Built by the last Spaniards Muslim rulers,The Alhambra is a regal palace which has glowed over the city for 800 years.. Its colors change throughout the day,Standing out as a beacon of terracotta orange under the midday Sun, before changing to red-pinkish hues with the fading light at dusk.
The palace’s interior, which is located in the Alhambra’s gilded halls has slowly been changing its color. After centuries of natural weathering, parts of the palace’s golden flanks and ornate, whitewashed walls are turning a patchy, dull purple – a stain two scientists think they can finally explain.
“Its source remained unknown up to now.” Send an emailCarolina Cardell, University of Granada Mineralogist and Isabel Guerra, Microscopy Specialist at Isabel Guerra publish their paper. It explains how technological advances allowed them to ‘peel away’ layers of the Alhambra’s weathered walls.
The least reactive metal, gold, should withstand the test of time. The precious metal is resistant to sunlight, humidity, air pollution, and baking temperatures, which is why it is such a prized material for crafting jewelry, coins, and more recently, electronic devices – all things you don’t want to degrade.
Gold, soft and malleable, was used to decorate ornaments and palaces. Armor and arms, and artworks that use a technique called “gilding”. Wafer-thin wafers of gold leaf were used to decorate the walls of Alhambra’s palace walls. They were overlaid with sheets of pliable zinc and covered with thin, waxy gold leaf. Over time, the surfaces took on a strange purple hue and were soon covered in white gypsum.
Since ancient times, chemistry has been able to transform gold’s warm glow into a purple-colored bruise. The technique was typically used by Roman alchemists to induce a mixture of hydrochloric and nitric acids, known as nitric Acid hydrochloride or aquia regula. color glassIt goes back to the 4th Century. The aqua regia reaction dissolves gold into tiny particles, which – as inventor and scientist Michael Faraday suggested in 1856 – scatter light into ruby-reds, purples and blues.
To date, there have not been any signs of nitric acids hydrochloride on the Alhambra’s walls. The hue change inside Alhambra had to be caused by a chemical process other than aqua regia.
Guerra and Cardell set out to investigate the Alhambra’s chemical composition using a scanning electron microscope with an array of spectrumrometers.
The researchers studied the Alhambra’s walls over centuries and then modelled the chemical weathering that might have occurred. They discovered an “unexpected combination” of electrochemical processes that could have made the purple-colored surfaces more vibrant.
Guerra and Cardell discovered fissures and voids in the gold leaf that were crater-shaped. These channels allowed moisture to reach the underlying zinc foil and corrode it. The walls were clean of any grime.
However, gold was found where grime covered the walls. Corroded instead. The gold was stripped of its electrons and formed spontaneously 70 nanometers wide gold nanoparticles. These nanoparticles are, according to Cardell and Guerra, the right size for scattering light waves that make it appear purple.
But not all people believe that the corrosion process caused the color change.
Catherine Louis, a chemist from the Surface Reactivity Laboratory in Paris, speaks with APS Physics. It’s incredible that gold can turn purple over time. However, the researchers didn’t perform any experiments to reproduce their corrosion process.
Cardell and Guerra argue that it would be impossible to replicate five centuries of weathering using lab experiments. However, it would not yield very informative results. In their paper.
“Our research is done using a real case study that covers more than five centuries of weathering in natural conditions. This limits our ability to identify the exact corrosion model,” says the duo. Writes.
They suspect that bimetallic gildings may be more prevalent than experts believe, as few surfaces would have a whitish coating like the Alhambra’s halls.
Cardell and Guerra stated that “the results shown here will hopefully assist experts of ancient goldded objects with information relevant to corrosion methods, materials of intervention and corrosion prevention.” Final conclusion.
The study was published in Science Advances.
