Science history: Fine china from China
The science and secrets of porcelain were much in demand.
By Jeff Glorfeld
The science of porcelain would make a good plot for a spy novel – a story of ancient industrial espionage involving a product prized throughout much of the known world
Kelly Haggart, writing in Canada for Toronto’s Globe and Mail newspaper, describes how China “was centuries ahead of the West in learning the secret of porcelain”, a product that was “hugely popular among Europeans”.
An article published by the Khan Academy through the British Museum explains how Chinese ceramics were first exported in large quantities during the Song dynasty (960-1279) and maintained throughout the succeeding Yuan (1279-1368), Ming (1368-1644) and Qing (1644-1911) dynasties.
Cups, saucers, and other products were being made in Europe but lacked the lustre, finesse and durability of Chinese porcelain.
The centre of China’s porcelain industry was in Jingdezhen, a city in Jiangxi province in north-eastern China.
In his paper titled “A History of the Porcelain Industry in Jingdezhen”, submitted to Britain’s University of Leeds in 1976, author Michael Dillon says Jingdezhen was “one of the first great industrial centres in China and probably one of the earliest in the world”.
The essential raw materials for porcelain manufacture, Dillon writes, are kaolinite, or kaolin clay, china stone (petuntse), and fuel for firing the kilns, all of which were found in good quality and quantity around Jingdezhen.
“Both china clay and china stone are formed when certain crystalline igneous rocks known as felspars (or feldspars) are decomposed by the action of sun, wind and rain,” he says.
Dillon cites British scholar AD Brankston’s description of this china clay, from his 1938 book Early Ming Wares of Chingtechen: “To state the case simply, kaolin is decomposed igneous rock from which the free quartz has been removed. "
Duncan Macintosh, in his 1986 book Chinese Blue and White Porcelain, says kaolin and pottery stone are mixed according to the grade of porcelain to be produced: equal quantities for the best and two-thirds petuntse to one-third kaolin for everyday ware.
An online resource, “Clay minerals soils to engineering technology to cat litter”, produced by the University of Southern California, says “kaolinite is the purest of clays, meaning that it varies little in composition. It also does not absorb water and does not expand when it comes in contact with water. Thus, kaolinite is the preferred type of clay for the ceramic industry.”
Another key part of China’s porcelain secret was heat. Dillon says the fuel that fired the kilns “was almost as important as the material that went into making the pots”.
“It was essential to have kindling that produced high, easily controllable temperatures and did not damage the porcelain. Light, resinous woods are best, as they produce a long flame and contain none of the impurities such as sulphur, that can discolour pots.”
He says the kilns consumed large quantities of fuel, one burning up to 60 kilograms of pine. By the end of the Qing dynasty, he says, “the local hills were so seriously depleted that timber had to be brought from further afield”. Eventually, the potters found that low-sulfur coal made a good substitute for pine wood.
Kelly Haggart adds that kaolin clay can withstand high temperatures during firing, and “Chinese artisans worked out how to build a structure that could be heated to 1300 degrees Celsius and, vitally, how to control the temperature. Baked at such a heat, Jingdezhen porcelain is extremely strong, yet almost translucent.”
In the early 16th century, the Encyclopedia of East Asian Art says, Portuguese traders brought back samples of kaolin clay from China, “which they correctly understood to be an essential ingredient in porcelain production”.
Even so, it wasn’t until 1722, through the efforts of a Jesuit missionary, Francois Xavier d'Entrecolles, that the secret Chinese methods of producing porcelain were released and published throughout Europe.
To break the monopoly on this prized commodity, Haggart says, France sent d'Entrecolles, to Jingdezhen, where he “persuaded the potters he’d converted to ‘divulge their secret to God’." He then wrote letters home, in 1722, detailing the secrets of making fine porcelain.