Porcelain

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).

Physical 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 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.

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. This follows the Wikipedia policy of drawing a line between technology and the fine arts, though in the case of porcelain the line is a difficult one to draw. Industrial and other uses are (or, it is hoped, will be) covered elsewhere in the encyclopedia.

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 and this puzzle has been and continues to be the subject of controversy. However, ceramics have an existance that is entirely independent of the words used to classify them and 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

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 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.

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).

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. The earliest ceramics were hand-built using a simple coiling technique in which clay was rolled into long threads that were then pinched and beaten together to form the body of a vessel. In the coiling method of construction, all of the energy required to form the body of a piece is supplied directly by the hands of the potter. This changed with the introduction of the fast-wheel, early forms of which utilised energy stored in the rotating mass of the heavy stone wheel itself. The wheel was wound-up and charged with energy by pushing it round with a stick, an arrangement that permitted the energy stored in the wheel to be finely directed to where it was required, at the point where the hands of the potter come into contact with the clay. Unlike hand-building, in wheel-throwing the bulk of the energy used does not come directly from the hands of the potter. The introduction of the fast-wheel brought benefits in the form of speed and a job that might have taken hours, or even days, to complete was reduced to one that could be done in minutes. However, the fast-wheel also brought with it significant problems and made new demands on the skills of the potter. 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, have a rapid and dramatic effect on the piece being thrown. The sight of a novice potter wrestling with an eccentric mass of rotating clay in an attempt to save both pot and face can produce mixed emotions in the minds of more experienced onlookers. However, the lesson should be well-learned; potters work within constraints set by the materials and methods that they use and these constraints cannot be overcome by the application of unfocused brute force.

Glazing. It 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.

Chinese porcelain

Chinese porcelain is made using China stone, China clay or a combination of the two materials. Both minerals derive from the weathering and decomposition of granitic rocks. China clay largely comprises the clay mineral kaolinite (Gaoling) and China stone, petunse (baidunzi) is a micaceous rock of variable composition whose componants include quartz and sericite. China stone also occurs kaolinised to a greater or lesser extent. China stone and China clay are both platy minerals, which is to say that they are composed to varying degrees of small platelets of high surface area (external and internal) and are capable of holding relatively large amounts of water. This is of importance because most of the methods used for forming the body parts of ceramic pieces (throwing on a wheel, for example) depend upon the application of compression to align the platelets and increase the plasticity and workability of the clay mixture. In the case of throwing, compression is applied by the hand of the potter.

Chinese ceramic wares are often classified as being either northern or southern. Present day China comprises two separate, and from the geological point of view, distinctly different, land masses: the northern and the southern. The two land masses were brought together by the action of continental drift, forming a junction that lies very approximately along the line of the present-day Yangtze river. Geological differences between the northern and the southern land masses have influenced the nature of the ceramic wares made in the two areas and, for example, in the north ceramic wares tend to have bodies made using mainly China clay, in the south ceramic wares tend to have bodies made using mainly China stone. In turn, this led to the development of coal-fuelled kilns suitable for the high-temperature firing of clay-rich wares in the north and wood-fuelled kilns more suitable for the lower-temperature firing of the stone-rich southern wares.

In the Western tradition ceramics are primarily divided into the categories of earthenware, stoneware or porcelain, depending upon the composition of the body material and the temperature at which the ware matures into a stable crystaline matrix. The Chinese tradition recognises only two primary categories of ceramic, high-fired (ci) and low-fired (tao). This can lead to confusion because, for example, in China no distinction is drawn between high-fired stonewares and porcelain. One result of this is that the property of resonance carries greater weight than that of translucence in the Chinese classification of high-fired ceramic wares. An unusual characteristic of Chinese porcelain is that in the main it is green-fired or once-fired, which is to say that the body and the glaze are fired together. After the body of a piece is formed and finished it is air-dried, coated with a glaze, dried again and fired. In the high temperature of the kiln the body and the glaze are fused together to become one unit. Chinese enamelled wares are also produced in this way, but the enamels are added after the first, high-temperature, firing and the pieces are sent for a second firing in a smaller, lower-temperature kiln.

Jingdezhen

The city of Jingdezhen has been an important centre for the production of ceramics in southern China since at least the early Han Dynasty. The early wares were low-fired but by the time of the Southern and Northern Dynasties (420 to 589) locally available raw materials were being used to produce a form of porcelain. In the year 1004, under the Song emperor Jingde, the newly re-named city of Jingdezhen was established as a centre for the production of imperial porcelain.

