April 1998 Perspectives on the History of Glass Composition s why and how we arrived at this position. Because of the priate colorants, such as copper, manganese, and iron salts many glasses that have been studied, we are forced to limit our There was no demonstrated interest in transparency at this omments to compositions that illustrate major discoveries or time. Beads also were made, and, later, small vessels were es;accordingly, some important glass compositions are constructed by coating sand cores with a glassy skin-the cores not discussed were removed after forming Besides the apparent limitations imposed by practical con The earlier and sometimes parallel development of cerami siderations and economics, physics appears to impose a real and metallurgical processes undoubtedly influenced and aided imitation to the variation of properties available in materials the growth of early glass technology, some furnac that lack a periodic crystalline lattice Inorganic glasses are ments and raw materials were applicable to glassmak extensive Egyptia making als less colorants are added; chemically durable and, therefore, had an effect, contributing knowledge of raw mate hemically inert; and brittle sintering techniques These general properties, however, are constrained only in rs ago, the blowing of glass articles with a those glasses that are normally thought of when we think of pipe was invented, probably in Syria, and this advance in tech- lasses. If we broaden our chemical viewpoint to include chal- nology was followed by a rapid increase in the use of glas cogenide, halide, and, especially, metallic glasses, a wide va ware. Glassblowing spread quickly through the roman empire riety of properties becomes available. Although the lack of a and soon glass bowls and drinking vessels were in use through crystalline lattice appears to impose a rather severe constraint out society, in both ordinary households and among the ruling with regard to some properties at the moment, in most cases we are not in a position to state unequivocally whether these this remarkable growth of blown glass production. Accord- constraints are absolute. For instance it had been thought to be ing sible to make bulk metallic glasses because their hard gly, strong efforts were directed toward the elimination of sphere structure results in rather simple dense packing, which glassware is, in itself, not conducive to extensive supercooling. However, this recently has been shown to be untrue. Also, it normally is (2) Raw-Material Preparation-The Search for considered that the lack of a crystalline lattice means that plas tic flow is not possible because the dislocations that result in Early glasses in the western world were almost all soda- stic flow cannot form. We have lime-silica compositions that varied depending upon the avail- this is completely true, or if it can be sensibly modified. ability of raw materials, but generally differed little from pres- There were few modern books in English on glass until the ent-day commercial glasses (Ta publication of George Morey's book, The Properties ofGlass, source of alkali were typical ingredients, with both the sand in 1938. After World War Il, and following the publication of nd the alkali me or magnesia to give the second edition of Morey in 1954, many other books ap- chemical durability adequate of the time. In peared. Recently, many edited bo the case of the sand, framer some have appeared regularly 6-12 The more general books usually and plant ash generally brou long with the present a short history of glass, a definition and description o soda and potash glass, and chapters that present properties and compositions of Two different sources of alkali affected the composition of simple glasses, where the chapters are arranged either by prod avated Na, CO3 )was usually the favored alkali, because it w erty or by glass composition. These books are important useful, especially as texts on the science and technology of avilla om norther Egypt(see, e.g., glas glass. There also have been many excellent review articles and Further east, in Mesopotamia and Persia, the alkali was usually book chapters that present special subjects. Examples are the provided by plant ash that contained more K,O(2%4%)and two excellent series by doremus and Tomozawa 2 and Uhl MgO(2%-6%)(see, e.g., glass 3 in Table 1). 21a The alkali mann and Kreidl. 3 In particular, in the Uhlmann and Kreidl content of the ash was influenced by the soil in which the plants ies, the Kreidl chapter on glass-forming systems is very rew. plants that grew in salty soil or near the sea were high in useful. It historically, scientifically, and technologically dis- soda, whereas those that grew inland had higher potash con usses almost every known glass-forming system. Here we tents.22 Agricola(1556)2 refers to the use of salts made from Attempt, by perhaps rather extreme simplification, to illustrate the ashes of salty herbs as well as to natron and"rock-salt. some of the issues having to do with property-composition When these were not available, he suggested the ashes of oak development. We present our simplified and personal view of could be used, or, as a last resort, the ashes of beech or pine some glass compositions-structures in order to make some The practice of using natron to produce higher-soda glasses simple generalizations. This hopefully leads to a better general continued in the Mediterranean region through early and me- understanding of what has been done, in many cases, empiri- dieval times. However, there was a surge in the use of potash cally, and hopefully leads to the possibility of predicting what in glassmaking during the 9th through 13th centuries, before remains possible. Such predictions were attempted at a meeting soda again became the predominant alkali. 