A micro-core sample taken from the panel “The histories of Joseph” of Ghiberti’s Gates of Paradise bas-relief was fully investigated during the last restoration in the eighties of the twentieth century. Recently, the systematic analysis via field emission scanning electron microscopy (FESEM) of the whole surface of this micro-sample allowed to well understand the artwork conservation state. Moreover, the modern advanced analytical technique allowed to find and localize two AuCu- and AuCu 3 -ordered phases due to diffusion phenomena, highlighted for the first time in an ancient artefact in gilt bronze exposed outdoors. Considerations on the thermodynamic stability of these ordered phases, along with estimation of reasonable diffusion coefficient values, seem to support K. Anheuser’s hypothesis on the fire-gilding manufacturing technique that the final step is a thermal treatment of a few minutes, conducted at a temperature certainly below 400 °C and with estimated values between 250 and 300 °C.