In the 18th century, a new purple overglaze enamel emerged from a factory near Dresden, Germany. Named after a German pioneer of porcelain and supposed alchemist, Johann Friedrich Böttger, this iridescent glaze became known as Böttger luster. Why this porcelain overglaze exhibited iridescence when other purple glazes of the time lacked a lustrous shine remained a mystery until now. This is the story of how a team of researchers solved a centuries-old artistic cold case.
Previously, no studies had been conducted on what leads to the purple shine of Böttger luster. Through translations of original records from the Meissen factory, where Böttger luster originates, it was known that the presence of gold is a key component to this purple lustrous effect. But the shape and size of the gold particles present in Böttger luster were unknown. A similar, purple-colored porcelain overglaze, known as Purple of Cassius, was also produced using gold at the Meissen factory but without any sort of lustrous effect. Why does one purple glaze appear lustrous while another purple glaze from the same factory shows no sign of metallic iridescence?
Research that started at Northwestern University over a decade ago and recently finished at Caltech under the leadership of Katherine Faber, Simon Ramo Professor of Materials Science, is the first to look at purple luster in detail. A paper describing this research was published by the Proceedings of the National Academy of Sciences (PNAS) on April 18. Celia Chari, Caltech materials science graduate student; Zane Taylor, fourth-year Caltech undergrad majoring in materials science; Sujing Xie, formerly of Northwestern University; Anikó Bezur of the Yale University Institute for the Preservation of Cultural Heritage; and Faber are co-authors.
Using transmission electron microscopy, it is possible to view the various layers of a fragment of a teapot with purple luster at the nanoscale (1 nanometer is 1 billionth of a meter; for reference, a single strand of DNA is 2.5 nanometers in diameter). By examining the teapot's overglaze enamel layer at such small scales, the researchers could assess how its chemical structure produces Böttger luster's characteristic iridescence.
This process allowed Faber and her colleagues to conduct a detailed comparison of the enamel layers of Böttger luster and non-iridescent Purple of Cassius overglazes. They found that the purple color of both overglazes is due to the presence of gold nanoparticles, but the difference in iridescence is due to the nanoparticle array. The overglaze of the Purple of Cassius sample contained gold nanoparticles with uniform sizes, while the overglaze enamel layer of Böttger luster contains a mixture of large and small gold nanoparticles, ranging from 30 to 500 nanometers across. This disparity in the sizes of the gold nanoparticle arrays of the two purple overglazes is the key to understanding what makes Böttger luster iridescent.
The challenges to solving this cold case were threefold: First, access to Böttger luster porcelain is limited, with only microscopic fragments available for study. Second, authentic Böttger luster uses gold hydrazide, which is highly explosive. Third, many materials used in the porcelain glazes evaporate when fired, so it is difficult to determine the pre-fired glaze ingredients based on an analysis of the finished product, which also could potentially have played a role in its iridescence.
To address these challenges, Chari recreated the glaze using a different, less explosive form of the gold while Taylor applied physics-based models to samples of historic Böttger luster.
"The recreation was really helpful," says Chari. "We originally called it reverse engineering, but I don't think that's an accurate term to use at all because it's not exactly the same recipe that was used in the Meissen factory. The way we like to think about it is we re-created purple luster in a safer way."
Meanwhile, Taylor's modeling showed that the presence of many large gold nanoparticles at the surface of the Böttger overglaze layer causes light that shines on the glaze to diffract differently depending on the angle of incidence, thus creating iridescence. Because the gold nanoparticles in Purple of Cassius glazes are small and uniform in size, these diffraction variations do not exist, and the glaze has a nonlustrous appearance.
"We found that the large gold nanoparticles don't contribute to color, but they do contribute to luster, and the small gold nanoparticles have the opposite effect," says Taylor.
"By comparing our re-creations and samples experimentally, we've come to a conclusive argument as to what gives something purple luster or just purple color," says Chari.
The case of Böttger luster is closed, but the door for cultural heritage science remains wide open. As DNA testing unlocked the mysteries of our ancestry, modern imaging technology can shine a light on the mysteries of our antiques.
The PNAS paper is titled "Nanoscale Engineering of Gold Nanoparticles in 18th Century Böttger Lusters and Glazes." The research was funded, in part, by an Art Institute of Chicago–Northwestern University Exploratory Research Grant.
Image: High-resolution photos show the difference in gold nanoparticles in the iridescent Böttger luster (top) and the non-iridescent Purple of Cassius (bottom) on a porcelain sample. (Credit: Faber Research Group)