Current Research on Artificial Arsenic Sulphide Pigments in Artworks: A Short Review

Abstract

A general re-examination of the artificial arsenic sulphide pigments orpiment, realgar and alacranite using polarised light microscopy (PLM), scanning electron microscopy (SEM) combined with energy and wavelength dispersive X-ray analysis (EDX, WDX) and X-ray diffraction analysis (XRD) revealed the following results: wet-process precipitation products of artificial orpiment consist of golden yellow, amorphous, spherical particles ranging from 0.2 to 2 ?m Ø. Dry-process roasting and/or sublimation products with an arsenolite/sulphur mixture consist mainly of amorphous golden yellow oval drops and spherules of amorphous arsenic sulphide glass (g-AsxSx) with a smooth surface ranging from 1 to 20 ?m Ø in association with arsenolite crystal aggregates and members of the alacranite solid solution series (As8S8)–(As8S9). A bright lemon yellow to orange-red arsenic sulphide pigment on a sixteenth/seventeenth-century South German polychrome recumbent sculpture and a seventeenth-century Dutch painting attributed to Rembrandt's studio has been identified as an artificial orpiment produced with dry-process (roasting/sublimation) methods. The recumbent figure contains (i) yellow, orange or red-brown amorphous splintery fragments and spherules of arsenic sulphide glass of up to 25 ?m Ø, (ii) yellow, brown or red-brown crystalline spherules and splintery fragments of alacranite, (iii) lemon yellow to red-brown agglomerates of arsenic sulphide glass, and (iv) colourless irregular fragments or octahedral crystals of arsenolite. This type can be classified as 'complex artificial orpiment'. The arsenic sulphide pigment in the yellowish paint layers of the Rembrandt studio painting proved to be very uniform bright yellow spherules (max. 9 ?m Ø) of arsenic sulphide glass, which can be classified as 'purified artificial orpiment glass'. A deep red historic arsenic sulphide cake from the collection of the Landesmuseum Joanneum (Graz, Austria) called 'Realgar' turned out to be an amorphous arsenic sulphide glass matrix (approx. g-As2S3) filled with numerous red crystal aggregates of alacranite (approx. As8S9) in the form of spherules, which fill gas bubbles and pores with an average 3 ?m Ø (max. 20 ?m Ø). A very fine-grained light red arsenic sulphide powder called 'Realgar' from an archaeological excavation of a historic arsenic smelter near Strassegg (Steiermark, Austria) turned out to be a member of the alacranite solid solution series (As8S8)–(As8S9) in the form of idiomorphic crystals and complex twinned crystal aggregates (average 10 ?m Ø, max. 30 ?m Ø). All red-colored crystals or crystal aggregates of artificial arsenic sulphides that have been called 'realgar' in the past are in fact members of the alacranite solid solution series. The occurrence of pararealgar as a secondary photochemical reaction product is not exclusively linked with realgar, but can also form on members of the alacranite solid solution series (As8S8)–(As8S9).

Synthetic arsenic sulfides in Japanese prints of the Meiji period

“…SEM-EDS analyses further strengthen this hypothesis, since the presence of alacranite spherules (Fig. 3b), with non-stoichiometric ratio between arsenic and sulfur, is indicated in literature as the evidence for synthetic arsenic sulfides produced by dry-process rather than a wetprocess method [2,14,15]. Interestingly, the analyses of the Aizu synthetic arsenic sulfide pigment carried out by Takamatsu in 1878 also show excess free sulfur [25].…”

“…Another arsenic sulfide, alacranite (As 8 S 9 ) [12] is a rather uncommon red mineral showing a complicated crystallographic structure where clusters of As 4 S 4 (realgar-type) and As 4 S 5 coexist in an ordered cagelike structure [3]. Although rare in nature, alacranite has been documented in works of art as a possible artificial product deriving from the arsenolite smelting [13][14][15]. The production of artificial orpiment in Western Europe was first mentioned by Johannes Alcherius in his recipe collection (1380-1420) [16], but the very first description of this synthetic pigment was made by Cennino Cennini in his treatise [17].…”

“…Techniques such as polarized light microscopy (PLM), micro-Raman spectroscopy (RM), X-Ray Fluorescence spectrometry, X-Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive microanalysis (SEM-EDS) [15] are best suited to this task: some of them allow noninvasive analysis while others require microsampling. An in-depth study carried out on natural and artificial orpiment, the latter obtained through both wet-and dry-process, pointed out how only the results from the combination of several techniques allowed to identify the actual nature of the pigment [2,14]. In particular, the presence of spherules of arsenic sulfide glass or alacranite solid solution series (As 8 S 8 )-(As 8 S 9 ) was shown to be conclusive evidence for an artificial origin for orpiment [14].…”

“…An in-depth study carried out on natural and artificial orpiment, the latter obtained through both wet-and dry-process, pointed out how only the results from the combination of several techniques allowed to identify the actual nature of the pigment [2,14]. In particular, the presence of spherules of arsenic sulfide glass or alacranite solid solution series (As 8 S 8 )-(As 8 S 9 ) was shown to be conclusive evidence for an artificial origin for orpiment [14].…”

Natural and synthetic arsenic sulfide pigments in Japanese woodblock prints of the late Edo period

“…2). The peaks at 193, 232, 330-340 and 493 cm −1 are assigned to various As-S vibrational modes and specifically, the broad peak centered at 340 cm −1 strongly indicates an amorphous arsenic sulfide, or artificial orpiment (g-As 2 S 3 or g-As x S x ) [24][25][26][27][28]. The peak centered at 471 cm −1 is characteristic of free sulfur.…”