The practice of biting a gold coin to test its authenticity is often seen in folklore and films. This seemingly crude method is based on a real physical property of the metal: its softness. The answer lies in the unique material science of gold itself, which explains why this historic test was once relevant, but also why it is completely unreliable in the modern world.
The Science Behind Gold’s Softness
Leaving a mark on gold stems from its inherent physical properties as a metal. Pure gold (24-karat) possesses extreme malleability and ductility, meaning it can be permanently shaped or deformed without fracturing. This characteristic makes it one of the softest metals. On the Mohs scale of mineral hardness, a system used to rank a material’s resistance to scratching, pure gold rates approximately 2.5 to 3.0. This low rating indicates that gold is easily scratched or dented by many common objects.
Human tooth enamel, the hardest substance in the body, ranks significantly higher on the Mohs scale, typically between 5 and 7. Since the tooth’s surface is considerably harder than pure gold, applying pressure causes the softer gold to yield and deform, leaving a noticeable indentation from the bite. This difference in hardness is the scientific basis for the historic bite test.
How Gold Purity Affects the Bite Test
The main factor influencing whether gold will mark easily is its purity, measured using the karat system. Pure gold is defined as 24 karats (24K). Because 24K gold is too soft for everyday use, it is almost always mixed with other metals like copper, silver, or zinc to create a stronger alloy. This alloying process directly increases the metal’s hardness and durability. Historically, the bite test was a simple way to gauge purity; a deep mark suggested high gold content, while resistance indicated a lower-karat alloy or a different metal.
For example, 18-karat gold is 75% pure gold and is noticeably harder than 24K gold. 14-karat gold contains only 58.3% pure gold, making it significantly more resistant to deformation.
Why Biting Gold is Not a Reliable Test
Despite its historical logic, biting gold is now an unreliable and potentially dangerous method for authentication. Modern counterfeiting techniques exploit this weakness, rendering the test useless. For instance, a common scam involves plating a soft, non-valuable metal like lead with a thin layer of real gold. A bite test on such an item would easily dent the soft core, falsely indicating pure gold. Counterfeiters also use high-density metals like tungsten, which, when plated, mimic the weight and initial softness of gold.
Furthermore, attempting the bite test carries a significant risk of dental injury. Biting down on a harder alloy, like 14K gold, can easily chip or crack tooth enamel. Safer, more accurate methods exist today, such as non-destructive X-ray fluorescence (XRF) analysis, precise density measurements, or chemical acid tests. These tools accurately determine the gold concentration without risking damage.