上海交通大学：《材料与文明》课程教学资源（参考资料）Understanding Mater_Chapter 15 - No Ceramics Age

15 No Ceramics s Age? 15.1.Ceramics and Civilization The utilization of ceramic materials by man is probably as old as human civilization itself.Stone,obsidian,clay,quartz,and min- eral ores are as much a part of the history of mankind as the prod- ucts which have been made of them.Among these products are tools,earthenware,stoneware,porcelain,as well as bricks,re- fractories,body paints,insulators,abrasives,and eventually mod- ern"high-tech ceramics"used,for example,in electronic equip- ment or jet engines.Actually,fired or baked ceramic objects are probably the oldest existing samples of handicraft which have come to us from ancient times.They are often the only archaeo- logic clues that witness former civilizations and habitats.More- over,there are scholars who believe that life took its origin from ceramics.And some ancient mythologies relate that man was cre- ated from clay.It might be of interest in this context that the He- brew word for soil,dirt,clay,or earth is "Adama."Taking all of these components into consideration,it might be well justified to ask why historians did not specifically designate a ceramics age. The answer is quite simple:stone,copper,bronze,and iron can be associated with reasonably well-defined time periods that have a beginning and frequently also an end during which these ma- terials were predominantly utilized for the creation of tools, weapons,and objects of art.In contrast to this,ceramic materi- als have been used continuously by man with essentially unbro- ken vigor commencing from many millennia ago until the pre- sent time.Thus,ceramics can be compared to a basso obstinato in a piece of music in which other instruments play the melody. Still,some identification of cultural stages through pottery is com- mon among historians and archaeologists,who distinguish a "pre- pottery"era from a pottery period.The latter one is classed by color,shape,hardness,and,notably,by decoration

15 The utilization of ceramic materials by man is probably as old as human civilization itself. Stone, obsidian, clay, quartz, and min￾eral ores are as much a part of the history of mankind as the prod￾ucts which have been made of them. Among these products are tools, earthenware, stoneware, porcelain, as well as bricks, re￾fractories, body paints, insulators, abrasives, and eventually mod￾ern “high-tech ceramics” used, for example, in electronic equip￾ment or jet engines. Actually, fired or baked ceramic objects are probably the oldest existing samples of handicraft which have come to us from ancient times. They are often the only archaeo￾logic clues that witness former civilizations and habitats. More￾over, there are scholars who believe that life took its origin from ceramics. And some ancient mythologies relate that man was cre￾ated from clay. It might be of interest in this context that the He￾brew word for soil, dirt, clay, or earth is “Adamá.” Taking all of these components into consideration, it might be well justified to ask why historians did not specifically designate a ceramics age. The answer is quite simple: stone, copper, bronze, and iron can be associated with reasonably well-defined time periods that have a beginning and frequently also an end during which these ma￾terials were predominantly utilized for the creation of tools, weapons, and objects of art. In contrast to this, ceramic materi￾als have been used continuously by man with essentially unbro￾ken vigor commencing from many millennia ago until the pre￾sent time. Thus, ceramics can be compared to a basso obstinato in a piece of music in which other instruments play the melody. Still, some identification of cultural stages through pottery is com￾mon among historians and archaeologists, who distinguish a “pre￾pottery” era from a pottery period. The latter one is classed by color, shape, hardness, and, notably, by decoration. No Ceramics Age? 15.1 • Ceramics and Civilization

288 15·No Ceramics Age? Beginning from early times,clay was of particular interest to man for a number of reasons.First,clay is abundantly found in many parts of the world,albeit in different compositions,quan- tities,and qualities.Second,clay is pliable if it has the right con- sistency (e.g.,water content).Third,the shape of an object made from clay is retained as long as it is not exposed to water for an extended period of time.Fourth,clay becomes hard when dried, for example,in the sun or near fires.A fifth and most important property of clay was eventually discovered in ancient times:clay permanently hardens to a virtually indestructible but brittle and porous material when heated above about 500C.At this tem- perature,an irreversible chemical reaction begins to take place which precludes the substance from returning to its original duc- tile state and makes it water-resistant.Moreover,the fired prod- uct is much less susceptible to environmental interactions than many metals and alloys such as iron.In primitive pottery- making the objects were placed in a shallow pit in the ground and the fire(using wood or dung)was built over them. Some early artifacts made of fired clay are at least 9,000 years old and consist mostly of pottery or building materials,such as bricks.Moreover,some figurines have been found (such as in French caves)portraying animals or human bodies.The oldest samples of baked clay include more than 10,000 fragments of statuettes which were found in 1920 near Dolni Vestonice, Moravia,in the Czech Republic.They portray wolves,horses, foxes,birds,cats,bears,or women with exaggerated female at- tributes(Figure 15.1).One of these prehistoric female figurines remained almost undamaged.It has been named the "Venus of Vestonice"and is believed to have been a fertility charm.Schol- ars date the statuette,which stands about 10 cm tall,as far back as 23,000 B.C.Much speculation has evolved about these frag- ments,which,incidentally,contain some mammoth bone ash in the clay.Specifically,one group of anthropologists proposes that the figurines served some divinational purpose and were designed to shatter in the fire (by wetting the clay)so that priests or shamans could foretell future events from the fragments.In any event,this practice apparently did not lead to the invention of pottery. A still earlier example for prehistoric pyrotechnology,dating back to about 50,000 B.C.,consists of ground-up iron oxide pow- ders which have been fired to yield various colors.Pigments of this kind,together with a lead oxide binder,have probably been used for millennia,for body painting and other decorations Unfired clay as building material was likewise rather common. In some cases,the mud was inserted between and around wooden

