(ORDO NEWS) — Built by the last of the Spanish Muslim rulers, the Alhambra is a royal palace that has dominated Granada for 800 years. During the day, its colors seem to change, s emerging as a terracotta-orange beacon in the midday sun, then giving way to reddish-pinkish hues at dusk. light fades.
From within, in the gilded halls of the Alhambra, the palace is also slowly changing color. After centuries of natural weathering, parts of the palace’s golden flanks and richly decorated whitewashed walls take on a mottled, dull purple hue – a stain the two scientists think can finally be explained.
“Its origin has remained unknown until now,” University of Granada mineralogist Carolina Cardell and microscopy specialist Isabel Guerra write in a published article that describes how technological advances allowed the pair to “peel off” the layers of the Alhambra’s weathered walls.
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The soft and malleable gold was also used to decorate palaces, adorn weapons and armor , as well as works of art using the gilding technique.
In the case of the Alhambra, thin sheets of gold superimposed on sheets of flexible tin originally decorated the walls of the palace. But over time, the surfaces took on a strange purple color, and in the 19th century they were quickly covered over with a white plaster coating.
The transformation of the warm glow of gold into a bluish-violet is a chemist’s trick known since ancient times. Commonly evoked by a mixture of nitric and hydrochloric acids known as nitric acid hydrochloride or aqua regia, Roman alchemists used the technique of staining glass as early as the 4th century.
The aqua regia reaction dissolves gold into tiny particles, which, as inventor and scientist Michael Faraday proposed in 1856, scatter light into ruby reds, purples and blues.
However, no traces of nitric acid hydrochloric acid were found on the walls of the Alhambra. With no aqua regia in the mixture, the change in hue inside the Alhambra must have been caused by another chemical process.
Cardell and Guerra set to work using a scanning electron microscope equipped with a set of spectrometers to reveal the chemical composition of the gold-plated parts of the Alhambra down to the nanoscale.
After studying the Alhambra’s centuries-old walls and modeling the chemical weathering that likely followed, the researchers found “an unexpected combination of electrochemical processes” could turn damaged surfaces purple.
Cardell and Guerra found crater-shaped voids and cracks in the gold leaf, channels through which moisture could enter under the tin foil and corrode it when the walls were free of dirt.
But where the walls were covered in mud, the gold corroded instead. When stripped of its electrons, the gold gradually degraded and spontaneously formed gold nanoparticles approximately 70 nanometers in diameter, which Cardell and Guerra say are just the right size to scatter the light waves that make it purple.
However, not everyone is convinced that it was this corrosion process that led to the discoloration.
Katherine Louis, a chemist at the Laboratory for Surface Reactivity (LRS) in Paris, told APS Physics that it is surprising how gold material can turn purple over time, but noted that the researchers did not conduct any experimental tests to try to replicate their proposed process. corrosion.
However, replicating five centuries of weathering in laboratory experiments would be challenging and not necessarily very informative, Cardell and Guerra argue in their paper.
“Our study is based on a real-world example of more than five centuries of natural weathering, which limits our ability to explain in an accurate model of corrosion,” the duo wrote.
They also suspect that the presence of gold nanoparticles and wear on bimetallic gilding is probably more common than architectural heritage experts have noticed, because few surfaces can be coated with a whitish coating. the same layer as the gilded halls of the Alhambra were.
“The results presented here will hopefully assist experts in ancient gilding with information related to corrosion techniques and intervention materials, as well as corrosion prevention,” Cardell and Guerra. conclusion.
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