Synchrotron radiation micro X-ray diffraction in transmission geometry for investigating the penetration depth of conservation treatments on Cultural Heritage stone materials

The assessment of the penetration depth of conservation treatments applied to cultural heritage stonematerials is a burning issue in conservation science. Several analytical approaches have been proposedbut, at present, many of them are not fully exhaustive to define in a direct way the composition andlocation of the conservation products formed after inorganic mineral treatments. Here, we explored, forthe first time, the analytical capability of synchrotron radiation m X-ray diffraction in transmissiongeometry (SR-mTXRD) for the study of the crystal chemistry and penetration depth of the consolidatingphases formed after the application of diammonium hydrogen phosphate (DAP) treatments on a porouscarbonatic stone (Noto limestone). The SR-mTXRD approach provided unambiguous information on thenature of the newly formed calcium phosphates (hydroxyapatite, HAP, and octacalcium phosphate, OCP)with depth, supplying important indications of the diffusion mechanism and the reactivity of thesubstrate. Qualitative and semi-quantitative data were obtained at the microscale with a non-destructiveprotocol and an outstanding signal-to-noise ratio. The SR-mTXRD approach opens a new analyticalscenario for the investigation of a wide range of cultural heritage materials, including natural and artificialstone materials, painted stratigraphies, metals, glasses and their decay products. Furthermore, it canpotentially be used to characterize the penetration depth of a phase"A" (or more crystalline phases) ina matrix"B"also beyond the cultural heritagefield, demonstrating the potential wide impact of the study.