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 ceramic products whose names are derived from the Italian city of Faenza and the city of Delft in Holland. Faïence is generally distinguished by its elaborate decorations which flourished particularly 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 stemming typically from sedimentary deposits that are low in iron content. Stoneware is hard and opaque and sometimes translucent. 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., respectively. 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 Englishman, 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 imitate) 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 materials, 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 property does not allow it to be fired to a hard and dense body at acceptable 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 ingredients produces porcelain that is white, dense, completely vitrified when fired above 1260°C, and translucent when thin. Interestingly 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 production 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 components is roughly the same. This one-step firing process is, however, 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 mixture 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. Following 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 better 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 produced 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 earliest pottery. Among them are impressing or stamping the clay before 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 painting the pottery with semi-liquid clay, called slip (with or without coloring metal oxides), has been quite popular over many millennia; see Plate 15.1. White slip, when covered with a transparent 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 ranging from bright red to purple and antimony yields yellow. To prevent 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, utilizing, 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 involving a colloidal suspension of gold, silver, or platinum to the glazed object, requiring an additional, gentle firing). Early pottery, 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 evolving from pottery over 9000 years is just overwhelming and cannot 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 materials 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 technique 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 concomitantly 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 knowing 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 understanding of why clay is such a remarkable material which is ductile 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 aluminum 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 environment in which they formed and the hydrological or climatic conditions. They are found in mudstones, shales, and soils almost everywhere on the earth. As was just indicated, clays are composed of silica (SiO2), alumina (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