Scientists Reveal the Chemical Secrets of the Nanoscale Iridescent Effect in 9th-Century Islamic Ceramics

A scientific team led by the Universitat Politècnica de Catalunya (UPC) in Barcelona and the European Synchrotron Radiation Facility (ESRF) has successfully revealed the nanoparticle chemical reactions that created the distinctive shimmering iridescent effect in 9th-century Islamic ceramic painting. The related research findings have been published in the academic journal Science Advances.

Centuries before the rise of modern nanotechnology, 9th-century Abbasid potters were already able to produce ceramics with vivid colors such as gold, red, brown, or yellow and a metallic luster. This technique for creating the iridescent effect originated in the Near East during the Abbasid period and subsequently spread throughout the Islamic world. UPC professor and co-corresponding author of the paper Trinitat Pradell explained that even today, it remains very difficult to perfectly reproduce this unique 9th-century effect, so the team wanted to understand exactly what chemical transformations the paint applied to the ceramic surface underwent.

To achieve this iridescent effect, ancient artisans first painted a mixture containing metal compounds such as silver and copper onto the surface of already glazed ceramics. During the subsequent firing process, these metal elements diffused from the paint into the vitreous glaze and underwent chemical changes including reduction reactions, ultimately forming nanoparticles of metallic silver and copper in the surface layer of the glaze. These particles were not simply deposited on the ceramic surface but were deeply embedded within the glaze layer, forming nanostructured regions whose chemical composition and microscopic morphology together determined how the surface interacted with light.

The chemical reactions during firing were crucial. The final color and luster of the ceramic depended not simply on whether silver or copper was present in the paint, but on the complex interactions between them and other elements in the glaze. In particular, the relative content of copper and silver directly influenced the chemical reactions that occurred during firing, thereby determining the formation and size characteristics of the metal nanoparticles.

The UPC team collaborated with Marine Cotte of the ESRF and colleagues, using the ESRF's ID21 beamline as well as X-ray spectroscopy, X-ray fluorescence, and X-ray diffraction techniques at the ALBA synchrotron light source to conduct in-depth analysis of these ancient materials. The researchers noted that a beam with micrometer-scale precision and targeted study of iron, copper, and silver elements were essential for uncovering the chemical mechanisms of the luster layer.

The study found that this metallic luster was obtained through an ion-exchange mechanism during the firing process. During this period, copper and silver ions diffused from the paint into the glaze layer and replaced alkali metal ions. To successfully complete this process, the firing temperature had to be precisely controlled between the glass transition temperature and the softening temperature of the glaze, thereby promoting ion diffusion while preventing the outer paint layer from sticking to the glaze surface.

The results also showed that the compositional and microstructural differences between different luster colors mainly stemmed from the relative proportions of copper and silver in the paint. The concentrations of silver and copper species and nanoparticles near the glaze surface were controlled not only by the copper-to-silver ratio of the paint itself but also regulated by reactions between the two, which together determined the type, quantity, distribution, and size of the nanoparticles.

Regarding future research plans, ESRF scientist and co-corresponding author of the paper Marine Cotte stated that this study was completed by analyzing tiny fragments of historical ceramics. The next step for the team will be to produce and analyze simulated samples to distinguish the specific effects of various production parameters, while also exploring the roles played by other metals such as iron and tin in this ancient craft.