Discovering the dual degradation pathway of emerald green in oil paints: The effects of light and humidity.

Abstract

Emerald green, a copper acetoarsenite pigment, enriched the 19th-century and 20th-century artists' palettes and caused darkening and fragilities in paintings. The accepted alteration mechanism involves the oxidation of triarsenite ions [(As₃O₆)^3-] to arsenates (AsO₄^3-), although questions about the promoting factors and the origin of the oxidized arsenic remain unanswered. This study investigates the primary environmental parameters inducing alteration in the oil binder and elucidates the associated degradation pathways through a multiscale analytical approach, including noninvasive spectroscopic techniques and synchrotron radiation-based x-ray methods. By combining results from a historical oil painting by James Ensor (1860 to 1949) and artificially aged paint mock-ups, we identified ultraviolet A-visible light and humidity (relative humidity ≥ 95%) as key driving factors, inducing a dual degradation pathway: Light promotes surface-stratified arsenic oxidation (As³⁺ → As⁵⁺) resulting in amorphous As⁵⁺-rich compounds, whereas a dark, high moisture environment favors arsenolite (As₂O₃) crystallization. In addition, a noninvasive analytical strategy is proposed for monitoring the conservation state of emerald green paints in historical artworks.