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Study Looks at Alhambra Gold Leaf Corrosion
Centuries after its construction, gold on the Alhambra, a palace and fortress of the Moorish monarchs of Granada, Spain, has spontaneously turned purple.
In a recent study by the University of Granada, researchers looked at the corrosion of the gilded tin plasterwork and the reasoning behind its color change.
About Alhambra
Built between 1238 and 1358 under the Islamic rule of Spain, the name Alhambra signifies in Arabic “the red,” which is believed to be derived from the reddish color of the rammed earth used to construct the outer walls.
The structure sits on a plateau overlooking Granada and, throughout its history, has witnessed war and natural disasters on several occasions. As a result of these instances, the Alhambra was repaired and rebuilt throughout much of the 14th and 15th centuries.
In the 1800s, architect José Contreras took on an extensive repair and rebuilding program which was endowed by Ferdinand VII in 1830. After the death of Contreras in 1847, his son Rafael continued his work for nearly four decades.
According to Britannica, additional restoration and conservation work continued through the 21st century. Despite the many changes the structure underwent, it managed to remain decorated in a wide variety of media.
One of the most notable materials, and what recently inspired a corrosion study, was the use of thin gold leaf. Reports indicate that the tin within the gold leaf, though more expensive, was much easier to work with and also offered the gold a thicker support.
In 1984, the Alhambra was designated a United Nations Educational, Scientific and Cultural Organization (UNESCO) world heritage site. The site was then expanded in 1994.
Today, visitors can witness the vast Alameda de la Alhambra (a park outside of the palace) and many historic architectural features throughout the site. Some of the feats include a massive triumphal arch known as the Gate of Pomegranates, several courtyards, the Hall of the Ambassadors, Comares Tower, the Generalife, and a citadel in addition to many other named courts, halls and rooms.
Gold Corrosion Study
Since published in the peer-reviewed journal Science, a team of researchers from the University of Granada recently explained how the gilded tin plasterwork in Granada's Alhambra spontaneously turned purple.
The study was co-authored by Carolina Cardell and Isabel Guerra.
While more expensive, the gold tin does not obtain enough structural integrity for heavy-duty structures like the Alhambra Palace, The Jerusalem Post shared. To explain the matter further, Cardell and Guerra closely analyzed plasterwork that had undergone water runoff and condensation throughout the palace.
By observing these environmental instances, the duo could better identify how an abnormal pattern of corrosion might corrupt the gold surface over time.
“Damaged gilded tin is found on plasterworks in (semi)-open areas of the Alhambra palaces at runoff/water condensation sites. Here, most of the gilding is lost and remains show gray-black, golden, or whitish color displaying an odd purple shade,” wrote the authors.
“Examination with a stereomicroscope (SM) and a polarized light microscope (PLM) of bulk and cross-section samples displays the structure and features of the decoration. The sequence of layers, described from interior to surface, is as follows. The gray-black color points to a porous corroded nonuniform metal foil...The golden color is due to a friable metallic yellow leaf...that contains fissures, flakes, and rough/rounded voids.”
As shown by the SM, the authors found that remnants in the materials appear as “an iridescent metallic purple shiny covering,” which is observed as a tiny grayish film under the PLM. On the surface, they also noted that a sponge-like texture adhered to the material. This material was reported to be whiteish in color with a purple tint.
The results of the study reveal that the gold/silver-tin ornament is due to sequential/coexisting galvanic corrosion, differential aeration corrosion, and dealloying of non-perfectly bonded and defect-based metals. The duo further explained in their study abstract that the damages were further enhanced by exposure to a chloride-rich atmosphere.
“The results shown here hopefully will help experts of ancient, gilded objects with the information relevant to corrosion methods and materials of intervention, as well as corrosion prevention,” concluded the study's authors.
A full copy of the study can be accessed here.
Gold Failures Elsewhere
Several years ago, a mysterious coating failure was causing 1,200 square feet of gold—on a then new, half-million-dollar paint job—to crack and peel on a massive historical monument in New York City’s Central Park.
Occurring in 2014, the 24-foot-tall rendering of Gen. William Tecumseh Sherman, sculpted by the late Augustus Saint-Gaudens in 1903, was regilded the year prior. According to PaintSquare Daily News, when the sculpture underwent rehabilitation, it was cleaned and coated with 23.75-karat gold leaf.
While traditional coated with three coats of polyurethane for protection, Michael Kramer, Founder of The Gilders' Studio (Olney, Maryland), which completed the project, said the team took a different approach.
Kramer said the statue “was toned, then coated with a urethane system,” not with polyurethane. “It was, in fact, an aliphatic urethane system which had been rigorously tested in the lab and the field.”
In any case, the $500,000 project started coming undone by the following October, when conservancy officials first noticed cracks forming at the base. First, they thought it was a moisture problem, but it continually got worse.
“The gold was just not adhering to the substrate as it's supposed to,” Christopher Nolan, Vice President for Planning, Design and Construction of the Central Park Conservancy, told The New York Times. “Something had gone wrong in the way the materials interact,” Nolan said, adding that in some areas, the coating “almost peels off like an orange.”
According to its website, The Gilders' Studio provides interior and exterior gilding, sculpture conservation, painted finishes and murals, and similar services. Kramer took a look at the Sherman cracking (also referred to as crazing).
“Even where there was no crazing evident to the eye, when you put it under the microscope, you could see it. So we knew it was systemic,” Kramer told The Times.
Since the discovery of the issue, The Gilders’ Studio accepted responsibility for the problem and paid for the regilding of the public artwork.
Officials do not know what caused the failure, but materials used were tested in the laboratory and onsite, Nolan said.
One possibility was an issue with the adhesive material, called size, that was used, or the urethane the conservancy opted to use to make the surface more resistant to pigeon damage than traditional wax.
Kramer told The Times that the size that was successfully tested came from a different lot than the size that was actually used. Perhaps, the report said, there was some discrepancy.
According to the conservancy, Sherman planned to regild the statue that summer. For the redo, it was shared that the urethane would be cut out, as would the intermediate glazing.
