Porcelain

Porcelain is a ceramic material made by heating materials, generally including clay in the form of kaolin, in a kiln to temperatures between 1,200 and 1,400 °C (2,200 and 2,600 °F). The toughness, strength, and translucence of porcelain arises mainly from the formation of glass and the mineral mullite within the fired body at these high temperatures.

Porcelain derives its present name from the old Italian porcellana (cowrie shell) because of its resemblance to the translucent surface of the shell. Porcelain can informally be referred to as "china" or "fine china" in some English-speaking countries, as China was the birthplace of porcelain making. Properties associated with porcelain include low permeability and elasticity; considerable strength, hardness, toughness, whiteness, translucency and resonance; and a high resistance to chemical attack and thermal shock.

The Combined Nomenclature of the European Communities defines porcelain as being "completely vitrified, hard, impermeable (even before glazing), white or artificially coloured, translucent (except when of considerable thickness), and resonant." However, the term porcelain lacks a universal definition and has "been applied in a very unsystematic fashion to substances of diverse kinds which have only certain surface-qualities in common" (Burton 1906).

Scope

The most common uses of porcelain are for utilitarian wares and artistic objects. It can be difficult to distinguish between stoneware and porcelain because this depends upon how the terms are defined. A useful working definition of porcelain might include a broad range of ceramic wares, including some that could be classified as a stoneware. Porcelain is used to make household wares, decorative items and objects of fine art amongst other things.

Materials

Kaolin is the primary material from which porcelain is made, even though clay minerals might account for only a small proportion of the whole. The word "paste" is an old term for both the unfired and fired material. A more common terminology these days for the unfired material is "body"; for example, when buying materials a potter might order an amount of porcelain body from a vendor.

The composition of porcelain is highly variable, but the clay mineral kaolinite is often a raw material. Other raw materials can include feldspar, ball clay, glass, bone ash, steatite, quartz, petuntse and alabaster.

The clays used are often described as being long or short, depending on their plasticity. Long clays are cohesive (sticky) and have high plasticity; short clays are less cohesive and have lower plasticity. In soil mechanics, plasticity is determined by measuring the increase in content of water required to change a clay from a solid state bordering on the plastic, to a plastic state bordering on the liquid, though the term is also used less formally to describe the facility with which a clay may be worked. Clays used for porcelain are generally of lower plasticity and are shorter than many other pottery clays. They wet very quickly, meaning that small changes in the content of water can produce large changes in workability. Thus, the range of water content within which these clays can be worked is very narrow and the loss or gain of water during storage and throwing or forming must be carefully controlled to keep the clay from becoming too wet or too dry to manipulate.

Methods

The following section provides background information on the methods used to form, decorate, finish, glaze, and fire ceramic wares.

Forming

Glazing

Unlike their lower-fired counterparts, porcelain wares do not need glazing to render them impermeable to liquids and for the most part are glazed for decorative purposes and to make them resistant to dirt and staining. Many types of glaze, such as the iron-containing glaze used on the celadon wares of Longquan, were designed specifically for their striking effects on porcelain.

Decoration

Porcelain wares may be decorated under the glaze using pigments that include cobalt and copper or over the glaze using coloured enamels. Like many earlier wares, modern porcelains are often biscuit-fired at around 1,000 degrees Celsius, coated with glaze and then sent for a second glaze-firing at a temperature of about 1,300 degrees Celsius or greater. Another early method is once-fired where the glaze is applied to the unfired body and the two fired together in a single operation.

Firing

In this process, green (unfired) ceramic wares are heated to high temperatures in a kiln to permanently set their shapes. Porcelain is fired at a higher temperature than earthenware so that the body can vitrify and become non-porous.

History

Chinese porcelain

Porcelain originated in China. Although proto-porcelain wares exist dating from the Shang Dynasty (1600â€"1046 BC), by the time of the Eastern Han Dynasty period (206 BC â€" 220 AD), glazed ceramic wares had developed into porcelain. Porcelain manufactured during the Tang Dynasty (618â€"907 AD) was exported to the Islamic world, where it was highly prized. Early porcelain of this type includes the tri-colour glazed porcelain, or sancai wares. There is no precise date to separate the production of proto-porcelain from that of porcelain. Porcelain items in the sense that we know them today could be found in the Tang Dynasty, and archaeological finds have pushed the dates back to as early as the Han Dynasty (206 BC â€" 220 AD). By the Sui Dynasty (581â€"618 AD) and Tang Dynasty (618â€"907 AD), porcelain was widely produced.

