The Alhambra Palace is the most visited monument in Spain, so we’re not surprised to see it’s also one of the most studied. But for a long time, researchers had yet to learn why the ceilings in certain parts of this famous palace were purple instead of gold.
This discolouration is due to centuries-old dirt building up on the ceilings from smoke and cooking oils from open kitchens below them. After years, they may have been freshly painted with gold paint or covered with marble or other decorations, but these coverings couldn’t prevent some stains from seeping through.
“The next time you see the Alhambra, take a look at the ceilings,” says Carlos Alvarado, a professor of engineering at the University of Granada who helped to expose this process. “The stains are more visible during certain times of day and in certain seasons.”
Alvarado and his former student, Miguel Angel Gallego, wanted to find out why the stains occurred in some places but not others. They collected images from Google Earth that showed interior rooms in the palace with dark streaks along their ceilings. Then they took an infrared camera that allowed them to observe light wavelengths invisible to human eyes and used it to see which mineral deposits were present on these surfaces.
Gallego and Alvarado said that all of the surfaces with traces of purple colouring contained calcium carbonate, which crystallizes into limestone over time. They also identified a spectrum of other minerals present at different levels on these surfaces: magnesium carbonate, magnesium phosphates, iron phosphate, and titanium oxides.
The researchers believe the stains are traced to smoke from cooking fires in kitchens below the floors above these ceilings. These fires may have fallen through gaps between buildings or downstairs to create a small fire in one area that would spread to others over time. The resulting smoke would carry soot, lime, and other particles and settle on the walls, ceilings and floorings where people walked.
“The smoke would go everywhere,” Gallego says. “You can see the paths where people walked. There must have been groups of workers cleaning all the time.”
Over time, this grime would turn into limestone deposits on surfaces throughout the palace. Still, in some instances, calcium carbonate deposits would also seep through marble or other materials covering the ceiling. The researchers suspect that these coverings are often cleaned before completely dry after being applied to make them smooth and workable — an unfortunate choice for preventing stains that may be worse than their alternatives.
“We believe that if the Alhambra’s upper ceilings were decorated with marble as we see in other buildings, the stains would potentially be much less noticeable”, Gallego says. “The Marrakech palace and the Ottoman baths in Istanbul are two great monuments with such ceilings.”
Gallego and Alvarado plan to investigate more castles and other historic buildings to see whether they could find similar stains. They also hope to apply their knowledge of these minerals and how they form to how modern homes are maintained by keeping them clean — whether roofs, walls, floors or other materials cover them.
“The technology we used to study the Alhambra can be applied to other buildings, allowing us to see the signs of use and dirt,” Gallego says. “We could also use it to find out who was walking around in a room that hasn’t been cleaned for many years.”
The recently restored ceilings at the Alhambra have long been known to be stained by smoke from cooking fires below them — but until now, no one knew what colours they were. At first, it was thought they were black due to carbon soot. But researchers used infrared cameras to show that they’re purple due to limestone deposits.