Pure white porcelain decorated with underglaze cobalt blue is one of the most successful combinations in the history of world ceramics.

From the Chinese Ming period (1368–1644) - to 15thcentury Meissen Zwiebelmuster (onion pattern) in Germany, many cultures have a popular tradition in blue and white. These include the Iranians, Turks, Indians, Vietnamese, Europeans and Americans as well as the better-known Chinese, Korean and Japanese.

The idea of painting ceramics using a cobalt blue pigment was not actually a Chinese invention but a process developed in the Middle East in Mesopotamia.

So what is cobalt?

Cobalt is a natural mineral that is refined by washing or by calcination (purification by the action of heat, usually in the region of 700C). It is ground and mixed with water to be used as a pigment for blue decoration. Modern refined cobalt oxides are very pure and very strong and they do not give the depth or character of natural cobalt.

There are various cobalt ores; for example, cobaltite, erythrite, glaucodot and skutterudite. However, Asbolite is a cobalt ore containing manganese and iron impurities, which was the source of underglaze blue for Chinese porcelain. It is the only cobalt ore that is an oxide that is safe to use: the other cobalt ores contain arsenic.

Asbolite blue pigment used at Jingdezhen in China during the 14th and 15th centuries was rich in iron, manganese, nickel, and copper. It was imported from Kashan in Persia and the Islamic regions in west Asia, and reached China in an already prepared state.

Cobalt was called sunmali in China; a transliteration of the term ‘huiqing’, literally meaning ‘Mohammedan blue’ during the Yuan dynasty (1279-1368).

Chinese potters experimented with this foreign cobalt, diluted with water, which they painted onto the ceramic body once it was dried. The colour in the first stage of the design was a grey black. The potter had to be very sure of his brushwork, as the porous surface of the body did not allow for corrections. After painting, the design it was covered with a transparent glaze and the vessel was fired to high temperature. During this glaze firing the grey turned blue.

The varying quality of Chinese cobalt blue ware is connected to the source of the cobalt ore and its availability. The first Ming Emperor Hongwu (1368 – 1398) had very restrictive trade policies, which resulted in a shortage of imported cobalt. What little they had was watered down and mixed with local materials, which resulted in the underglaze blue of the period having a dull greyish tone.

The early cobalt ores imported from Iran and other Islamic regions of west Asia were lower in manganese but higher in iron compared to the Chinese cobalt found in deposits of the western regions of China during the 16th century.

This absence of manganese accounted for the absence of the purple or red tinge in the blue, which fired to a bright sapphire colour instead. A higher iron content in imported cobalt resulted in an uneven blue ranging from a darker tone with black specks to a pale blue. One important difference between the manganese-rich pigments and the iron-rich cobalt ores that preceded them was the tendency of the iron-rich cobalts to diffuse through to the surfaces of the thick porcelain glazes during cooling. This gave the famous ‘heaped and piled’ effect so characteristic of 14th and early 15th century underglaze blue painting.

What is ‘underglaze’?

The term underglaze can often lead to confusion because of the variety of techniques, oxides, underglaze colours, stains and pigments used. To look at the technique in its basic definition, it is the application of oxide onto green ware or more traditionally onto biscuit ware. The decoration is covered with a transparent glaze and then glaze fired.

The fact that the decoration is applied ‘under the glaze’ produces a level of permanence and durability, particularly in comparison to ‘onglaze enamel’ or ‘lustre’ decoration. Oxides that are also strong fluxes are usually more successful in the underglaze technique ie. iron / cobalt /copper. Modern underglaze stains tend to have fluxes added to them and are used with a medium (fat oil and turpentine or gum) and it is traditional to apply them to the biscuit ware. Overloading of the medium with stains results in thick application and will cause crawling of the glaze.

Modern underglaze products on the market today are very accessible and easy to use; they usually contain polyvinyl acetate adhesive which also acts as an extender to assist flow.

Japanese Cobalt pigments:  ‘ Gosu’

‘Gosu’ is the Japanese name for natural cobalt. The ground cobalt powder was traditionally applied with a thick green tea solution that had been boiled down to a syrup. This mixture would be combined with the ‘Gosu’, enabling it to be applied freely and smoothly during painting. It was allowed to dry and then glazed. The tannin in the tea kept the decoration from spreading when the glaze was applied.