This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Scientists led by the Universitat Politècnica de Barcelona and the ESRF, the European Synchrotron, have revealed the chemical reactions in nanoparticles that created a unique, shimmering effect in the painting on ninth-century Islamic ceramics. The results are published in Science Advances.
Many centuries before the rise of nanotechnology, ninth-century Abbasid potters were producing ceramics with striking colors (golden, red, brown or yellow) and a metallic appearance resulting from nanoparticles. This technology, which created the iridescent effect, emerged in the Near East during the Abbasid period and subsequently spread across the Islamic world. "Today we still find it very difficult to reproduce the effects they did in the ninth century, so we wanted to find out what chemical transformation the paint applied to the ceramics went through to create such effects," explains Trinitat Pradell, professor at the Universitat Politècnica de Barcelona and co-corresponding author of the publication.
To achieve the effects, artists would start by painting a glazed ceramic surface with a mixture containing metallic compounds, notably silver and copper. During a subsequent firing, these elements diffused from the paint into the glassy glaze and underwent chemical reactions, including reduction, leading to the formation of metallic silver and copper nanoparticles within a thin surface layer. The particles were therefore not simply deposited on the ceramic: They became embedded in the glaze, creating a nanostructured region whose composition and morphology determined how the surface interacted with light.
The chemistry of the firing process was particularly important. The final appearance was not determined simply by the presence of silver or copper in the paint, but by the complex interplay between them and other elements present in the glaze. In particular, the relative amounts of copper and silver influenced the chemical reactions taking place during firing and, consequently, the formation and characteristics of the metallic nanoparticles.
The UPC team joined forces with ESRF scientist Marine Cotte and colleagues to study these ancient materials using X-ray spectroscopy and X-ray fluorescence on the ID21 beamline at the ESRF, the European Synchrotron, and X-ray diffraction at ALBA Synchrotron. "The small size of the beam and the study of Fe, Cu and Ag have been essential to unveil the chemistry of the luster layers, and this would not have been possible without Marine Cotte and the people at the ID21 beamline adapting the beamline to our problem," Pradell explains. They found that luster is obtained through an ion-exchange process during firing, whereby copper and silver ions diffuse from the paint into the glaze, replacing alkali ions.
To enable this process, the luster must be fired at a temperature between the glass transition temperature and glaze softening to favor ionic diffusivity and prevent the luster paint from sticking to the glaze. The results also showed that differences in composition and microstructure among different luster colors are primarily due to the relative amounts of copper and silver present in the luster. The concentration of silver and/or copper species and nanoparticles near the glaze surface is controlled by the relative amounts of copper and silver in the paint itself, as well as by the reactions between them.
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