Investigating sulfur crystal formation in arsenic sulfide pigments by using advanced mobile X-rays methods

INVESTIGATING SULFUR CRYSTAL FORMATION IN ARSENIC SULFIDE PIGMENTS BY USING ADVANCED MOBILE X-RAYS METHODS

Eva Luna Ravan1,2, Rosario Andolina1, Irene Barba Castagnaro3, Michela Botticelli1, Costanza Miliani1, Claudia Caliri1 and Francesco Paolo Romano1.

1ISPC-CNR, Via Biblioteca 4, 95124, Catania, Italy
2Sapienza University of Rome, Piazzale A. Moro 5, 00185 Rome, Italy
3Department of Physics, University of Calabria, Via Ponte Bucci, Cubo 33B, 87036 Rende, CS, Italy

Arsenic sulfide minerals have been used as artists’ pigments since antiquity, with realgar for red-orange tones and orpiment yielding yellow hues. These materials are intrinsically unstable and progressively degrade, eventually losing their characteristic colours. To evaluate the potential of non-invasive, mobile X-ray-based methods for investigating their physicochemical stability, simplified paint layers were prepared and subjected to controlled environmental conditions through different artificial aging protocols. Advanced X-ray techniques developed at the XRAYLab of ISPC-CNR in Catania were employed to characterize the morphological, structural, and chemical transformations associated with degradation. These included mobile 2D micro-XRF, XRPD mapping, and 3D confocal XRF. Complementary SEM and micro-Raman analyses were performed to validate this approach. Besides identifying the well-known degradation products of arsenic sulfide pigments, the study revealed the unexpected formation of elemental sulfur crystals on the surface of aged mock-ups. This phenomenon has been rarely documented in literature [1]. Typically, the alteration of orpiment or realgar leads to sulfur conversion into hydrogen sulfide, sulfates, or organosulfur compounds [2]. However, recent studies indicate that elemental sulfur release from crystalline orpiment is dependent on pH, dissolved oxygen, and temperature [3]. A comparable phenomenon emerged during a MOLAB campaign at the Egyptian Museum in Turin, where non-invasive analyses of the Book of the Dead of Kha detected sulfur aggregates in areas painted with orpiment, spatially uncorrelated with arsenic. These results provide new insights into the degradation pathways of arsenic sulfide pigments and underscore the need for further research into the implications of sulfur formation in painted artworks.

Acknowledgements: CHANGES Spoke 5 PNRR PROJECT PE 0000020; E-RIHS.it Italian national node of the European Infrastructure of Heritage Science.

[1] M. Leona, Archetype Publications with the Freer Gallery of Art, 2005.
[2] F. T.H. Broers, Journal of the american chemical society, pp. 8847-8859, 2023.
[3] M. Mirazimi, Minerals Engineering, 2021.