The mystery of the gold turning purple in the Al-Hamra Palace has finally been solved

As Bilbo Baggins said, not everything gold shines like gold. Sometimes it can turn purple, as in the Alhambra Palace, where a layer of gold used in wall and ceiling decorations has changed color over the centuries.

This color change was seen as a mysterious event until now, but the chemistry of the transformation has finally been clarified.

The Alhambra Palace in Granada, Spain is considered one of the largest examples of Islamic architecture in the world. Construction of the palace began in 1238, however, it was modified and expanded many times, especially in the 14th century.

Parts of Al-Hamra’s ceilings are gilded with gold leaf, but in some places these have developed unusual purple spots. Purple may have been associated with emperors since Roman times, but neither the rulers nor the builders of the palace expected this.

Professor Carolina Cardell of the University of Granada and Dr. As Isabel Guerra notes in Science Advances, a series of chemical reactions that cause gold to become nanoparticles produces this purple color.

These purple fragments were confusing the observers. Because, as the article states, “pure gold (Au) is the least reactive metal in natural and industrial environments. Au does not change color under sunlight or in general environmental conditions such as humidity, air pollution, corrosive gases and high temperatures.”

Although the Al-Hamra belonged to the period sometimes known as Spain’s Islamic golden age, the ornaments were not of solid gold. Such production would be both impractical and ridiculously expensive. It would also likely be a constant target of thieves for centuries. Instead, it was made of tin that was gilded with a thin layer of gold.

(A) General view of the Lions palace in the Alhambra. (B) Multi-colored residues showing traces of purple in closed wet areas (black arrows). (C) Damaged gilded tin with purple places in muqarnas. Cardell and Guerra/Science Advances

A 2006 article described the color purple, stating that it only occurs in moist areas, but could not explain why.

Cardell and Guerra used a high-resolution field-emission scanning electron microscope to examine the purple spots and compare them to areas that retained their original golden hue.

This color comes from the fact that gold particles, like other metals, are only nanometers wide and have different properties compared to the bulk material form. In fact, it is possible to make the vast majority of rainbows from gold particles suspended in water by simply changing their size and shape. Particles similar in size to the wavelengths of visible light absorb certain photons and reflect longer and shorter ones to be picked up by our eyes.

The authors found that exposure to chlorine-rich water disintegrates the glitter into nanoparticles about 70 nanometers wide. This size was just the right size to reflect the violet part of the light spectrum while absorbing most other light we can see. The effect was considered unattractive in the 19th century and was covered with a plaster coating, but some areas began to show itself.

(D) Gilded tin structure visible from the inside out: worn gray-black metallic foil, damaged metallic gold leaf (layer 2); iridescent purple-grayish coating and purple whitish coating fragments on the surface. (E) Polarized light microscope image of the gilding section. Note the irregular surface of the gray metallic foil (layer 1) and the crater-shaped voids in the gilded tin (circles). C. Cardell et al. Guerra, University of Granada, Spain.

Besides the presence of water, the breakdown of gold to nanoparticle size also depends on how porous the gold glitters are and how they adhere to the underlying tin. The presence of small cracks can oxidize tin acting as an anode, triggering galvanic corrosion. The electrons released by the tin act as an inactive electrode and move towards the unreacted gold.

However, due to pollution, the gold was partially contaminated. By dissolving the oxygen-deficient areas into microflakes, differences in oxygen concentration occurred on the surface of the gold. The fact that silver was added to the mix along with gold during the production of the gildings helped speed up this process.

Although spherical gold nanoparticles can be produced from microflakes with many different reducing agents, the authors consider tin ions in chloride solutions to be the most likely cause. The chlorine ions that kick-start the process are also thought to be a product of air pollution, probably rather than salt from the sea.