Novel insights on the study of a fifteenth-century oro di metà/Zwischgold gilding by means of ion and electron microscopy: characterization of the stratigraphy avoiding cross-sections preparation

In this work, an analytical methodology to identify and thoroughly characterize the oro di metà/Zwischgold gilding is reported. Gildings in paintings are commonly studied by means of optical microscopy (OM), Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectrometry (SEM-EDS), as well as other techniques, but they all require sample preparation. In this study, a sample of Zwischgold taken from an Italian panel painting of the fifteenth century was thoroughly characterized using Focused Ion Beam (FIB)-SEM sectioning, accessing the layered structure from the surface into depth without further sample preparation. The results achieved were compared to those of a further sample taken from the same area, which was prepared by conventional resin embedding and polishing techniques in order to evaluate the effects of the mechanical stresses induced by resin drying and polishing along the latter process. A more uniform morphology and sharper outlines of the metal layers were observed after FIB milling, likely due to the more accurate and less invasive cutting process. Moreover, since SEM-EDS analysis is limited in terms of resolution to be performed straight on the milled area relative to the thinness of the metal layers, Time of Flight - Secondary Ion Mass Spectrometry (ToF-SIMS) analysis was performed, and allowed revealing two distinct metal layers in the gilding consisting of Au and Ag thus confirming a Zwischgold structure.