Porcelain

UNESCO Courier, Oct, 1988

Ordinary pottery 'is made from clay baked in a kiln at temperatures rang, from 500 degrees C to 1150 degrees C, and is called earthenware. Porcelain is something quite different: it consists of a body of fused clay covered by a glaze, a glassy substance, and is fired at a high temperature-about 1280 degrees C. The secret of making porcelain lies in the use of a pure clay, kaolin or China clay, which when fired at a sufficiently high temperature changes its physical composition, a process known as vitrification, and becomes translucent and totally impervious to water. The reason why China was able to "invent" porcelain at a very carly age compared to the rest of the world was that the Chinese potters both found the clay and were able to produce the high temperature necessary to fuse it.

It now seems that archaeological finds push back the date of true porcelain to the first century AD. By the third century AD, in any case, true porcelain was undeniably in use.

By the Song Dynasty (960-1279), porcelain had reached a very high degree of artistry. Porcelain manufacture by this time was a highly organized trade employing hundreds of thousands of people. There were teams of men who specialized in washing the clay, others who concerned themselves only with glazes, others who maintained the kilns, and so on. One kiln of this period which has been excavated could accommodate 25,000 pieces of porcelain at a single firing. It was built on the slope of a hill, the gentle incline of about 15 degrees reducing the speed of the flames through the kiln. The sophistication of the kilns was most impressive. Some were fired by burning wood, while others were down-draught burners of charcoal. Control of the firing process was of the utmost precision. In the Mung Dynasty (1369-1644), when the famous blue and white ware was largely produced, the best lustrous quality of the cobalt blue pigment could only be obtained at certain specific temperatures, and in a reducing (de-oxidizing) flame.

The secrets of porcelain manufacture were jealously guarded, and visitors from Europe such as Marco Polo could but gape and wonder. Porcelain objects were still a very great rarity in Europe by the fifteenth century. They were gifts for kings and potentates. Not until 1520 did the first sample of kaolin clay reach Europe, brought by the Portuguese. Europeans then thought that if only they could find deposits of this white clay, they would be able to make porcelain. But kaolin clay alone is far from sufficient for the making of porcelain.

The countless experiments carried on with various earths and solid substances in furnaces eventually had a most unpredictable result. Scientists and craftsmen began to notice that upon cooling down again, molten minerals could crystallize. Until this began to be observed, Western scientists had been convinced that crystals could only be formed from liquids. About the middle of the eighteenth century in Europe, the idea began to gain ground that perhaps the Earth's rocks could have been formed from the cooling of molten masses of lava. In 1785 the geologist James Hutton presented his revolutionary new theory of the Earth based on this idea. And so, one of the great scientific advances in the Western world took place as a direct consequence of the attempts by Europeans to find the secret of procelain manufacture.