Eventually, porcelain and the expertise required to create it began to spread into other areas of East Asia. During the Song Dynasty (960â€"1279 AD), artistry and production had reached new heights. The manufacture of porcelain became highly organised, and the kiln sites excavated from this period could fire as many as 25,000 wares. While Xing Ware is regarded as among the greatest of the Tang Dynasty porcelain, Ding Ware became the premier porcelain of Song Dynasty. By the time of the Ming Dynasty (1368â€"1644 AD), porcelain wares were being exported to Europe. Some of the most well-known Chinese porcelain art styles arrived in Europe during this era, such as the coveted blue-and-white wares. The Ming Dynasty controlled much of the porcelain trade, which was expanded to Asia, Africa and Europe via the Silk Road. In 1517, Portuguese merchants began direct trade by sea with the Ming Dynasty, and in 1598, Dutch merchants followed.

Some porcelains were amore highly valued than others in imperial China. We can identify the most valued types by their association with the court, either as tribute offerings, or as products of kilns under imperial supervision. Some of the best-known examples are of Jingdezhen porcelain. During the Ming dynasty, Jingdezhen porcelain become a source of imperial pride. The Yongle emperor erected a white porcelain brick-faced pagoda at Nanjing, and an exceptionally smoothly glazed type of white porcelain is peculiar to his reign. Jingdezhen porcelain's fame came to a peak in the Qing dynasty.

European porcelain

These exported Chinese porcelains were held in such great esteem in Europe that in the English language china became a commonlyâ€"used synonym for the Franco-Italian term porcelain. Apart from copying Chinese porcelain in faience (tin glazed earthenware), the soft-paste Medici porcelain in 16th-century Florence was the first real European attempt to reproduce it, with little success.

Early in the 16th century, Portuguese traders returned home with samples of kaolin, which they discovered in China to be essential in the production of porcelain wares. However, the Chinese techniques and composition used to manufacture porcelain were not yet fully understood. Countless experiments to produce porcelain had unpredictable results and met with failure. In the German state of Saxony, the search concluded in 1708 when Ehrenfried Walther von Tschirnhaus produced a hard, white, translucent type of porcelain specimen with a combination of ingredients, including kaolin and alabaster, mined from a Saxon mine in Colditz. It was a closely guarded trade secret of the Saxon enterprise.

In 1712, many of the elaborate Chinese porcelain manufacturing secrets were revealed throughout Europe by the French Jesuit father Francois Xavier d'Entrecolles and soon published in the Lettres édifiantes et curieuses de Chine par des missionnaires jésuites. The secrets, which d'Entrecolles read about and witnessed in China, were now known and began seeing use in Europe.

Meissen

Von Tschirnhaus and Johann Friedrich Böttger were employed by Augustus the Strong and worked at Dresden and Meissen in the German state of Saxony. Tschirnhaus had a wide knowledge of science and had been involved in the European quest to perfect porcelain manufacture when in 1705 Böttger was appointed to assist him in this task. Böttger had originally been trained as a pharmacist; after he turned to alchemical research, he claimed to have known the secret of transmuting dross into gold, which attracted the attention of Augustus. Imprisoned by Augustus as an incentive to hasten his research, Böttger was obliged to work with other alchemists in the futile search for transmutation and was eventually assigned to assist Tschirnhaus. One of the first results of the collaboration between the two was the development of a red stoneware that resembled that of Yixing.

A workshop note records that the first specimen of hard, white and vitrified European porcelain was produced in 1708. At the time, the research was still being supervised by Tschirnhaus; however, he died in October of that year. It was left to Böttger to report to Augustus in March 1709 that he could make porcelain. For this reason, credit for the European discovery of porcelain is traditionally ascribed to him rather than Tschirnhaus.

