Science1 publisher2 min readPublished
Synchrotron X-rays trace ninth-century Abbasid luster to an ion exchange in the glaze
A team led by the Universitat Politecnica de Barcelona found copper and silver ions swapping into the glaze during firing and reducing to metal nanoparticles. Separating the parameters that set each color will take mock-ups.
The Scientist · Science desk
What happened
- A team led by the Universitat Politecnica de Barcelona and the ESRF has reported in Science Advances the chemical reactions that produced the shimmering painted surfaces of ninth-century Islamic ceramics.
- The mechanism is an ion exchange during firing, in which copper and silver ions diffuse out of the painted layer into the glaze and replace the alkali ions already there.
- Firing had to stay above the glaze's glass transition temperature so the ions could move, and below the softening point, which kept the luster paint from sticking to the glaze.
- The measurements combined X-ray spectroscopy and fluorescence on the ESRF's ID21 beamline with X-ray diffraction at ALBA Synchrotron, on a beamline adapted to read iron, copper and silver.
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Why it matters
- constraint Because the analysed material is historical fragments with no recorded firing history, the paper can name the mechanism but cannot hand anyone the ratio, temperature or hold time that yields a given color.
- capability Ion exchange during an ordinary firing puts metal nanoparticles inside the glass surface itself, so an optically active nanostructured layer can be formed without depositing anything on top.
- decision A reproduction attempt now knows which variable to fix first, the copper-to-silver ratio in the paint, before tuning kiln schedule or glaze chemistry.
Trinitat Pradell, professor at the Universitat Politecnica de Barcelona and co-corresponding author, said the work started from something modern labs still cannot do. "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," Pradell said [3].
The particles do not sit on the surface. Ions that cross into the glaze are reduced to metal there and grow into nanoparticles within a thin layer near the surface [4]. Composition and shape of that nanostructured region decide how the surface handles light [5].
Both ends of the firing window are properties of the specific glaze [7]. The glass has to be above its transition temperature for ions to move at all, and the paint bonds to the glaze once softening starts, so a modern attempt has to find the window for its own glaze composition [7].
Reading that chemistry out of a fragment took a small beam. The team used X-ray spectroscopy and X-ray fluorescence on the ID21 beamline at the ESRF, plus X-ray diffraction at ALBA Synchrotron: three techniques across two facilities [9][13]. "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 said [10].
The color finding is the one an experimentalist will want to test. Relative amounts of copper and silver in the paint, together with the reactions between the two metals, set the type, amount, distribution and size of the nanoparticles [8]. But the samples are historical fragments with no recorded firing history, so the copper-to-silver ratio varies alongside everything else that differed between workshops [14]. Marine Cotte, ESRF scientist and co-corresponding author, said the next stage addresses that. "This study was carried out by analyzing tiny fragments from historical ceramics. The next step will be to produce and analyze mock-up samples to distinguish the effects of each manufacturing parameter. We'd also like to explore the role of other metals, such as iron and tin," Cotte said [11].
The thing this doesn't tell you is how to fire a gold luster on purpose. A named mechanism narrows the search; it does not supply a ratio, a temperature or a hold time, and fragments whose kiln history nobody wrote down cannot supply them either [14]. The work is published as a Cu, Fe, Ag micro XANES study in Science Advances [12].
What to watch
- Whether the planned mock-up firings reproduce the fragment color series when only the copper-to-silver ratio is varied.
- Whether iron and tin take part in the luster reactions, as Cotte wants to test, or sit inert in the glaze.
- Publication of the firing window in degrees for named glaze compositions, the form a workshop or a lab could use.