Identifying a rare die variety is not a matter of having a keen eye, for rare die varieties can be detected by utilising a number of different reference catalogues, physical measurement and magnification.
Consulting Variety Catalogues
1) Match the coin against a recognised and authoritative reference catalogue of die varieties. For example, all milled British coins are listed in the English Silver Coinage (ESC) and every documented die variety is included with a detailed description. The person identifying the variety must know which reference system they are using before they start to examine the coin.
Examining Die Characteristics Under Magnification
A loupe, handheld magnifier or digital microscope can reveal subtle characteristics on a coin’s surface that are not visible to the naked eye. The spacing of the letters and the positioning of the design elements on the coin can help to confirm whether a coin is a normal issue or a rare variety. The shape of individual digits is also very important. For example, a dated coin that has been overdated will show evidence of the earlier date being scratched into the die and left in the metal of the coin when the new date was punched in.
Checking Weight and Diameter
Variety tolerances are slight and physical measurement of weight and diameter can be critical in confirming or exposing the true nature of a suspected variety. The weight is stated in grams to the last two decimal places, and the diameter is read off using callipers. Occasionally a suspect coin is found to be of a known minting anomaly; more often, however, a deviation from the published average for a known die combination will be a cause for concern.
Reading Die Polish and Flow Lines
Heavy die polish will leave raised striations in the fields of a coin, and these can be so individualized as to be unique to a single die. Additionally, surface flow lines from the metal of the die, can be traced on the surface of a coin. These lines can also be individualized and even tell the specialist the striking position of the coin.
Cross-Referencing Provenance Records
The records of previous sales of coins of the same type, the labels on coins in collections, and the invoices of dealers who have previously sold the same type of coin all form part of the evidence which specialists use to confirm the variety of a particular coin. For example, the records of coins of the British types in the British Museum’s collection are published and form an authoritative baseline against which any claimed variety of a published type can be checked.
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Each of these methods can on its own give clues about the coins being studied, but it is really the combination of them all which can allow a confident attribution to be made.