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Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and . The toughness, strength, and translucence of porcelain arise mainly from the formation of glass and the mineral mullite within the fired body at these high temperatures.

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

For the purposes of trade, 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).

Porcelain is used to make table, kitchen, sanitary, and decorative wares; objects of fine art; and tiles. Its high resistance to the passage of electricity makes porcelain an excellent insulator. Dental porcelain is used to make false teeth, caps, crowns and veneers.

Scope, materials and methods

Scope

The most common uses of porcelain are the creation of artistic objects and the production of more utilitarian wares. It is 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.

Materials

Clay is generally thought to be 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 significant component. Other materials can include feldspar, ball clay, glass, bone ash, steatite, quartz, petuntse and alabaster; further information on these formulations is given at "soft-paste porcelain".

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. The relatively low plasticity of the material used for making porcelain make shaping the clay difficult. In the case of throwing on a potters wheel it can be seen as pulling clay upwards and outwards into a required shape and potters often speak of pulling when forming a piece on a wheel, but the term is misleading; clay in a plastic condition cannot be pulled without breaking. The process of throwing is in fact one of remarkable complexity. To the casual observer, throwing carried out by an expert potter appears to be a graceful and almost effortless activity, but this masks the fact that a rotating mass of clay possesses energy and momentum in an abundance that will, given the slightest mishandling, rapidly cause the workpiece to become uncontrollable.

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. Great detail is given in the glaze article. 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 bisque-fired at around 1000 degrees Celsius, coated with glaze and then sent for a second glaze-firing at a temperature of about 1300 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.
A porcelain doll from the Czech Republic


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.

Categories of porcelain

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

Hard paste

Main article Hard-paste porcelain


Some of the earliest European porcelains were produced at the Meissen factorymarker in the early 18th century; they were formed from a paste composed of kaolinite, quartz, and alabaster and fired at temperatures in excess of , producing a porcelain of great hardness and strength. Later, the composition of the Meissen hard paste was changed and the alabaster was replaced by feldspar, 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

Main article Soft-paste porcelain


Its history dates from the early attempts by European potters to replicate Chinese porcelain by using mixtures of china clay and ground-up glass or frit; soapstone and lime were known to have also been included in some compositions. As these early formulations suffered from high pyroplastic deformation, or slumping in the kiln at raised temperature, they were uneconomic to produce. Formulations were later developed based on kaolin, quartz, feldspars, nepheline syenite and other feldspathic rocks. These were technically superior and continue in production.

Bone china

Main article Bone China
Although originally developed in England to compete with imported porcelain, Bone china is now made worldwide. It has been suggested that a misunderstanding of an account of porcelain manufacture in China given by a Jesuit missionary was responsible for the first attempts to use bone-ash as an ingredient of Western porcelain (in China, china clay was sometimes described as forming the bones of the paste, while the flesh was provided by refined porcelain stone) . For whatever reason, when it was first tried it was found that adding bone-ash to the paste produced a white, strong, translucent porcelain. Traditionally English bone china was made from two parts of bone-ash, one part of china clay kaolin and one part china stone (a feldspathic rock), although this has largely been replaced by feldspars from non-UK sources.

History

Chinese porcelain

Porcelain is generally believed to have originated in Chinamarker. Although proto-porcelain wares exist dating from the Shang Dynasty about 1600 BC, by the Eastern Han Dynasty (100-200 CE) high firing glazed ceramic wares had developed into porcelain, and porcelain manufactured during the Tang Dynasty period (618–906) was exported to the Islamic world where it was highly prized. Early porcelain of this type includes the tri-color glazed porcelain, or sancai wares. Historian S.A.M. Adshead writes that true porcelain items in the restrictive sense that we know them today could be found in dynasties after the Tang, during the Song, Yuanmarker, Mingmarker, and Qingmarker Dynasties.

By the Sui ( about 580 AD ) and Tang ( about 620 AD ) dynasties, porcelain had become widely produced. Eventually, porcelain and the expertise required to create it began to spread into other areas; by the seventeenth century, it was being exported to Europe.

Korean and Japanese porcelain also have long histories and distinct artistic traditions.

European porcelain

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

The European search for the secret of porcelain manufacture ended in 1708 with the discovery by Ehrenfried Walther von Tschirnhaus and Johann Friedrich Böttger of a combination of ingredients, including Colditzmarker clay (a source of kaolinite), calcined alabaster, and quartz, that produced a hard, white, translucent porcelain. It appears that in this discovery technology transfer from East Asia played little part.

Meissen

Meissen porcelain - 19th Century pair of candelabras and a clock.
Tschirnhaus and Böttger were employed by Augustus the Strong and worked at Dresdenmarker and Meissenmarker in the German state of Saxony. Tschirnhaus had a wide knowledge of European 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, it was his claim that he knew the secret of transmuting dross into gold that 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 the red stoneware of Yixingmarker.

A workshop note records that the first specimen of hard, white European porcelain was produced in January 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 true white porcelain. For this reason, credit for the European discovery of porcelain is traditionally ascribed to him rather than Tschirnhaus.

The Meissen factorymarker 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 greater than 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 Technologymarker.

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 Rouenmarker produced the earliest soft-paste in France, but the first important French porcelain was made at the Saint-Cloud factory before 1702. Soft-paste factories were established in Chantilly 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 fifteen years after Briand's demonstration, half a dozen factories were founded in England to made soft-paste table-wares and figures:



Other developments

William Cookworthy discovered deposits of china clay in Cornwallmarker, 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 Cornish china clay and china stone to make porcelain with a body composition similar to that of the Chinese porcelains of the early eighteenth century.

As an electric insulating material

porcelain insulator for medium high voltage


Porcelain is an excellent insulator for use at high voltage, especially in outdoor applications. Examples are: terminals for High voltage cables, bushings of power transformers, insulation of high frequency antennas and many other cases.

As a building material

Demonstration of the translucent quality of much porcelain.
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 Capodimontemarker, Naples, the Royal Palace of Madridmarker and the nearby Royal Palace of Aranjuezmarker. and the Porcelain Tower of Nanjingmarker in China. More recent noteworthy examples include The Dakin Buildingmarker in Brisbane, Californiamarker and the Gulf Buildingmarker in Houston, Texas which, when constructed in 1929, had a seventy-foot long 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.”

European and American porcelain



See also



References

  • Combined Nomenclature of the European Communities - EC Commission in Luxembourg, 1987 .
  • Burton, William. Porcelain, its Nature, Art and Manufacture. Batsford, London, 1906.


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