Wednesday, February 28

We finally know the origin of the purple stains of the Alhambra

For more than a century, strange blue spots on the decorations of the Alhambra had puzzled experts. Now, thanks to new microscopy tools, two researchers from the University of Granada (UGR) have solved the mystery. Like so many times, it can be said that the correct answer was the simplest.

The mystery of the persistent blue.
During the more than seven centuries that have passed since the beginning of the construction of the Granada palatial complex, it has seen many changes. The most important ones, such as improvements, extensions and restorations, do not take away from the existence of other small changes that did not come from the hand of man.

One of these changes was the appearance of a series of purple spots in some decorated areas of the complex. The blue stains were revealed after a work carried out in the 19th century, in which some of the gilt reliefs were plastered in order to cover the deterioration of the original decoration.

Deciphering the riddle.
The first investigations indicated that this color was produced by small gold nano-spheres with a size of approximately 70 nanometers. But there was still a question to be resolved: how was it possible?

An article in the magazine Science Advances explains the phenomenon of apparent transmutation. The authors of the piece resorted to an electron microscope to solve the mystery. Through these tools they were able to analyze the nano-spheres that this pigment was made of and determine how they had been formed.

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Gold does not seem.
Gold is chemically inert, the least reactive of the metals, but there are chemical processes that can alter and dissolve it. “Since the Middle Ages, it has been known that gold can be dissolved in aqua regia (a mixture of concentrated nitric acid and concentrated hydrochloric acid), which was used to create the pigment Purple of Cassio,” said Carolina Cardell and Isabel Guerra, authors of the study. in a press release.

The blue color is formed after an oxidation-reduction process in which aqua regia dissolves metallic gold to transform it into gold ions that form auric chloride complexes that, when joined to a tin chloride solution, create gold nanoparticles like those present in the Alhambra. These resulting nano-spheres give the blue color.

Natural reaction.
But in the Alhambra no one applied aqua regia, rather the process occurred naturally, as an interaction between the tin on which the decorative gold layer had been applied and environmental conditions such as an atmosphere rich in marine aerosols, specifically in chloride , as Cardell points out.

Finally, the blue nano-spheres ended up passing through the tin layer, generating the blue-white contrast that can be seen today in the plasterwork of the Alhambra located in the Patio de los Arrayanes and in the Patio de los Leones.

Beyond art.
Understanding the process behind the formation, artificial or natural, of Cassio purple has opened new doors to the use of the material. As explained by Maite Maguregui, professor at the Department of Analytical Chemistry at the University of the Basque Country, who has not taken part in the study, this phenomenon can have various applications, “for example, tuning or modifying the color of materials to taste, detecting nanoplastics in the environment through a color change due to its interaction with metallic nanoparticles, etc.”

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Cassio purple, an ancient nanomaterial.
The pigment known as Cassio purple can be classified as a nanomaterial as it is composed of gold nanoparticles. This is not so rare, as Josefina Pérez Arantegui, Professor of the Department of Analytical Chemistry at the University of Zaragoza, points out: “mankind has used nanomaterials since ancient times, produced naturally or artificially.”

Arantegui points out that a very early example of this is ruby ​​glass, a material whose color is also given by the presence of gold nanoparticles inside, although this time the color conferred is not blue but red.

Image | University of Granada

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