Investigating the coloring of a masterwork of Chinese ceramics, researchers are testing glaze formulas and firing conditions to reproduce the distinct milky blue of Ru celadon. 

Celadon is a pottery term that refers to both a transparent, greenish glaze and the wares to which the glaze is applied, which were first developed in China. Celadons come mostly in some shade of green, but shades of pale blue—notably Ru celadon—are highly valued, and in historical times were reserved more or less exclusively for use in the Chinese Imperial court.

Table 1. Glaze composition in this study.

In the open-access paper, “Analysis of structural effects on coloring mechanism of Ru celadon porcelain,” Yen‐Yu Chen(Chinese Culture University) and Yi‐Wun Bai and Wen‐Cheng J. Wei (National Taiwan University) investigate methods to reproduce the unique color and milky opalescence of ancient Ru celadon–glazed ceramics.

The color of celadon glaze is generated by two mechanisms: chemical coloring by iron species in calcium aluminosilicate compositions; and structural coloring by inhomogeneities, specifically crystallites and voids in the glass. While there is some information available about the material composition of ancient celadon ware—both from analysis of ancient shards and from prior studies—the kiln firing methods are not well understood. For example, it is believed the porcelain was fired in a reducing environment, but there is no way of knowing the composition of the gases or their temperature—the technology simply did not exist for those measurements 1000 years ago.

2 Appearances of (A) the shard of Ru celadon porcelain, (B) top-view of L-2a sample after fired at 1250°C for 5 hours in the reducing atmosphere, and (C) top-view of RW1-1 sample as fired at 1250°C for 5 hours in the reducing atmosphere.

In this paper, the authors created their own celadon by varying a range of experimental conditions, including composition relative to phase-stability data for the complex chemical system, as well as firing temperatures, environments, and holding times. They measured the model systems they created against a shard of ancient celadon ceramic for color, microstructure, and chemical content of the glass and crystallites.

In the end, they came close to the ancient celadon color and opalescence, giving insight into ancient firing protocols. Their work supports the combination of the chemical coloring and structural coloring mechanisms. Specifically, the dual-phase nature of the glass contributes to Rayleigh scattering (dispersion of light due to extremely small particles—it’s the phenomenon behind the colors of the sky (blue during day, red-yellow at sunrise/sunset)), crystallites and voids contribute to the milky color, and the ratio of Fe2+ and Fe3+ oxidation states of iron contribute to chemical coloration.

 3 TEM micrographs of the shard of Ru celadon porcelain, including (A) the glaze layer of Ru celadon porcelain, and (B) anorthite surrounded by the dual‐phasic glass matrix. 4 SEM micrographs of (A) the shard of Ru celadon porcelain, and L-2a samples fired at (B) 1200°C, (C) 1250°C, and (D) 1300°C for 5 hours in the reducing atmosphere after etched in the 3% HF solution for 30 seconds. 5 SEM micrographs of the cross-section of (A) the shard of Ru celadon porcelain, and L-2a samples fired at (B) 1200°C for 5 hours, (C) 1250°C for 5 hours, (D) 1300°C for 5 hours, and (E) 1250°C for 50 hours after fired in the reducing atmosphere. The spherical bubbles can be observed in all samples. 1–5 Photos: Chen, Bai, and Wei; International Journal of Ceramic Engineering & Science (CC BY 4.0).

The researchers’ methods show examples of using predictive modeling techniques in ceramics. Some models predict behaviors on first principles (electronic and atomic behaviors) while some explore how microstructure can be manipulated to produce desired outcomes.

The open-access paper, published in International Journal of Ceramic Engineering & Science, is “Analysis of structural effects on coloring mechanism of Ru celadon porcelain” (DOI: 10.1002/ces2.10058).

This article originally appeared in Ceramic Tech Today September 22, 2020.

the author Jonathon Foreman is the managing editor for journals at The American Ceramic Society.

Topics: Ceramic Artists