Natural and amorphous arsenic sulfide pigments: characterization, degradation and influence of the binding medium
18 September 2017
Campus Middelheim, A.143 - Middelheimlaan 1 - 2020 Antwerpen (route: UAntwerpen, Campus Middelheim
Organization / co-organization:
Department of Chemistry
Koen Janssens, Jana Sanyova
PhD defence Marc Vermeulen - Faculty of Science, Department of Chemistry
Arsenic sulfides correspond to group of pigments used on a wide variety of objects ranging from Egyptian antiquities to early 20th-century decorative arts and which color varies from bright yellow to vivid orange/red. However, their vibrant colors can sometime change over time, modifying the visual aspect of the artwork. Therefore, to preserve the objects in which they are used, it becomes crucial to understand the chemical reactions happening in these artists’ pigments. This represents the base of this research.
Historically, arsenic sulfide pigments have been considered easy to identify due to the limited numbers of arsenic-containing pigments found in artists’ palettes. The identification of natural orpiment (As2S3) or natural realgar (As4S4) was often based on elemental analysis in yellow and orange/red areas respectively. However, with the description of amorphous arsenic sulfide pigments (g-AsxS100-x) as early as the 14th century and the identification of pararealgar (As4S4), the yellow light-induced degradation product of realgar, it became clear that new ways to characterize arsenic sulfide pigments were required. Among all available techniques, Raman spectroscopy appeared as one of the most interesting due to its ability to differentiate between the various crystalline and amorphous forms based on their vibrational signatures. In this work, Raman spectroscopy was also used to gain access to important information regarding the amorphous arsenic sulfide pigment production processes. This information appears crucial to understand the pigment’s history of use and technical innovations.
Along with the characterization of the various forms of arsenic sulfide pigment, this work also investigated, through a multi-analytical approach, the chemical behavior of the pigments, from the formation of arsenate species in humid environments to the influence of extrinsic factors such as binding medium or light on the pigment itself or other compounds found in the paint system. Therefore, by combining conventional methods (Raman or infrared spectroscopies, optical and electronic microscopies) and advanced techniques (SR-XANES, electrochemistry), it was shown that all pigments and binders do not behave equally in relation to the aging process. Furthermore, a light-induced structural modification of natural orpiment, similar as the one observed for the transformation of realgar into pararealgar, was identified as the cause for the arsenic sulfide-induced darkening of copper- and lead-based pigments.
The increased knowledge on arsenic sulfide characterization and degradation presented in this Ph.D. is believed to help drawing better diagnostic, conservation, exhibition and storage guidelines which should allow future generations to enjoy the artworks in conditions as close as possible to their original forms.