Once upon a time, Spain’s hilltop Alhambra palace glittered with gold. Over the centuries, though, the Islamic citadel’s ornate, gilded structures on its ceilings and elsewhere fell into disrepair, with curious purple splotches marring them. The stains’ origins were a mystery. But scientists say they now understand the chemistry behind the purple tinge. Analyses reveal that as the gilding decayed, it formed gold spheres invisible to the naked eye that are responsible for the purple color , the researchers report online September 9 in Science Advances . The finding may have implications for understanding how other art and architecture degrades with time. Medieval artisans crafted some Alhambra ceilings to look like a cave’s stalactites, then gilded them with a layer of tinfoil topped by a gold-and-silver alloy. In the 19th century, people covered the degrading gilding with gypsum, a white mineral found in plaster. Geologist Carolina Cardell of Spain’s University of Granada first noti...
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