Beginning from early times, clay was of particular interest to man for a number of reasons. First, clay is abundantly found in many parts of the world, albeit in different compositions, quan￾tities, and qualities. Second, clay is pliable if it has the right con￾sistency (e.g., water content). Third, the shape of an object made from clay is retained as long as it is not exposed to water for an extended period of time. Fourth, clay becomes hard when dried, for example, in the sun or near fires. A fifth and most important property of clay was eventually discovered in ancient times: clay permanently hardens to a virtually indestructible but brittle and porous material when heated above about 500°C. At this tem￾perature, an irreversible chemical reaction begins to take place which precludes the substance from returning to its original duc￾tile state and makes it water-resistant. Moreover, the fired prod￾uct is much less susceptible to environmental interactions than many metals and alloys such as iron. In primitive pottery￾making the objects were placed in a shallow pit in the ground and the fire (using wood or dung) was built over them. Some early artifacts made of fired clay are at least 9,000 years old and consist mostly of pottery or building materials, such as bricks. Moreover, some figurines have been found (such as in French caves) portraying animals or human bodies. The oldest samples of baked clay include more than 10,000 fragments of statuettes which were found in 1920 near Dolní Veˇstonice, Moravia, in the Czech Republic. They portray wolves, horses, foxes, birds, cats, bears, or women with exaggerated female at￾tributes (Figure 15.1). One of these prehistoric female figurines remained almost undamaged. It has been named the “Venus of Veˇstonice” and is believed to have been a fertility charm. Schol￾ars date the statuette, which stands about 10 cm tall, as far back as 23,000 B.C. Much speculation has evolved about these frag￾ments, which, incidentally, contain some mammoth bone ash in the clay. Specifically, one group of anthropologists proposes that the figurines served some divinational purpose and were designed to shatter in the fire (by wetting the clay) so that priests or shamans could foretell future events from the fragments. In any event, this practice apparently did not lead to the invention of pottery. A still earlier example for prehistoric pyrotechnology, dating back to about 50,000 B.C., consists of ground-up iron oxide pow￾ders which have been fired to yield various colors. Pigments of this kind, together with a lead oxide binder, have probably been used for millennia, for body painting and other decorations. Unfired clay as building material was likewise rather common. In some cases, the mud was inserted between and around wooden 288 15 • No Ceramics Age?

15.1.Ceramics and Civilization 289 FiGURE 15.1.Baked clay fig- urine called the "Venus of Vestonice"found in 1920 in the Czech Republic.Approxi- mate age:23,000 B.C. structures.In other instances,pressed clay bricks formed in bas- kets were used (called adobe)which were sometimes fortified with straw.(This constitutes one of the first examples of the pro- duction of a composite material,that is,a technique to strengthen clay by fibers.)Additionally,asphalt from natural oil wells was sometimes used as mortar.However,buildings of this type were highly vulnerable to the weather.They easily crumbled and thus needed constant renewal (except for cliff-dwellings;see Plate 15.11).As a consequence,layers of settlements were often built on top of previous ones,creating a mound of occupation debris (called a tell or a tall).This is characteristic for ruins found in Mesopotamia.Mass production of bricks was not performed in this region until the sixth millennium B.C.,probably because of a shortage of fuel.Later,however,that is,in the sixth century B.C.,the buildings of Babylon utilized fired and glazed bricks(see Plate 15.5). As outlined above,pottery-making is one of the oldest forms of crafts.The resulting vessels are quite useful for storing prop-

structures. In other instances, pressed clay bricks formed in bas￾kets were used (called adobe) which were sometimes fortified with straw. (This constitutes one of the first examples of the pro￾duction of a composite material, that is, a technique to strengthen clay by fibers.) Additionally, asphalt from natural oil wells was sometimes used as mortar. However, buildings of this type were highly vulnerable to the weather. They easily crumbled and thus needed constant renewal (except for cliff-dwellings; see Plate 15.11). As a consequence, layers of settlements were often built on top of previous ones, creating a mound of occupation debris (called a tell or a tall). This is characteristic for ruins found in Mesopotamia. Mass production of bricks was not performed in this region until the sixth millennium B.C., probably because of a shortage of fuel. Later, however, that is, in the sixth century B.C., the buildings of Babylon utilized fired and glazed bricks (see Plate 15.5). As outlined above, pottery-making is one of the oldest forms of crafts. The resulting vessels are quite useful for storing prop- 15.1 • Ceramics and Civilization 289 FIGURE 15.1. Baked clay fig￾urine called the “Venus of Veˇstonice” found in 1920 in the Czech Republic. Approxi￾mate age: 23,000 B.C

