Ancient Porcelain

This article accompanies the textbook The Real World of Chemistry 6th ed by Lois Fruen Kendall/Hunt Publishing ISBN 0-7872-9677-5

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Porcelain, which we call china, was produced in China 1500 years before the techniques for making it were discovered in Europe. Chinese potters produced pure white translucent porcelain that had the consistency of polished seashells. The word porcelain was coined by Marco Polo in the 13^th century from a Portuguese word porcellana, meaning "little pig." He likened Chinese porcelain to pig-shaped cowry shells that were used as currency in parts of East Asia (Hamer 247).

The first porcelain was made in Zhejiang during the Han Dynasty (25-219 AD). By the time of the Sung Dynasty (960-1279 AD), Chinese potters had discovered a way to use underglazes to produce the characteristic blue and white and red and white porcelains associated with the Chinese vases.

Major pottery production began during the Yuan Dynasty (1280-1368 AD) at Jingdezhen. By the 1700s, Chinese potters had developed methods of overglazing with enamels to produce multicolored designs on their porcelain. Not long after Vasco da Gama opened a sea route to China in 1497, Chinese porcelain became the most valuable commodity in Europe.

The discovery of china stone (or petuntse) in southern China was the key to beginnings of porcelain production. China stone contains quartz, fledspar in the form of sericite, and a form of mica called muscovite. The mica in the china stone caused the clay to be somewhat plastic, but it still was difficult for the early Chinese potters to shape their pieces. The mica also acted as an alkali flux to vitrify (partially melt) the fledspar during firing (Bowman 28), but it depended on a very narrow range of firing temperatures to vitrify properly, so early porcelain potters must have had a many pieces fail (Lambert 69).

As the demand for larger pieces grew, Chinese potters found that mixing 10-20% kao ling with porcelain-stone clay produced better results. The type of clay that the Chinese called kao ling is called kaolin or china clay today. Kaolin contains mostly kaolinite (Al₂O₃•2SiO₂•2H₂O which should be written Al₂Si₂O₅(OH)₄ to better describe the crystal structure). Chinese kaolin is high in alumina, which gives the clay its white color, makes the clay more plastic, and helps to control vitrification. Alumina also cuts down on the shrinkage of the raw clay and enables the clay body to stand higher firing temperatures (Hamer 6). When the Chinese potters mixed kaolin with china stone and fired it at 1450°C, the feldspar in the china stone partially melted (vitrified) and infused into the pores of the kaolin, reforming as microscopically fine crystals known as mullite needles. These mullite needles are unique to porcelain and give it its strength, translucence, and resonance (Gleeson 49).

Potters in during the Ming (1368-1644 AD) and Qing Dynasties (1644-1912 AD) increased the amount of kaolin even more (Lambert 69). As a result, their porcelains gained more translucence, smoothness, and glossiness. The best porcelains contain equal amounts of kaolin and porcelain stone.

To decorate their porcelain, Yuan Dynasty potters developed underglazes. They painted these underglazes in patterns onto the pots in the dry "biscuit" state before glazing and firing. Then, they applied a colorless, transparent glaze, which, when fired, formed a protective layer over the decoration. They used cobalt oxide for blue decorations and cupric oxide for red. The pigments were resistant to the very high temperatures in the kilns and remained stable, stayed sharp, and did not bleed into the glaze when the porcelain was fired (Cheng’an 83).

Analysis of Chinese porcelain has shown that Chinese glazes produced the optimum transparency with absence of crazing, so the underglaze would be displayed to the best effect (Bowman 29). These glazes were made by crushing and powdering limestone with the same ingredients used to make porcelain (Lambert 69).

Red underglazed bottle

By the Qing Dynasty, potters were adding enamel overglaze decorations to porcelain that had already been fired with blue underglaze. The materials for the overglazes contained iron, copper, and cobalt (Cheng’an 80). The overglazes were drawn on the baked white porcelain and then fired a second time at 700-800°C (84).

Porcelain is considered to be the ultimate achievement in the history of pottery production. Its beauty, translucency, strength, and resonance have not been rivaled.

Chinese potters today still describe their wares using the verse used to describe ancient Jingdezhen pottery (Hamer 247).

Qing Dynasty vase

As white as jade,

As thin as paper,

As bright as a mirror,

As sound as a bell.

Today, we take china cups and dishes for granted, but in 17^th and 18^th century Europe, nothing was produced locally that was suitable from which to drink the newly fashionable tea, coffee, and chocolate beverages. Mugs made from pewter were fine for beer but did not insulate the drinker from the boiling water used to make the new drinks. Porcelain from China was perfect, because it was an insulator rather than conductor of heat.

Faience makers in France and delft makers in Holland tried to imitate Chinese porcelain by covering their earthenware with white tin glazes. Unlike porcelain, faience and delftware were fired at low temperatures and resulted in porous vessels that were not watertight. The white tin glazes made the vessels watertight, but one chip of the glaze and the vessel would not hold liquids (Gleeson 59). The secret for making porcelain remained with the Chinese until 1708.

Chinese potters produced porcelain so far in advance of other civilizations because of their ingenuity and willingness to experiment to improve their product. Their achievement stands as one of the most important advances in science and art.

Sources

Bowman, Sheridan. Science and the Past. Toronto: University of Toronto Press, 1991.

Cheng’an, Jiang ed. Artefacts of Ancient Chinese Science and Technology. Beijing: Morning Glory Publishers, 1998.

Denio, Allen A. "Chemistry for Potters." The Chemistry of Art. Washington D.C.: American Chemical Society, 1980.

Gleeson, Janet. The Arcanum: The Extraordinary True Story. New York: Warner Books, 1999.

Hamer, Frank and Janet. The Potter’s Dictionary of Materials and Techniques 3^rd ed. Philadelphia: University of Pennsylvania Press, 1993.

Lambert, Joseph B. Traces of the Past: Unraveling the Secrets of Archaeology through Chemistry. Reading: Addison-Wesley, 1997.