A History of Pottery The production of pottery is one of the most ancient arts. The oldest known body of pottery dates from the Jomon period (from about 10,500 to 400 BC) in Japan; and even the earliest Jomon ceramics exhibit a unique sophistication of technique and design. Excavations in the Near East have revealed that primitive fired-clay vessels were made there more than 8,000 years ago. Potters were working in Iran by about 5500 BC, and earthenware was probably being produced even earlier on the Iranian high plateau. Chinese potters had developed characteristic techniques by about 5000 BC. In the New World many pre-Columbian American cultures developed highly artistic pottery traditions. After general sections on basic pottery types and decorating techniques this article focuses on the development of Western pottery since the beginning of the Renaissance. For detailed treatment of ancient Western and non-Western pottery, see Chinese art and architecture; Egypt, ancient; Greek art; Islamic art and architecture; Japanese art and architecture; Korean art; Mesopotamia; Minoan art; Persian art and architecture; pre- Columbian art and architecture. TYPES OF WARES Pottery comprises three distinctive types of wares. The first type, earthenware, has been made following virtually the same techniques since ancient times; only in the modern era has mass production brought changes in materials and methods. Earthenware is basically composed of clay--often blended clays--and baked hard, the degree of hardness depending on the intensity of the heat. After the invention of glazing, earthenwares were coated with glaze to render them waterproof; sometimes glaze was applied decoratively. It was found that, when fired at great heat, the clay body became nonporous. This second type of pottery, called stoneware, came to be preferred for domestic use. The third type of pottery is a Chinese invention that appeared when feldspathic material in a fusible state was incorporated in a stoneware composition. The ancient Chinese called decayed feldspar kaolin (meaning "high place," where it was originally found); this substance is known in the West as china clay. Petuntse, or china stone, a less decayed, more fusible feldspathic material, was also used in Chinese porcelain; it forms a white cement that binds together the particles of less fusible kaolin. Significantly, the Chinese have never felt that high-quality porcelain must be either translucent or white. Two types of porcelain evolved: "true" porcelain, consisting of a kaolin hard-paste body, extremely glassy and smooth, produced by high temperature firing, and soft porcelain, invariably translucent and lead glazed, produced from a composition of ground glass and other ingredients including white clay and fired at a low temperature. The latter was widely produced by 18th-century European potters. It is believed that porcelain was first made by Chinese potters toward the end of the Han period (206 BC-AD 220), when pottery generally became more refined in body, form, and decoration. The Chinese made early vitreous wares (protoporcelain) before they developed their white vitreous ware (true porcelain) that was later so much admired by Europeans. Regardless of time or place, basic pottery techniques have varied little except in ancient America, where the potter's wheel was unknown. Among the requisites of success are correct composition of the clay body by using balanced materials; skill in shaping the wet clay on the wheel or pressing it into molds; and, most important, firing at the correct temperature. The last operation depends vitally on the experience, judgment, and technical skill of the potter. DECORATING TECHNIQUES In the course of their long history potters have used many decorating techniques. Among the earliest, impressing and incising of wares are still favored. Ancient potters in Egypt, Mesopotamia, Greece, northern India and the high regions of Central Asia (where primitive terra-cotta figures, associated with religious cults were produced) frequently decorated wares with impressed or incised designs. A notable incising technique developed more recently was that of Korean potters working in the Koryo period (918-1392). These artisans began by ornamenting their celadon wares with delicately incised and impressed patterns and later developed elaborate inlaying by filling incised lines with colored slip (semiliquid clay). Black and white slip was used most effectively for inlaying colored porcelains. Decoration of this sort generally depends more on the skill of the artisan than on the complexity of the tools being used. An especially popular type of decoration involved the sgraffito, or "scratched," technique used by Italian potters before the 15th century. This technique, which is thought to have reached Italy from the Near East, was probably derived from China, where it was first used during the Song (Sung) dynasty (960-1279). By the 16th century Italian potters working mainly in Padua and Bologna had developed great skill in sgraffito, which entailed the incising of designs on red or buff earthenware that had been coated with ordinary transparent lead glaze, usually toned yellow or, sometimes, brown, copper, or green. After firing, the wares were dipped into white clay slip so that a dark pattern could be cut on the surface. By cutting through the white slip, the artist produced a design on the exposed red or buff body. Pigments were also sometimes applied. After a further coating of lead glaze the ware was fired a second time. A sound knowledge of glazes--both utilitarian and decorative--is vital to the potter. The origin of glazes and glazing techniques is unknown, but the fine lustrous glazes developed in China surely began with a simple glaze that served to cover earthenware and render it watertight. Chinese potters used two kinds of glazes, one composed basically of