Pseudoboleite as an alteration product of azurite in the painted stone of the mullioned windows of the Bargello Palace (Florence)

During the recent restoration of the Bargello courtyard and façades (13th-14th century), initiated in 2020, traces of painted polychrome decoration were identified on Pietraforte intrados and extrados of the mullioned windows. On the stone intrados, blackish lozenge-shaped impressions –likely remnants of a metallic leaf– were against a pale blue background. To investigate the original materials and painting technique, a combination of non-invasive in situ methods (digital microscopy, energy-dispersive X-ray fluorescence spectroscopy, and reflectance spectrometry) and micro-invasive analyses (optical microscope, micro-Raman, SEM-EDS, XRD, FT-IR, GC–MS, PY-GC–MS) was applied. The integrated analyses of the blue decorations revealed the presence of azurite, pseudoboleite and rare particles of lead white, along with Cu-oxalates and Ca-oxalates. The occurrence of pseudoboleite, a rare greenish mineral, suggests it formed as a result of alteration processes involving azurite and lead white, potentially triggered by chlorine-based cleaning treatments. Rare Hg-S-based particles, likely associated with vermilion, were identified in the extrados. Additionally, trace amounts of egg and a non-drying fat were detected.