Coin collectors and dealers utilize various methods to analyze their coins, including analytical and non-analytical methods, and ascertain their precious metal content. Non analytical methods involve examining the overall presentation of the coin with simple tools such as a loupe or a magnifying glass. The mint year, any surface alterations, special marks, and visible damages will often provide information regarding the general condition of the coin and its authenticity. 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
Analytical testing methods include the usage of electrical conductivity devices for gold content measurements. These devices take advantage of the fact that gold is an excellent electrical conductor, and therefore measuring the electrical conductivity of a coin can provide information regarding its gold content. This method is typically restricted to the measurement of just a few metals, typically Au (gold) and Pt (platinum). Unfortunately, the Ag (silver) content — which is of high interest to numismatics professionals — cannot be detected by electrical conductivity testers, nor can they provide clear indication of the presence of any alloying metals such as copper or zinc. So typically they provide only qualitative results. These devices are also prone to false readings due to cross-contaminations of the testing probes.
Acid tests are 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, namely 10k, 14k, 18k, and 22k. Based on chemical reactions that take place, or the lack 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 high-value numismatic coins. The acids used are corrosive and require special protection. In fact, we have come up with eight reasons jewelers should not use acid to test jewelry that we have outlined in an infographic.
How Can XRF Analysis Assess a Coin’s Precious Metal Content?
X-ray fluorescence (XRF) instrumentation provides a fast, accurate, and most importantly, nondestructive alternative for assessing the metal composition of coins, and are currently used by jewelers, manufacturers, and most importantly, numismatic professionals.
How does portable XRF work? Each of the elements present in a sample produces a unique set of characteristic x-rays that is a “fingerprint” for that specific element when illuminated by primary x-rays. XRF analyzers determine the elemental composition of a sample by measuring the energy of the characteristic x-ray emitted by the different elements in the sample when it is illuminated by primary x-rays. By simultaneously measuring the fluorescent x-rays emitted by the different elements in the sample, a portable XRF analyzer can rapidly determine those elements present in the sample and their relative concentrations.This is referred to as the elemental composition of the sample. Almost instantly you will have an accurate purity analysis for on-the-spot sorting and valuation of gold, silver, and all precious metals…as well as quantification of other desirable and undesirable elements.
Regardless of the method used to evaluate coins, there is a great value in avoiding any damage to a collectable coin as this may diminish its value by thousands of dollars in certain cases.
Take a look at this application note that discusses in more detail the various analytical methods of numismatic testing.