In antiquity, Pb was a common element added in the production of large bronze artifacts, especially large statues, to impart fluidity to the casting process. As Pb does not form a solid solution with pure Cu or with the Sn–Cu alloy phases, it is normally observed in the metal matrix as globular droplets embedded within or in interstitial positions among the crystals of Sn-bronze (normally the α phase) as the last crystallizing phase during the cooling process of the Cu–Sn–Pb ternary melt. The disequilibrium Sn content of the Pb droplets has recently been suggested as a viable parameter to detect modern materials [Shilstein, Berner, Feldman, Shalev & Rosenberg (2019). STAR Sci. Tech. Archaeol. Res. 5, 29–35]. The application assumes a time-dependent process, with a timescale of hundreds of years, estimated on the basis of the diffusion coefficient of Sn in Pb over a length of a few micrometres [Oberschmidt, Kim & Gupta (1982). J. Appl. Phys. 53, 5672–5677]. Therefore, Pb inclusions in recent Sn-bronze artifacts are actually a metastable solid solution of Pb–Sn containing ∼3% atomic Sn. In contrast, in ancient artifacts, unmixing processes and diffusion of Sn from the micro- and nano-inclusions of Pb to the matrix occur, resulting in the Pb inclusions containing a substantially lower or negligible amount of Sn. The Sn content in the Pb inclusions relies on accurate measurement of the lattice parameter of the phase in the Pb–Sn solid solution, since for low Sn values it closely follows Vegard's law. Here, several new measurements on modern and ancient samples are presented and discussed in order to verify the applicability of the method to the detection of modern artwork pretending to be ancient.

The composition of lead inclusions in modern and ancient leaded tin bronzes was studied by X-Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) in Scanning Electron Microscope (SEM). Lattice parameter of lead inclusions in all bronzes was smaller than the lattice parameter of pure lead. This determination indicates that lead inclusions in bronzes are nothing else but Pb-Sn solid solutions. Tin concentration in lead inclusions in modern bronzes was not less than 3 at% in accordance with the Pb-Sn phase diagram, tin concentration in lead inclusions of ancient bronzes was as low as 1 at%. This difference enables a distinction between ancient bronze artefacts and modern products including the most sophisticated fakes. On the other hand, our work demonstrates that the generally accepted Pb-Sn phase diagram corresponds to an incomplete equilibrium state and only after centuries-long aging at ambient temperatures does the Pb-Sn solid solution reach real equilibrium.