Fire gilding investigation on early medieval copper-based jewellery by focused ion beam (FIB) on FEG-SEM

•Focused ion beam on a field emission gun scanning electron microscope (FIB FEG-SEM) is a remarkable tool for sampling thin, fragile and complex surface layers of cultural materials.•Applied on medieval mercury gilded copper-based elite jewellery from Prague Castle, it allows to samplein situcross-sections of a few tens of µm and to perform high resolution observations at the nanometer scale.•The fire-gilding investigated is characterized by a two-layer structure related to different metallurgical phases.•Heating the gilding layer not only allows evaporation of mercury but also promotes a sintering process between the gold grains.