feldspar, and another produced by fusing silica of quartz or sand by means of a flux, generally of lead oxide. Chinese potters regarded glazes and glazing techniques as having prime importance; under the Han emperors they made great efforts to improve this technology. The use of lead glaze increased, and wood ash was incorporated to impart a dullish brown or gray green coloring, somewhat blotchy and occasionally iridescent. These effects were entirely natural, as no coloring matter was added to the composition. Glazing techniques were modified under successive dynasties. Colored glazes were developed and used to brilliant effect by Tang (T'ang) and Song potters, and a great diversity of brightly hued wares appeared over the centuries. Many connoisseurs feel that the pure white porcelain, called blanc de chine, which first appeared during the Ming dynasty, is the most serenely beautiful of all Chinese ceramics. Dehua (Te-hua) potters in Fujian (Fukien) province, working during the 17th century, produced their blanc de chine masterpieces in the purest white porcelain coated with a thick white glaze. Salt glaze, used by English potters during the early 1700s, may well have been known to the Chinese but was not used by them. Near Eastern potters glazed wares in ancient times. Potters in Mesopotamia and Iran commonly used an alkaline glaze made of quartz mixed with sodium and potassium. An admixture of colored metallic oxides, mostly lead, was introduced later. Painting on pottery and porcelain became richly colorful in many regions and periods. Decorative brush painting directly on the baked clay reached its zenith in China during the Ming dynasty (1368-1644), whose artists were highly skilled at painting in fired colors. For a long period Chinese ceramic artists had used only black or brown pigment to decorate wares that were then covered with clear glaze. It is believed that the appearance in China of 13th-century brush-decorated wares from Persia sparked a change. These works, painted in blue cobalt under the glaze, inspired the brushwork of the Chinese and the resulting so-called blue-and-white style. Ming artists also excelled in painting over the glaze, using brilliant enamel colors. The overglaze technique, which evolved over two centuries, demanded correct preparation of the enamels, skill in application, and the proper (low) firing temperature. The overglaze enamel decorations executed during the reign of Chenghua (1465-87), which were never surpassed in China, incorporated flowers, foliage, and figure subjects against backgrounds of arabesques and scrollwork. Designs enclosed within dark blue outlines were filled in with brilliant color. Enamel decoration of superb quality was also executed in Japan during the Edo period (1615-1868) by celebrated artists and potters of the caliber of Kenzan, Kakiemon, and Ninsei. In the ancient Aegean the potter's art developed continuously from the Neolithic period and through the periods of the Minoan and Mycenaean civilizations, culminating, in ancient Greece, in a unique type of painted pottery, which reached its height between the 6th and 4th centuries BC. The finest Greek pottery, especially Attic vases, was exquisitely proportioned and often decorated with finely painted relief work. Unlike artisans in Egypt, Mesopotamia, and Persia, the Attic potters did not apply heavy glaze to their wares. The unique gloss commonly seen on Attic pottery and similar wares made elsewhere in Greece still baffles those who have tried to determine its formula and method of application. Neither a glaze nor a varnish, it is more marked on some areas, such as those painted black, than on others. Some experts conjecture that it may be attributed to illite or a similar clay mineral in a weak solution that was thinly applied to the surface of wares or mixed into the black "paint" used by the artists. In the Islamic world ceramic decorative art flowered with the creation of a great diversity of painted wares. Painted luster decoration on pottery originated in Mesopotamia and spread to ancient Egypt; later, under Islam in Persia, this type of decoration on white-glazed wares became incredibly brilliant. Islamic luster-painted wares were later imitated by Italian potters during the Renaissance. MAJOR TRADITIONS IN THE WEST After the fall of the ancient Roman Empire potters in Europe produced little other than repetitive utilitarian wares until the end of the Middle Ages. Earthenware A distinctive type of earthenware known as majolica, which was derived from Chinese porcelain, appeared in Italy during the last quarter of the 14th century. It is now believed that this type of painted earthenware was inspired by the Hispano-Moresque luster-decorated ware of Spanish origin introduced to Italy by Majorcan seagoing traders. Majolica ware, whether thrown on the wheel or pressed into molds, was fired once to obtain a brown or buff body, then dipped in glaze composed of lead and tin oxide with a silicate of potash. The opaque glaze presented a surface that was suitable to receive decoration. A second firing after decoration fixed the white glaze to the body and the pigments to the glaze, so that the colors became permanently preserved. Frequently, the beauty of these wares was increased by dipping them in a translucent lead glaze composed of oxide of lead mixed with sand, potash, and salt. When certain luster pigments and enamels were used in all-over painting, wares had to be specially fired at low temperature. Application of metallic luster pigments required great skill because these colors were extremely volatile and needed special handling. Luca della Robbia (see della Robbia, family) did not, as has been held, invent the enamel tin-glazing process; nevertheless, his work raised majolica production from a craft to high art in Italy. Not only did he use blue and white enamels in decorative work, but, as a sculptor, he