Investigation of surface silver enrichment in ancient high silver alloys by PIXE, EDXRF, LA-ICP-MS and SEM-EDS

Abstract

Surface elemental analysis presenting high silver (Ag) contents have been regarded as reliable for original bulkcomposition of silver coins which are accordingly referredas being produced from very pure Ag alloys. Also, the ex-tent in which the surface elemental composition analytical results are influenced by surface Ag enrichment hasbeen disregarded in coins alloys with Agfinenesses greater than the Ag content indexed to the maximum valueof copper (Cu) solid solubility in Ag–91.2 wt.%. PIXE and EDXRF results obtained on Portuguese11 dinheirossilvercoins issued in the 16th century with a legal Agfineness of 916.6 thousandths, show 4 to 7% higher Ag surface con-tents. As identified by LA-ICP-MS andμ-PIXE through coins thicknesses, the overestimation of thesuperficial mea-sured Ag is associated to a Cu depletion gradient representing as much as 50% less Cu on surface than on the bulk ofthe coins. The subsurface microstructural characterization by SEM-EDS reveals the presence of an important super-ficial layer depleted of Cu-phase, with a thicknessb70μm, resulting primarily from intergranular corrosion duringthe coin manufacturing process. These layers canonly be accessed bydestructive or invasive analysis, as thicknessesbeyond the information emission depth obtained with EDXRF could not be identified by Ag Kα/Lαratios. Asshowed by the combination of different methods of analysis,EDXRF, PIXE, SEM-EDS and LA-ICP-MS, the results ob-tained by surface analytical techniques may differ from the original bulk compositions for these high Ag content coins and should be taken cautiousness together with historical context and information