Learning from the past: Rare ε-Fe₂O₃ in the ancient black-glazed Jian (Tenmoku) wares

Ancient Jian wares are famous for their lustrous black glaze that exhibits unique colored patterns. Some striking examples include the brownish colored "Hare's Fur" (HF) strips and the silvery "Oil Spot" (OS) patterns. Herein, we investigated the glaze surface of HF and OS samples using a variety of characterization methods. Contrary to the commonly accepted theory, we identified the presence of ε-Fe₂O₃, a rare metastable polymorph of Fe₂O₃ with unique magnetic properties, in both HF and OS samples. We found that surface crystals of OS samples are up to several micrometers in size and exclusively made of ε-Fe₂O₃. Interestingly, these ε-Fe ₂O₃ crystals on the OS sample surface are organized in a periodic two dimensional fashion. These results shed new lights on the actual mechanisms and kinetics of polymorphous transitions of Fe₂O ₃. Deciphering technologies behind the fabrication of ancient Jian wares can thus potentially help researchers improve the ε-Fe ₂O₃ synthesis.