Porcelain

Porcelain is a ceramic material made by heating refined materials, often including clay in the form of kaolinite, to high temperatures in a kiln at 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 at these high temperatures of glass and the mineral mullite within the fired body.

Porcelain was named after its resemblance to the white, shiny cowry, called in old Italian porcella (little pig), because the curved shape of its upper surface resembles the curve of a pig's back. Properties associated with porcelain include low permeability and elasticity; high strength, hardness, glassiness, durability, whiteness, translucence, resonance, brittleness; 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 universally agreed 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 insulating material and it is widely used for high-voltage insulators. It is also used in dentistry to make false teeth, caps and crowns.

Scope, materials and methods

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. A difficult line to draw is that which divides high-fired stoneware from porcelain because this 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.

Materials

Chinese porcelain from the reign of the Qianlong Emperor (1735-1796)

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. This property also contributes to porcelain's use as a slipcasting body.^{[dubious – discuss]}

Methods

File:CzechdollS.jpg

A porcelain doll from the Czech Republic

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

Forming. Porcelain wares can be formed by any of the shaping methods listed in the Pottery article.

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 has been speculated that the first glazes were accidental and resulted from the presence in the kiln of lime-rich wood ash ^{[citation needed]} , 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. 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. 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. The porcelain is fired at a higher temperature than earthenware or stoneware so that the clay can vitrify and become non porous.

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).

Hard paste

Main article Hard-paste porcelain

One of the earliest European porcelains was produced at the Meissen factory and was compounded from china clay kaolin, quartz and alabaster and fired at temperatures in excess of 1,350 °C (2,462 °F) 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.

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 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 of Cornish china stone (a feldspathic rock), although this has largely been replaced by feldspars from non-UK sources

History

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.

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 Cornish china stone to make a form of porcelain the body of which in character closely resembled the Chinese porcelains of the early eighteenth century.

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. Schmidt was the creative force behind KT&K's famed Lotus Ware, commonly acknowledged to be the finest porcelain ever produced in the United States.^{[dubious – discuss]}

As a building material

Dakin Building, Brisbane, California using porcelain panels

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.

References

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

External links