Non-invasive evidence of mercury soaps in painted miniatures on ivory

Abstract

AbstractThis work focuses on a non-invasive study of two selected painted miniature portraits on ivory to describe the degradation processes resulting from the interaction between mercury-based and lead-based pigments with oils. In the studied miniatures, containing both lead white and cinnabar, the formation of metal soaps has been clearly detected. However, until now they have been identified exclusively as lead soaps. In this study, the formation of mercury soaps has been evidenced in painted artworks for the first time, together with the well-known lead soaps. The fully non-invasive analytical approach involved the use of large-area X-ray fluorescence scanning, X-ray powder diffraction and Fourier transform infrared spectroscopy accompanied by Raman spectroscopy. It enabled complex description of the miniatures, leading to both the identification and the deduction of approximate composition of mercury soaps. In addition, approximate calculations of the amount of pigments consumed by saponification unveiled the complex processes taking place in the oil-based paint containing both cinnabar and lead white, which were further studied within long-term model experiments monitored by both Fourier transform infrared spectroscopy and X-ray powder diffraction.