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Art & Artifacts

Scientists uncover the mysterious origins of purple plaster in a Spanish palace

Once adorned in gold leaf, the ceilings of the Alhambra palace complex owe their royal hues to gold nanoparticles

by Ariana Remmel
September 13, 2022

A flat chip of corroded material about 2 mm tall and 1.5 mm wide. The chip has irregular, rough grey edges. The middle includes a couple swatches of a chalky purple material embedded in shiny yellow gold.
Credit: Sci. Adv.
Gold nanoparticles about 70 nm in diameter make the white plaster covering this gilded tin appear purple.

Purple splotches sprawling across the ceiling of a palace in southern Spain have puzzled art conservators for decades. Now, scientists searching for the splotches’ origin have finally struck gold, according to a new study.

Originally built in the 13th century, the Alhambra palace complex in Granada, Spain, once featured intricately sculpted ceilings gleaming with golden adornments. This opulent effect was achieved using a type of gold leaf made from fragile sheets of gold alloy reinforced with tin foil, says Carolina Cardell, a geologist at the University of Granada who studies art conservation and restoration. But these gilded surfaces eventually deteriorated to cauliflower-textured grime that conservators in the 19th century hid under white plaster, she says. In the 1990s, Cardell noticed purple stains permeating some of these plastered patches where no pigment could explain the new hue. “We thought immediately this is because the metals are suffering corrosion, but we had to wait until we applied the correct analytical techniques to figure out the origin of the purple,” Cardell says.

To investigate, Cardell and Isabel Guerra at the University of Granada analyzed samples of the corroded material layer by layer, including the tin foil, gold leaf, and purple-stained plaster, using high-resolution scanning electron microscopy and transmission electron microscopy. The researchers traced the origins of the lumpy black grime to compounds resulting from degradation of the tin foil. They also discovered gold nanoparticles about 70 nm in diameter embedded in the plaster, which appear purple to the eye. These nanoparticles likely arose from a series of electrochemical reactions triggered by marine aerosols wafting in from the coast that carried water vapor and salts into the gaps between the tin and gold layers, Cardell says. The white plaster that was meant to obscure the degraded tin also highlighted the purple nanoparticles left behind by the deteriorated gold alloy (Sci. Adv. 2022, DOI: 10.1126/sciadv.abn2541).

Cardell suspects this type of corrosion has already afflicted gilded tin decorations beyond the Alhambra and hopes her team’s discovery will help restoration experts to preserve other cultural monuments.



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