290 15·No Ceramics Age? erty and dry food,for cooking,and for transporting water.Still, pottery-making is seldom found among nomadic tribes in par- ticular as long as natural materials,such as gourds,skins,and large leaves,can be found or when baskets can be woven for the above-mentioned purposes.Moreover,and most importantly, potters must live within the reach of their raw materials and their kilns.Finally,ceramic pots are heavy and might break during travel.It is therefore not surprising that the art of pottery did not commence or was not practiced at all locations of the world at the same time and with equal sophistication,even though the raw materials were certainly available.Instead,pottery-making has been mainly exercised in areas in which agriculture has been firmly established,that is,where the population was reasonably settled.It is interesting to know in this context that pottery ap- pears in the Americas approximately 5,000 years later than in the 'old world"and that glazes,as well as the potter's wheel,and fre- quently also the kiln,were not known there in pre-Columbian times. The discovery that fired clay objects are water-resistant and sturdy eventually led to a systematic development of kilns with permanent walls and open tops.Certain modifications on the out- lay of the kilns and the type of fuel used eventually allowed in- creasingly higher temperatures.This has been discussed already in Chapter 1 in the context of the interrelationship between pottery-making and copper-smelting. 15.2·Types of Pottery Pottery is broadly divided into vitrified ware and unvitrified ware, a distinction that is based on whether or not clay composition and firing temperature cause the clay to melt or fuse into a glassy (vitreous)substance.Earthenware is made from "earthenware clay"fired at relatively low temperatures,that is,between 800 and 1200C,depending on the raw material;see Plate 15.1.It is porous when not subsequently glazed (see below)and is rela- tively coarse and often red or buff-colored,even black after fir- ing.Bricks and other construction materials,such as tiles,as well as terra cotta vessels are the major products in this category. Earthenware was probably the earliest kind of ceramics that was made,dating back to about 7,000 or possibly 8,000 B.C.Speci- mens of this age were found,for example,in Catal Huyuk in Ana- tolia (today's Turkey). Because of its porosity,the water which is stored in vessels made of unglazed earthenware percolates eventually through the

erty and dry food, for cooking, and for transporting water. Still, pottery-making is seldom found among nomadic tribes in par￾ticular as long as natural materials, such as gourds, skins, and large leaves, can be found or when baskets can be woven for the above-mentioned purposes. Moreover, and most importantly, potters must live within the reach of their raw materials and their kilns. Finally, ceramic pots are heavy and might break during travel. It is therefore not surprising that the art of pottery did not commence or was not practiced at all locations of the world at the same time and with equal sophistication, even though the raw materials were certainly available. Instead, pottery-making has been mainly exercised in areas in which agriculture has been firmly established, that is, where the population was reasonably settled. It is interesting to know in this context that pottery ap￾pears in the Americas approximately 5,000 years later than in the “old world” and that glazes, as well as the potter’s wheel, and fre￾quently also the kiln, were not known there in pre-Columbian times. The discovery that fired clay objects are water-resistant and sturdy eventually led to a systematic development of kilns with permanent walls and open tops. Certain modifications on the out￾lay of the kilns and the type of fuel used eventually allowed in￾creasingly higher temperatures. This has been discussed already in Chapter 1 in the context of the interrelationship between pottery-making and copper-smelting. Pottery is broadly divided into vitrified ware and unvitrified ware, a distinction that is based on whether or not clay composition and firing temperature cause the clay to melt or fuse into a glassy (vitreous) substance. Earthenware is made from “earthenware clay” fired at relatively low temperatures, that is, between 800 and 1200°C, depending on the raw material; see Plate 15.1. It is porous when not subsequently glazed (see below) and is rela￾tively coarse and often red or buff-colored, even black after fir￾ing. Bricks and other construction materials, such as tiles, as well as terra cotta vessels are the major products in this category. Earthenware was probably the earliest kind of ceramics that was made, dating back to about 7,000 or possibly 8,000 B.C. Speci￾mens of this age were found, for example, in Catal Hüyük in Ana￾tolia (today’s Turkey). Because of its porosity, the water which is stored in vessels made of unglazed earthenware percolates eventually through the 290 15 • No Ceramics Age? 15.2 • Types of Pottery

