Two Spanish researchers solve the mystery of the violet color in the Alhambra

It is due to an electrochemical process that has given rise to the formation of gold nanospheres The color can be seen thanks to the plaster used to restore monuments in the 19th century His research has been published in an article in the journal Science Advances El plaster that was placed in the 19th century in the Alhambra was essential to show the mystery and Carolina Cardel and Isabel Guerra’s tenacity to solve it. The two researchers from the University of Granada have discovered why the gold of the plasterwork of the Alhambra (in the Pomares and Leones palaces) is turning purple. A change in color that Cardell as a geologist and Guerra as an expert in microscopy , they appreciated about 30 years ago, but that, although it aroused their curiosity then, they have not been able to solve until now. Once the technical, labor and personal “stars aligned” they found the culprit and, what is reason for the color change. The person responsible is an electrochemical process that has led to the formation of gold nanospheres with a size of 70 nanometers. At NIUS we have spoken with Professor Carolina Cardell so that she can explain step by step how this electrochemical process occurs and how this detective work was that has led them to solve the enigma. Question: They have just published an article where they reveal what is happening so that the gold changes color in the Alhambra, can you tell us how you have achieved it? A: We began to investigate the composition of the polychrome painting in the Alhambra almost 30 years ago. At that time we saw that there was indeed a violet color on the surface of the plasterwork in the palaces of Pomares and in the Palace of the Lions that did not correspond to a deliberate application. The type of decoration in the plasterwork consists of the application of a very thin sheet of gold on a thicker sheet of tin. Q: But something was altering its color because in certain areas what you see are dark, grayish spots and what is more surprisingly, there are purple or violet areas next to the gold.R. That is, in 2006 we published the structure of that gilding, which was a very thin layer of gold on a layer of tin and all of that covered by a layer of plaster. We saw the violet color but we did not know what produced it, where it came from and what produced that process. Q. And they decided to find out.R. The microscopic techniques of today did not exist then, in addition to the fact that we had other professional and family responsibilities. But when all the stars aligned we were able to find out. Q. And what did they discover? A: About ten years ago the university acquired high-resolution electron microscopes that allowed us to study all this with greater precision, so we were able to discover that this violet color is due to the formation of nanospheres gold with a size of 70 nanometers. It is the size that provides this violet color.P. And how did this phenomenon occur, was it spontaneous in nature? A. The mechanism was more difficult to decipher than to know what had caused it. The universal solvent for gold is aqua regia, this is a mixture of concentrated nitric acid and concentrated hydrochloric acid. It never occurs to anyone to pour aqua regia on the plaster, we had to know what mechanism had acted naturally in these golds to cause that color. We appreciate that the areas that had that color are semi exposed to the elements where there is humidity and are also affected by the atmosphere of Granada, which is rich in marine aerosols, specifically in chlorides because the city is close to the Mediterranean. That is to say that the humidity of the air and other natural factors have had to do.R. Its not that easy. When we obtained images with the optical and electron microscopes, we saw that gaps had formed between the gold foil and the tin in the form of a crater. When the gilding is done by burnishing on the tin sheet, imperfections or holes usually occur, and those little holes are the channels through which the water with the chlorides accesses the tin sheet. So the first thing to corrode is the tin. Nothing happens to gold at first, but when the tin corrodes, tin oxides are formed and these come out through those crater-shaped holes and are deposited on the surface of the gold sheet. They do not form a continuous layer but irregular, in one part there is an oxide deposit. This deposition of “dirt” on the gold foil favored the dissolution of the gold. Q. You mean that corrosion and external factors favored the dissolution of gold. A. It is a combination of electrochemical corrosion processes. To understand us, this is like a galvanized battery, the least noble element is tin, the most noble is gold that does not change. But with a solution rich in chloride, electrochemical corrosion occurs. The tin is introduced through these channels, but the gold finds areas where it has tin and others where it does not. This dissolves the gold that has areas with oxygen and others without oxygen, which causes the electrochemical process of corrosion.Detail of what is happening in the Alhambra.niusdiario.esP. And is this how the nanospheres that give rise to the purple color are produced? When gold is dissolved, it wants to return to its metallic state immediately, so it precipitates in the most stable form, which are spheres. Therefore nanospheres are formed that with a size of 70 nanometers gives that violet or purple color. P. His research has just been published in the prestigious journal Science Advances, why is it important? It is important for several elements, on the one hand it advances scientific research on gold attack processes, on the other it helps restorers to know what is happening so that these deteriorated plasterwork can be intervened. I imagine that there will also be other monuments around the world where something similar is happening. Here we have been able to see it because in the 19th century in the Alhambra, to somehow restore the deterioration that was being seen, it was decided to apply a layer of white plaster and that layer has been decisive in showing us what was happening. plaster that today probably no restorer would approve of putting on the monument but that has somehow served as a revealing role for the violet allowing it to stand out. “At the time it was a valid solution and has allowed us to decipher the secret that we now reveal,” he confesses the researcher who now leaves the value of her finding to restoration experts. “We have discovered what is happening and why, now it is up to others to decide what to do with the information we give them.”