A new study published in the journal Scientific Reports has revealed that the unusual coating on ancient Chinese tea bowls, known as Jian tea ware, is actually an extremely rare form of iron oxide, according to a report in Live Science .

Jian ware temmoku tea bowls are Chinese stoneware made for domestic use chiefly during the Song dynasty (960–1279 AD) and into the early 14th century. Jian ware was made in Fujian province, first in kilns at Jian’an and later at Jianyang, and have long been appreciated in Japan; indeed, the term temmoku itself is derived from the Japanese pronunciation of the Chinese Tianmu Shan, a mountain where, according to tradition, Japanese Buddhist priests visited a Buddhist temple and acquired some of these bowls to take back to Japan. The Jian tea bowls are fairly uniform in potting, with dark, coarse-grained stoneware bodies and lustrous bluish black or brownish black glazes that generally are shot through with brownish streaks likened to "hare's fur", “oil spots” and “partridge spots.” Occasionally, as in this fine bowl, the glaze exhibits a multi-colour surface iridescence as light plays across it.

The black porcelain of Jian ware had a purplish black paste and a thick lustrous black glaze. A silver gloss showing through the black glaze resembled rabbit hair, partridge feathers, or oil spots. Photo source .

A team of scientists, led by Catherine Dejoie of Lawrence Berkeley National Lab in California, analysed the microstructure and local chemical composition of the ancient pottery and unexpectedly found that the characteristic patterns of the Jian ware were actually produced accidently by the iron-rich clay oxidising at extremely high temperatures, causing molten iron flux in the glaze to flow down the sides of the bowls and crystallize into very rare iron oxides while cooling in the kiln.

The result is the presence of pure epsilon-phase iron oxide, which has only been fully characterized within the last ten years and which scientists have never been able to produce in the lab in its pure form, as modern synthesis techniques have only managed to grow tiny crystals contaminated with hematite.

“What is amazing is that the ‘perfect synthesis conditions’ for epsilon-phase iron oxide were encountered 1000 years ago by Chinese potters,” says Ms Dejoie.

Marked by extremely persistent magnetization, epsilon-phase iron oxide could hold the key to better, cheaper permanent magnets used in data-storage and other electronics. Moreover, the epsilon phase is non-toxic and highly resistant to corrosion. “The next step will be to understand how it is possible to reproduce the quality of epsilon-phase iron oxide with modern technology,” says Dejoie. “And to identify and extract synthesis conditions and other factors to obtain large crystals of pure epsilon phase.”

Featured image: Tea Bowl with “Hare’s-Fur” Decoration from Southern Song Dynasty. Photo source .

By April Holloway