Corroding gold leaf in the world-famous Alhambra palace in Grenada results in purple nanoparticles.

Gold is known as an inert material, yet gold alloys can certainly degrade. Researchers from the University of Grenada analysed gilded tin on the ceilings of the Alhambra palace, which turns a purplish colour in some places. They show that the plaster corrodes electrochemically, creating nanoparticles about 70 nm in size from the gold in the gilding. And that seems to be the optimal size to give a purple colour, they write in Science Advances.