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- Cobalt Blue Colouration
Cobalt has been used to provide blue colouration for glass and glazes on ceramics in various parts of the world for at least four thousand years. China has its own sources of cobalt in the form of asbolite, a mixture of cobalt and manganese oxide. However, from scientific tests of early glazes on Chinese ceramics, it was found that the cobalt was markedly free of manganese. Therefore, it was concluded that the cobalt ore must have been imported from Persia or other places in the Middle East, where it has been used in the past for such things as Egyptian glass of the XVIII dynasty (1400 b.c.), Mesopotamian pottery with blue designs of the 9th and 10th centuries a.d., and Persian ceramics of Kashan with underglaze blue decorations of the 13th century. The latter had a major influence on the development of Chinese blue and white porcelain.
The first use of cobalt as a colourant on Chinese ceramics occurred during the Tang dynasty (618-906). It was applied in a splashed fashion on earthenware objects as either a single colour or in combination with yellow, green and white in the tri-colour lead glaze known as sancai. Also, there are a few recently excavated examples of Tang ceramics with painted designs in underglaze blue, which will be discussed later.
After the Tang dynasty, its use appears to have been discontinued, only to be revived again for the underglaze blue porcelain of the Yuan dynasty (1271-1368). The cobalt ore was again imported directly or indirectly from Persia for use in these early blue and white porcelains. Hence, it is often referred to as 'Mohammedan blue� (huihui qing) or sumani, referring to Sumatra, which was a trade centre for merchants from the East and the West, where Muslim merchants sold much of their wares. On various occasions in the 15th century, the envoys from Sumatra to China brought with them Mohammedan blue as part of their tribute.
It was not until the 17th century after much experimentation that the Chinese developed a reliable method to separate the impurities from their own previously unsatisfactory cobalt ore. This new method produced cobalt sufficiently pure to give the desired colour on their porcelains making the use of expensive imported cobalt less essential. If impurities such as iron and manganese are present in the cobalt ore, the resulting colour is greyish, but when the ore is subjected to a purification process, a pure blue colour can be obtained.
Origins of Blue and White Porcelain
It is thought that Chinese blue and white porcelain received a significant impetus to its development from porcelain wares known as qingbai or yingqing of the Song (960-1279) and Yuan (1271-1368) dynasties, and from a white ware known as shufu, also of the Yuan dynasty, which was developed from yingqing wares. Yingqing ware is a fine porcelain with a pale blue or green glaze, while shufu wares have an opaque, thick white glaze, sometimes with the Chinese characters �sh� and 'fu' moulded or incised on the piece. The most characteristic feature of shufu ware is its opaque luanbai (egg-white) glaze. The earliest blue and white wares have similarities to shufu pieces in the clay body, glaze and construc�tion. Shufu ware was the first ceramic ware to be made at Jingdezhen by official order. Jingdezhen in Jiangxi province was destined to become the single most important centre for the manufacture of Chinese porcelain due to its fortunate location near large supplies of high-grade raw materials, like kaolin and petuntse (china stone), essential for the manufac�ture of porcelain. Also, it was situated next to a river system, which allowed for easy transport of products to the capital and to ports in the south and east for export.
The exact date for the first manufacture of blue and white wares in China is still being debated, but undoubtedly over the next several decades, archaeological excavations will determine a more precise date. The main reasons for the controversy in dating are some relatively recent excavated items found in the Tang dynasty (618-906) remains of Yangzhou city in Jiangsu province. A few fragments of Tang porcelaneous stoneware with
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