An innovative electron paramagnetic resonance and statistical analysis approach to investigate the geographical origin of multi-layered samples from a Renaissance painting

The present study aims at unravelling whether an innovative, integrated method based on electron paramagnetic resonance (EPR) spectroscopy and statistical analysis could shed light on the mineralogical attribution of calcium carbonate, CaCO3, microcrystalline powders found in an unusual preparation layer of a Renaissance painting on wood. Moreover, the undertaken investigation was performed to assess, if possible, the geographical provenance of the same carbonate powders. A specifically designed procedure was operated based on previous methods applied to the investigation of ancient white and colored marbles. Microscopic samples were taken from the painting with the aim of keeping both their size and mass at a minimum possible. All investigated samples consist of a highly complex, multi-layered, heterogeneous material, where the calcium carbonate aliquot is very limited. The investigation has been carried out in a non-destructive way, registering the EPR spectra of the fragments, identifying the diagnostic Mn(II) spectra, parameterizing them, and comparing them with the most extensive available database through a set of robust statistical methods. The main results include the attribution to calcite (marble rather than chalk), the mineralogical speciation of the carbonate layer and the reduction of the possible calcite provenance sites to few localities where marble was exploited. Among them, the marble from Apuan Alps is one of the likely provenances of this calcite.