Sources of cobalt over time
Recent research suggests that there were five sources of cobalt common to traditional Chinese ceramics. Imported cobalt is distinguished by a high proportion of lead (the ratio of FE2O3 to CoO is between 2.21 and 3.02) and low manganese content (the ratio of MnO to CoO is 0.01 to 0.06) which yields a rich blue with obvious iron-rust spots.
Xiu Ban (oxide spots on surface) and Yun San (faint evasion) are observed features of the decoration which can help identify cobalt sources and to give support in dating. The cause for these effects are under debate.
For cobalt, there were several kinds and all carried different characteristic and reactions in the firing process. I do not think its possible to date a piece just by the "rust compositions" and "faint evasions", rather all things have to be considered. Short summary up to my present understanding (will be expanded).
1) Tang Qinghua - high copper, high iron, low manganese, rich and dark blue with faint evasion, source not yet identified. Several sorces are possibly like Persia (Iran, Afghanistan), India, Egypt, and western Asia Minor along the shores of the Mediterranean.
2) The Song dynasty used domestic cobalt principally from Zhejiang (Zhejiang Jiang Shan Xian)in which is rich in manganese and low in iron, so the resultant blue is grayish with some black, some appeared with light/dark blue layers and with faint evasion.
3) Yuan Qinghua (underglaze blue) differs much between early and late period. Large wares used imported cobalt. The imported cobalt used in the Yuan came from the Middle East. Correct firing appears jade blue. Over-firing appeared blue with purple tint, under-firing grayish blue. No faint evasion. Obvious brush heaping strokes. Small Yuan wares used local cobalt, Grayish blue, with rust composition.
4) Ming Hongwu. Due to civil war the import of cobalt seems to have come to a halt and the quality of underglaze blue is genearlly very low. Only local cobalts were used.
Imperial Hongwu wares has three kinds:
a) Light blue, bright with layers, no faint evasion.
b) Light blue, with dark blue dots, little faint evasion.
c) Grayish blue with blackish blue tints to black blue rust.
Early Hongwu folk wares gray and dark, late Hongwu became much brighter blue, with black / brown rust composition.
5) Yongle to Chenghua used both imported Su Ne Bo Qing from south east countries and local cobalts, imperial wares used mainly "Su-ni-bo"-blue, folk ware used imported and local cobalts mixed. That used in the Ming Yongle and Xuande was known as "su-ma-li" blue or "su-ni-bo" blue (transliterations of foreign words), and presumed to come from the Middle East. Since there are no cobalt minerals in present-day Iran the "su-ma-li" blue may have come from somewhere near Syria. Su-ma-li might also refer to "Sumatra", where a large Chinese population had established themselves in the city of Palembang possibly as early as during Southern Song.
6) Zhengtong, Tianshun, Jingtai similar to Yongle and Xuande.
7) Chenghua to early Zhengde "Su-ni-bo"-blue came to a full stop in mid Chenghua, and switched to Ping Deng Qing, also named Bo Tang Qing, found in Jingdezhen Le Ping Xian, a elegant mid hue blue with rich and light several distinct layers.
Zhengde used Mo Min Zhi cobalt from Jiangxi Shang Ko Xian, grayish blue, bright and clear, stable and clean.
8) Late Zhengde to early Wanli Imperial ware switched to Hui Qing from Yunnan ( some say from middle east countries ) blue with purple tint, lack of layers. Folk ware used mainly local cobalts, bluish gray or blackish gray.
9) Wanli to Late Ming Hui Qing was used until Wanli 13th year (1586). After 1586, Imperial ware used Zhejiang cobalts from Zhejiang Ju Xian, blue with gray tints, bright and clear. Folk wares used many sources of cobalts now widely discovered in many parts of Zhejiang province. The process of purifying local cobalts have matured leading to the invention of the great Kangxi Qinghua in the Qing Dynasty.
Regarding the term "Mohammedan blue", this first showed up in the Jaijing period and was probably imported from Iraq. It was also during this period that the cobalt started to be highly refined. As near This dark toned blue, commonly called violet ink, was used until the eleventh year of Wanli in 1605 when the suply are said to have ran out. At this time cobalt form Zhejian was used. Along with the extra refining methods used during Jiajing, magnets where incorperated during Wanli to remove iron form the cobalt. This process made the cobalt almost as good as the Mohammedan import of the past.
A
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