The image of a prospector or an Olympic athlete biting into a gold object to test its authenticity is an enduring cultural trope. This common practice raises a fundamental question about the physical nature of the metal. Whether this action is a legitimate test or a dangerous myth hinges entirely on the specific properties of gold and the real-world risks involved in using teeth as an assay tool.
Gold’s Unique Physical Characteristics
The possibility of leaving a tooth mark on gold stems from its unique atomic structure, resulting in extreme softness. Pure gold, known as 24-karat, registers a low value of approximately 2.5 on the Mohs scale of mineral hardness. For comparison, common steel measures between 4 and 4.5 on the same scale.
This softness is directly related to gold’s exceptional malleability, meaning it can be hammered or pressed into thin sheets without cracking. The atoms within the metallic structure can be moved without breaking their bonds, allowing the metal to easily deform under pressure. This characteristic is why a solid object made of pure gold will readily accept an impression from a human tooth. However, this property changes significantly when gold is mixed with other metals to create alloys.
The Historical Purpose and Limitations of the Bite Test
The bite test emerged as a crude, on-the-spot method for verifying the purity of gold, primarily during periods when official testing was unavailable. The logic behind the test was that if a coin or bullion was pure gold, its inherent softness would allow a clear indentation to be made by a person’s teeth. A lack of a mark suggested the object was made of a harder, less valuable base metal.
This method was particularly relevant historically to detect counterfeits where a heavy but inexpensive metal, like lead, was gold-plated. Lead is also a very soft metal, so it would also accept a bite mark. However, its lower density compared to gold meant a trained individual could often detect the forgery by the object’s weight.
The test quickly became unreliable and is largely irrelevant today due to modern alloying and counterfeiting techniques. Gold used in contemporary commerce, such as 10-karat or 14-karat jewelry, contains high percentages of harder metals like copper or nickel, making it significantly more resistant to indentation. A bite on these common alloys would likely leave no mark, incorrectly suggesting the item is fake. Furthermore, modern sophisticated counterfeits can mimic the density and properties of gold far better than historical methods, rendering a dental impression useless for authentication.
Safety Concerns: Toxicity and Dental Health
Attempting the bite test carries two main risks: potential damage to the mouth and exposure to toxic substances. Human tooth enamel has a Mohs hardness rating between 5 and 5.5, which is considerably harder than pure 24-karat gold. Biting down on an unknown object can result in a cracked or chipped tooth if the item is an alloy or has an uneven, sharp surface. The force applied by a bite can easily exceed the fracture strength of the tooth.
While pure elemental gold is biologically inert, the metals often alloyed with it pose a health risk. Gold items, especially those of lower karat values, contain metals like copper, nickel, and zinc, which can leach into the mouth when bitten. Nickel and copper are known to cause allergic reactions or exhibit toxic effects if ingested in high quantities. The safety concern is the possibility of ingesting or absorbing hazardous metal components from non-pure gold items.