"Sumali," as an imported cobalt ore from overseas, was a sort of precious and valuable pigment used for imperial kilns only, which produces characteristic "iron spot" to blue-and-white porcelain in early Ming Dynasty (A.D. 14th-15th century). Although there were some old studies on it, the morphology and formation of iron spot has not been fully investigated and understood. Therefore, five selected samples with typical spot from Jingdezhen imperial kiln in Ming Yongle periods (A.D. 1403-1424) were analyzed by various microscopic analysis including 3D digital microscope, SEM-EDS and EPMA. According to SEM images, samples can be divided into three groups: un-reflected "iron spot" without crystals, un-reflected "iron spot" with crystals and reflected "iron spot" with crystals. Furthermore, 3D micro-images revealed that "iron spots" separate out dendritic or snow-shaped crystals of iron only on and parallel to the surface of glaze for which "iron spot" show strong metallic luster. Combining with microscopic observation and microanalysis on crystallization and non-crystallization areas, it indicates that firing oxygen concentration is the ultimate causation of forming reflective iron spot which has a shallower distribution below the surface and limits crystals growing down. More details about characters of "iron spot" used "Sumali" were found and provided new clues to coloration, formation mechanism and porcelain producing technology of imperial kiln from 14th to 15th centuries of China.
In this paper, optical microscopy, SEM-EDS, Raman and FT-IR analyses were used to investigate different types of corrosion product between layers of glaze and body of Kraak porcelain objects that were excavated from "Nan'ao I" shipwreck, the South China Sea. Several contaminants including NaCl, CaSO 4 •2H 2 O, FeOOH and FeS 2 , were found in many of the pores and cracks of the porcelain objects from Jingdezhen Kiln and Zhangzhou Kiln, even after conservation. While compared with the sample of Jingdezhen Kiln, the contaminants in the sample of Zhangzhou Kilns much more severely. The research pointed out that these contaminants were formed due to the marine environment influence and cross-contamination with other metal relics located in the shipwreck, and more contaminants formed in porous structure, hence reduce the porcelain quality. As regards to conservation, the fragility of ceramic objects from underwater contexts are likely to be caused by various kinds of insoluble ferrous salts and its phase transition pressure. Previous studies on this kind of remains tend to focus on the macro-insoluble salts on the enamel surface. In this paper, preliminary investigations were performed for the first time on different kinds of inner micro-corrosion product in China.
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