Electron microscopy reveals the reason for the purple stains on Alhambra ceilings

Bob Yirka is a research scientist at Phys.org.

There are purple stains on the ceilings of the famous Alhambra palace in Spain. Carolina Cardell and Isabel Guerra wrote a paper about their study of the ceiling of the fortress and what they found.

Improvements were made over hundreds of years after construction of the Alhambra began. Artists added materials to the ceiling in order to make it look like stalactites. They were gilded with a mix of gold and silver. Light gold was the initial color. Many of the features began to become blue or purple over time, giving the ceiling a unique look. Workers applied gypsum over the gilding to make the ceiling white. The purple color was visible again after the gypsum was gone. The purple color was thought to be caused by age and air pollution. They knew that gold doesn't oxidize, so it had to be something else.

The researchers were able to take a closer look at the ceiling with the help of an electron microscope. At a small size, gold appears to be a purple metal after forming in the material over time. Over time, flaws in the original gilding allowed contact between the metals in the gilding, and the tin to push its way to the surface. The gold spheres were formed by additional chemical reactions.

The researchers suggest that their work could be used by other historians to help solve the mystery of the purple stains.

More information: Carolina Cardell et al, Natural corrosion-induced gold nanoparticles yield purple color of Alhambra palaces decoration, Science Advances (2022). DOI: 10.1126/sciadv.abn2541 Journal information: Science Advances

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