While cobalt blue was much admired in Tang dynasty China it was usually employed sparingly, since good quality cobalt was expensive. The cobalt available in China contained manganese and tended to give a rather dull blue. In order to get a blue of the brilliance of that seen in the glazes of these two jars it was necessary to use imported cobalt, which was rather pure, containing only traces of copper and nickel, and quite different from the imported high-iron cobalt of the Yuan dynasty, or the high-manganese cobalt found in China itself. The cobalt brought into Tang China had to travel considerable distances from the Middle East and would have been very costly. However, when combined with a lead-fluxed glaze, this cobalt produced the beautiful sapphire blue seen on the current jars. Like all Tang lead-glazed wares, the glaze on these jars stops well before the foot, to allow for glaze running during firing.
Fragments of Tang blue-glazed vessels excavated from the kiln site at Gongxian in Henan province are illustrated in Zhongguo taoci quanji -7 - Tang sancai, Shanghai, 1983, pl. 156. A Tang cobalt blue-glazed jar of similar size and shape as the current vessels, but apparently with less even glaze, in the collection of the St. Louis Art Museum is illustrated in Chinese Art in Overseas Collections - Pottery and Porcelain, Palace Museum, Taipei, 1986, p. 62. Another is illustrated by R. Krahl, The Anthony de Rothschild Collection of Chinese Ceramics, Eranda Foundation, 1996, pp. 18-9, no. 10. Monochrome blue ceramics were scarce even in the Tang period, and the current pair of jars is particularly rare.
The result of Oxford Authentication Ltd. thermoluminescence test no. C104j48 is consistent with the dating of this lot.