Detailed descriptions of the manufacture of porcelain at Jingdezhen during the Qing dynasty exist, including, from the European perspective, the letters of Père d'Entrecolles and from the Chinese perspective, a memoir written by Tang Ying. Two letters written by Père Francois Xavier d'Entrecolles, a Jesuit missionary (and industrial spy) who lived and worked in Jingdezhen described in detail the methods and materials used in the manufacture of porcelain wares in the later years of the reign of the Kangxi emperor; an important period in the history of Chinese ceramics. In his first letter, dated 1712, d'Entrecolles describes the way in which China stone was crushed, refined and formed into little white bricks known in Chinese as petuntse or baidunzi. He then goes on to describe the refining of China clay, kaolin or Gaoling, the preparation of glazes, the stages of forming porcelain wares, glazing and firing. Père d'Entrecolles, explaining his motives for describing what he had seen at Jingdezhen, states that "Nothing but my curiosity could ever have prompted me to such researches, but it appears to me that a minute description of all that concerns this kind of work might, somehow, be useful in Europe" but in the event his first letter came too late to be of much help in the European search for the secret of making porcelain. In 1743, during the reign of the Qianlong emperor, Tang Ying, the imperial supervisor at Jingdezhen produced a memoir entitled "Twenty illustrations of the manufacture of porcelain." Unfortunately, the original illustrations have been lost but the text of the memoir may be found here, together with photographs replacing the missing illustrations and an additional commentary.

History

In the context of Chinese ceramics the term porcelain lacks a universally accepted definition. This in turn has led to confusion about when the first Chinese porcelain was made. Claims have been made for the late Eastern Han period (100 to 200 AD) the Three Kingdoms period (220 to 280 AD) the Six Dynasties period (220 to 589 AD) and the Tang Dynasty (618 to 906 AD). A strong body of Chinese scholarly opinion is currently of the view that the first true porcelain was made in the Chinese province of Zhejiang during the Eastern Han period, but this opinion is controversial. However, Chinese experts emphasise the presence of a significant proportion of porcelain-building minerals (China clay, China stone or a combination of both) as an important factor in defining porcelain and shards recovered from Eastern Han kiln sites in Zhejiang, estimated to have been fired at a temperature of between 1260 to 1300 degrees Celsius, were found that met this condition.

During the Sui and Tang periods (581 to 906) a wide range of ceramics, low-fired and high-fired, were produced. These included the well-known Tang lead-glazed sancai (three-colour) wares, the high-firing, lime-glazed Yue celadon wares and low-fired wares from Changsha. In northern China, high-fired, translucent porcelains were made at kilns in the provinces of Henan and Hebei.

During the Song and Yuan dynasties porcelain was made at Jingdezhen and other kiln sites in southern China using crushed and refined China stone alone, but by the early eighteenth century China clay was being added to the China stone, in about equal proportions. Porcelain bodies made from China stone fire at a lower temperature, in the region of 1200 degrees Celsius, than those made with a mixture of China clay and China stone, which require firing in the region of 1350 degrees Celsius. China clay when added to the body material produced a porcelain of great strength and whiteness (whiteness, in particular, was a much sought after property of porcelain, especially that used for blue and white wares). The temperatures within a typical large, southern egg-shaped kiln varied greatly, from hot, at the firebox end, to cooler, at the chimney end. One advantage gained by the addition in varying amounts of China clay was that the composition of the mix could be varied to suit the position that the wares made from it would occupy in the kiln, with a clay-rich mix being used for wares to be fired at the hot end of the kiln and a stone-rich mix being used for wares to be fired at the cooler end of the kiln.

Some notable Chinese porcelain wares

Qingbai wares

Qingbai wares were made at Jingdezhen and at many other southern kilns from the time of the Northern Song until their almost complete eclipse, starting early in the fourteenth century, by underglaze-decorated blue and white wares. The qingbai glaze is a porcelain glaze, so-called because it was made using China stone, an important constituent of the porcelain body. The qingbai glaze is clear, but contains iron in small amounts. When applied over a white porcelain body the glaze produces a greenish-blue colour that gives the glaze its name (qingbai in Chinese means greenish-blue). Bowls, some with incised or moulded decoration and varying from the everyday to more finely made pieces represent the overwhelming bulk of surviving qingbai wares.

The Song dynasty qingbai bowl illustrated was probably made at the Jingdezhen village of Hutian, which was also the site of the Imperial kilns established in the year 1004. The bowl has incised decoration, probably representing clouds or the reflection of clouds in the water. The body is white, translucent and has the texture of very fine sugar, indicating that it was made using crushed and refined China stone, rather than a mixture of China stone and China clay. The glaze and the body of the bowl would have been fired together, in a saggar, in a large, wood-burning dragon-kiln or climbing-kiln typical of southern kilns of the period. Though not the case with the bowl illustrated, many Song and Yuan qingbai bowls were fired upside down in special segmented saggars, a technique first developed at the Ding kilns in Hebei province. The rims of such wares were left unglazed but were often bound with bands of silver, copper or lead.

Blue and white wares

Following in the tradition of earlier qingbai porcelains, blue and white wares are glazed using a transparent porcelain glaze. The blue decoration is painted onto the body of the porcelain before glazing, using very finely ground cobalt oxide mixed with water. After the decoration has been applied the pieces are glazed and fired.

