Silver and tin plating as medieval techniques of producing counterfeit coins and their identification by means of micro-XRF

Archaeological surveys and metal detector prospecting yield a great amount of coins from the medieval period. Naturally, some of these are counterfeit which an experienced numismatist can determine without using chemical methods. The production of counterfeit coins in the middle ages took place in castles, caves or other remote areas where waste from this activity can still be found today – copper sheets, technical ceramics and counterfeit coins. Until recently, it has been assumed that medieval counterfeit coins are made by silver-plating copper blanks using an amalgam. However, the performed analyses reveal that there are many more techniques of counterfeiting of coins. Other techniques were based on e.g. tin amalgam plating of the blanks or alloying so-called white metal with silver-like appearance from which the coins were minted. Current chemical analyses indicate that the coins were often tinned by hot dipping with no amalgamation. Micro-X-ray fluorescence analysis has been chosen as a suitable non-destructive method to identify present chemical elements in investigated artifacts and to quantify their concentrations. In addition, a quick technique telltale the plating was applied. This technique utilizes the detected fluorescence ratio Kα/Kβ of copper, which is the main ingredient of a lot of historical metallic materials. - Highlights: • Tiny remains of plating on the surface of coins were recognized with micro-XRF. • More techniques of counterfeiting of coins were revealed. • The determined low fluorescence ratio Kα/Kβ of copper was an evidence of plating.