be broken down into simpler materials by ordinary chemical means. However, the gold encountered in everyday items like jewelry, coins, and electronics is almost always a mixture of gold and other metals. This combination is known as an alloy. The intentional creation of gold alloys is a standard practice driven by the need to modify the element’s natural properties for practical applications.
Defining the Terms: Element, Compound, and Alloy
Understanding the nature of gold requires clear definitions of the terms used to classify matter. An element is a pure substance made up of only one type of atom, such as gold (Au) or oxygen (O). This substance cannot be changed into anything simpler through chemical reactions.
A compound consists of two or more different elements that are chemically bonded together, forming a new substance with unique properties, like water (H2O) or table salt (NaCl). The atoms in a compound are combined in a fixed ratio.
An alloy is a specific type of mixture composed of two or more elements, where at least one is a metal. Alloying involves physically mixing the components, usually by melting them together. Gold mixed with metals like copper or silver is classified as an alloy.
The Nature of Pure Gold
Pure gold, known as 24-karat gold, is classified as a noble metal, meaning it exhibits exceptional resistance to corrosion, oxidation, and tarnishing. This chemical inertness is why gold jewelry recovered from ancient sites often appears in the same condition as when it was first made.
Despite its chemical stability, pure gold has a very low hardness, registering between 2 and 2.5 on the Mohs scale. This makes the metal extremely soft, ductile, and malleable, meaning it can be easily scratched, bent, or shaped. A single ounce of pure gold can be hammered into a sheet that is nearly transparent or drawn into a wire many miles long.
These properties make pure gold unsuitable for most items requiring structural integrity and daily wear resistance. A ring made of 24-karat gold would quickly show signs of damage and deformation. Pure gold is typically reserved for investment bullion, such as bars and coins, rather than functional objects.
Why Gold is Alloyed for Commercial Use
Alloying is a deliberate engineering choice to overcome the limitations of pure gold, primarily by enhancing its durability. Adding harder metals, such as copper, silver, nickel, palladium, or zinc, increases the structural integrity of the gold, making it significantly more resistant to scratching and deformation.
The alloying process also allows for modification of the metal’s appearance, which is important for the jewelry industry. Pure gold has a distinctive, deep yellow color, but adding specific metals changes its hue. For example, rose gold uses a high proportion of copper, while white gold is created by mixing gold with metals like palladium or nickel to achieve a silvery-white color.
A final practical reason for alloying is to manage the material cost of the final product. Mixing gold with less expensive metals reduces the overall proportion of gold in the item. This allows manufacturers to create a range of products at different price points while retaining gold’s desirable qualities, such as its luster and resistance to corrosion.
Understanding Purity: Karats and Fineness
The gold content within an alloy is measured using two primary systems: karats and millesimal fineness. The karat system is based on a fraction of 24 parts, where 24 Karat (24K) represents pure gold (100% content). Lower karat numbers indicate a higher proportion of other metals in the alloy.
For example, 18K gold means 18 parts out of 24 are pure gold, corresponding to 75% gold content. Similarly, 14K gold is 14 parts gold out of 24 (approximately 58.3% gold), a common purity level used for durable daily-wear jewelry.
The millesimal fineness system expresses purity as parts per thousand and is frequently used in Europe and for gold bullion. To convert karats to fineness, the percentage of gold is multiplied by ten. Thus, 18K gold (75% pure) is marked as 750 fineness. Common fineness markings include 999 for investment-grade gold, 750 for 18K, and 583 or 585 for 14K gold.