CHARACTERIZATION OF THE MICROSTRUCTURE, CHEMICAL COMPOSITION AND STATE OF CONSERVATION OF COINS OF THE “ALBERONE TREASURE”

The trove of Alberone di Ro Ferrarese consists of about one thousand coins of different alloys, issued by Italian and foreign mints and dating back, from the XV to the XVI century. The discovery of the treasure was made in December 1923 when a farmer dug up a buried pottery jar containing the coins, while plowing a field on the right bank of the Po river close to Ferrara. Seven quattrini minted in Ferrara in the XV century, belonging respectively to the authorities of Niccolò d'Este (1393-1441), Leonello d'Este (1441-1450), Borso d'Este (1450-1471) and Ercole I d'Este (1471-1505), have been studied. Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) microprobe and X-ray diffractometry (XRD) have been used to evaluate the morphology and the nature of the surface patinas. Subsequently, some of the coins have been cleaned up to remove the surface corrosion products and the microstructural and chemical analysis of the alloys has been carried out by the same techniques. The investigations have shown that for all coins the alloy composition is 15.5-16.2% Ag, less than 1 % Pb and balance Cu. The coins exhibit an inclusion-rich biphasic dendritic microstructure, often plastically deformed due to the minting operations. SEM-EDS analyses have demonstrated the presence of a silver-rich and a copper-rich phase, which do not correspond to the thermodynamically stable structural components. The microstructures are compatible with a minting technique based on the production of cast blanks, followed by hammer cold striking. As-received coins are irregularly covered by reddish and green corrosion products, mainly constituted by cuprite and malachite. The growth of this film of corrosion products induces a selective copper dissolution at the expenses of the copper-rich phase, as confirmed by EDS element mapping on mildly cleaned coin surfaces