AIC's 52nd Annual Meeting

Arsenic sulfide pigments, broadly ranging in color from red to yellow, have been used since prehistoric times in their natural, mineral form.1 The most widely known of these arsenic sulfides are orpiment (As2S3), realgar (α-As4S4), and the light-induced alteration product of realgar known as pararealgar (γ-As4S4). The poisonous quality and reactivity of such pigments has been known for centuries, with Cennino Cennini warning that “there is no keeping company with [the arsenic sulfides]” and to “look out for yourself” when working with them. Although arsenic sulfides lost popularity and became commonly replaced by less-toxic yellow colorants over time, they are heavily used in works of art dated prior to the 19th century. Much research has been carried out on understanding the alteration of realgar into pararealgar, both from an atomic-level perspective2 and from a museum perspective.3 Nevertheless, there are many other mineralogical arsenic-bearing phases that have scarcely been identified in works of art, including dimorphite (As4S3), bonazziite (β-As4S4, which is the high-temperature counterpart of α-As4S4), and alacranite (As8S9).4 As a result, the light-stability of these pigments has not been fully assessed from a conservation viewpoint. Recently, as part of Harvard University’s Mapping Color in History project, an arsenic sulfide pigment identified as β-As4S4 was collected from the workshop of the traditional Indian painter, Mr. Babulal Marotia, based in Jaipur, Rajasthan. It is known that β-As4S4 can be obtained from realgar by heat-treating the mineral at temperatures approximating 250 ºC.5 Considering that naturally occurring bonazziite is particularly rare, the identification of this arsenic sulfide phase suggests that the pigment was procured by roasting natural realgar. In the current study, the lightfastness of the Indian pigment will be evaluated and compared to paint outs of natural realgar, orpiment, pararealgar, and artificial realgar from the Forbes Pigment Collection housed at the Harvard Art Museums. The limitations of using a microfading tester on realgar-type pigments will be explained and compared to results from fading experiments carried out in natural lighting conditions. The light-induced alteration of the Indian pigment will be further assessed in-situ using a combination of Raman spectroscopy and photocrystallography. Insights on the photochemical reactions taking place will be evaluated against the natural light fading colorimetry measurements, providing a thorough review on the light-induced degradation pathways of the roasted pigment, and its implications for art conservation.

References

(1) Daniels, V.; Leach, B. The Occurrence and Alteration of Realgar on Ancient Egyptian Papyri. Stud. Conserv. 2004, 49, 73–84.

(2) Bonazzi, P.; Menchetti, S.; Pratesi, G. The Crystal Structure of Pararealgar, As4S4. Am. Mineral. 1995, 80 (3–4), 400–403. https://doi.org/10.2138/am-1995-3-422.

(3) Trentelman, K.; Stodulski, L.; Pavlosky, M. Characterization of Pararealgar and Other Light-Induced Transformation Products from Realgar by Raman Microspectroscopy. Anal. Chem. 1996, 68 (10), 1755–1761. https://doi.org/10.1021/ac951097o.

(4) Gliozzo, E.; Burgio, L. Pigments—Arsenic-Based Yellows and Reds. Archaeol. Anthropol. Sci. 2022, 14 (1), 4. https://doi.org/10.1007/s12520-021-01431-z.

(5) Bonazzi, P.; Menchetti, S.; Pratesi, G.; Muniz-Miranda, M.; Sbrana, G. Light-Induced Variations in Realgar and beta-As4S4: X-Ray Diffraction and Raman Studies.

(6) Zheng, S.-L.; Wang, Y.; Yu, Z.; Lin, Q.; Coppens, P. Direct observation of a photoinduced nonstabilized nitrile imine structure in the solid state J. Am. Chem. Soc. 2009, 131 (50), 18036–18037. https://doi.org/10.1021/ja9094523