Study of dry‐ and wet‐process amorphous arsenic sulfides: Synthesis, Raman reference spectra, and identification in historical art materials

Abstract

This paper reports the dry and wet synthetic procedures and characterization by Raman spectroscopy of amorphous arsenic sulfide reference pigments. Reference spectra of two amorphous materials obtained by wet process methods and four dry process references of amorphous arsenic sulfide pigments of known composition are presented and discussed.While all materials present a main band characteristic for the amorphous pigment centered on 341 cm −1 , additional small contributions indicate the presence of sulfur, arsenic oxide, and crystalline nano phases embedded in the amorphous matrix. Although only the broad 341-cm −1 peak is necessary to identify the arsenic sulfide as an amorphous material, the smaller additional features allow for the characterization of the various manufacturing processes and initial materials used. In ideal conditions, these small features also enable to assess the As/S ratio of the studied amorphous arsenic sulfide pigments based on their relative intensity. In this context, the latter reference spectra were used to characterize the amorphous arsenic sulfide pigments and their arsenic to sulfur elemental composition in four 18th-to 20th-century historical samples and compared with scanning electron microscopy with energy dispersive X-ray semiquantitative analyses. The identification of the amorphous arsenic sulfide used in these historical samples was compared with the description of the manufacturing processes reported in historical sources of the time, allowing for a better understanding of the evolution of the amorphous arsenic sulfide pigments manufacturing methods.

Orange for gold? Arsenic sulfide glass on the V&A Leman Album

“…The comparison with spectra published in recent literature (see, for example, Grundmann and Richter, Vermeulen et al, and Vermeulen et al) showed a good correspondence between our spectra and those from samples recently classified as arsenic sulfide glass . As reported by Vermeulen et al, the broad band at 340 cm ‐1 corresponds to the Raman shift of the As‐S‐As stretching vibration in AsS 3 in amorphous arsenic sulfide pigments, whereas those at 186 and 233 cm ‐1 are attributable to realgar‐like crystalline nano‐phases embedded in the amorphous matrix.…”

“…Moreover, although the naturally occurring mineral forms of arsenic sulfides are well studied (see Grundmann and Richter and Bonazzi et al), their artificially manufactured equivalents are largely unexplored, and often no difference between natural and artificial sulfides is made in the literature. Recent investigations involving both the analysis of pigmented artefacts and the experimental preparation of pigment powders and cakes according to the historical recipes are finally clarifying the nature, production, and use of artificial arsenic pigments …”

“…The analytical techniques commonly used to characterise these compounds are optical microscopy, SEM‐EDS, XRD, and X‐ray fluorescence (XRF), but Raman spectroscopy has recently proven to be a powerful tool to discriminate amongst different arsenic sulfides, both of mineral and artificial origin …”

The evolution of the materials used in the yun technique for the decoration of Burmese objects: lacquer, binding media and pigments

“…The spectra for these particles also showed minor bands at ca. 233 and 495 cm −1 , which are attributed to realgar-like nano phase and are considered to be indicative of a heat process used to produce the amorphous pigment [51]. This hypothesis was also supported by the fact that some other particles in these samples showed mixed features between natural orpiment and amorphous arsenic sulfide.…”

“…This hypothesis was also supported by the fact that some other particles in these samples showed mixed features between natural orpiment and amorphous arsenic sulfide. These observations can be taken as indication of the production of amorphous arsenic sulfide by sublimation of natural orpiment [50,51].…”