SR-based μXRD–μXRF 2D mapping to study Mg-rich historical frescoes subjected to inorganic conservation treatments

This paper proposes the novel application of synchrotron radiation (SR) micro X-ray diffraction (μXRD) and micro X-ray fluorescence (μXRF) mapping to explore the interaction of inorganic-mineral conservation treatments (ammonium oxalate, AmOx) with the decayed, magnesium-containing, layered carbonatic matrix of Cultural Heritage (CH) fresco paintings. The high quality of SR μXRD–μXRF datasets was qualitatively and semi-quantitatively investigated and the complex mixture and stratigraphy of both Ca- and Mg-oxalate phases formed within an Italian fresco painting (XV century) – displaying a high degree of compositional and microstructural heterogeneity – were 2D localised at the microscale. The comparison of the different phase maps and elemental maps showed how the phase composition of reaction products varies as a function of the (i) Mg-rich or Ca-rich carbonatic regions of the fresco, (ii) Ca²⁺ availability, and (iii) microstructure and state of conservation of the fresco. Moreover, the crystallisation of the new phases within single layers and between contiguous ones of the fresco painting was highlighted, demonstrating the synergic protective, passivating, and consolidating action of AmOx treatment across the fresco stratigraphy. Above all, this study proves the high potential of SR μXRD–μXRF mapping in the conservation of stone materials and opens new analytical perspectives in heritage science and materials science for the advanced and non-destructive elemental and structural investigations of heterogeneous, layered, multiphase micrometric systems.