Porcelain

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Image:Thoughtful porcelain.jpg Porcelain is a hard ceramic substance made by heating at high temperature selected and refined materials often including clay in the form of kaolinite. Porcelain clay when mixed with water forms a plastic paste which can be worked to a required shape or form that is hardened and made permanent by firing in a kiln at temperatures of between about 1200 degrees Celsius and about 1400 degrees Celsius. The toughness, strength and translucence of porcelain arises mainly from the formation at high temperatures within the clay body of the mineral mullite and glass.

Porcelain was so-named after its resemblance to the white, shiny Venus-shell, called in old Italian porcella. The curved shape of the upper surface of the Venus-shell resembles the curve of a pig's back (Latin porcella, a little pig, a pig).

Properties associated with porcelain include those of low permeability, high strength, hardness, glassiness, durability, whiteness, translucence, resonance, brittleness, high resistance to the passage of electricity, high resistance to chemical attack, high resistance to thermal shock and high elasticity.

Porcelain is used to make wares for the table and kitchen, sanitary wares, decorative wares and objects of fine art. Its high resistance to the passage of electricity makes porcelain an ideal insulating material and it is used in dentistry to make false teeth, caps and crowns.

The earliest porcelains originated in China. The reader is referred to the Wikipedia article on Chinese porcelain for a discussion on the early history of the material.

1 Scope, materials and methods
2 Categories of porcelain
3 East Asian porcelain & pottery
4 European porcelain
- 4.1 Meissen
5 Wikipedia links on porcelain
6 External links

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Scope, materials and methods

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Scope

Porcelain has many uses, but this article is concerned mainly with its employment as a material used to make objects of craft and fine art, including decorative and utilitarian household wares. This follows the Wikipedia policy of drawing a line between technology and the arts, though in the case of porcelain the line is a difficult one to draw. Industrial and other uses are not covered here. Another difficult line to draw is that which divides high-fired stonewares from porcelain. Where this line is drawn depends upon how the terms porcelain and stoneware are defined. In this article the term porcelain is taken to encompass a broad range of high-fired ceramic wares, including some that might according to some systems of classification fall into the category of stoneware.

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Materials

When referring to the materials that they use, potters often employ words and names in a way that can be confusing for the layman. Reference should be made to the Wikipedia article on Pottery for an explanation of some of these terms, but it might be helpful to note that the material used to form the body of porcelain wares is often referred to as clay, even though clay minerals might account for only a small proportion of its whole. The porcelain clay body, unfired or fired, is sometimes spoken of as the paste and porcelain clay is itself sometimes described as the body (for example, when buying materials a potter might order such an amount of porcelain body from a vendor).

The composition of porcelain is highly variable, but China clay, comprising mainly or in part the platey clay mineral kaolinite is often a significant component. Other materials mixed with China clay to make porcelain clay have included feldspar, ball-clay, glass, bone ash, steatite, quartz, petuntse and alabaster.

The clays used by potters are often described as being long or short according to plasticity. Long clays are cohesive (sticky) and of high plasticity and short clays are less cohesive and are of 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. Porcelain clays are of lower plasticity (shorter) than many other clays used for making pottery and wet very quickly, which is to say that small changes in the content of water can produce large changes in workability. Thus, the range of water contents within which porcelain 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.

Some clays used for making ceramic wares are too cohesive to be thrown on the wheel, including for example, the brown clays used to form the bodies of the red stonewares of Yixing in the Chinese province of Jiangsu and as a result of this Yixing-wares are almost always hand-built.

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Methods

The Wikipedia article on Pottery provides much useful background information on methods used for forming, decorating, finishing, glazing and firing ceramic wares.

Forming. Porcelain wares are formed by hand-building, moulding, pressing, slip-casting or by throwing on a potter's wheel. Sometimes a combination of these methods is used and, for example, it would not be uncommon for a piece to have a thrown body, moulded handles and slip-cast decoration, the parts being luted together before firing (lute is a thick liquid mixture of clay and water used to join unfired parts together).

The relatively low plasticity of the clays used for making porcelain can cause difficulties for the potter, particularly in the case of wheel-thrown wares. To the spectator, throwing is often 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. It is generally supposed that the first glazes to appear on ceramic wares resulted from the unavoidable presence in the kiln of lime-rich wood ash, which acted on the surface of the wares as a flux. 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. In common with many earlier wares, modern porcelain wares 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 of glazing particularly associated with Chinese and early European porcelains the glaze was 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. Firing is the operation of heating green (unfired) ceramic wares at high-temperatures in a kiln to make permanent their shapes.

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Categories of porcelain

Western porcelain is generally divided into the three main categories of hard-paste, soft-paste and bone china, depending on the composition of the paste (the paste is the material used to form the body of a piece of porcelain).

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Hard paste

One of the earliest European porcelains was produced at the Meissen factory and was componded from China clay kaolin, quartz and alabaster and was fired at temperatures in excess of 1350-degrees Celsius to produce a porcelain of great hardness and strength. At a later date the composition of Meissen hard paste was changed and the alabaster was replaced by feldspar, lowering the firing temperature required. China clay, feldspar and quartz (or other forms of silica) continue to this day to provide the basic ingredients for most continental European hard paste porcelains.

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Soft paste

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 pryoplastic 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.

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Bone china

Although orginally 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 it was first tried, it was found that when bone-ash was added to the paste it 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 of China stone (a feldspathic rock), although this has largely been replaced by feldspars from non-UK sources

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East Asian porcelain & pottery

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European porcelain

Porcelain was first made in China and it is a measure of the esteem in which the Chinese export porcelains of the seventeenth and eighteenth centuries were held in Europe that china became a commonly used synonym for the term porcelain. After a number of false starts the European search for the secret of porcelain manufacture ended in 1708 with the discovery by Ehrenfried Walther von Tschirnhaus assisted by 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. It appears that this discovery was one in which technology transfer from the Orient played little if any part.

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Meissen

Tschirnhaus and Böttger worked at Dresden and at Meissen, in the German state of Saxony, for the highly eccentric Polish Elector, 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, whose own talent for performing the exact opposite of this operation was very well developed. 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 Dresden, 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 to achieve translucence (greater than 1350 degrees Celsius). 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. The truth of this famous but widely disbelieved story was finally established in the 1980s when the procedure was repeated in an experiment at the Massachusetts Institute of Technology.

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