Linking indigo use in Mexican colonial paintings to Antonio Palomino’s historical recipes for color permanence

Indigo pigments identified in the paint layers of eighteenth-century Mexican colonial oil paintings display exceptional chromatic stability, contradicting European Early Modern treatises that warned against their use due to poor lightfastness. This study evaluates the hypothesis that local artists deliberately modified indigo to improve its permanence by experimentally reproducing the purification process under thermal treatments described by Antonio Palomino (1655–1726) in El museo pictórico y escala óptica (1724). A synthetic indigo reference (95% purity) and three natural indigo sources —one from the Viluppuram District, India, and two from Niltepec, Oaxaca, Mexico—were selected to investigate their chemical composition, microstructure and degradation pathways via SEM–EDS, ATR–FTIR, NMR, GC–MS, and XRD. The results indicate that thermal treatment promotes partial oxidation and molecular stabilization of the indigo in raw samples retaining impurities and inorganic additives from artisanal extraction processes. This finding highlights innovative color-preparation technologies in Mexican colonial artistic practices.