15.2·Types of Pottery 291 walls and evaporates on the free surface,thus cooling the water in the container.For the same reason,terra cotta containers can- not be used for storing milk or wine.Examples of more recent unglazed earthenware include Chinese teapots,first made dur- ing the Ming dynasty,as well as red stoneware made in MeiBen (Germany)or by Wedgwood in England at the beginning of the eighteenth century A.D. In order to hermetically seal the pores of goods made of earth- enware,an additional processing step called glazing was intro- duced around or probably even before 3000 B.c.by the Egyp- tians.It involved the coating of the fired objects with a water suspension of finely ground quartz sand mixed with sodium salts (carbonate,bicarbonate,sulfate,chloride)or plant ash.This was followed by a second firing,during which the glassy particles fused into an amorphous layer.The second firing is often at a lower temperature,being just sufficient to fuse the glaze.Inci- dentally,the Egyptians also glazed beads and bowls made of steatite,a soft stone which could be easily shaped,drilled,and abraded.Another technique to produce beads involved hand- molding a mixture of crushed quartz with sodium salts and mala- chite (Plate 1.3),which was then fired. Two other types of glazes which have been applied to earth- enware are likewise several millennia old.One of them is a trans- parent lead glaze.Lead reduces the melting or fusion point of the glaze mixture,which allows the temperature of the second firing to be even lower.Lead glaze was invented in China during the Han dynasty (206 B.C.-A.D.200)and was subsequently widely used by many civilizations.However,some lead from the glaze on tableware may be leached by the food.It is believed that this poi- soned a large number of Roman nobility and thus contributed (together with the lead from water pipes)to the fall of the Ro- man empire.Lead glazing for tableware is outlawed today by many countries unless fritted glazes are utilized which convert lead into a nontoxic form. An alternative technique involves an opaque,white tin glaze which hides possible color blemishes,for example,from iron im- purities.Tin glazing was probably first discovered by the Assyr- ians who lived in Mesopotamia(today's Northern Iraq)during the second millennium B.C.It was utilized for decorating bricks but fell eventually into disuse possibly because of the sudden in- terruption of the tin supply,as explained already in Chapter 7. In the ninth century A.D.,tin glazing was reinvented and again extensively utilized in Mesopotamia.From there it spread to Italy via the Spanish island of Maiolica (Majorca),after which the product was later named (Plate 15.7).French earthenware

walls and evaporates on the free surface, thus cooling the water in the container. For the same reason, terra cotta containers can￾not be used for storing milk or wine. Examples of more recent unglazed earthenware include Chinese teapots, first made dur￾ing the Ming dynasty, as well as red stoneware made in Meißen (Germany) or by Wedgwood in England at the beginning of the eighteenth century A.D. In order to hermetically seal the pores of goods made of earth￾enware, an additional processing step called glazing was intro￾duced around or probably even before 3000 B.C. by the Egyp￾tians. It involved the coating of the fired objects with a water suspension of finely ground quartz sand mixed with sodium salts (carbonate, bicarbonate, sulfate, chloride) or plant ash. This was followed by a second firing, during which the glassy particles fused into an amorphous layer. The second firing is often at a lower temperature, being just sufficient to fuse the glaze. Inci￾dentally, the Egyptians also glazed beads and bowls made of steatite, a soft stone which could be easily shaped, drilled, and abraded. Another technique to produce beads involved hand￾molding a mixture of crushed quartz with sodium salts and mala￾chite (Plate 1.3), which was then fired. Two other types of glazes which have been applied to earth￾enware are likewise several millennia old. One of them is a trans￾parent lead glaze. Lead reduces the melting or fusion point of the glaze mixture, which allows the temperature of the second firing to be even lower. Lead glaze was invented in China during the Han dynasty (206 B.C.–A.D. 200) and was subsequently widely used by many civilizations. However, some lead from the glaze on tableware may be leached by the food. It is believed that this poi￾soned a large number of Roman nobility and thus contributed (together with the lead from water pipes) to the fall of the Ro￾man empire. Lead glazing for tableware is outlawed today by many countries unless fritted glazes are utilized which convert lead into a nontoxic form. An alternative technique involves an opaque, white tin glaze which hides possible color blemishes, for example, from iron im￾purities. Tin glazing was probably first discovered by the Assyr￾ians who lived in Mesopotamia (today’s Northern Iraq) during the second millennium B.C. It was utilized for decorating bricks but fell eventually into disuse possibly because of the sudden in￾terruption of the tin supply, as explained already in Chapter 7. In the ninth century A.D., tin glazing was reinvented and again extensively utilized in Mesopotamia. From there it spread to Italy via the Spanish island of Maiolica (Majorca), after which the product was later named (Plate 15.7). French earthenware, 15.2 • Types of Pottery 291

292 15·No Ceramics Age? Faience,and Dutch earthenware,Delft,are likewise tin-glazed ce- ramic products whose names are derived from the Italian city of Faenza and the city of Delft in Holland.Faience is generally dis- tinguished by its elaborate decorations which flourished partic- ularly in the sixteenth and seventeenth centuries (Plate 15.9). (Some people apply the word Faience to a much broader range of ceramic products dating back to the Egyptians.)It should be noted that unglazed ceramic products are called bisqueware. Stoneware is fired at temperatures around 1200 to 1300C, which causes at least partial vitrification of certain clays stem- ming typically from sedimentary deposits that are low in iron content.Stoneware is hard and opaque and sometimes translu- cent.Its color varies from black via red,brown,and grey to white. Fine white stoneware was made in China as early as 1400 B.C. (Shang dynasty)and was glazed with feldspar.Korea and Japan followed at about 50 B.C.and the thirteenth century A.D.,respec- tively.The first European stoneware was produced in Germany after Johann Friedrich Bottger,an alchemist looking for gold (Chapter 17),together with E.W.von Tschirnhaus,rediscovered red stoneware in 1707(Plate 15.8).Josia Wedgwood,an Eng- lishman,followed somewhat later with black stoneware called basalte,and with white stoneware,colored by metal oxides,called jasper.Stoneware may remain unglazed or may receive lead or salt glazes.The latter one (first used in the Cologne region in Germany)involves NaCl which is tossed into the kiln when it has reached its highest temperature,allowing sodium and silica from the clay to form sodium silicate.This yields a pitted appearance like an orange peel.Alternatively,the objects are dipped into a salt solution before firing.Salt glazes give off poisonous chlorine gas during firing and are thus environmentally objectionable. Large-scale salt glazing was therefore discontinued some years ago. The climax of the art of pottery was reached when the Chinese invented porcelain,a white,thin,and translucent ceramic that possesses a metal-like ringing sound when tapped.It is believed that Marco Polo,when seeing it in China (about 1295),named it porcellana (shell)because of its translucency.In its initial form, porcelain was produced during the T'ang dynasty (A.D.618-907) but was steadily improved to the presently known configuration starting with the Yuan dynasty (A.D.1279-1368).Many western and Islamic countries tried in vain to duplicate (or vaguely imi- tate)this ultimate form of tableware until eventually,in 1707-1708,the above-mentioned J.F.Bottger succeeded,which laid the ground for the MeiRen porcelain manufacture in Saxony (Germany)in 1710

