Porcelain
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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 1,200 °C (2,192 °F) and 1,400 °C (2,552 °F). 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 its resemblance to the cowry, which in old Italian was called porcella, meaning little pig. [1] Properties associated with porcelain include low permeability andelasticity; considerable strength, hardness, glassiness, and brittleness; whiteness, translucence, and resonance; and 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. It is also used in dentistry to make false teeth, caps and crowns.
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[edit] Scope, materials and methods
[edit] 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 stoneware.
[edit] Materials
- Further information: Pottery
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 "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. This property also contributes to porcelain's use as a slipcasting body.[dubious ]
[edit] Methods
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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. In an alternative method particularly associated with Chinese and early European porcelains, the glaze is applied to the unfired body and the two fired together in a single operation. Wares glazed in this way are described as being green-fired or once-fired.
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 or stoneware so that the clay can vitrify and become non-porous.
[edit] Categories of porcelain
Western porcelain is generally 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.
[edit] Hard paste
- Main article Hard-paste porcelain
Some of the earliest European porcelains were produced at the Meissen factory in the early 18th century; they were formed from a paste composed of kaolinite, quartz, and alabaster and fired at temperatures in excess of 1,350 °C (2,462 °F), 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.
[edit] 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.
[edit] 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 what ever 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
[edit] History
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The earliest porcelains originated in China possibly during the late Eastern Han Dynasty. The reader is referred to the article on Chinese porcelain where the history of early porcelain is discussed.
[edit] European porcelain
Porcelain was first made in China, and it is a measure of the esteem in which the exported Chinese porcelains of the seventeenth and eighteenth centuries were held in Europe that in English china became a commonly used synonym for the Franco-Italian term porcelain. After a number of false starts, such as the so-called Medici porcelain, 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 Colditz clay (a type of kaolin), calcined alabaster and quartz, that proved to be suitable for making a hard, white, translucent porcelain, first produced at Meissen. It appears that in this discovery technology transfer from East Asia played little part.
William Cookworthy is credited with finding china clay in Cornwall which made 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 a form of porcelain the body of which in character closely resembled the Chinese porcelains of the early eighteenth century.
[edit] Meissen
Tschirnhaus and Böttger worked at Dresden and at Meissen, in the German state of Saxony, for Augustus the Strong. Tschirnhaus had a wide knowledge of European science and had also worked on the search for porcelain for more than a decade. In 1705 Böttger was appointed to assist him in this task. After training as a pharmacist, Böttger turned to alchemy and 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 research, Böttger was obliged to work with other alchemists in the futile search for transmutation, but his work in this area ended in 1705, when he was appointed to assist Tschirnhaus in the search for the secret of making porcelain. However, one of the first results of the collaboration between Tschirnhaus and Böttger was the development of a red stoneware that resembled the red wares of Yixing, and a factory was established to make these wares at Meissen, in 1707.
A workshop note records that the first specimen of hard, white European porcelain was produced in January, 1708. At this time the research was still being carried out under the direction of Tschirnhaus, who died in October of that year. It was left to Böttger to report to Augustus in March, 1709 that he could make good, white porcelain and for this reason credit for the European discovery of porcelain is traditionally given to him, but unjustly, in the view of many of those who point to the essential role played by Tschirnhaus.
The Meissen factory was established in 1710, following the development of a kiln and a glaze suitable for use with Böttger's porcelain, which required firing at very high temperatures (greater than 1,350 °C (2,462 °F) to achieve translucence. Meissen porcelain was once-fired or green-fired in the Chinese manner and was noted for its great resistance to thermal shock; so much so that 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.
Heinrich Schmidt, a designer from the Meissen factory, went on to establish his reputation in America when he joined the Knowles, Taylor & Knowles factory in East Liverpool, Ohio in the 1890s.
[edit] As a building material
In unusual modern cases porcelain has also been used as a building material for exterior surfaces. Generally the porcelain is formed into large rectangular panels . An award winning building using porcelain is the Dakin Building, Brisbane, California. An older example is the Gulf Building, Houston, Texas, built in 1929, which had a seventy-foot long logo of porcelain [1]
For a short time in America, porcelainized steel homes were produced in a Columbus, Ohio factory and constructed throughout the US. These Lustron homes had porcelain coated steel ceilings, walls, exterior siding and roofs. About 2500 were built and many remain standing today. The homes were advertised as maintenance free, never needing painting.
[edit] See also
[edit] Europe and The Americas
[edit] East Asia
[edit] References
- ^ porcellaneous - definition of porcellaneous by the Free Online Dictionary, Thesaurus and Encyclopedia
- Combined Nomenclature of the European Communities - EC Commission in Luxembourg, 1987 .
- Burton, William. Porcelain, it's Nature, Art and Manufacture. Batsford, London, 1906.
[edit] External links
- Chinese and Japanese Porelain Collector's Help and Info Page
- How porcelain is made
- How bisque porcelain is made
