The news outlets and social media posts were buzzing this month about a gold coin that was discovered in a field by someone combing the area with a metal detector1. The coin, a 13th-century gold penny found in southwestern Britain, fetched about a half million dollars at auction. The coin was valuable not only for the gold content, but also, of course, for the history.
Can you imagine taking a metal detector to a field or beach and finding such a treasured coin? It’s not often one comes across a penny made from gold, never mind one that is 800 years old. Can you imagine being a jewelry store owner, pawnbroker, or coin shop operator and having someone walk in with this coin asking to trade it for cash? Counterfeiters are getting better and better at replicating real coins – including weight, appearance, and strike cleanliness – so precious metals and numismatics industry experts might consider it a fake until it was verified by several methods.
In modern times, most coins are made from a base metal (like tungsten, lead, copper, and nickel) plated with gold or silver. To be absolutely certain of the composition of a gold or silver coin, one would have to assay and verify the precious metals content of the coin.
Coin collectors and dealers utilize various methods to analyze their gold and silver coins, including analytical and non-analytical methods. Non analytical methods involve using magnifying tools to examine the overall presentation of the coin, its mint year, any surface alterations, special marks, etc. Other non-analytical testing methods include a ping test, in which the coin is struck with a piece of metal; based on the sound reflected, information can be ascertained as to whether a coin is alloyed or consists of a high concentration of gold or silver (the pure, or near-pure gold will make a dull, non-lasting sound, whereas pure silver produces a sustained ring).
Analytical testing methods include the usage of electrical conductivity devices. This method is typically restricted to the measurement of just a few metals. The acid test method is often used to determine the Au karat-value of a coin. Acid tests, by their nature, are qualitative and have limited accuracy. The tests work by applying a series of acids to determine specific karat value ranges. Based on chemical reactions that take place, or the lack of thereof, the user may conclude that the karat value of a coin or jewelry item lies within a certain range, e.g., “above 14k and below 18k”. What the test does not reveal is the actual gold concentration, the existence and concentration of other precious metals or alloying elements, or any indication of gold plating. The acid test is also destructive and by no means is acceptable when it comes to evaluating historic and high-value numismatic coins. The acids used are corrosive and require special protection.
Fire assay is the most accurate precious metal verification technique, and is considered the legal method for gold hallmarking, but it is a destructive method because you have to melt the piece in order to determine the gold or other metal concentration — totally out of the question for coins that would be valuable for their historic value.
Portable X-Ray Fluorescence (XRF) technology is the technique commonly used in pawn shops, cash-for-gold operations, and jewelry stores to obtain accurate precious metal content. With XRF precious metals analysis technology the shop workers can measure the content of all gold and precious metals, as well as determine the presence and concentration of other trace, alloying elements, and dangerous heavy elements, which could impact health and the valuation of the pieces. Most importantly, it is a non-destructive technology, so the piece — whether a piece of jewelry or a coin — does not lose any value during testing. This is an especially important factor if you’re dealing with high value items.
We’ve gone into more details about precious metals, alloys, and their testing methods in our webinar Not All that Glitters is Gold. You can download the recording anytime and see a demo of the analyzers in action.
XRF instruments provide a fast, accurate, and most importantly, nondestructive alternative for assessing the metal composition of coins, but you will need the numismatic professionals to confirm the historic value.
References and Additional Resources
- 1Smithsonian Magazine
- On-Demand Webinar: Not All that Glitters is Gold.
- Application Note: Adding Value to the Numismatic Profession with Portable XRF
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