The Meissen factory was established in 1710 after the development of a kiln and a glaze suitable for use with Böttger's porcelain, which required firing at temperatures of up to 1,400 °C (2,552 °F) to achieve translucence. Meissen porcelain was once-fired, or green-fired. It was noted for its great resistance to thermal shock; a visitor to the factory in Böttger's time reported having seen a white-hot teapot being removed from the kiln and dropped into cold water without damage. Evidence to support this widely disbelieved story was given in the 1980s when the procedure was repeated in an experiment at the Massachusetts Institute of Technology.

Soft paste porcelain

The pastes produced by combining clay and powdered glass (frit) were called Frittenporzellan in Germany and frita in Spain. In France they were known as pâte tendre and in England as "soft-paste". They appear to have been given this name because they do not easily retain their shape in the wet state, or because they tend to slump in the kiln under high temperature, or because the body and the glaze can be easily scratched.

Experiments at Rouen produced the earliest soft-paste in France, but the first important French soft-paste porcelain was made at the Saint-Cloud factory before 1702. Soft-paste factories were established with the Chantilly manufactory in 1730 and at Mennecy in 1750. The Vincennes porcelain factory was established in 1740, moving to larger premises at Sèvres in 1756. Vincennes soft-paste was whiter and freer of imperfections than any of its French rivals, which put Vincennes/Sèvres porcelain in the leading position in France and throughout the whole of Europe in the second half of the 18th century.

The first soft-paste in England was demonstrated by Thomas Briand to the Royal Society in 1742 and is believed to have been based on the Saint-Cloud formula. In 1749, Thomas Frye took out a patent on a porcelain containing bone ash. This was the first bone china, subsequently perfected by Josiah Spode.

In the twenty-five years after Briand's demonstration, half a dozen factories were founded in England to make soft-paste table-wares and figures:

• Chelsea (1743)
• Bow (1745)
• St James's (1748)
• Bristol porcelain (1748)
• Longton Hall (1750)
• Royal Crown Derby (1750 or 1757)
• Royal Worcester (1751)
• Lowestoft porcelain (1757)
• Wedgwood (1759)
• Spode (1767)

Other developments

William Cookworthy discovered deposits of kaolin in Cornwall, making a considerable contribution to the development of porcelain and other whiteware ceramics in the United Kingdom. Cookworthy's factory at Plymouth, established in 1768, used kaolin and china stone to make porcelain with a body composition similar to that of the Chinese porcelains of the early 18th century.

Types

Porcelain can be divided into the three main categories: hard-paste, soft-paste and bone china depending on the composition of the paste, the material used to form the body of a porcelain object and the firing conditions.

Hard paste

These porcelains that came from East Asia, especially China, were some of the finest quality porcelain wares. The earliest European porcelains were produced at the Meissen factory in the early 18th century; they were formed from a paste composed of kaolin and alabaster and fired at temperatures up to 1,400 °C (2,552 °F) in a wood-fired kiln, producing a porcelain of great hardness, translucency, and strength. Later, the composition of the Meissen hard paste was changed and the alabaster was replaced by feldspar and quartz, allowing the pieces to be fired at lower temperatures. Kaolinite, feldspar and quartz (or other forms of silica) continue to provide the basic ingredients for most continental European hard-paste porcelains.

Soft paste

Soft-paste porcelains date back from the early attempts by European potters to replicate Chinese porcelain by using mixtures of clay and ground-up glass (frit) to produce soft-paste porcelain. Soapstone and lime were known to have been included in these compositions. These wares were not yet actual porcelain wares as they were not hard and vitrified by firing kaolin clay at high temperatures. As these early formulations suffered from high pyroplastic deformation, or slumping in the kiln at raised temperature, they were uneconomic to produce and of low quality. Formulations were later developed based on kaolin with quartz, feldspars, nepheline syenite or other feldspathic rocks. These were technically superior and continue in production. Soft-paste porcelains are fired at lower temperatures than hard-paste porcelain, therefore these are in general less hard than hard-paste porcelains.

