More than a thousand years ago, in the ninth century, potters in the Abbasid world were doing something that still stumps scientists today: they painted ceramics in shimmering gold, red, brown, and yellow that glowed with a metallic, iridescent beauty. Now, researchers in Barcelona have finally cracked the secret of how they did it — and the answer lies in nanoparticles, the same kind of tiny structures that power modern nanotechnology, centuries before the word even existed.
"We still find it very difficult to reproduce the effects they did in the ninth century," says Trinitat Pradell, a professor at the Universitat Politècnica de Barcelona and co-author of the study published in the journal Science Advances. So she and her colleagues teamed up with scientists at the ESRF, the European Synchrotron, to find out exactly what chemical magic was happening inside those ancient glazes.
The technique, called luster, worked like this. Artists painted a glazed ceramic surface with a mixture containing metallic compounds, especially silver and copper. Then they fired the piece. During that firing, the metals diffused from the paint deep into the glassy glaze he glaze, where chemical reactions — including a process called reduction — turned them into tiny metallic silver and copper nanoparticles buried in a thin surface layer. Crucially, the particles weren't just painted on top. They became embedded inside the glaze, creating a nanostructured region whose shape and makeup determined how light bounced off the surface. That's what produced the dazzling play of color.
To see these microscopic secrets, the team used X-ray spectroscopy and X-ray fluorescence on the ID21 beamline at the ESRF, plus X-ray diffraction at the ALBA Synchrotron. The beam was so tiny that it could study single fragments of the ancient pottery. "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," Pradell explains.
The firing had to be just right — hot enough to let ions move, but not so hot the paint stuck to the glaze. And the balance between copper and silver was everything: their relative amounts governed what kind of nanoparticles formed, how big they were, and how they were arranged want, and that's what created the different colors.
Now the researchers want more. "The next step will be to produce and analyze mock-up samples to distinguish the effects of each manufacturing parameter," says Marine Cotte of the ESRF. "We'd also like to explore the role of other metals, such as iron and tin."
Nine centuries later, a lost art is finally giving up its secrets — and it turns out the ancient potters were nanoscientists ahead of their time.