also used the majolica technique to add brilliance to the surface of his productions. By the beginning of the 15th century Italian potters had abandoned the old familiar processes, and a revolution in style and techniques was under way. The severe style as followed principally in the school of Tuscany continued to the end of the 15th century, but rules and principles slackened until the inclusion of human figures in designs, previously frowned upon, was accepted. At the end of the 15th century Faenza became the thriving center of a reinvigorated pottery industry in Italy. A new, rich decorative style, known as istoriato, fired the imagination of potters, reaching its zenith in the workshops of Urbino. In early 17th-century England, attractive slipwares were produced, including the slip- decorated earthenware that was a speciality of the Toft family of potters. A kind of tin- glazed earthenware was also produced in the Netherlands, principally at Delft, beginning in the mid-17th century. Termed delftware, it was among the first European wares to be decorated with motifs inspired by Chinese and Japanese models. Continental Porcelains Eventually, European potters, who much admired the porcelain of the Far East, attempted to imitate it, but the formula remained elusive. Francesco de Medici, grand duke of Tuscany, produced an inferior type of soft-paste porcelain in his Florence workshop during the 16th century. In March 1709, Augustus II of Saxony announced that his ceramist Johann Bottger (1682-1719) had discovered how to make porcelain. The first European royal porcelain manufactory was consequently established at Meissen (see Meissen ware) near Dresden, Germany. Throughout the century following the discovery of the porcelain formula--when, despite the utmost precautions at Meissen, the secret leaked out--many rival factories were set up in Europe. Germany, Austria, Italy, France, and England soon had factories engaged in the production of wares much like those of Meissen. Porcelain figures were first produced in Meissen as table ornaments; the earliest examples were formed as part of sweetmeat dishes. Many splendid wares issued from the royal factory, but none were more admired than the finely modeled and decorated porcelain figures imitated by almost every German, Austrian, Italian, and English factory of note. Widespread interest in figures of both pottery and porcelain has continued to the present. Johann Joachim Kandler (1706-75), a master modeler, was the most notable of the artisans engaged in this work at Meissen and rivaled the famous Franz Anton Bustelli (1723-63) of Nymphenburg (see Nymphenburg ware). The methods used to produce porcelain figures as developed by Kandler imparted a new dimension to the art. German porcelain figures were usually produced from molds, which, in turn, were cast from an original master model made of wax, clay, or, occasionally, wood. The use of molds facilitated unlimited reproduction. Because the figures shrank during firing, allowances had to be made in their sizes; they were also provided with a small venthole in the back or base to permit excess heated air to escape. Because different factories placed these holes differently, their positions help determine the provenance and authenticity of given pieces. When considerable undercutting was necessary, porcelain figures were usually made in sections, using separate molds. Portions of elaborate groups and single figures were later joined by a specially trained assembler (known as a "repairer") who usually worked from a master model. Europe's second hard-paste porcelain factory began operations at Vienna in 1717. In the late 1700s at the royal Sevres (see Sevres ware) factory in France, potters experimented until they developed a remarkably white, finely textured body. Sevres wares were painted in unique colors that no other European factory could duplicate. The bleu de roi and rose Pompadour of Sevres wares captivated all Europe and, with the products of Meissen and Vienna, inspired English potters. English Wares The finest English porcelain--both soft- and hard-paste--was made between about 1745 and 1775. The first English porcelain was probably produced at Chelsea (see Chelsea ware) under Charles Gouyn, but his successor Nicholas Sprimont, a Flemish silversmith who took over management in 1750, was responsible for the high-quality wares, especially the superb figures, for which the factory became famous. Factories at Worcester (see Worcester ware), Bow, and Derby also produced wares that rival those of the Continent. Led by the ambitious, energetic, and enterprising Josiah Wedgwood and his successors at the Etruria factory, English potters in the late 18th and early 19th centuries became resourceful and inventive. Wedgwood's contributions consisted mainly of a much improved creamware, his celebrated jasperware, so-called black basalt, and a series of fine figures created by famous modelers and artists. After Wedgwood, other potters of the first half of the 19th century developed a number of new wares. Of these, Parian ware was the most outstanding and commercially successful. The name of this ware was derived from Paros, the Greek island from which sculptors in ancient times obtained the creamy or ivory-tinted marble that Parian ware resembled. The first examples of this new product, described as "statuary porcelain," issued from Copeland and Garret's factory in 1842 and were immediately acclaimed. Two varieties of Parian ware were produced: statuary parian, used in the making of figures and reproductions of sculpture, and hard-paste, or standard, parian, from which hollowware was made. Statuary parian, incorporating a glassy frit, is classified as soft porcelain. Standard parian, with a greater proportion of feldspar in the composition but no frit, is hard porcelain. Early parian statuary was ivory-tinted due to the presence