Egyptian blue in the Castelseprio mural painting cycle. Imaging and evidence of a non-traditional manufacture

The mural paintings in Santa Maria foris portas church at Castelseprio (Lombardy) are a precious early medieval cycle datable between 9th or 10th century CE and are therefore among the oldest Carolingian in Italy. The painting cycle has been analysed in situ, to characterise the palette used in its decoration, with non-invasive molecular and elemental techniques. In addition, Visible Induced Luminescence (VIL) has been applied to assess the extended use of Egyptian blue within the painting cycle. The palette, beside the use of the expected pigments, is characterised by the diffuse presence of Egyptian blue, more than 500 years after the fall of the Western Roman Empire. A remarkable feature in the composition of Egyptian blue is the presence of non-negligible amounts of zinc, indicating a non-traditional production of it. In fact, the use of a copper/zinc alloy rather than the more common copper/tin or copper/arsenic proper of bronze residues as source of copper, has never been previously reported.