Once upon a time, the Alhambra Palace on top of the Spanish hill glittered in gold. Over the centuries, however, the ornate, gilded structures of the Islamic citadel on the ceilings and elsewhere had disintegrated with curious purple stains darkening them. The origin of the spots was a mystery. But scientists say they now understand the chemistry behind the purple hue.
Analyzes revealed that as the gilding decomposes, it forms golden spheres, invisible to the naked eye, that are responsible for the purple color, the researchers reported online Sept. 9 in Scientific progress. The discovery could have implications for understanding how other art and architecture deteriorate over time.
Medieval craftsmen crafted some Alhambra ceilings to look like stalactites from a cave, then gilded them with a layer of tinfoil coated with an alloy of gold and silver. In the 19th century, people covered the humiliating gilding with gypsum, a white mineral found in gypsum.
Geologist Carolina Cardel of Spain’s University of Granada first noticed purple spots on the gypsum in 1993, but at the time she and her colleagues didn’t have the tools to understand the spots. Things changed when the university acquired two types of electron microscopes. Microscopes connect to other instruments that reveal the chemical elements and compounds of the sample at the nanoscale.
Cardel’s colleague Carmen Navarrete, former head of restoration at the Alhambra, died before the team could get answers. Cardel and electron microscopy expert Isabel Guerra, also of the University of Granada, participated without Navarrete to examine layers of gilding, plaster and stains from the Alhambra. “We said we have to finish this and dedicate this work to her,” Cardell says.
The dots in the microscopic images of the plaster turned out to be nanospheres of pure gold, most of which are about 70 nanometers wide. Nanoparticle colors depend on their size, which affects how they interact with light, and 70 nanometers is the right size for a purple hue.
Based on the elements and compounds found, Cardel and Guerra concluded that multiple corrosion processes formed the nanoparticles (SN: 03/21/15). Although pure gold is resistant to corrosion, the Alhambra’s gold-silver alloy is not. Defects in the gilding let moisture in, including the chloride-rich Mediterranean spray in the air. This creates chemical contacts between the metals of the plating, similar to those in a battery. As a result, the lower tin corrodes, working its way through the defects in the alloy and coating some of the gold as grayish dirt.
Different parts of the gold were thus exposed to different concentrations of oxygen. This caused further chemical reactions that dissolved some of the gold, paving the way for the formation of the spheres. These spheres eventually settle into the plaster, Cardell says.
“The level of detail of the research is phenomenal,” says Francesca Casadio, who directs the Department of Conservation Sciences at the Art Institute of Chicago. “Others will see these purple hues and have a rubric to understand the phenomenon.”
There are few reports of purple gold on damaged works of art and architecture. Cardel believes that the white plaster coating added to the Alhambra in the 19th century made the purple easily noticeable. “We think this purple color … is more common than people realize.”