of iron in the feldspar devoid of iron silicate. Suitable deposits were eventually located in Sweden and Ireland. Both English and American potters either obtained details of the original formula or worked out their own, and the resulting production of Parian wares on both sides of the Atlantic was enormous. Among the most beautiful and successful wares invented by 19th-century potters were those decorated in what came to be known in England as pate-sur-pate, a paste-on-paste technique devised sometime after 1870 by Marc-Louis Solon (1835-1913) of Minton's in England. Pate-sur-pate, involving both modeling and painting techniques, was stained Parian ware decorated with reliefs in translucent tinted or white slip, the colors being laid one upon the other. Solon was inspired by a Chinese celadon case decorated with embossed flowers that he had admired in the museum at Sevres, where he worked for a time. At first his slip painting on biscuit porcelain simply peeled off; he was successful, however, when he applied layers of slip to a damp surface. Minton wares decorated with pate-sur-pate became the most costly and coveted ceramic ornaments produced in England in the last quarter of the 19th century. Only a few English potters mastered Solon's complex technique, although the work of his pupil, Alboin Birks, rivaled that of the master. 20th-Century Developments By the late 19th century, with the development of machinery and the introduction of new technologies, the age of mass production dawned and the potter's art consequently suffered. Western ceramic wares declined markedly in quality of materials and decoration. Florid designs, gaudy coloring, and inartistic shapes became fashionable, and the resulting decadence continued into the 20th century. Not until the 1930s were signs of revival in the form and decoration of ceramics discernible, principally in the productions of artist-potters who were active in Western Europe and the United States. Many of these artist-potters arrived at their innovations by way of continuous experiment with materials and techniques. Others sought inspiration from primitive types of Japanese pottery or in the forms of ancient American Indian traditions. Since the end of World War II, the design and decoration of ceramics in both Europe and the United States, especially ornamental wares, has been largely influenced by individual artist-artisans. Commercial products, such as tablewares, have tended to reflect the styles and patterns developed by these potters, whose work has often shown striking originality. Various Applications of Ceramics Ceramics, pronounced suh RAM ihks, are one of the three most important types of engineering materials that are primarily synthetic. The other two are metals and plastics. Ceramics include such everyday materials as brick, cement, glass, and porcelain. They also include unusual materials used in electronics and spacecraft. Most ceramics are hard and can withstand heat and chemicals. These properties give them a variety of uses in industry. Artists also create ceramics. This article discusses the use of ceramics in industry. For information on how artists make ceramics, see POTTERY. Properties of ceramics. Manufacturers make common ceramics from such minerals as clay, feldspar, silica, and talc. These minerals, called silicates, form most of the earth's crust. Clay is an important silicate. But it is not used in all ceramic materials. Glass, for example, is made from sand. Chemists make materials called advanced ceramics in the laboratory from compounds other than silicates. These compounds include alumina, silicon carbide, and barium titanate. Most ceramic products, like their mineral ingredients, can withstand acids, gases, salts, water, and high temperatures. But not all ceramic products have the same properties. Common ceramics are good insulators--that is, they conduct electricity poorly. However, certain ceramics lose their electrical resistance and become superconductors when they are cooled (see SUPERCONDUCTIVITY). Some ceramic materials are magnetic. Engineers control the properties of ceramics by controlling the proportion and type of materials used. Kinds of ceramic products: The properties of ceramics make them especially suitable for certain products. Products made of ceramic materials include abrasives (materials used for grinding), construction materials, dinnerware, electrical equipment, glass products, and refractories (heat- resistant materials). Abrasives: Manufacturers use some extremely hard ceramic materials for cutting metals and for grinding, polishing, and sanding various surfaces. These ceramic materials include alumina and silicon carbide. Construction materials: Clay and shale are used in making strong, durable bricks and drainpipes for homes and other buildings. Tiles are made of clay and talc. Cement consists chiefly of calcium silicates and is used primarily in making concrete. Gypsum is used to produce plaster for the surfaces of walls and ceilings. Bathtubs, sinks, and toilets are made of porcelain, which consists chiefly of clay, feldspar, and quartz. Dinnerware: Ceramics make excellent containers for food and drinks. They do not absorb liquids, and they resist acids, salts, detergents, and changes in temperature. Most ceramic dinnerware is made from a mixture of clays, feldspar, and quartz. Electrical equipment: Ceramics that do not conduct electricity are used as insulators in automobile spark plugs, on electric power lines, and in television sets. Such ceramics include alumina and porcelain. Another ceramic material, barium titanite, is used in making capacitors, which store electric charges in electronic equipment. Magnetic ceramics are used in electronic circuits and in electric motors. Complex electronic circuits are bonded on thin layers of alumina. Glass products: Glass is one of the most important materials, chiefly