More than half a millennium ago, artists adorned plasterwork and ornaments in the Alhambra palace in what is now Granada, Spain, with magnificent gold-leaf decorations. Over the centuries, the decorations experienced the normal wear and tear that art conservationists are familiar with. Yet none of the common damage mechanisms could explain strange purple blotches (see the photo below) that have stained the gold, a metal treasured for its inertness. Now, with the help of latest-generation electron microscopy, Carolina Cardell, a geologist, and Isabel Guerra, a biologist and instrument specialist, have laid out how, in specific environments, gold can corrode and turn purple.
Cardell and Guerra, both at the University of Granada, analyzed samples of the plasterwork decoration with a high-resolution scanning electron microscope and identified compounds with the help of built-in x-ray and Raman spectroscopic equipment. The researchers focused on the deteriorated gold leaf and on the chemically damaged layer behind it. Medieval artists commonly employed gilding, in which they used a layer of gold to decorate another material. In this case, the Alhambra artists adhered flimsy sheets of gold leaf to a thicker foil composed of tin.
The researchers’ imaging and spectroscopy revealed significant corrosion of the tin backing and a layer of grayish, tin-rich grime overlaying some of the gold leaf. The outermost layer of the decorations was a white gypsum coat that was applied in the 19th century to mask the worn regions of gilding. Embedded in that surface layer, the researchers found tiny gold spheres (shown in the micrograph below), most with a diameter of about 70 nm. Incident visible light interacts with gold nanoparticles through surface plasmons (see the article by Mark Stockman, Physics Today, February 2011, page 39) and emerges with a purple color.
Having found the culprit for the stains, Cardell and Guerra worked out how the gold nanoparticles formed. The story that underlies their model begins with the application of the gold leaf, which contained imperfections that exposed the vulnerable layer of tin behind it to the humid, chlorine-rich air that is found in the coastal Granada region. Over time, metallic tin oxidized to Sn2+ ions and combined with oxygen, hydrogen, and chlorine to form compounds that pushed up through the gold leaf as the gray-colored grime. Along with silver impurities in the gold leaf, the grime was a key ingredient for spurring the chemical breakdown of the gold. The parts of the gold leaf covered in grime had less exposure to atmospheric oxygen than the uncovered parts, triggering another round of redox reactions through a process called differential aeration corrosion. Some of the gold dissolved in the chlorinated moist medium and eventually precipitated out as metallic gold nanospheres.
Cardell and Guerra suspect that conservationists will find similar stains in other gold-leaf artworks if they look closely enough. The splotches at the Alhambra, they say, stand out because of the gypsum layer: The bright white color makes the embedded gold nanoparticles far more visible to the naked eye than they are in uncoated areas. (C. Cardell, I. Guerra, Sci. Adv. 8, eabn2541, 2022.)
Related: X-ray imaging shows how a 17th-century painting lost its color
When an arsenic sulfide pigment chemically degraded, it stripped the painting’s yellow rose of visible details.
