"Its origin remained unknown until now," write University of Granada mineralogist Carolina Cardell and microscopy specialist Isabel Guerra in their published paper, which outlines how technological advances made it possible for the pair to 'peel back' the layers of the Alhambra's weathered walls.
After studying the Alhambra's centuries-old walls and modelling the chemical weathering that likely ensued, the researchers found an "unexpected combination of electrochemical processes" might have shaded the damaged surfaces purple.
Cardell and Guerra found crater-shaped voids and fissures in the gold leaf, channels through which moisture could reach the underlying tin foil and corrode it, when the walls were free of grime.
Stripped of its electrons, the gold gradually degraded and spontaneously formed gold nanoparticles roughly 70 nanometers in diameter that, Cardell and Guerra say, are the right size to scatter a spread of light waves that make it appear purple.
They also suspect that the presence of gold nanoparticles and the deterioration of bimetallic gildings are likely more widespread than architectural heritage experts have noticed because few surfaces would be covered with a whitish layer like the Alhambra's gilded halls were.