Faïence, and Dutch earthenware, Delft, are likewise tin-glazed ce￾ramic products whose names are derived from the Italian city of Faenza and the city of Delft in Holland. Faïence is generally dis￾tinguished by its elaborate decorations which flourished partic￾ularly in the sixteenth and seventeenth centuries (Plate 15.9). (Some people apply the word Faïence to a much broader range of ceramic products dating back to the Egyptians.) It should be noted that unglazed ceramic products are called bisqueware. Stoneware is fired at temperatures around 1200 to 1300°C, which causes at least partial vitrification of certain clays stem￾ming typically from sedimentary deposits that are low in iron content. Stoneware is hard and opaque and sometimes translu￾cent. Its color varies from black via red, brown, and grey to white. Fine white stoneware was made in China as early as 1400 B.C. (Shang dynasty) and was glazed with feldspar. Korea and Japan followed at about 50 B.C. and the thirteenth century A.D., respec￾tively. The first European stoneware was produced in Germany after Johann Friedrich Böttger, an alchemist looking for gold (Chapter 17), together with E.W. von Tschirnhaus, rediscovered red stoneware in 1707 (Plate 15.8). Josia Wedgwood, an Eng￾lishman, followed somewhat later with black stoneware called basalte, and with white stoneware, colored by metal oxides, called jasper. Stoneware may remain unglazed or may receive lead or salt glazes. The latter one (first used in the Cologne region in Germany) involves NaCl which is tossed into the kiln when it has reached its highest temperature, allowing sodium and silica from the clay to form sodium silicate. This yields a pitted appearance like an orange peel. Alternatively, the objects are dipped into a salt solution before firing. Salt glazes give off poisonous chlorine gas during firing and are thus environmentally objectionable. Large-scale salt glazing was therefore discontinued some years ago. The climax of the art of pottery was reached when the Chinese invented porcelain, a white, thin, and translucent ceramic that possesses a metal-like ringing sound when tapped. It is believed that Marco Polo, when seeing it in China (about 1295), named it porcellana (shell) because of its translucency. In its initial form, porcelain was produced during the T’ang dynasty (A.D. 618–907) but was steadily improved to the presently known configuration starting with the Yüan dynasty (A.D. 1279–1368). Many western and Islamic countries tried in vain to duplicate (or vaguely imi￾tate) this ultimate form of tableware until eventually, in 1707–1708, the above-mentioned J.F. Böttger succeeded, which laid the ground for the Meißen porcelain manufacture in Saxony (Germany) in 1710. 292 15 • No Ceramics Age?

15.2·Types of Pottery 293 The secret of porcelain was found in a combination of raw ma- terials,namely,in pure,white,kaolin clay (see Section 15.4) which was mixed with quartz and feldspatic rock.Pure kaolin, having a melting point of 1260C,is,however,too difficult to shape due to its poor ductility.Further,its high refractory prop- erty does not allow it to be fired to a hard and dense body at ac- ceptable temperatures.The combination with other ingredients, such as "ball clay,"increases the plasticity of kaolin and reduces its firing temperature.In addition,alumina and silica serve as glazing ingredients.Only the careful balance between the ingre- dients produces porcelain that is white,dense,completely vitri- fied when fired above 1260C,and translucent when thin.Inter- estingly enough,kaolin was used in Bottger's time to powder wigs,and it is said that this inspired him to experiment with kaolin. Bottger probably utilized for his exploratory experiments the solar furnace developed by von Tschirnhaus,who reported on it in 1699.In this device the sunlight was focused with a large, 1-m-diameter lens which allowed it to reach at least 1436C,that is,the melting temperature of a sand-lime mixture.Such a high temperature could not be achieved at that time in Europe with conventional means.Later endeavors by Bottger,however,to build a horizontal high-temperature kiln allowed mass produc- tion of porcelain.During the firing,the feldspar vitrifies while the clay ensures that the vessel maintains its shape.In other words,the body and glaze of most hard porcelains can be fired in one operation since the fusion temperature of both compo- nents is roughly the same.This one-step firing process is,how- ever,not always performed,in particular if colored decorations need to be added. Early Western attempts to imitate porcelain included milk glass (a mixture of glass and tin oxide),soft porcelain (a mix- ture of clay and ground glass)manufactured particularly in Italy and Egypt,and an English version of soft porcelain in which bone ash(from cattle)was added to ground glass and clay.Fol- lowing this practice,the British also added in later years some bone ash to the true,hard porcelain which renders an ivory-white color.Bone china is somewhat easier to manufacture and bet- ter resists chipping.A different type of translucent ware was made in Persia during the seventeenth century and was called Gombroon.In Italy,under the patronage of the Grand Duke Francesco I de Medici,a hard,white,translucent ware was pro- duced between 1575 and 1587 which,however,because of its high content in alkali and alkali earth (total 13%)and alumina (9.5%),liquefied rapidly when the temperature was raised.Prob-

