Morphological and structural analysis of iron-rich porcelains excavated from the Jian kiln site of Song dynasty.

Iron-rich porcelains generally embrace relatively high iron concentration in the glaze and body, which have an important position in the history of ancient Chinese high-temperature ceramics. The colour and lustre of glaze patterns are closely related to the orientation and order of crystallisation. In this work, three representative types of iron-rich porcelains (persimmon red-glazed sample, oil spot-glazed sample and mirror black-glazed sample) were analysed by portable energy-dispersive X-ray fluorescence (PXRF), optical coherence tomography (OCT), optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and laser Raman spectroscopy (RS) to determine the morphology, chemical composition and microstructure. Results showed that layered structure was observed in both persimmon red and oil spot glazes, however with respective thickness. Besides, iron-enriched crystals mostly precipitated on glaze surface. For the persimmon red sample, multilayer microstructure consisting of three sublayers beneath glaze surface was identified. Crystals and dark red substrates were all made of ε-Fe O , implying that it was fired at a strong reducing atmosphere. But for the oil spot sample, large-scale leaf-shaped crystals were well characteristic of ε-Fe O , while small snowflake-shaped crystals were assigned to α-Fe O , indicating that the firing environment was partially reduced. In addition, there was no crystalline layer existing on surface of mirror black-glazed porcelain, and plain black glaze layer was featured by a glassy matrix. The Jian kiln is a famous ancient folk kiln in the southern China that is currently located in Jianyang county of Fujian province. Its production of black-glazed porcelain began in the Tang dynasty (618-907 AD), became prosperous in the Southern Song dynasty (1127-1279 AD), and declined and ended in the Yuan dynasty (1271-1368 AD). Black-glazed Jian tea bowls were perfect for highlighting the rich white tea decoction. The thick and lustrous black glaze of Jian bowls sometimes were featured by streaked or mottled patterns, usually recognised by 'hare's fur (HF)', 'oil spot (OS)', 'partridge spot (PS)' and so on, which were the crystalline markings of iron oxide precipitated during firing in the dragon kiln. Iron oxide acted as a colorant for black-glazed porcelain and, thus, was called as iron-rich porcelain. The unique patterns originated from local iron-enrichment raw materials and firing process, and the locally iron-enriched areas generated due to the supersaturated crystallisation of different iron oxide in the cooling period. It was generally believed that research studies on the crystallographic nature and growth mechanism of surface crystals would provide help for the manufacturing of ancient Chinese glazes. With this aim, three types of iron-rich porcelain excavated from the Jian kiln were selected to further study the forming cause and growth process of glaze patterns using a series of characterisation methods, like portable energy-dispersive X-ray fluorescence (PXRF), optical coherence tomography (OCT), optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and laser Raman spectroscopy (RS).