because of its transparency. Products made of glass include food containers, light bulbs, windows, and lenses for eyeglasses and telescopes. Fiberglass insulates the walls of many homes. Cables made of glass fibers transmit telephone calls and other information. The main ingredient in glass is silica. A glasslike coating called porcelain enamel serves as a protective surface on many metal products. These products include such appliances as refrigerators, stoves, and washing machines. Porcelain enamel also makes outdoor signs weather-resistant. Refractories: The property of heat-resistance makes refractories suitable for the manufacture of industrial boilers and furnaces, such as the furnaces used to make steel. Refractories shaped into tiles cover the surface of space shuttles, which must withstand the intense heat created by high speeds. Ceramics used in making refractories include alumina, magnesium oxide, silica, silicon carbide, and zirconium oxide. Other products: Ceramic engineers continually develop new uses for ceramics. For example, porcelain is used to make false teeth and artificial bone joints. Uranium oxide ceramics serve as fuel elements for nuclear reactors. Cutting tools are made from silicon nitride. Refractories made from carbides are used to make parts for aircraft engines. Alumina is used in making certain types of lasers (instruments that produce intense light beams). Making ceramics. The clays and other minerals used in ceramics are dug from the earth and refined to improve their purity. Machines crush and grind the materials into fine particles. The particles are mixed in the proper proportion, and water or other liquid is added to produce a mixture that can be shaped. A gluelike substance is sometimes added to mixtures that do not contain clay. Glass and some refractory products are made by melting the particles and shaping them when they are molten. The most common methods for shaping clay ceramics are slip casting, jiggering, extrusion, and pressing. In slip casting, the liquid mixture is poured into a mold that absorbs water. As the water is absorbed, a layer of ceramic particles is deposited onto the mold, forming such hollow items as teapots and vases. The excess liquid is then poured out of the mold. In jiggering, a machine presses the clay onto a rotating mold. Jiggering is used to make dinnerware. Extrusion shapes items into rods or tubes by forcing ceramic paste through a shaping tool called a die. In pressing, ceramic powder is pressed in a steel die or a rubber mold. After the product has dried, it is strengthened by firing, a process that takes place in special furnaces called kilns. Ceramics are fired at temperatures ranging from about 1200 to 3000 _F (649 to 1649 _C). Firing hardens the product permanently and gives it strength, durability, and other desired qualities. Manufacturers cover many ceramic products with a glassy coating called glaze. Glaze prevents the item from absorbing liquids and makes it smoother and easier to clean. Glazes are also used for decoration. History. Pottery, the oldest form of ceramic products, dates back to prehistoric times. Examples of pottery about 13,000 years old have been found in several parts of the world. Industrial uses of ceramics began in the 1900's. Military requirements of World War II (1939-1945) created a need for high-performance materials and helped speed the development of ceramic science and engineering. During the 1960's and 1970's, advances in atomic energy, electronics, communications, and space travel required new kinds of ceramics. For example, the introduction of high-speed spacecraft required heat- resistant ceramic coatings and engine linings. Since the early 1980's, researchers have worked to develop ceramic engines for cars, trucks, pumps, and electric generators. The discovery of ceramic superconductors in 1986 has spurred intense worldwide research to develop superconducting ceramic parts for electronic devices, electric motors, and transportation equipment. Ceramics in Manufacturing Materials made of clay are among the most ancient manufactured articles and have played a vital role in human civilization. Although clay, as a ceramic material, is still widely used, modern ceramics include a wide range of nonorganic, nonmetallic materials whose manufacture requires heating at high temperatures. Important ceramics products include brick and tile, clay pipe, refractory brick, pottery and porcelain articles and enamels, ferrites in computer memories, barium titanate and alumina in electronics, uranium dioxide as nuclear fuel, and garnets in lasers. Glass and cement are also major ceramic materials The raw materials used to make ceramics are inexpensive and widely available, and include clay, feldspar, quartz sand, iron oxides, and alumina. Clay is made up of fine, platelike crystals of hydrated aluminosilicates. The crystals are usually from about 1 to 10 microns (.001 to .010 mm) in their longest dimension. A thin film of water binds the crystals together and with their platelike shape gives the clay its easy working properties. The platelike form of clay crystals reflects the molecular layer structure of the silicon-oxygen and aluminum-oxygen groups in the clay compounds. CERAMICS MANUFACTURE Dust Press Method: Most American manufacturers use the press method for forming tile. Powdered raw materials are mixed with water or other binding liquids, and are formed by a hydraulic press. The clay is usually pressed damp, with about 10 percent water, into dies or molds under moderate pressures. Ceramics made of purified powders such as alumina and ferrites are pressed dry at higher pressure with an organic binder (for example, 1 percent polyvinyl alcohol). In isostatic pressing, the powder is held in a rubber mold, and the pressure is applied with a fluid such as glycerine giving uniform pressure throughout the sample, with less