Engineers Resurrect a 900-year-old Technology

The last Chinese potter who knew how to create a translucent, blue-green glaze known as “Ru glaze,” died more than 900 years ago.

The secrets of this highly prized glaze died with him.

Today, fewer than 100 Ru-glazed ceramics exist. Not many were made because Ru ware was reserved for the 10th to 12th century imperial court, and none have been made during the past 900 years because no one has been able to reproduce the technology that created this delicate, opalescent finish.

That may soon change. University of Arizona engineers are using scanning electron microscopes, molecular-level understanding of materials and a knowledge of high-tech ceramics to time travel through bits of existing Ru glaze for a peek back into 12th century China.

“People have tried to produce Ru glazes, but haven’t succeeded,” said Alix Deymier, a junior in Materials Science and Engineering, who is working with MSE Professor Pamela Vandiver to unravel the secrets of Ru glazing.

“But we hope to bring back that knowledge,” she said.

Extensive analysis of the Jun glazes — which were used both before and after Ru glazing — has given Deymier and Vandiver some clues. Many thought the Jun glazes, which are bluer than Ru glazes, were produced with manganese or other chemicals. But Vandiver discovered that the color is caused by what materials scientists call a “liquid-liquid phase separation.”

Jun glazes are actually a form of glass, and materials scientists know that glass behaves more like a liquid than a solid at the molecular level. Glass doesn’t have a lattice structure and is more amorphous, so it has many of the same properties as a liquid.

A liquid-liquid phase separation is like mixing oil and water, Deymier explained. “If you mix them, the oil makes small droplets within the water,” she said. “Well, this is the same thing that’s happening with Jun glaze. There are two glasses and one is forming inside the other. That’s causing scattering centers and refraction at interfaces between the glasses, which are scattering the light and making it look blue. So it’s not a chemical, but the microstructure of the glaze that’s creating the color.”

Deymier and Vandiver put a tiny sample of Ru glaze under a scanning electron microscope after etching the glaze with acid and found the same kind of pits they see in Jun ware, which are the telltale signs of a liquid-liquid phase separation.

Vandiver tried to replicate the composition of Jun glaze and then fired these samples at different temperatures. After that, she analyzed the microstructure of the various samples and produced a phase diagram that shows where in the compositional range the liquid-liquid phases occur and how to relate their microstructure to the firing temperature.

“We’re now trying to look at the microstructure of the Ru glaze to determine where we can place it on this phase diagram,” Deymier said.

But first they need to know the chemical composition of Ru glaze. Glazed ceramics are fired twice. They’re fired before the glaze is added to harden the ceramic. Then the chemicals that will create the glaze are brushed on the ceramic’s surface or the ceramic may be dipped in the chemicals. Then the object is fired a second time to create the glazed surface.

Deymier and Vandiver plan to use an electron microprobe in UA’s Lunar and Planetary Laboratory to determine the exact composition of their Ru glaze samples. “We are using the electron beam microprobe because we need to know the composition as well as the microstructure of the glaze before we can place the Ru ware on the phase diagram,” Deymier said.

“We’ll analyze the glaze from the outside to the inside to see if there’s a variation in composition through the sample and then we’ll test the different samples to see if the composition varies from sample to sample,” she added.

“We want to see if the Ru ware from different locations is different,” she said. “We want to see how much variation there is in the pottery that’s all classified as Ru ware.”

The researchers are using Ru pottery samples that were donated by museums and by other researchers who hope the UA team will be able to bring this technology back from the past.

Unraveling the mysteries of Ru glazing began as a class project for Deymier when she was taking a scanning electron microscope lab in the MSE department. It’s now turned into an ongoing research project.

“We’re hoping that by discovering these things — the temperature at which the glaze was fired and its chemical composition — that we’ll be able to replicate and bring back the knowledge of how Ru ware was made,” Deymier said.

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