EVIDENCE OF ABNORMAL HIGH SILVER CONTENT IN A POLISH POLOTRAK COIN ISSUED IN 1624 AS CONSEQUENCE OF ADVANCED COPPER CORROSION

Abstract

Long ground resting of coins small denomination might affect their microstructure due to advanced corrosion. It is well known that cleaning of silver-copper alloy artifacts leads to a surface enrichment of the silver amount due to the partial removal of the copper grains which are more corroded. We found a półtorak issued in 1624 with abnormal characteristics like lower weight and dull sound when it is thrown on the solid surfaces. It resulted to be made of very high silver content about 97 wt. % according SEM-EDX correlated with XRD in great discordance with the common półtoraks from 1624 having about 70 wt.% silver. Advanced investigation of the abnormal półtorak reveals a porous internal structure indicating complete dissolution of β grains and copper sheets within the eutectic. All observation pleads for a progressive corrosion involving partial leaching of product through the wet resting ground which might ensure the dissipation of the corrosion-product and affecting deeper layers. Such dynamic-corrosion for 300 years might explain this intragranular corrosion which transforms an Ag-Cu hypereutectic alloy into almost pure silver. This preliminary hypothesis must be further investigated on larger półtoraks hoards which might have such abnormally corroded coins and on suspicious random finds.