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

ABSTRACT

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.

KEYWORDS:

• Orpiment
• arsenolite
• acanthite
• xanthoconite
• XANES mapping
• Pietro Lorenzetti
• fourteenth-century paintings
• pigment degradation
• mordant

Acknowledgements

This work was carried out with the support of Diamond Light Source (Beamline I18) under Proposal SP17644: Atomic-scale mechanisms for the degradation of the mordant layers in paintings by Pietro Lorenzetti (fourteenth century). Our particular thanks are due to Professor Fred Mosselmans for his support with the synchrotron work. We are also grateful to the Philadelphia Museum of Art and the Ferens Art Gallery, Hull for permitting study of the paintings.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data set: raw data can be made available on request by email to H. Howard, Jens Najorka ([email protected]), and Paul F. Schofield ([email protected]). Synchrotron data are available from the Diamond Light Source Archive.

Notes

1 Detailed studies using μ-XRPD (micro X-ray powder diffraction) on a paint cross-section has also shown that the secondary arsenate minerals schultenite (PbHAsO₄), and mimetite (Pb₅(AsO₄)₃Cl), have precipitated at the interface between an orpiment layer and the underlying lead white-rich layer (Simoen et al. 2019). Mimetite has also been identified in later, lead white containing additions to Cimabue’s Maestà and has been interpreted as a secondary product of the degradation of orpiment (Monico et al. 2022).

Additional information

Funding

This work was supported by Diamond Light Source under Grant SP17644.