MA-XRF analysis on paintings and physico-chemical characterization of smalt

Abstract

The aim of this research was, on the one hand, the investigation of two paintings using a mobile scanning macro-XRF systems (MA-XRF) while, on the other hand, a chemical-physical characterization of smalt, a blue potash glass pigment containing cobalt that is present in the two paintings,, was carried out. For this purpose, scanning electron microscopy coupled to an energy dispersive X-Ray spectrometer (SEM-EDS) and X-ray absorption near-edge spectroscopy with synchrotron radiation as source (SR-XANES) measurements were performed.

Two paintings, Ritratto di Michelangelo attributed to S. del Piombo (1485-1547) and The Education of Mary painted by P.P. Rubens (1577-1640), were analyzed using a mobile scanning macro-XRF instrument (MA-XRF). The first painting revealed the presence of an underpainting representing Mary with Child and, possibly, young St. John. This underpainting has a strong resemblance to a work of art by Andrea del Sarto (1486-1530), Madonna and Child with the Young St John, supporting in a significantly new manner hypothesis about the already difficult attribution of the painting to del Piombo. A digital imaging reconstruction based on the elemental maps allowed to clearly visualize the hidden painting and, to some extent, to give an idea of its original colors. In a very convincing manner the reconstruction highlights the similarity between the covered painting and the work of del Sarto. The analyses on the Rubens painting helped in the identification of the pigments used by the artist and in the visualization of the original shape of Mary’s scarf, painted with (now discolored) smalt. A few samples were taken from each of the paintings to characterize in greater detail the smalt grains, present in Mary’s hidden dress in Ritratto di Michelangelo and in Mary’s scarf in The Education of Mary. This pigment, a blue potash glass containing cobalt, has a complex degradation process, that involves the leaching out of potassium from the silica matrix, the consequent change of coordination of Co2+ from tetrahedral (blue) to octahedral (discolored), and the formation of carboxylates soaps between the potassium and the oil (medium), giving rise to a brownish-gray appearance of the paint layer. Due to its conservation problems, smalt is a well-studied pigment. Nevertheless, many elements are present in the pigment as impurities and their role in the degradation process has not been considered previously. In order to supply to this lack of knowledge, a number of scanning electron microscopy energy dispersive X-Ray spectrometry (SEM-EDS) measurements were carried out. For comparative purposes, other historic paint samples containing smalt were analyzed as well. These samples come from different works of art belonging to different periods and provenances. An interesting phenomenon was observed during the analysis: sometimes a lead-enriched rim is present in the degraded smalt grains. This points to an interaction between the silica matrix and the lead ions, coming from the environment (lead white) or present inside the grains, has occurred. The interpretation of the results was possible thanks to a chemometrical analysis on the quantified EDS spectra, allowing to understand the correlation between the smalt composition and its degradation. Moreover, synchrotron radiation X-ray absorption near-edge spectroscopy (SR-XANES) measurements were performed on both degraded and well preserved smalt, to certify the change in the coordination of Co2+ due to the leaching out of potassium, and trying to better understand the phenomenon of lead-enriched rim formation.