Microstructural evolution of Cu-based alloys in Roman mirrors

Roman mirrors have been characterized by means of different analytical techniques, OM, SEM+EDS, GDOES, XRD. They are made with the classic bronzes used by Romans for manufacturing luxurious mirrors (high tin amount, 27-30%; lead amount, 7-8%); on ancient mirrors the following three intermetallic compounds may be foud:?-Cu6Sn5, 61.5 wt%Sn;?-Cu3Sn, 38.2 wt% Sn; ?-Cu31Sn8, 32.6% Sn. These latter phases and a+d eutectoid may be indentified when deliberate tinning of low tin bronzes or copper is carried out, while the mirrors produced with high tin leaded bronzes are characterised by a more complex microstructure and this is especially true if the alloy has been quenched. The mirrors show a front side, shiny, and a back side, dull. The microstructure of the front side is characterized by the presence of a noticeable surface tin oxide enrichment, by an inner layer with small rounded lead globules dispersed in a very fine homogeneous a+d eutectoid cast structure. The microstructure of the back side is characterised by a homogeneous and mineralised a+d eutectoid ghost structure throughout, with deep corrosion attacks. The Sn surface enrichment seems to be responsible for the dark grey silvery lustrous appearance of the front side of the mirrors still reflecting after more than 2000 years of burial.