By Kyle Clifford Knapp for PCGS ……
Experts at PCGS use a variety of technical methods when identifying and authenticating coins. One essential and often revealing examination is X-ray fluorescence spectroscopy (XRF), a form of non-destructive elemental analysis providing information about the metallic composition of a coin’s surface. XRF can be thought of as a more refined version of the acid-based assays common in coin and jewelry stores, or “test cuts” used since ancient times to verify the purity of precious metal coins. Using an electron beam in place of these harsher methods is one of the great luxuries of numismatics in the modern age and has yielded invaluable information for authenticators and researchers.
The first task for any cataloger or grader is that of proper identification. Unfortunately, experimental phases at the United States Mint do not always make this easy.
During the mid-19th century, many new compositions were tested for efficacy and expense as the Mint sought to provide consistent alternatives to both the foreign specie that remained prevalent in antebellum circulation and the spate of private currency substitutes unleashed by hoarding during the Civil War. The 1864 pattern cent pictured above is Judd-356a, a rendering of the circulating Indian Cent design struck in pure copper. Versions also exist in copper-nickel (Judd-356b) and copper with successive levels of aluminum alloy (Judd-353 to 355). As handling and patination can subtly change the appearance and coloration of the surface, these metallurgical differences are often not visually distinguishable.
Following a proper identification, XRF can be used as an advanced authentication tool by comparing a suspect piece to the known values of a population of previously authenticated or well-pedigreed examples. While published composition data are roughly accurate for most United States issues, idiosyncrasies exist, especially within the rare and often-counterfeited early American, colonial, and pioneer series. Access to such experimental knowledge is one of the many advantages available to PCGS experts when evaluating the authenticity of a newly discovered specimen.
XRF technology has also been used to expand our numismatic knowledge. The “New Haven” restrikes of Fugio Cents from the 1860s can have colorations ranging from dark chocolate to brassy yellow and were largely categorized by appearance alone for many decades. Elemental analysis of a large number of specimens submitted to PCGS over the past several years reveals two distinct groupings: one nearly pure copper, the other alloyed with roughly 8% zinc. The pure copper pieces appear far scarcer than previously thought, suggesting the possibility of a distinct origin.
Other mysteries remain unsolved. The relationship between the two sizes or “denominations” of the St. Patrick New Jersey coinage has long been a point of debate amongst colonial specialists. Colloquially known as farthings and half-pence, their relative weights do not fall into such a convenient ratio, while coloration ranges from dark chocolate to beige. One promising theory resolved this problem by holding that the “farthings” must be debased; experimental data now shows this not to be the case, with several pieces found to be nearly pure copper aside from their splashers. Back to the drawing board!
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