Scientists from the Universitat Politècnica de Barcelona (UPC) and the European Synchrotron Radiation Facility (ESRF) have uncovered the chemical processes behind the shimmering, metallic appearance of ninth-century Islamic ceramics. Their findings, published in Science Advances, reveal how ancient potters created these striking effects using nanoparticles. This technique, which emerged during the Abbasid period in the Near East, spread across the Islamic world and is still admired today for its vibrant colors and reflective surfaces. The ceramics were made by applying a paint containing metallic compounds—mainly silver and copper—onto glazed surfaces. When fired at high temperatures, these elements reacted within the glaze. Through a process called reduction, the metals transformed into tiny nanoparticles, forming a thin layer on the ceramic's surface. These nanostructured regions interact with light in a way that creates the metallic sheen. The final appearance of the ceramics depended on the balance of silver, copper, and other elements in the glaze, as well as the firing conditions. To study the materials, the UPC team worked with scientists at the ESRF, using advanced techniques such as X-ray spectroscopy and X-ray fluorescence. These methods allowed them to analyze the distribution and composition of elements like iron, copper, and silver within the glaze. They discovered that the shimmering effect, or "luster," was created through an ion-exchange process during firing. Copper and silver ions from the paint moved into the glaze, replacing other ions, and formed nanoparticles that gave the surface its unique optical properties. The team emphasized that the firing temperature was crucial—too low, and the ions wouldn’t move properly; too high, and the paint could stick to the glaze. The differences in color and appearance among various luster types are mainly due to the varying amounts of copper and silver in the paint. These elements not only influence the formation of nanoparticles but also determine their size, distribution, and overall effect on light. Future research will focus on creating mock-up samples to test different manufacturing conditions and explore the roles of other metals, such as iron and tin, in the process.