Probing Attic Black Glaze

In “Ode on a Grecian Urn,” John Keats pays homage to the beauty of Attic—that is, Athenian—pottery. The black glaze that helped give such urns their distinctive beauty was a technological achievement of the 6th and 5th centuries B.C. that scientists and archaeologists are still trying to understand today.

The familiar Attic pottery was made of porous reddish-brown ceramic with a glossy black layer on top, both of which contained iron. In a three-stage firing process performed under oxidizing, reducing, and oxidizing conditions, the black glaze vitrified and adhered to the ceramic. The result was a material that was both beautiful and durable.

The molecular origin of those properties was the focus of a recent study of the materials found in ancient Greek vases and in modern reproductions (Anal. Chem. 2014, DOI: 10.1021/ac500990k). The work, which used a method called confocal X-ray absorption near edge spectroscopy, or XANES, was carried out by an interdisciplinary team led by Lars Lühl, then a postdoc in the research group of Birgit Kanngiesser at the Technical University of Berlin.

XANES is a nondestructive technique. A lens focuses an X-ray beam to a 30-μm spot that strikes a sample and excites fluorescence, which is measured at a 45-degree angle to the surface. Conventional XANES is performed without an additional lens in front of the fluorescence detector. But that configuration results in measurements that can’t distinguish among layers in the sample. Lühl and his coworkers used a second lens in front of the detector. The overlap in the focus of the two lenses defines the probe volume. In this way, they can tell which layer an analyte is in.

Modern reproductions of the pottery were produced by Lühl’s collaborator Eleni Aloupi-Siotis, a chemist and archaeological scientist, at her company, Thetis Authentics, in Athens. The modern black glaze material—like the ancient black glaze—was produced by using a colloidal suspension of specially selected Attic clay particles smaller than 330 nm that was fired under oxidizing-reducing-oxidizing conditions at 890–900 °C. The result, Aloupi-Siotis says, is a “highly resistant potash-alumino-silicate glass, which is colored by magnetite nanocrystals synthesized in situ.”

Working with geochemist Max Wilke of the GFZ German Research Centre for Geosciences, in Potsdam, the team used confocal XANES to determine the oxidation state of iron within the reproductions’ material and to quantify the ratio of Fe³⁺ to total iron. They found that the highest-quality Attic black glaze, which has a characteristic bluish hue, has the lowest percentage of Fe³⁺, between 20 and 30%. They were also able to make confocal XANES measurements of ancient pottery shards unearthed during recent archaeological excavations from the Acropolis and Kerameikos in Athens.

The authors say that “the possibility to compare ancient black glaze with modern samples offered us a unique opportunity to fully unravel the nature and properties of an ancient nanomaterial with unique properties that result in its perfect conservation in hostile burial environments for almost two millennia.”

“The study of ancient Athenian pottery is of interest because the production of these objects was a significant technological advancement requiring sophisticated understanding and control of material properties and the conditions required to transform them into a smooth, glossy black surface,” says Karen Trentelman, a conservation scientist at the Getty Conservation Institute in Los Angeles. She is the director of a collaborative project on Athenian pottery with Aerospace Corp. and Stanford University’s synchrotron radiation source.

“The only evidence we have of how these objects were made lies within the vessels themselves, and, being valuable historic artifacts, opportunities to carry out scientific analyses can be limited,” Trentelman says. “The work using confocal XANES to probe the glaze layer without having to remove a sample would potentially allow more vessels and more areas on vessels—not just broken edges—to be studied.”

The Getty project “illustrates the dual nature of present-day interest in the Attic black glaze,” Aloupi-Siotis says. The work could lead to modern coatings while also improving archaeologists’ understanding of ancient materials and methods. “Both of these interests are best served if the analytical research is based on well-documented archaeological artifacts and on modern reproductions.”