Natural corrosion-induced gold nanoparticles yield purple color of Alhambra palaces decoration

Scientists have uncovered the natural corrosion process behind the striking purple coloration observed on gold decorations in the Alhambra palaces, Granada. The study reveals that electrochemically corroded gilded tin plasterwork spontaneously generates gold nanospheres, approximately 70 nm in size, which are optimal for producing this rare and visually distinctive hue. This phenomenon is driven by a combination of galvanic corrosion, differential aeration corrosion, and dealloying processes, exacerbated by exposure to chloride-rich atmospheres. A 19th-century gypsum coat applied to the artwork further accentuates the purple coloration by obscuring the underlying gilding. The research demonstrates how gold dissolution, migration, and deposition can occur over centuries under natural environmental conditions, providing new insights into the degradation mechanisms of historical metalwork.

Cardell, C., & Guerra, I. (2022). Natural corrosion-induced gold nanoparticles yield purple color of Alhambra palaces decoration. Science Advances, 8(36). https://doi.org/10.1126/sciadv.abn2541