Photoluminescence Micro-imaging Sheds New Light on the Development of Metal Soaps in Oil Paintings

Abstract

This paper describes a photoluminescence imaging approach coupled to a synchrotron source (SR-PL imaging) that provides a new, powerful way to probe the processes involved in metal soap formation in paint films, processes that play an important role in the deterioration of paint films in traditional and modern oil paintings.The technique couples multispectral PL mapping with sub-micrometer spatial resolution with the ability to tune the excitation radiation from UV-C to the visible range. False-color RGB images of the distribution of PL from three historical paint cross sections (two with lead soaps and one with zinc soaps) using a range of excitation and emission wavelengths were obtained. The resulting maps of PL from lead soaps within the cross sections provide new insight about their formation, which can be visualized occurring around lead white particles or flocs of particles as an initial step. Inhomogeneity within an individual lead soap aggregate is revealed and it can be seen that the aggregated forms have complex structures. A single aggregate in one sample was found to comprise five phases.High spatial resolution images of the distribution of zinc soaps throughout a ZnO-containing paint film show that the periphery of small fissures containing soaps show luminescence of zinc oxide particles and no sign of a gradient in PL into the paint film, and thus allow inferring that Zn soap formation is not initiated at cracks, but rather soaps accumulate in them. Haloes of high luminescence around particles of emerald green implicate it in the development of Zn soaps.These results show the potential of SR-PL imaging to provide improved characterization of Pb and Zn soap structures, leading to a better understanding of the kinetics of their formation and the development of macroscopic aggregates.