It is believed that underglaze blue and white porcelain was first made in the Tang Dynasty. No complete piece of Tang blue and white is known to exist, but shards dating to the eighth or ninth century have been unearthed at Yangzhou in Jiangsu province. It has been suggested that the shards originated from a kiln in the province of Henan. In 1957 excavations at the site of a pagoda in the province Zhejiang uncovered a Northern Song bowl decorated with underglaze blue and further fragments have since been discovered at the same site. In 1970 a small fragment of a blue and white bowl, also dated to the eleventh century, was also excavated in the province of Zhejiang. In 1975 shards decorated with underglaze blue were excavated at a kiln site in Jiangxi and, in the same year, an underglaze blue and white urn was excavated from a tomb dated to the year 1319, in the province of Jiangsu. It is of interest to note that a Yuan funerary urn decorated with underglaze blue and underglaze red and dated 1338 is still in the Chinese taste, even though by this time the large-scale production of blue and white porcelain in the Yuan, Mongol, taste had started at Jingdezhen.

Starting early in the fourteenth century, blue and white wares rapidly became the main product of Jingdezhen, reaching the height of its technical excellence during the later years of the reign of the Kangxi emperor and continuing in present times to be an important product of the city.

The tea caddy illustrated shows many of the characteristics of blue and white porcelain produced during the Kangxi period. The translucent body showing through the clear glaze is of great whiteness and the cobalt decoration, applied in many layers, is of a fine blue hue. The decoration, a sage in a landscape of lakes and mountains with blazed rocks is typical of the period. The potting is good and the porcelain body is finely textured, indicating the presence of a significant proportion of China clay in the paste. The piece would have been fired in a saggar (a lidded ceramic box intended to protect the piece from kiln debris, smoke and cinders during firing) in a reducing atmosphere in a wood-burning egg-shaped kiln, at a temperature approaching 1350 degrees Celsius.

Fakes and reproductions

Chinese potters have a long tradition of borrowing design and decorative features from earlier wares. Whilst ceramics with features thus borrowed might sometimes pose problems of provenance, they would not generally be regarded as either reproductions or fakes. However, fakes and reproductions have also been made at many times during the long history of Chinese ceramics and continue to be made today in ever-increasing numbers. A few examples are given below.

• Reproductions of Song dynasty Longquan celadon wares were made Jingdezhen in the early eighteenth century, but outright fakes were also made there, using special clay and artificially aged by boiling in meat broth, refiring and storage in sewers. Père d'Entrecolles records that by this means the wares could be passed off as being hundreds of years old.

• At Jingdezhen the two remaining wood fired, egg-shaped kilns produce convincing reproductions of earlier wares and at Zhejiang province good reproductions of Song Longquan celedon wares continue to be made in large, side-stoked dragon kilns.

• Before World War II, the English potter Bernard Leach found what he took to be genuine Song dynasty cizhou rice-bowls being sold very cheaply on the dock of a Chinese port and was surprised to learn that they were in fact freshly made.

• In modern times the market for Song dynasty Jian tea-bowls has been severely depressed by the appearence in large numbers of modern fakes good enough to deceive even expert collectors. It is reported that some of these fakes show evidence of having had genuine Song dynasty iron-foot bases grafted onto newly made bodies.

• In the late nineteenth century fakes of Kangxi period famille noire wares were made that were convincing enough to deceive the experts of the day. Many such pieces may still be seen in museums today, as may pieces of genuine Kangxi porcelain decorated in the late nineteenth century with famille noire enamels. A body of modern expert opinion holds that porcelain decorated with famille noire enamels was not made at all during the Kangxi period, though this view is disputed.

• A fashion for Kangxi period blue and white wares grew to large proportions in Europe during the later years of the nineteenth century and triggered the production at Jingdezhen of large quantities of porcelain wares that looked back to the ceramics of the earlier period. Such blue and white wares were not fakes or, in the main, convincing reproductions, even though some pieces carried four-character Kangxi reign-marks that continue to cause confusion to this day.

The value of testing in the authentication of Chinese porcelain is disputed. The most widely-known test, the thermoluminescence test (TL-test) can be used to provide an estimate, within very wide limits, of the date of last firing. The test is carried out on small samples of porcelain drilled or cut from the body of a piece, which can be risky and disfiguring. For this reason the test is rarely used for dating finely-potted, high-fired ceramics. Other tests can be used to determine the composition of glazes and body materials, for comparison with the results of analyses carried out on reference specimens of known provenance. It is however widely held that at best, testing can only be of use when combined with other, more traditional, methods for helping to establish provenance. Such methods might including comparative techniques, expert opinion and the evaluation of written and verbal records, where these are available.

Chinese porcelain references

• He Li, Chinese Ceramics. The New Standard Guide. Thames and Hudson, London, 1996. ISBN: 0 500 23727 1.
• Rose Kerr and Nigel Wood, Science and Civilisation in China, Volume 5, Part XII: Ceramic Technology. Cambridge University Press, 2004. ISBN: 0 521 83833 9.
• Stacey Pierson, Earth, Fire and Water: Chinese Ceramic Technology. Percival David Foundation of Chinese Art, University of London, 1996. ISBN: 07286 0265 2.
• Rosemary Scott and others, Imperial Taste. Chinese Ceramics from the Percival David Foundation. Chronicle Books, San Francisco, n.d. ISBN: 087701 612 7.