Degradation of Fourteenth-century Mordant Gilding Layers: Synchrotron-based Microfocus XRF, XRD, and XANES Analyses of Two Paintings by Pietro Lorenzetti.

Analyses of the degraded mordant gilding on two early fourteenth-century paintings by Pietro Lorenzetti were undertaken at the UK synchrotron facility Diamond Light Source. Previous studies revealed a complex orpiment-tinted mordant and its application for adhering two separate layers of silver and gold leaf and demonstrated degradation of the original materials. A new study involving synchrotron radiation (SR) microfocus X-ray fluorescence (µ-XRF) maps on cross-sections of the discolored mordant allowed imaging of the extent of migration of mobile As-Ag-S bearing species and provided ideal positions from which to collect microfocus X-ray absorption near edge structure (µ-XANES) maps of As, Ag, and S for oxidation state analysis. SR µ-XANES indicates that arsenolite (As₂O₃) is present throughout the mordant suggesting light-induced photooxidation of orpiment. Both As³⁺ (as oxide) and As⁵⁺ have been detected and these are known signs for orpiment degradation. SR microfocus X-ray diffraction (µ-XRD) was used to reveal the nature of altered phases in the mordant layer and identified the presence of acanthite Ag₂S, xanthoconite Ag₃AsS₃, and arsenolite As₂O₃. The analysis confirmed that the darkening of the mordant is most probably caused by finely dispersed grey-colored Ag ⁺ -bearing acanthite particles (Ag₂S). The results are discussed in the context of analysis of contemporary paintings and recent studies on the alteration of orpiment. [ABSTRACT FROM AUTHOR]