The secret of porcelain was found in a combination of raw ma￾terials, namely, in pure, white, kaolin clay (see Section 15.4) which was mixed with quartz and feldspatic rock. Pure kaolin, having a melting point of 1260°C, is, however, too difficult to shape due to its poor ductility. Further, its high refractory prop￾erty does not allow it to be fired to a hard and dense body at ac￾ceptable temperatures. The combination with other ingredients, such as “ball clay,” increases the plasticity of kaolin and reduces its firing temperature. In addition, alumina and silica serve as glazing ingredients. Only the careful balance between the ingre￾dients produces porcelain that is white, dense, completely vitri￾fied when fired above 1260°C, and translucent when thin. Inter￾estingly enough, kaolin was used in Böttger’s time to powder wigs, and it is said that this inspired him to experiment with kaolin. Böttger probably utilized for his exploratory experiments the solar furnace developed by von Tschirnhaus, who reported on it in 1699. In this device the sunlight was focused with a large, 1-m-diameter lens which allowed it to reach at least 1436°C, that is, the melting temperature of a sand–lime mixture. Such a high temperature could not be achieved at that time in Europe with conventional means. Later endeavors by Böttger, however, to build a horizontal high-temperature kiln allowed mass produc￾tion of porcelain. During the firing, the feldspar vitrifies while the clay ensures that the vessel maintains its shape. In other words, the body and glaze of most hard porcelains can be fired in one operation since the fusion temperature of both compo￾nents is roughly the same. This one-step firing process is, how￾ever, not always performed, in particular if colored decorations need to be added. Early Western attempts to imitate porcelain included milk glass (a mixture of glass and tin oxide), soft porcelain (a mix￾ture of clay and ground glass) manufactured particularly in Italy and Egypt, and an English version of soft porcelain in which bone ash (from cattle) was added to ground glass and clay. Fol￾lowing this practice, the British also added in later years some bone ash to the true, hard porcelain which renders an ivory-white color. Bone china is somewhat easier to manufacture and bet￾ter resists chipping. A different type of translucent ware was made in Persia during the seventeenth century and was called Gombroon. In Italy, under the patronage of the Grand Duke Francesco I de Medici, a hard, white, translucent ware was pro￾duced between 1575 and 1587 which, however, because of its high content in alkali and alkali earth (total 13%) and alumina (9.5%), liquefied rapidly when the temperature was raised. Prob- 15.2 • Types of Pottery 293

294 15·No Ceramics Age? ably all specimens of this production sustained some distortion during firing which led eventually to the abandonment of this technique.Likewise,French soft-paste porcelain (with high lime and low clay content)were difficult to form and fire. For the interested reader some recipes for porcelain and glazes are listed below.The quantities are given in mass percent. White porcelain (for casting bodies) Kaolin 46% Silica 34.2% Potassium feldspar 19.8% Add Sodium Carbonate 0.4% Firing temperature 1285-1325°C Shrinkage 11.5% White/light-gray porcelain(for throwing bodies) Kaolin 40% Silica 25% Potassium feldspar 25% Ball clay (Kentucky #4) 7% Bentonite (volcanic ash clay) 3% Firing temperature 1260-1325°C Shrinkage 15% Clear semi-mat glaze Feldspar 58% Silica 12.5% Whiting (natural CaCo3) 12.5% Kaolin 11% Zinc oxide 6% Add:C.M.C.2 1 tsp. Firing temperature: 1170-1250°C 15.3.Shaping and Decoration of Pottery One of the earliest methods for shaping clay included pressing the clay into a basket which was eventually consumed by the fire. Other techniques utilized paddling clay over the exterior of a mold pot to form the base,whereby the upper portion was formed with a series of coils laid layer upon layer.Also,hand-modelling was frequently practiced.The potter's wheel is,in contrast,a rel- Adapted from J.Chappell,Clay and Glazes (see Suggestions for Further Study). 2Carboxymethyl cellulose (acts as thickener and binder)

