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

• Pseudoboleite was identified for the first time in a paint layer as an alteration product. • Lead white and azurite mixture alters to pseudoboleite when exposed to chlorine. • Copper and calcium oxalates were detected as degradation product. • A non-drying fat of animal origin has been applied to the stone substrate. • Egg was probably used as a pictorial binder. 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.