Portuguese silver from the 15th to the 17th century, the 11 dinheiros silver coins

Rui Borges

CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal

High silver surface contents of high fineness silver coins have been considered in some cases as deriving from very pure silver alloys, being reliable for original bulk composition. My PhD investigation focused on microstructural and compositional characterization of Portuguese 11 dinheiros silver coins, and has revealed important surface silver enrichments in high silver fineness coins. In these silver alloys, coin manufacturing process induces a subsurface microstructurally modified layer resulting from intergranular dry and wet corrosion in annealing operations, primarily related to preferential leaching of Cu-rich phase in subsurface depth. This study shows, through the combination of different analytical methods, EDXRF, PIXE, SEM-EDS and LA-ICP-MS, that important metallurgical information resulting from the minting process may be missed, when relying only on the judgment of high silver alloys surface analysis.

Albeit the existing minor/trace elements compositional gradients between coins surface and bulk, surface contents correlations discriminate distinct silver sources processed during the 15th to 17th centuries in Portugal, from different historical periods and mints, Lisbon and Porto. Gold/bismuth ratios related to the processed silver initial composition and lead/bismuth ratios related to the silver metallurgical processes.

Portuguese minting relied on different silver sources during the 15th to 17th centuries being that low Au and high Bi contents probably derived from argentiferous copper ore sources and during Philippine chronologies the presence of the new discovered Potosí American silver is distinguishable from the European by Au contents < 100 ppm and very low Bi contents.

Abstract presented in the Conference Materiais 2019, 14-17 Abril 2019, Lisboa, Portugal