Deadly Red: Photochemical Alterations in Bonazziite, Realgar, Pararealgar, and Other Arsenic Sulfide Pigments

Abstract This study evaluates the light sensitivity of arsenic sulfide pigments, including realgar (α-As4S4), bonazziite (β-As4S4), pararealgar (γ-As4S4), orpiment (As2S3), and red arsenic sulfide glass (g-As43S57). A particular emphasis is placed on understanding the reaction kinetics behind light-induced color changes of realgar and related mineral pigments: realgar, bonazziite, and pararealgar. Light-exposure experiments are combined with Raman spectroscopy, two-dimensional powder X-ray diffraction, and spectrophotometry to systematically understand phase and color changes as a function of light dose. To apply these studies to paints, the effects of different binders are also examined, with gum showing the least influence on the light-induced alterations. Changes in color are related to photoinitiated chemical and structural evolutions of the mineral pigments. Time-resolved Raman spectroscopy of pigments exposed to 633, 532, and 475 nm lasers provides information on the reaction kinetics of realgar and bonazziite as they alter into pararealgar (with associated color changes from red to yellow) and on the photosensitivity of pararealgar when excited by high-energy visible light (with color changes from yellow to red). Results are linked back to traditional pigment processing techniques in South Asia as well as a case study of Portrait of Francis I, King of France by the 16th century Ottoman painter Haydar Reis. The latter is believed to feature pararealgar as an original artist’s material, which is present throughout the painting in a variety of yellow and red hues, reflecting its light-sensitive nature.