Diagnostic characterisation of masonry and decorative materials of the apsidal area of the Cathedral of Cefalù (Sicily, Italy)

Abstract The Cathedral of Cefalù (Sicily, Italy), a UNESCO World Heritage Site since 2015, was built during the Norman rule and is famous for its magnificent mosaic. The mosaic apparatus, located in the apsidal area of the Cathedral, recently underwent extensive conservation work. Besides the mosaic tesserae , this area of the Cathedral was subjected to interventions aimed at preserving the integrity of masonry plasters, mortars, stuccoes and coloured plasters. However, basic information such as the chemical and mineralogical composition and microstructural arrangement of original materials was still lacking. In this perspective, in situ and ex situ analyses were carried out for the first time on the masonry and decorative materials of Cefalù’s Cathedral apse. Chemical analyses were carried out on original mosaic tesserae by means of portable X-ray fluorescence, highlighting that their manufacture involved the recycling of glass, excluding the use of natural glass and identifying the chromophores responsible for the wide colour variety ; X-ray diffractometry was performed on masonry plasters and mortars allowing to recognise the use of predominantly carbonatic binders and the presence of degradation products such as calcium oxalates and sodium chloride; finally, petrographic observations carried out on stuccoes enabled the identification of various types of aggregates (carbonatic, gypsum-based, and quartz-feldspathic) embedded in a carbonatic binder, while petrographic analysis of the coloured plasters allowed the identification of three pigmentation techniques. Overall, these analyses provided very useful information on the nature of building and decorative materials as well as their mineralogical composition and degradation products, laying the foundations for future conservation work that takes into account the characteristics of each material. Graphic abstract