Bone china

Although originally developed in England since 1748 to compete with imported porcelain, bone china is now made worldwide. The English had read the letters of Jesuit missionary Francois Xavier d'Entrecolles, which described Chinese porcelain manufacturing secrets in detail. One writer has speculated that a misunderstanding of the text could possibly have been responsible for the first attempts to use bone-ash as an ingredient of English porcelain, although this is not supported by researchers and historians. In China, kaolin was sometimes described as forming the bones of the paste, while the flesh was provided by the refined rocks suitable for the porcelain body. Traditionally English bone china was made from two parts of bone-ash, one part of kaolin and one part china stone, although this has largely been replaced by feldspars from non-UK sources.

Other uses

Electric insulating material

Porcelain and other ceramic materials have many applications in engineering, especially ceramic engineering. Porcelain is an excellent insulator for use at high voltage, especially in outdoor applications, see Insulator (electricity)#Material. Examples include: terminals for high voltage cables, bushings of power transformers, insulation of high frequency antennas and many other components.

Building material

Porcelain can be used as a building material, usually in the form of tiles or large rectangular panels. Modern porcelain tiles are generally produced to a number of recognised international standards and definitions. Manufacturers are found across the world with Italy being the global leader, producing over 380 million square metres in 2006. Historic examples of rooms decorated entirely in porcelain tiles can be found in several European palaces including ones at Capodimonte, Naples, the Royal Palace of Madrid and the nearby Royal Palace of Aranjuez. and the Porcelain Tower of Nanjing. More recent noteworthy examples include The Dakin Building in Brisbane, California, and the Gulf Building in Houston, Texas, which when constructed in 1929 had a 21-metre-long (69 ft) porcelain logo on its exterior. A more detailed description of the history, manufacture and properties of porcelain tiles is given in the article “Porcelain Tile: The Revolution Is Only Beginning.”

Bathroom fittings

Because of its durability, inability to rust and impermeability, glazed porcelain has been in use for personal hygiene since at least the third quarter of the 17th century. During this period, porcelain chamber pots were commonly found in higher-class European households, and the term "bourdaloue" was used as the name for the pot.

However bath tubs are not made of porcelain, but of porcelain enamel on a metal base, usually of cast iron. Porcelain enamel is a marketing term used in the US, and is not porcelain but a form of glass-based vitreous enamel. The process was developed in the 1880s by David Buick, a Scottish-born entrepreneur. Before this many baths were made of copper or tin and would frequently rust or corrode. Baths with vitreous enamel coatings do not scratch, rust, stain or corrode (and are resistant to damage from chemical cleaners), and thus became widely used. The contemporary process is broadly the same as developed by Buick.

Loudspeakers

Hard-paste porcelain is ideal for loudspeaker casings. Thanks to its toughness and density it is possible to achieve very thin, yet sturdy and robust walls. And it is almost vibration-free so that it is not the casing that sounds but the sound source. And since porcelain can be molded into any shape or form, it is even possible to produce perfect spherical shapes. With an explosive form, the cast can be “blown up” into two perfect hemispheres. Only when the two halves fit together seemlessly, the porcelain ball is fit to be a speaker.

Manufacturers

See also

• Lithophane
• Sea pottery
• Faience

References

Further reading

• Combined Nomenclature of the European Communities - EC Commission in Luxembourg, 1987 .
• Burton, William (1906). Porcelain, its Nature, Art and Manufacture. Batsford, London
• Le Corbeiller, Clare (1985). Eighteenth-century Italian porcelain. New York: The Metropolitan Museum of Art. ISBN 0870994212. 
• Finlay, Robert (2010). The Pilgrim Art: Cultures of Porcelain in World History. Volume 11 of California World History Library (illustrated ed.). University of California Press. ISBN 0520945387. Retrieved 24 April 2014. 
• Guy, John (1986). Guy, John, ed. Oriental trade ceramics in South-East Asia, ninth to sixteenth centuries: with a catalogue of Chinese, Vietnamese and Thai wares in Australian collections (illustrated, revised ed.). Oxford University Press. Retrieved 24 April 2014. 
• Valenstein, S. (1998). A handbook of Chinese ceramics, Metropolitan Museum of Art, New York. ISBN 9780870995149

External links

• How porcelain is made
• How bisque porcelain is made
• ArtLex Art Dictionary - Porcelain

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