ably all specimens of this production sustained some distortion during firing which led eventually to the abandonment of this technique. Likewise, French soft-paste porcelain (with high lime and low clay content) were difficult to form and fire. For the interested reader some recipes for porcelain and glazes are listed below.1 The quantities are given in mass percent. White porcelain (for casting bodies) Kaolin 46% Silica 34.2% Potassium feldspar 19.8% Add Sodium Carbonate 0.4% Firing temperature 1285–1325°C Shrinkage 11.5% White/light-gray porcelain (for throwing bodies) Kaolin 40% Silica 25% Potassium feldspar 25% Ball clay (Kentucky #4) 7% Bentonite (volcanic ash clay) 3% Firing temperature 1260–1325°C Shrinkage 15% Clear semi-mat glaze Feldspar 58% Silica 12.5% Whiting (natural CaCo3) 12.5% Kaolin 11% Zinc oxide 6% Add: C.M.C.2 1 tsp. Firing temperature: 1170–1250°C One of the earliest methods for shaping clay included pressing the clay into a basket which was eventually consumed by the fire. Other techniques utilized paddling clay over the exterior of a mold pot to form the base, whereby the upper portion was formed with a series of coils laid layer upon layer. Also, hand-modelling was frequently practiced. The potter’s wheel is, in contrast, a rel- 294 15 • No Ceramics Age? 1Adapted from J. Chappell, Clay and Glazes (see Suggestions for Further Study). 2Carboxymethyl cellulose (acts as thickener and binder). 15.3 • Shaping and Decoration of Pottery

15.3.Shaping and Decoration of Pottery 295 atively late invention.It appeared in the Near East around 3500 B.C.and in China between 2600 and 1700 B.C.Some areas of the world,such as the Western Hemisphere,never used the potters wheel until contact with European settlers was made. A number of decorations have been applied even to the earli- est pottery.Among them are impressing or stamping the clay be- fore firing with fingernails,pointed sticks,or ropes (Japanese Jomon ware of the second and first millennium B.c.)or rolling a cylinder with a design over the clay body,thus producing relief ornaments(Etruscans,first millennium B.c.).Washing or paint- ing the pottery with semi-liquid clay,called slip (with or without coloring metal oxides),has been quite popular over many mil- lennia;see Plate 15.1.White slip,when covered with a transpar- ent glaze,looks quite similar to tin glazing.Ancient Egyptians, for example,painted animals and scenic motifs with slip on red potteries.Metal oxides have often been added to glazes or slip for color.Specifically,tin oxide provides a white color,cobalt oxide and cupric oxide yield various bluish hues,and cuprous oxide,a series of greens.The colors obtained from ferric iron vary from pale yellow via orange-red to black.Manganese gives colors rang- ing from bright red to purple and antimony yields yellow.To pre- vent intermingling of the different hues,patterns are outlined with clay threads,thus exercising a cloisonne technique.The colors can be applied either under the glaze or over the glaze.When the decoration is painted on a white tin glaze,a third firing,utiliz- ing,for example,a transparent lead glaze,needs to be applied. Many more decoration techniques are (and have been)used including luster decoration(invented by early Islamic potters in- volving a colloidal suspension of gold,silver,or platinum to the glazed object,requiring an additional,gentle firing).Early pot- tery,dating back as far as 6500 B.c.,was polished or burnished after firing by rubbing with a soft,smooth stone (Turkey,6500 B.C.,Incas A.D.500,North American Indians A.D.1000 Plate 15.10)or varnished (Fiji islands).The wealth of art work evolv- ing from pottery over 9000 years is just overwhelming and can- not be done justice in a few paragraphs as presented here.The interested reader is referred to the art books listed at the end of this chapter. It might be of interest to know how the age of ancient pottery can be determined.Certainly,the common carbon fourteen method which requires organic material and which measures the radioactive decay of C-14,cannot be applied for inorganic ma- terials such as clay.However,in some cases pottery has been added to human burial sites which allows an estimate of the age of ceramics by knowing the age of the bones.In other cases where

atively late invention. It appeared in the Near East around 3500 B.C. and in China between 2600 and 1700 B.C. Some areas of the world, such as the Western Hemisphere, never used the potters wheel until contact with European settlers was made. A number of decorations have been applied even to the earli￾est pottery. Among them are impressing or stamping the clay be￾fore firing with fingernails, pointed sticks, or ropes (Japanese Jomon ware of the second and first millennium B.C.) or rolling a cylinder with a design over the clay body, thus producing relief ornaments (Etruscans, first millennium B.C.). Washing or paint￾ing the pottery with semi-liquid clay, called slip (with or without coloring metal oxides), has been quite popular over many mil￾lennia; see Plate 15.1. White slip, when covered with a transpar￾ent glaze, looks quite similar to tin glazing. Ancient Egyptians, for example, painted animals and scenic motifs with slip on red potteries. Metal oxides have often been added to glazes or slip for color. Specifically, tin oxide provides a white color, cobalt oxide and cupric oxide yield various bluish hues, and cuprous oxide, a series of greens. The colors obtained from ferric iron vary from pale yellow via orange-red to black. Manganese gives colors rang￾ing from bright red to purple and antimony yields yellow. To pre￾vent intermingling of the different hues, patterns are outlined with clay threads, thus exercising a cloisonné technique. The colors can be applied either under the glaze or over the glaze. When the decoration is painted on a white tin glaze, a third firing, utiliz￾ing, for example, a transparent lead glaze, needs to be applied. Many more decoration techniques are (and have been) used including luster decoration (invented by early Islamic potters in￾volving a colloidal suspension of gold, silver, or platinum to the glazed object, requiring an additional, gentle firing). Early pot￾tery, dating back as far as 6500 B.C., was polished or burnished after firing by rubbing with a soft, smooth stone (Turkey, 6500 B.C., Incas A.D. 500, North American Indians A.D. 1000 Plate 15.10) or varnished (Fiji islands). The wealth of art work evolv￾ing from pottery over 9000 years is just overwhelming and can￾not be done justice in a few paragraphs as presented here. The interested reader is referred to the art books listed at the end of this chapter. It might be of interest to know how the age of ancient pottery can be determined. Certainly, the common carbon fourteen method which requires organic material and which measures the radioactive decay of C-14, cannot be applied for inorganic ma￾terials such as clay. However, in some cases pottery has been added to human burial sites which allows an estimate of the age of ceramics by knowing the age of the bones. In other cases where 15.3 • Shaping and Decoration of Pottery 295