warping and fewer defects. Samples in bar, rod, or tube form can be extruded through a die. The fired bisque, or clay body can absorb water, and is generally regarded as a "soft" tile, another common name of this type of tile is whiteware. Hand Made tile - Slip casting: In slip casting, a suspension of ceramic powder, usually in water, is poured into a mold made of plaster of paris. Water is absorbed by the mold, and a hard lining on the mold wall is built up; excess liquid is poured out of the mold. Using slip casting, a number of complex shapes can be made economically, since the cost of the molds is low. These tiles are also considered soft. After forming, the ceramic ware must be carefully heated for a few hours at about 100 degrees-200 degrees C (about 200 degrees-400 degrees F) to remove excess water or binder. The rate of drying must be carefully controlled so that warping and defects do not form as the sample shrinks. After drying, the article is fired at a high temperature (800 degrees-2000 degrees C/1500 degrees-3500 degrees F) to sinter or bind together the individual crystals of the ceramic powder into a solid, coherent mass. The higher the firing temperature, the more dense and less porous the material becomes. A wide range of properties in ceramics is possible with different firing temperatures and times. IMPORTANT CERAMIC CATEGORIES The most common ceramic articles of pottery, porcelain, brick, and pipe form complex mixtures of several different solid phases after firing. Traditional whitewares and porcelains contain at least three starting materials--clay, feldspar, and silica sand. When a mixture of these materials is heated at high temperatures (above 1200 degrees C/2200 degrees F), the feldspar (potassium-sodium aluminosilicate) melts and coats the clay and sand crystals. As firing proceeds, tightly bound water in the clay structure is removed, and fine, needlelike crystals of an aluminosilicate called mullite are formed from the clay. The grains of silica sand are partly dissolved in the viscous liquid feldspar. In the cooled structure there is a glassy phase from the liquid feldspar that binds together the sand grains and mullite crystals. This glassy phase may also give the ware a smooth, polished surface. Firing at an intermediate temperature (about 1100 degrees C/2000 degrees F) produces stoneware, a heavy, opaque ceramic, nonporous and glazed. At lower firing temperatures (less than 1000 degrees C/1832 degrees F), a more porous ware with a rough surface results and is usually called earthenware. To make fine, translucent porcelain requires a higher firing temperature (up to 1400 degrees C/2500 degrees F) so that more glass is formed. NEW CERAMICS APPLICATIONS "High technology" ceramics are new types of materials that surpass earlier ceramics in strength, hardness, light weight, or improved heat resistance. For example, ceramic powders can now be made from particles of absolutely uniform size. When sintered, these powders produce ceramics that are far less vulnerable to fracture or thermal shock than ordinary ceramics. Added to a matrix of metal or ceramic, thin ceramic fibers increase the tensile strength of the material (see composite material). Barium titanate has a high dielectric constant and consequently is used in capacitors. It is also strongly piezoelectric (see piezoelectricity), which means that it develops an electrical voltage when stressed in a particular crystallographic direction. It is used in microphones, phonograph pickups, strain gauges, and ultrasonic devices. The apatites are a family of calcium phosphate minerals that have been widely used as phosphors in fluorescent lamps. Special impurities in the apatite--such as manganese-- fluoresce and thereby change the ultraviolet light of a mercury arc into visible light. In a lamp this change increases the output of visual radiation for a given input. Hydroxylapatite is a bone mineral, and has recently been developed as a bone and tooth implant material. The most convenient form of nuclear fuel is uranium dioxide. Uranium ores are reacted and purified to form uranium dioxide, which is then sintered into pellets that serve as nuclear fuel. The pellets are packed into long tubes and are especially stable even with the severe radiation and thermal conditions they encounter. Beta-alumina (a compound of 1 part sodium oxide with 11 parts aluminum oxide) has been traditionally used as a refractory material; recently it was found to have high electrical conductivity at low temperatures. This conductivity results from high mobility of the sodium ions, and beta-alumina is being used as an electrolyte in high-temperature batteries. The beta-alumina separates liquid sodium and a liquid sulfur-sodium polysulfide mixture; as the battery is charged or discharged, the sodium ions in the electrolyte transfer charge from one liquid to another. This battery is highly efficient and should find application in electric vehicles and for storage of electric power during off-peak hours. Ceramics have proven to be ideal hosts for the fluorescent ions needed in lasers. Ruby, which is alumina containing some chromium impurity, is one of the most used laser materials, and garnets are also excellent laser hosts. Amorphous ceramics are produced by firing ceramic material for a short time at low temperatures, to produce substances that lack the usual crystalline ceramic structure. Like plastics, these ceramics can be sprayed onto surfaces or injection-molded before they are fired. They are used to make complex shapes and thin ceramic films. Ultrahard ceramic layers are built into the steel in tank bodies. When a projectile penetrates a layer, it pulverizes the ceramic, breaking the bonds that bind the molecules together. This chemical change causes the ceramic fragments to expand. In expanding, they grind up the softer material of the projectile, making it