The Porcelain Industry
As with the industries of printing and paper making, the Tang (618–906) and Song (960–1279) dynasties supported important industrial advances in pottery manufacture. The origin of porcelain pottery-making technology dates back 6000 years to the Neolithic period, and the related technologies and skills were continuously developed throughout early Chinese history.
During the Tang, a systematic approach to the industry of pottery making marked a spectacular increase in quantity and quality of work. Porcelain kilns were constructed all over the country and improvements were instituted in nearly every aspect of production. By the time the industry had matured during the Song, porcelain was an indispensable item for daily use in all sectors of society and was exported by land and sea. By the eleventh century, it was being shipped to Japan, India, Arabia, Turkey, and Africa. The Portuguese began exporting porcelain directly to Western Europe in the sixteenth century, and by the late seventeenth century Europeans had developed a passion for porcelain “china.” Chinese artisans succeeded in keeping the technology for porcelain production secret from Europeans for two hundred years. Even after successful industrial espionage led to the rise of a European porcelain industry in the early eighteenth century, Chinese porcelain remained a popular import item.
Porcelain base is made from a combination of porcelain clay, orthoclase, and quartz. The external surface of a porcelain base is coated with vitreous glaze and the base is baked in a porcelain kiln at 1200°C. The finished porcelain, after the baking process, has a very low water absorption coefficient (below 0.004) and is extremely hard. In addition to these practical strengths, porcelain is highly valued because of its beautiful glaze. Glazing is divided into two groups: underglaze and overglaze. Underglaze is applied to the porcelain base before it is baked. Overglaze is applied after the porcelain base is baked, when it is then rebaked with the glaze.
Glaze created from a combination of mineral substances was originally invented in the Shang and Zhou dynasties (eleventh to tenth centuries BCE). The glazes produced in ancient China were generally green in color, until the Sui dynasty (581–618) when white glaze was first created. Manipulation of minerals to produce various colors can be observed in the use of iron in glaze production. Iron has two oxides: ferrous oxide (green color) and ferric oxide (dark brown or terracotta color). Iron in the glaze transforms into ferrous oxide when the baking process is a reduction reaction, and into ferric oxide when the baking process is an oxidization reaction. The final color of finished porcelain is determined by the percentage of iron in the glaze. The finished glaze is light green if the content of iron in the glaze is below 0.8%, and dark green if the iron is over 0.8%. When iron content exceeds 5%, the glaze will be dark brown or nearly black. The same principle applies similarly to other color agents. In the Tang dynasty, the porcelain industry was classified into green porcelains, mainly in north China, and white porcelains, mainly in south China.
During the Song dynasty, comprehensive improvements were achieved in almost every aspect of the porcelain production cycle. The whole manufacturing process was standardized into several working procedures including base-making, glazing, ingredient control, and temperature maintenance. Each procedure was supervised by a specialist. In addition the industry was divided into five types of kilns: Ding Kiln, Ru Kiln, Guan Kiln, Ge Kiln, and Jun Kiln. Each kiln had distinguishing features for glaze coloring and pattern design.