296 15·No Ceramics Age? such a comparison is not possible,the thermoluminescence tech- nique is utilized.This method makes use of the fact that many clays and soils contain minute amounts of radioactive elements such as uranium,thorium,or potassium.Their emitted a-,B-,or y-radiations excite under certain circumstances some electrons of the clay into higher energy states where they might be trapped in impurity states(see Chapter 13).If thermal energy is supplied to this substance,the electrons may be forced to leave their metastable positions and revert to a lower energy state by con- comitantly emitting light.In other words,thermoluminescence unlocks the stored energy that has been radiation-induced over time by slightly heating the object under investigation.For this it is essential that the clay has been fired at some time in order that "the time clock is reset to zero."The age is then assigned to an object by measuring the amount of emitted light and by know- ing its and the surrounding soil's radioactive content as well as by taking into consideration how susceptible the material is to radiation damage.Another dating technique that is,however,still in its developmental state involves electron spin resonance. At this point the reader probably wants to know about the chemistry and physical properties of ceramics.Specifically,why is clay pliable and what is the composition of clay?This will be explained in the next section. 15.4.The Science Behind Pottery The principal purpose of this section is to provide some under- standing of why clay is such a remarkable material which is duc- tile when wet and hard after firing.First of all,"clay"is not just one substance but a whole family of minerals whose common characteristic is that they have a sheetlike crystal structure,as we shall see momentarily,which allows the platelets (that are <1 um in diameter)to slide easily past one another even when only little force is applied.Further,clays are so-called hydrous alu- minum or magnesium silicates,which are distinguished by the property that they lose physically adsorbed or structural water when heated.Clays have formed as a result of marine sediments or from hydrothermal activities during all ages.Clays vary in composition and additional constituents depending on the envi- ronment in which they formed and the hydrological or climatic conditions.They are found in mudstones,shales,and soils al- most everywhere on the earth. As was just indicated,clays are composed of silica(SiO2),alu- mina (Al203),possibly magnesia (Mgo),and water,along with impurities of iron,alkalies,or alkaline earths.They are some-

such a comparison is not possible, the thermoluminescence tech￾nique is utilized. This method makes use of the fact that many clays and soils contain minute amounts of radioactive elements such as uranium, thorium, or potassium. Their emitted -, -, or -radiations excite under certain circumstances some electrons of the clay into higher energy states where they might be trapped in impurity states (see Chapter 13). If thermal energy is supplied to this substance, the electrons may be forced to leave their metastable positions and revert to a lower energy state by con￾comitantly emitting light. In other words, thermoluminescence unlocks the stored energy that has been radiation-induced over time by slightly heating the object under investigation. For this it is essential that the clay has been fired at some time in order that “the time clock is reset to zero.” The age is then assigned to an object by measuring the amount of emitted light and by know￾ing its and the surrounding soil’s radioactive content as well as by taking into consideration how susceptible the material is to radiation damage. Another dating technique that is, however, still in its developmental state involves electron spin resonance. At this point the reader probably wants to know about the chemistry and physical properties of ceramics. Specifically, why is clay pliable and what is the composition of clay? This will be explained in the next section. The principal purpose of this section is to provide some under￾standing of why clay is such a remarkable material which is duc￾tile when wet and hard after firing. First of all, “clay” is not just one substance but a whole family of minerals whose common characteristic is that they have a sheetlike crystal structure, as we shall see momentarily, which allows the platelets (that are 1 m in diameter) to slide easily past one another even when only little force is applied. Further, clays are so-called hydrous alu￾minum or magnesium silicates, which are distinguished by the property that they lose physically adsorbed or structural water when heated. Clays have formed as a result of marine sediments or from hydrothermal activities during all ages. Clays vary in composition and additional constituents depending on the envi￾ronment in which they formed and the hydrological or climatic conditions. They are found in mudstones, shales, and soils al￾most everywhere on the earth. As was just indicated, clays are composed of silica (SiO2), alu￾mina (Al2O3), possibly magnesia (MgO), and water, along with impurities of iron, alkalies, or alkaline earths. They are some- 296 15 • No Ceramics Age? 15.4 • The Science Behind Pottery