inoperative. Ceramic fibers combined with epoxy glues produce a composite fabric that is lightweight but stronger than steel. It is currently used in making small airplane bodies. A History of Porcelain Porcelain, pronounced POUR suh lihn, is a type of ceramics highly valued for its beauty and strength. It is often called china, or chinaware, because it was first made in China. Porcelain is characterized by whiteness, a delicate appearance, and translucence (ability to let light through). Because it is the hardest ceramic product, porcelain is used for electrical insulators and laboratory equipment. However, porcelain is known primarily as a material for high-quality vases and tableware, as well as for figurines and other decorative objects. The type of porcelain that is used for such purposes produces a bell- like ring when struck. Porcelain differs from other types of ceramics in its ingredients and in the process by which it is produced. Two common types of ceramics--earthenware and stoneware--are made from a single natural clay, which is then fired (baked). In many cases, the object is coated with a glassy substance called glaze. Firing at a low temperature produces earthenware, a porous material. Earthenware can be made waterproof by glazing. Firing at a high temperature produces stoneware, a hard, heavy material. Stoneware is nonporous without glazing. Unlike earthenware and stoneware, porcelain is basically made from a mixture of two ingredients--kaolin and petuntse. Kaolin is a pure white clay that forms when the mineral feldspar breaks down. Petuntse is a type of feldspar found only in China. It is ground to a fine powder and mixed with kaolin. This mixture is fired at temperatures from about 2280 _F (1250 _C) to 2640 _F (1450 _C). At these extreme temperatures, the petuntse vitrifies--that is, it melts together and forms a nonporous, natural glass. The kaolin, which is highly resistant to heat, does not melt and therefore allows the item to hold its shape. The process is complete when the petuntse fuses itself to the kaolin. Kinds of porcelain There are three main kinds of porcelain: (1) hard-paste porcelain, (2) soft-paste porcelain, and (3) bone china. The differences between these types of porcelain are based on the material from which they are made. This material is called the body or paste. Hard-paste porcelain, which is sometimes called true porcelain or natural porcelain, has always been the model and ideal of porcelain makers. It is the type of porcelain first developed by the Chinese from kaolin and petuntse. Hard-paste porcelain resists melting far better than other kinds of porcelain. For this reason, it can be fired at higher temperatures. These hot temperatures cause the body and the glaze to become one. When hard-paste porcelain is broken, it is impossible to distinguish the body from the glaze. The proportions of kaolin and petuntse in hard-paste porcelain may vary. The porcelain is said to be severe if the percentage of kaolin is high, and mild if the percentage of kaolin is low. Most collectors of porcelain prefer mild porcelain because of its mellow, satiny appearance. In comparison, severe porcelain may seem harsh and cold. Soft-paste porcelain, sometimes called artificial porcelain, was developed in Europe in an attempt to imitate Chinese hard-paste porcelain. Experimenters used a wide variety of materials in their efforts to produce a substance that was hard, white, and translucent. They eventually developed soft-paste porcelain by using mixtures of fine clay and glasslike substances. These materials melt at the high temperatures used in making hard-paste porcelain. For this reason, soft-paste porcelain is fired at lower temperatures and does not completely vitrify--that is, it remains somewhat porous. Breaking a piece of soft-paste porcelain reveals a grainy body covered with a glassy layer of glaze. Although soft-paste porcelain was invented in imitation of true porcelain, it has merits of its own. Most of it is creamy in tone, and some people prefer this color to pure white. In addition, the colors used to decorate it merge with the glaze to produce a soft, silky effect that appeals to many collectors. Bone china is basically made by adding bone ash (burned animal bones) to kaolin and petuntse. English porcelain makers discovered this combination of ingredients about 1750, and England still produces nearly all the world's bone china. Though not as hard as true porcelain, bone china is more durable than soft-paste porcelain. The bone ash greatly increases the translucence of the porcelain. Decorating porcelain A piece of porcelain is shaped on a potter's wheel or in a mold. After this stage, the porcelain worker may decorate it by (1) surface modifications, (2) painting, or (3) transfer printing. Surface modifications are achieved by incising (carving), perforating (poking holes), and embossing (applying raised designs). A well-known method of embossing porcelain is to apply a mixture of water and clay, called slip, to the item with a brush. Relief designs (three-dimensional effects) are usually molded separately and then attached to the porcelain. Painting the porcelain surface may be done in several ways. One method is to use a colored glaze, such as the famous Chinese celadon. This glaze is a soft gray-green color. Another type of decoration is underglaze (designs painted on a piece before it is glazed). A deep blue made from the metal cobalt is the most dependable color used for underglazing. Cobalt blue has been widely used both in China and in Europe. Paints that are applied over the glaze are commonly called enamels. A large variety of enamel colors were perfected at an early period. Most of them are made from metallic oxides, such as iron, copper, and manganese. Enamel colors require a second firing to make them permanent. Porcelain painting in Europe differed greatly from porcelain