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

Abstract

Despite its fame as a chemically inert noble metal, gold (alloys) may suffer degradation under specific scenarios. Here, we show evidence of electrochemically corroded gilded tin plasterwork in the Alhambra (Granada, Spain) driving spontaneously made gold nanospheres with the optimal size (ca. 70 nm) to impart purple color at the surface. Purple gold on damaged artworks is found sparsely, and its formation is not fully explained yet. We prove that our decayed gold/silver-tin ornament is due to sequential/coexisting galvanic corrosion, differential aeration corrosion, and dealloying of nonperfectly bonded and defect-based metals. Damage is enhanced by exposure to a chloride-rich atmosphere. A white gypsum coat applied during the 19th century to overlap the unaesthetic gilding assists observation of the gold-based purple color. Our work demonstrates gold dissolution, millimetric migration, physical translocation, and deposition as secondary pure gold nanospheres over a centurial time scale under natural environmental conditions.

Why a Gold Palace is Turning Purple

“…Now Carolina Cardell and Isabel Guerra from the University of Granada, Spain, have found a possible explanation. They show that the makeover comes predominantly from environmental corrosion of the gold [1]. Guerra says that this work was made possible thanks to technological advances that were unavailable when the purple coloration was first noticed in the 1990s.…”

Adsorption of multiple NO molecules on Au10−and Au9Zn−planar clusters. A comparative DFT study

“…[1][2][3][4][5][6] Functionalized pure and doped gold nanoparticles are used in diverse fields, such as nanomedicine 7,8 or decorative artworks. 9 It is known that the natural corrosion of gold/tin nanoparticles yields the purple colour of the Alhambra palace's plasterwork. 9 Ma and coworkers 10,11 have recently studied the adsorption and reactions of NO on anionic gold clusters Au n À (n r 80) using a mini flow-tube reactor running at 150 K. A strong size dependence of the reactions was observed: all sizes with n = 4-11 were reactive, as well as all the even sizes (except n = 32, 56, and 58) and three odd ones (n = 17, 49, and 61).…”

“…9 It is known that the natural corrosion of gold/tin nanoparticles yields the purple colour of the Alhambra palace's plasterwork. 9 Ma and coworkers 10,11 have recently studied the adsorption and reactions of NO on anionic gold clusters Au n À (n r 80) using a mini flow-tube reactor running at 150 K. A strong size dependence of the reactions was observed: all sizes with n = 4-11 were reactive, as well as all the even sizes (except n = 32, 56, and 58) and three odd ones (n = 17, 49, and 61). Anions with n r 11 mainly form complexes containing several NO units (up to six), and most of the reactive ones with nZ12 formed Au n (NO) 3 À complexes.…”

“…1–6 Functionalized pure and doped gold nanoparticles are used in diverse fields, such as nanomedicine 7,8 or decorative artworks. 9 It is known that the natural corrosion of gold/tin nanoparticles yields the purple colour of the Alhambra palace’s plasterwork. 9…”

“…9 It is known that the natural corrosion of gold/tin nanoparticles yields the purple colour of the Alhambra palace’s plasterwork. 9…”