Material of the month – Porcelain

Materials World magazine
,
1 Jun 2016

It is a material most associated with the Great British tea party. This month, Anna Ploszajski discovers the history of porcelain. 

The term porcelain refers to a broad range of ceramic materials, the common denominator being the clay mineral kaolin. It is frequently mixed with the mineral feldspar, or ingredients such as ball clay, powdered glass, bone ash, soapstone, quartz, petuntse or alabaster. The malleability of the pre-fired material provides the maker with infinite possibilities of shapes, aside from the classic crockery we have come to know. 

Once formed, the porcelain object is heated in a kiln to temperatures between 1,200°C and 1,400°C. The combination of toughness, strength, impermeability and translucence of porcelain wares is mainly thanks to the vitrification of the ingredients, meaning that the material becomes non-porous and glassy during firing. Meanwhile, needle-shaped crystals of the mineral mullite are formed at high temperatures that interlock to provide the remarkable mechanical strength and thermal shock resistance of the final product.  

With seemingly such advanced materials engineering going into the production of porcelain, you’d be forgiven for thinking that porcelain is a relatively new material. But, the story of porcelain begins more than 2,000 years ago in China, the country with which this material has become synonymous. The exact moment of porcelain’s invention is not known, although experts can be certain that by the time of the Eastern Han Dynasty, corresponding to the years 25–220 AD, recognisable porcelain ceramics had emerged from their earlier iterations. The production of these ‘celadon’ wares continued into the following Three-Kingdoms period, the Jin Dynasty and the Sui Dynasty in regions south of the Yangtze River, while a more translucent, white variety of porcelain was developed to the north.  

By the time of the Tang Dynasty, (618–907 AD), tea drinking had gained huge popularity, and this practice promoted the production and export of Chinese porcelain wares, along the northern Silk Road, the southern Tea Horse Road, and by sea. In the year 1004, during the subsequent Song Dynasty (960–1279 AD), the production of porcelain became organised and centralised to the town of Jingdezhen in the Jiangxi Province, which remained the main production centre for 900 years. Jingdezhen was perfectly situated next to a plentiful supply of kaolin, and the tall hill from which it was extracted gave kaolin its name – ‘kow’ or ‘gao’, meaning tall, and ‘ling’ meaning hill. This was a truly enormous operation – with kilns that could fire as many as 25,000 wares at once. 

The supply of porcelain 

The impact that routes like the Silk Road had on all aspects of life around this time is impossible to underestimate, facilitating not only the trade of goods, but ideas and culture too. An example is the development of blue and white porcelain – the cobalt pigment coming from Iran and the porcelain expertise from China. Later, during the Ming Dynasty (1368–1644 AD), the addition of manganese allowed for more intricate designs by preventing the blue colour bleeding into the white during firing. During this time, the Chinese closely controlled the porcelain supply to Europe, Asia and Africa. But, in 1603, the Dutch captured a Portuguese cargo ship bearing thousands of porcelain pieces and brought it back to Europe. This caused a frenzy of porcelain auctions in Europe, and items sold for such extortionate prices that porcelain became known as white gold.

Such high demand and low supply made the Europeans keen to produce their own porcelain goods.This turned out to be easier said than done. The solution remained evasive until 1708, when a German physicist and chemist, Ehrenfried Walther von Tschirnhaus, together with the alchemist Johann Friedrich Böttger, succeeded in producing a hard, white, translucent ceramic using a mixture of kaolin and alabaster, fired at 1,400°C in a wood-fired kiln. Unfortunately, von Tschirnhaus died later that year, so Böttger alone reported the finding to Augustus II, King of Poland, in March 1709.

The secret uncovered

Böttger’s success resulted in the founding of a factory the following year at Meissen, near Dresden in the east of Germany. Like the Chinese, the factory kept the method and recipe a closely-guarded secret. Two years later, in 1712, the French Jesuit priest Francois Xavier d’Entrecolles, who discovered the secret to Chinese porcelain while working in the Jiangxi region, relayed it in letters to his contacts in Europe. In 1771, a French nobleman called Nicolas-Christiern de Thy blew the lid off the secret for good by widely publishing L’art de la porcelaine, detailing the production process. Porcelain factories quickly sprung up across Europe, rendering the prestige of porcelain as a scarce commodity well and truly over.

The porcelain from China and the Meissen factory is so-called hard paste porcelain. But the European efforts to crack the code of the Chinese porcelain led to the development of a whole new group of materials, broadly called soft-paste porcelain because of the slightly lower firing temperatures used.

Porcelain worldwide

Italy had had a long love affair with porcelain, ever since Marco Polo brought back the first examples of Chinese porcelain in 1295. In fact, the English word porcelain derives from the Italian word for ‘little piglet’, porcellana, after the little white cowry sea- shells it resembles. So it was in Florence that Francesco de Medici, the Grand Duke of Tuscany, began his attempts at mimicking the Chinese method. He was equipped with kaolin samples brought back by Portuguese traders, but without the secret Chinese techniques, his early efforts proved unsuccessful. Eventually, in 1575, he hit upon a formulation comprising feldspar, calcium phosphate, wollastonite and frit, which produced a ceramic similar to the Chinese examples, and this turned out to be the first soft-paste porcelain.

In France, experiments in Rouen, near the north coast, yielded successful soft-paste porcelain in 1673. In 1693, the Saint-Cloud factory opened, producing fine soft-paste porcelain wares that mimicked the attractive Chinese blue and white design. 

By the mid 1700s, other factories had sprung up across France, tweaking the formula in an attempt to tackle the troubles with pyroplastic deformation, or slumping in the kiln, which was limiting high throughput production.

A great British tradition

Across the channel, Thomas Briand exhibited his own soft-paste porcelain to the Royal Society in 1742, said to be based on the Saint-Cloud formula. The first porcelain factory in England was the Chelsea Porcelain Manufactory, founded in 1743, which specialised in trinkets for the aristocracy. But it wasn’t long until a rival sprung up in the East End – the Bow Porcelain Factory, in 1747 – owned by Thomas Frye. Its location close to the cattle markets and slaughterhouses of Essex gave Frye ample supply of bone ash, which he patented for use in porcelain in 1748. This recipe for the ambiguously named fine porcelain was later improved on by Josiah Spode in Stoke-on-Trent, and introduced to the market as bone china in 1796.

The product quickly caught on with other newly-founded English pottery manufactures, and there was bitter rivalry among them. One of the founding partners of the Lowestoft Porcelain Factory is rumoured to have hidden in a barrel to catch a glimpse of the mixing of the porcelain ingredients by their opposition in Bow. But, despite the theatrics, bone china was a huge success, and has become a staple of the quintessentially British afternoon tea.

In the domestic setting outside the dining room, glazed porcelain goods have been used for centuries for bathroom pieces because of their impermeability, hardness and durability. Similarly, bathroom and kitchen tiles are frequently made from porcelain, thanks to its attractive finish, wipeable surface, and ability to be finished in any colour or pattern. Some buildings have even been coated with porcelain tiles, such as the Royal Palace of Madrid, Spain, the Dakin Building, USA,
and the Porcelain Tower of Nanjing, China.

I’d bet that most people would be hard pressed to name applications for porcelain outside of the domestic setting. However, porcelain can be found in a critical role as a high-voltage electrical insulator, particularly outdoors. Look up under a power line and you might well glimpse porcelain supports keeping the lights on by insulating the lines. So let’s raise a glass, or, should I say, a nice cup of tea, to this ancient, astounding and no longer elusive material.