Cobalt in Chinese Pottery and Porcelain

Cobalt is the principal colouring element responsible for the blue decoration and many blue glazes found on Chinese ceramics. In blue-and-white porcelain, a cobalt-bearing pigment is painted onto the unfired body, covered with a transparent glaze and fired at high temperature. During firing, the cobalt becomes incorporated into the glassy glaze and develops its characteristic blue colour.

How cobalt was used

Cobalt was employed in several different ways:

• Underglaze blue: decoration painted beneath a transparent glaze, as on blue-and-white porcelain.
• Monochrome blue glaze: cobalt added to the glaze to cover most or all of a vessel. Examples include sacrificial blue, powder blue and pale cobalt glazes such as clair-de-lune.
• Overglaze blue enamel: cobalt used in a separately formulated enamel and fired at a lower temperature. This is technically different from underglaze blue.

Only a relatively small amount of cobalt is needed to produce a strong colour. Its appearance nevertheless depends on more than concentration. The composition of the glaze, impurities in the pigment, firing temperature, kiln atmosphere, brushwork and the thickness of application all influence the finished blue.

Cobalt ores and their appearance

Traditional cobalt pigments were prepared from naturally occurring ores rather than chemically pure cobalt oxide. These ores commonly contained varying amounts of iron, manganese, nickel, copper, arsenic and other elements.

Imported pigments used during certain periods were often comparatively rich in iron and low in manganese. They could produce an intense blue with dark concentrations where the pigment accumulated. Domestic Chinese ores frequently contained more manganese and could give softer, greyer or slightly violet tones. These are broad tendencies rather than fixed rules.

The dark spots and concentrated brush marks seen on some Yuan and early Ming porcelains are often described by collectors as “heaped and piled.” They are associated with the behaviour of iron-rich cobalt during firing. However, similar effects can be imitated, and their presence alone cannot establish a date or source.

Historical development

Tang dynasty

Cobalt blue was used sparingly on some Tang dynasty lead-glazed pottery. Archaeological evidence from the Gongyi kilns also includes late Tang ceramics painted in cobalt beneath a glaze, among the earliest known Chinese blue-and-white wares.

The cobalt used on Tang ceramics appears to have differed chemically from most later domestic Chinese pigments and was probably obtained through long-distance trade.

Song dynasty

Blue-and-white production during the Song dynasty was extremely limited. A small number of excavated fragments have been identified, particularly in Zhejiang, but no major Song production centre for blue-and-white porcelain has been securely established.

Analysed examples generally contain manganese-rich cobalt consistent with a domestic Chinese source, although the number of tested objects remains too small for broad conclusions.

Yuan dynasty

Blue-and-white porcelain became a major ceramic tradition during the Yuan dynasty, particularly at Jingdezhen. Large dishes, jars and vases were produced for the Chinese court and for export across Asia and the Middle East.

Many important Yuan wares were painted with an imported, iron-rich and manganese-poor cobalt. This material could produce a deep, saturated blue with darker areas where the pigment was heavily applied. Domestic cobalt was also used, especially on smaller or less costly wares.

Early Ming dynasty

Changes in supply during the Hongwu period resulted in considerable variation in cobalt colour. Some wares show pale or greyish blue, while others contain darker concentrations.

During the Yongle and Xuande periods, imported cobalt was widely used for prestigious Jingdezhen porcelain. These wares are admired for their powerful blue colour, tonal variation and characteristic dark pigment deposits. Domestic and mixed pigments may also have been employed, so cobalt appearance should never be treated as an absolute dating test.

Historical sources use names transcribed in several ways, including Sumali, Su-ma-li and Su-ni-bo, for imported blue pigment. Its precise linguistic and geographical origin remains debated.

Middle and late Ming dynasty

Domestic cobalt became increasingly important during the middle Ming period. Chenghua porcelain is particularly associated with a softer, more controlled blue, produced through careful pigment preparation and refined painting.

Names such as pingdeng qing, shizi qing and huiqing appear in historical discussions of Ming cobalt. These terms may refer to different materials, grades or places of supply, but they cannot always be matched securely to a single modern ore source.

During the Jiajing and Wanli periods, imported, domestic and mixed pigments were used. Some produced strong blue with a violet cast, while others gave greyer or more restrained tones. The colour varies considerably according to the glaze and firing conditions.

Qing dynasty

Qing potters drew cobalt from several Chinese regions, including Zhejiang, Jiangxi and Yunnan. Improved selection, washing, grinding and preparation allowed greater control over the finished colour.

Kangxi blue-and-white porcelain is celebrated for clear, bright cobalt and its wide range of tones, from delicate washes to strong sapphire blue. This effect depended on skilful pigment preparation, painting and firing rather than on one cobalt source alone.

Under the Yongzheng and Qianlong emperors, the imperial kilns produced exceptionally even monochrome blue glazes as well as carefully controlled underglaze decoration. Cobalt continued to be used throughout the Qing dynasty and into modern production.

Can cobalt be used to date porcelain?

Cobalt can provide useful evidence, but it cannot date a ceramic by itself. Colour and visible pigment effects must be considered together with:

• the porcelain body and glaze;
• vessel form and proportions;
• painting style and brushwork;
• foot construction and kiln characteristics;
• reign marks and inscriptions;
• provenance and archaeological context.

Scientific methods such as X-ray fluorescence, XANES and trace-element analysis can help compare cobalt pigments and identify broad compositional groups. Results must still be interpreted carefully because ores were mixed, refined and reused, while contamination from the glaze or ceramic body can affect measurements.

Claims that a particular blue, dark spot or chemical ratio proves one exact reign should therefore be treated with caution.

A note about painting media

Some workshop traditions mention tea water or other organic liquids being used while preparing cobalt pigment. These may have affected the handling or suspension of the paint. However, claims that stale tea prevented deformation or chemically stabilised cobalt through sodium carbonate are not sufficiently established to be presented as standard ceramic technology.

Collector’s summary

Cobalt is one of the most important materials in the history of Chinese ceramics. It created the blue-and-white tradition, contributed to imperial monochrome glazes and connected Chinese kilns with extensive mineral and trading networks. Its colour can offer valuable clues, but reliable attribution always depends on examining the complete object rather than the blue pigment alone.

Jar with dragon, Xuande mark and period, 1426–1435. Porcelain painted with cobalt blue beneath a transparent glaze, Jingdezhen. The Metropolitan Museum of Art, object 37.191.1. Public Domain.