Once upon a time, the Alhambra palace on a hilltop in Spain shone with gold. Over the centuries, however, the Islamic citadel’s gilded and ornate structures on their roofs and elsewhere fell into disrepair, with curious purple stains marring them. The origins of the spots were a mystery. But scientists say they now understand the chemistry behind the purple dye.
Analysis reveals that as the dorado decayed, it formed golden spheres invisible to the naked eye that are responsible for the purple colorresearchers report online Sept. 9 at Progress of science. The finding may have implications for understanding how other works of art and architecture degrade over time.
Medieval craftsmen fashioned some Alhambra ceilings to look like cave stalactites, then gilded them with a layer of aluminum foil topped with a gold-silver alloy. In the 19th century, people covered the gold gradient with plaster, a white mineral found in plaster.
Geologist Carolina Cardell of the University of Granada in Spain first noticed purple spots on the plaster in 1993, but she and her colleagues didn’t have the tools to understand the spots back then. Things changed when the university obtained two types of electron microscopes. Microscopes are coupled with other instruments that reveal the elements and chemical compounds in a sample at the nanometer scale.
Cardell’s colleague, Carmen Navarrete, a former head of restoration at the Alhambra, died before the team could get any answers. Cardell and electron microscopy expert Isabel Guerra, also from the University of Granada, continued without Navarrete to examine layers of gilding, plaster, and dyes from the Alhambra. “We said we had to finish this and dedicate this work to him,” says Cardell.
The dots in the microscopic images of the plaster proved to be nanospheres of pure gold, most of which are about 70 nanometers wide. The colors of nanoparticles depend on their size, which influences their interactions with light, and 70 nanometers is the right size for a purple hue.
Based on the elements and compounds detected, Cardell and Guerra conclude that multiple corrosion processes formed the nanoparticles (Serial number: 03/21/15). Although pure gold is resistant to corrosion, the Alhambra’s gold-silver alloy is not. Flaws in the dorado let in moisture, including sea spray in the chloride-rich Mediterranean air. That created chemical contacts between the metals in the gold similar to those in a battery. As a result, the underlying tin corroded, eating its way through flaws in the alloy and coating some of the gold as a grayish grime.
Thus, different parts of the gold were exposed to different concentrations of oxygen. That triggered more chemical reactions that dissolved some of the gold, paving the way for the spheres to form. Those spheres ended up settling into the plaster, says Cardell.
“The level of detail in the study is phenomenal,” says Francesca Casadio, director of the conservation sciences department at the Art Institute of Chicago. “Others will see these purplish hues and have a rubric to understand the phenomenon.”
There are few reports of purple gold in damaged works of art and architecture. Cardell believes that the white plaster cladding that was added to the Alhambra in the 19th century made the purple easily noticeable. “We think this purple color…is more widespread than people realize.”