The fate of neutral verdigris in wall paintings: chromatic, micro-morphological, mineralogical and chemical transformations

This study fills the gap regarding the behaviour of neutral verdigris pigment (Cu(CH₃COO)₂·H₂O) in traditional wall-painting techniques, an area far less studied than its degradation in manuscripts or easel paintings. Verdigris was applied using three historical methods—a secco (egg yolk or rabbit glue), mezzo-fresco (lime water), and fresco (lime paste)—and complementary pigment–binder mixtures were prepared to isolate binder effects. Multimodal analyses (stereomicroscopy, spectrophotometry, OM, SEM, XRD, ATR-FTIR) revealed a striking contrast between acidic and alkaline environments: verdigris remained chemically and microstructurally stable in a secco but underwent rapid and extensive alteration in lime-based techniques. These transformations produced marked colour shifts, micro-textural heterogeneity, and micrometric secondary compounds such as copper and calcium acetates (e.g. paceite), and tenorite. The findings provide new insights into historical wall-painting practices as they re-examine the use of verdigris with certain painting techniques, also with significant implications for conservation strategies involving verdigris-containing artworks.