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