painting in China. Chinese decorators separated each color from the next with a dark outline, but European artists blended colors together with no separating line. In addition, Europeans used decorations purely for their artistic value, but Chinese decorations were symbolic. For example, a pomegranate design symbolized a wish for many offspring because a pomegranate has many seeds. Transfer printing revolutionized the porcelain industry in 1756 by enabling workers to decorate wares much faster than they could by hand. In this process, a design is engraved on a copper plate, inked with ceramic color, and transferred to tissue paper. While the color is still wet, the tissue paper is pressed against a porcelain object, leaving the design on its surface. History of porcelain Oriental porcelain. The Chinese probably made the first true porcelain during the Tang dynasty (618-907). The techniques for combining the proper ingredients and firing the mixture at extremely high temperatures gradually developed out of the manufacture of stoneware. During the Song dynasty (960-1279), Chinese emperors started royal factories to produce porcelain for their palaces. Since the 1300's, most Chinese porcelain has been made in the city of Jingdezhen. For centuries, the Chinese made the world's finest porcelain. Collectors regard many porcelain bowls and vases produced during the Ming dynasty (1368-1644) and Qing dynasty (1644-1912) as artistic treasures. Porcelain makers perfected a famous blue and white underglazed procelain during the Ming period. Painting over the glaze with enamel colors also became a common decorating technique at this time. During the Qing period, the Chinese developed a great variety of patterns and colors and exported porcelain objects to Europe in increasing numbers. By the 1100's, the secret of making porcelain had spread to Korea and to Japan in the 1500's. Workers in these countries also created beautiful porcelain objects. A Japanese porcelain called Kakiemon was first produced during the 1600's. It features simple designs on a white background. Another well-known Japanese porcelain called Imari ware, or Arita, is famous for its dense decorations in deep blue and red. European porcelain. As early as the 1100's, traders brought Chinese porcelain to Europe, where it became greatly admired. However, it was so rare and expensive that only wealthy people could afford it. As trade with the Orient grew during the 1600's, porcelain became popular with the general public. The custom of drinking tea, coffee, and chocolate became widespread and created a huge demand for porcelain cups and saucers. European manufacturers responded by trying to make hard-paste porcelain themselves, but for a long time they failed to discover the secret. Nevertheless, some of their experiments resulted in beautiful soft-paste porcelain. The first European soft-paste porcelain was produced in Florence, Italy, about 1575. By the 1700's, porcelain manufactured in many parts of Europe was starting to compete with Chinese porcelain. France, Germany, Italy, and England became the major centers for European porcelain production. French porcelain. France became famous during the 1700's as the leading producer of soft-paste porcelain. The first factories were established at Rouen, St. Cloud, Lille, and Chantilly. The most celebrated type of soft-paste porcelain was first produced at Vincennes in 1738. In 1756, the factory was moved to the town of Sevres. Its soft-paste porcelain became known as Sevres. The earliest Sevres had graceful shapes and soft colors. Sevres pieces produced from 1750 to 1770 were decorated with brilliant colors and heavy gilding. Many of these pieces had richly colored backgrounds and white panels painted with birds, flowers, landscapes, or people. Sevres is also noted for its fine figurines of biscuit (unglazed porcelain). Beginning in 1771, a hard-paste porcelain industry developed near Limoges, where kaolin deposits had been discovered. By the 1800's, Limoges had become one of the largest porcelain centers in Europe. An American named David Haviland opened a porcelain factory at Limoges in 1842 to make tableware for the American market. Haviland porcelain features soft colors that blend together and small floral patterns. German porcelain. A German chemist named Johann Friedrich Bottger discovered the secret of making hard-paste porcelain in 1708 or 1709. This discovery led to the establishment of a porcelain factory in Meissen in 1710. Meissen porcelain is sometimes called Dresden because Bottger first worked near the city. For nearly a century, it surpassed in quality all other hard-paste porcelain made in Europe. The great success of Meissen porcelain can be partly attributed to the fine artists who decorated it. They painted the wares with an amazing variety of colors and designs. Johann Horoldt (or Herold), who became chief painter in 1720, produced beautiful Chinese and Japanese as well as European designs. Johann Kandler, who worked from about 1730 to 1770, is famous for his exquisite figures of animals and people. Political disorder in Germany and competition from Sevres porcelain drove the Meissen factory into decline during the late 1700's. It continued to operate but did not make wares of the same artistic quality. English porcelain. England is well known as the center for the production of bone china. Before the invention of bone china, the English manufactured fine soft-paste porcelain at Chelsea, Bow, and Derby. Most of this English porcelain was styled after Oriental and Continental designs. Worcester porcelain, first produced in 1751, is one of the oldest and best English porcelains. During its early years, the Worcester factory produced soft-paste porcelain, much of it decorated with Chinese designs in blue underglaze. Since the 1760's, it has manufactured bone china in a wide variety of colors and patterns.