Study of Medieval Artistic Stained Windows: The Case of the Rose Window of Sant’Ambrogio Chapel in the Basilica of San Petronio in Bologna—Italy

Within the framework of an extensive conservation project involving multiple stained-glass windows of the Basilica of San Petronio in Bologna, Italy, this study reports the results of the diagnostic campaign on the rose window depicting Sant’Ambrogio between two angels holding the coats of arms of the Marsili family. The rose window is located in the homonymous chapel and, based on recent studies attributing the cartoon to the Bolognese painter Biagio Pupini, who was active in San Petronio from 1519, is dated to the early sixteenth century. No evidence was found regarding the workshop responsible for the production of the stained-glass window. The window showed no significant conservation issues, either in the glass elements or in the lead cames. However, the extensive degradation of the grisaille—likely caused by a low-quality mixture, improper firing, or aggressive cleaning—resulted in the loss of the original drawing. This study presents the results of non-invasive investigations on the glass tiles of the rose windows and the analyses of deposits present on their surfaces. Fiber Optic Spectroscopy (FOS) in transmittance, X Ray Fluorescence (XRF), and Hyper Spectral Imaging (HIS) in transmittance were used to investigate the glass composing the rose window. Fourier Transform Infrared Spectroscopy (FT-IR) was applied to study deposit samples collected from the external surface of the window. Additionally, only four glass samples, obtained from hidden areas or already detached fragments, were analyzed using Scanning Electron Microscope with Energy-Dispersive Spectroscopy (SEM-EDS). In addition, a photographic processing method is described, which enabled the recovery of the ghost image, the faint trace or imprint left by the grisaille on the glass during firing, allowing the conservators to faithfully reintegrate the original drawing.