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).