12 September 2022

Purple ceiling secrets revealed

The Alhambra palace in Spain recently gave up some secrets with the help of an electron microscope at the University of Granada.

Alhambra, Spain
© JJ Montalban/Unsplash

Over its long history the Alhambra has seen many changes and modifications. One such change was the addition of ceiling decorations intended to resemble stalactites.

Gilded tin was common during the Italian Renaissance and the Alhambra used the process too. The creations were covered with tin and gilded with a mix of gold and silver. Originally light gold, as time wore in the structures became purple or blue.

The 19th century solution to this discolouration was to apply gypsum over the gilding, making the ceiling white. As the gypsum wore away, the purple resurfaced.

The new research, published in Science Advances, found that nanospheres made of pure gold (around 70 nm) had formed in the material over time—the nano gold appears purple.

Flaws in the original gilding let moisture enter, which carried in salt from sea spray allowing contact between the metals in the gilding. As the tin corroded and pushed its way to the surface, additional chemical reactions led to the formation of the gold spheres, which eventually settled into the gypsum.

The paper can be accessed using DOI: 10.1126/sciadv.abn2541