SR micro X-ray fluorescence (µXRF) mapping of decayed, magnesium-containing, carbonatic matrix of Cultural Heritage (CH) fresco paintings treated by ammonium oxalate

This study presents a novel application of synchrotron radiation (SR) micro X-ray fluorescence (µXRF) mapping to investigate the interaction between ammonium oxalate (AmOx), an inorganic-mineral conservation treatment, and the decayed, magnesium-rich, layered carbonate matrix of Cultural Heritage (CH) fresco paintings.
High-quality SR µXRF datasets enabled a qualitative and semi-quantitative elemental analysis, providing microscale 2D mapping of the complex spatial arrangement of calcium (Ca) and magnesium (Mg) within a highly heterogeneous 15th-century Italian fresco. Additionally, the technique offered insights into the spatial distribution of silicon (Si) and aluminium (Al), as well as iron (Fe) grains associated with iron-based pigments in the outermost intonachino layer.
By comparing elemental maps and via RGB correlations, this study examined variations in elemental composition in relation to stratigraphy, microstructure, and conservation state of fresco samples. Notably, results revealed localized Mg enrichment within specific layers and on the fresco’s surface. Combined with synchrotron-based micro X-ray diffraction (µXRD), these findings provided a deeper understanding of AmOx’s protective, passivating, and consolidating effects across the fresco’s stratigraphy.