05-13 Bottger luster and Purple of Cassius enamel

[Image above] 18th century Meissen teapot (Art Institute of Chicago) with Böttger luster (top) and Purple of Cassius enamel (bottom) noted with cross-sectional maps of gold nanoparticle signatures from energy dispersive X-ray spectroscopy. Credit: Celia Chari

As the chill of winter slowly gives way to warmer temperatures, I am also slowly working up the urge to begin my annual spring cleaning before allergies hit in full force. And though I dislike some of the chores on my list, I do enjoy sorting through my files and seeing old school assignments that I saved from as far back as elementary school.

These old assignments often surprise me because I rarely remember learning the information they contain. Much of the information we learn in primary school has little bearing on everyday life, so it is no wonder that adults struggle to recall the trivia on the show “Are You Smarter than a 5th Grader?”—there often is no reason to retain that knowledge beyond those long-ago exams.

Individual knowledge is not the only thing that can be lost in time—collective knowledge is also routinely lost, sometimes through extreme circumstances such as civilization collapse but more often through lack of passing down the information to a younger generation. Roman concrete, vulcanization of rubber, and curing scurvy with vitamin C are all examples of knowledge that was lost before being rediscovered years (often centuries) later.

Yet for every rediscovered technique, there are countless others we still cannot replicate (the sculpted glass invertebrate marine models by Bohemian father-and-son team Leopold and Rudolf Blaschka is a ready example). Fortunately, new analysis methods are being developed and refined every day, and a recent paper published in Proceedings of the National Academy of Sciences demonstrates how these methods provide previously elusive insights.

Katherine T. Faber, Simon Ramo Professor of Materials Science at the California Institute of Technology and adjunct professor of materials science and engineering at Northwestern University, is senior author on the new paper. She is joined by Caltech students Celia Chari and Zane Taylor, along with Anikó Bezur, Wallace S. Wilson director of the Technical Studies Laboratory at Yale University’s Institute for the Preservation of Cultural Heritage, and former Northwestern University postdoc research associate Sujing Xie.

In their study, Faber and her colleagues used advanced analytical techniques to determine what gives Böttger luster, a historical purple overglaze, its distinctive iridescence.

The history of Böttger luster traces to the early 18th century and the development of porcelain outside of China. In the 17th century, Chinese porcelain was widely sought after by European aristocrats. However, the recipe was closely guarded by Chinese potters, so Europeans were unable to create porcelain themselves.

In 1702, Augustus II “the Strong” (Elector of Saxon, King of Poland, and Grand Duke of Lithuania) summoned alchemist Johann Friedrich Böttger to his palace and ordered him to discover the secret to creating porcelain. After a series of failed experiments, unsuccessful attempts to flee, and forceful recaptures, Böttger successfully produced the first continental European hard-paste porcelain, which was distinct from contemporary porcelain formulations.

In 1710, August II established the Meissen manufactory, located near Dresden in present-day Germany, to continue producing this European porcelain. Alchemists at the manufactory initially struggled to produce overglaze polychrome decorations that were compatible with the required high firing temperatures, but they ultimately developed the Böttger luster iridescent overglaze that is famous today.

Like other historical red and purple glazes, Böttger luster gets its color from gold particles within the glaze. However, while alchemists of the time were aware that gold produced reddish/purplish colors, the ability to visualize the particles and investigate the nanoscience at work did not occur until the early 20th century with the development of the ultramicroscope by Nobel laureates Richard Zsigmondy and Henry Siedentoph.

In an email, Faber explains that the decision to understand the nanoscience behind Böttger luster is due to an observation by coauthor Bezur, who was working at the Art Institute of Chicago when the investigation started. She was studying Meissen porcelains with respect to Du Paquier porcelains made in Vienna when she became curious as to why Böttger luster, unlike other historical gold-based glazes, was iridescent. She teamed up with Faber and the others through NU-ACCESS, a collaboration between Northwestern University and the Art Institute of Chicago.

To understand the mechanics behind Böttger luster’s iridescence, the researchers compared it to Purple of Cassius, another gold-based purple colorant used at the Meissen manufactory that is noniridescent. First, they analyzed historical samples of the two glazes using several spectroscopy and microscopy methods to reveal the glazes’ composition and structure. Then, they experimentally synthesized Böttger luster in hopes of replicating the distinct optical effects.

Through these experiments, they showed that the distinct optical properties of Böttger luster and Purple of Cassius are due to the different ways gold is applied to each glaze.

“In Böttger luster, as with other types of lusterware, the metal nanoparticles develop in the top glaze through a firing step that allows metal ions to diffuse, nucleate, and grow into particles of several sizes inside the glaze. In Purple of Cassius, the gold nanoparticles are precipitated using tin(II) chloride prior to being incorporated into the leaded frit that is applied to create an overglaze enamel, providing a more homogenous particle dispersion and smaller particle sizes,” they write.

Faber says now that their work on historic glazes is concluded, they are conducting experiments on other aspects of historic artifacts, specifically corrosion of earthenware in acidic environments.

“Low-fired pottery, such as earthenware, is known to have poor mechanical properties due to its porous nature, which can easily incorporate salts that can cause the delicate material to fracture. To preserve earthenware objects of cultural heritage for the ages, it is critical to understand how these materials degrade in corrosive atmospheres, like acid rain or acidic soils, and how this corrosion can be mitigated through careful environmental control,” she says.

The paper, published in Proceedings of the National Academy of Sciences, is “Nanoscale engineering of gold particles in 18th century Böttger lusters and glazes” (DOI: 10.1073/pnas.2120753119).