While a resurgence of interest in monochrome-glazed porcelains can be seen in the Kangxi reign (1662-1722) of the Qing dynasty, a concerted effort to produce new and interesting glazes was seen in the reign of the Yongzheng emperor (1723-35). The man primarily responsible for the raft of new glaze, and indeed enamel, colors was Tang Ying. Having served in the Imperial Household Department since the age of 16, on the accession of the Yongzheng emperor in 1723 Tang Ying was appointed vice-Director of the Imperial Household Department. In the sixth year of Yongzheng (1728) Tang Yin was sent to Jingdezhen. At first he worked as an assistant under the then director, Nian Xiyao, but after Nian's death in 1738, the full responsibility fell to Tang Ying. For further discussion of Tang Ying's life and work see P. Lam, 'Tang Ying 1682-1756 - The Imperial Factory Superintendent at Jingdezhen', TOCS, vol. 63, 1998-99, pp. 65-82.
Tang Ying himself worked as a potter learning all the processes necessary to produce fine porcelain, and under his direction the Imperial kilns produced some of their finest pieces, while a host of innovations were introduced. He also left a considerable corpus of writings on ceramics and other material, and is credited with the development of a number of the new colored glazes used on imperial porcelain. Among the new glaze colors developed right at the end of the Yongzheng reign were the group incorporating the so-called 'tea-dust' and 'ancient bronze' glazes, to which the glaze on the current basin belongs. Yongzheng vessels bearing these glazes are therefore extremely rare, since the majority of extant examples date to the succeeding Qianlong reign (1736-95).
The origins of these glazes can be traced at least as far back as the Tang dynasty, when glazes of 'tea-dust' type were produced at a number of kiln sites in north China. It seems likely that this type of glaze initially resulted from the under-firing of a black glaze made with the loessic clays. In addition to having a relatively high iron content (4-6, these loessic clays also contained a significant amount of magnesium oxide (2-3. This magnesium oxide is an important constituent of the pyroxene family of minerals, and it is members of this family, that appear in fine crystalline form during the cooling process of underfired glazes of this type. These pyroxene micro-crystals tend to give the glazes a yellowish or greenish color. For further discussion see N. Wood, Chinese Glazes - their Origins, Chemistry and Recreation, London, 1999, p. 140.
Such glazes continued to find favor during the Song dynasty, but seem to have largely fallen out of favor thereafter, only coming to prominence again with the renewed interest in monochrome glazes in the early 18th century. The Qing dynasty glazes were not, of course, made using loessic clay, but, according to Georges Vogt, appear to have been made from a mixture of glaze stone, 'glaze ash', and a siliceous red clay, see N. Wood, op. cit., pp. 156-7, which contained the right amounts of iron and of magnesium oxide to produce speckled crystalline greenish glazes if fired at a relatively low temperature and cooled slowly. The range of colors and textures amongst these yellowish and brownish-green glazes had led them to be given a number of evocative names. In addition to 'tea-dust', these include 'snakeskin green', 'eel-skin yellow', 'old monk's habit', 'turtle skin green', and 'crab-shell green'. Interestingly one version of this glaze appears to imitate the patina on ancient bronzes and was known by the name Yu yao chang yi zhong guan fang de you sei (see National Palace Museum, Qingdai danseyou ciqi, Taipei, 1981, p. 18), which implies that it was reserved for imperial use only. The authors of the Taipei catalogue note that this glaze has not been found at kilns other than imperial kilns. Even today in Beijing, the authors of the The Complete Treasures of the Palace Museum - 37 - Monochrome Porcelain, Hong Kong, 1999, designate porcelains with this type of glaze changguan you - 'official workshop glaze', pp. 242-251. However, one of these vessels, with a particularly speckled green glaze, is described as 'turtle skin green' in another Beijing Palace Museum publication, Gugong bowuguan cang - Qingdai yuyao ciqi, juan 1, xia, Beijing, 2005, pp. 446-7. Actually, the current basin is possibly most accurately described as 'crab-shell green'.
A jardiniére or deep bowl of this shape and size, also inscribed with a Yongzheng four-character mark on an unglazed concave base, covered with what is described as a brown bronze glaze is illustrated in The Complete Collection of Treasures of the Palace Museum - 35 - Monochrome Porcelain, Hong Kong, 1999, p. 66, pl. 60. See, also, ibid., p. 276, no. 253.