The interior of the Alhambra
The interior of the Alhambra in Granada, 2016

Gold is desirable for three simple reasons. It is scarce, inert and yellow. Other metals just can’t compete: copper turns green when it oxidises, silver tarnishes as it reacts with sulfurous compounds. Platinum is more promising, but you’d need to be a trained metallurgist or jeweller to pick it out from a line-up of other base metals. Only gold survives the ravages of chemistry and history, all the while being easily recognisable. Well, most of the time.

The Alhambra in Granada is one of the world’s most famous and well-preserved examples of Islamic architecture. Its magnificent ceilings were originally gilded with gold, but over the years these deteriorated. In the 19th century, to save on the expense of renovation, the glorious gilding was simply covered with gypsum.

Then, in 1993, Dr Carolina Cardell from the University of Granada spotted purple patches forming on the ceiling. It is only in recent years, however, that the university acquired the tools and techniques to investigate.

Dr Cardell and her team knew that whilst pure gold may well be inert, when mixed with other metals to make alloys some chemical reactions can occur. The conditions in the Alhambra made this chemistry possible; the gilding used is actually a leaf of tin coated with a gold/silver alloy. Meanwhile, the salt-rich spray from the nearby sea added chlorides to the mix. These conditions set up electrochemical reactions which dissolved some of the gold within the moist gypsum.

Using electron microscopic techniques, Dr Cardell spotted signs of this gold embedded within the gypsum. But how does this explain the purple patches? Well, gold nanoparticles are not actually yellow. Instead, they take on a red to purple colour, depending on their size. In fact, you have almost certainly come across this phenomena in recent years. This is because gold nanoparticles are also responsible for the purple smears and lines on the Covid lateral flow tests that we are all too familiar with.

So, all that is gold does not glitter. Sometimes, it turns purple.

This piece is a preview from our New Humanist winter edition. Subscribe here.