24 to celebrate Kreidl's 80th birthday 14 Much glass made in the Middle Ages was dark green, dark The sections that follow immediately have to do with the brown, or almost black as a result of the impurities present early history of glass. The reader is directed to the papers of This"waldglas, or forest glass, often was used for bottles and Cables-l7 and symposia arranged by Kingery, 9 for other drinking vessels, but interest grew in preparing clearer, more- interesting insights into this history transparent glass. Although little is known about glass technol- ogy in the middle ages, we do know that some attention was IL. Early Glasses given to the purification of raw materials. One of the major sources of glass technology information in this period comes from L'Arte Vetraria,s written by Antonio Neri, an Italian ( Middle Eastern Origins and Roman Growth priest and glassworker, in 1612, and translated to English in The earliest known synthetic glasses were created in Asia 1662 by Christopher Merrett, an English physician and one of Minor several millennia ago. Some isolated examples may be the founders of the Royal Society. (It also was translated by as early as 7000 BC, but it is clear that, by 2500 BC, there were Johann Kunckel in 1679; both Merrett and Kunckel added lany sources, probably first in Mesopotamia, then in Egypt valuable personal observations on glassmaking Agricola and The first glassmakers were motivated to create decorative ob- Neri devoted considerable space to raw-material preparation, jects, possibly to simulate gems and semiprecious stones, using discussing the careful selection of crystals(quartz) and clean sintered bodies of silica and desert soda(natron)with appro- white stones free of black or yellow veins''to be used inas why and how we arrived at this position. Because of the many glasses that have been studied, we are forced to limit our comments to compositions that illustrate major discoveries or changes; accordingly, some important glass compositions are not discussed. Besides the apparent limitations imposed by practical con￾siderations and economics, physics appears to impose a real limitation to the variation of properties available in materials that lack a periodic crystalline lattice. Inorganic glasses are generally considered to be isotropic; dielectric; transparent, un￾less colorants are added; chemically durable and, therefore, chemically inert; and brittle. These general properties, however, are constrained only in those glasses that are normally thought of when we think of glasses. If we broaden our chemical viewpoint to include chal￾cogenide, halide, and, especially, metallic glasses, a wide va￾riety of properties becomes available. Although the lack of a crystalline lattice appears to impose a rather severe constraint with regard to some properties at the moment, in most cases, we are not in a position to state unequivocally whether these constraints are absolute. For instance, it had been thought to be impossible to make bulk metallic glasses because their hard sphere structure results in rather simple dense packing, which is, in itself, not conducive to extensive supercooling. However, this recently has been shown to be untrue. Also, it normally is considered that the lack of a crystalline lattice means that plas￾tic flow is not possible because the dislocations that result in plastic flow cannot form. We have yet to determine whether this is completely true, or if it can be sensibly modified. There were few modern books in English on glass until the publication of George Morey’s book, The Properties of Glass,5 in 1938. After World War II, and following the publication of the second edition of Morey in 1954, many other books ap￾peared. Recently, many edited books and edited proceedings have appeared regularly.6–12 The more general books usually present a short history of glass, a definition and description of glass, and chapters that present properties and compositions of simple glasses, where the chapters are arranged either by prop￾erty or by glass composition. These books are important and useful, especially as texts on the science and technology of glass. There also have been many excellent review articles and book chapters that present special subjects. Examples are the two excellent series by Doremus and Tomozawa12 and Uhl￾mann and Kreidl.13 In particular, in the Uhlmann and Kreidl series, the Kreidl chapter on glass-forming systems is very useful. It historically, scientifically, and technologically dis￾cusses almost every known glass-forming system. Here we attempt, by perhaps rather extreme simplification, to illustrate some of the issues having to do with property–composition development. We present our simplified and personal view of some glass compositions–structures in order to make some simple generalizations. This hopefully leads to a better general understanding of what has been done, in many cases, empiri￾cally, and hopefully leads to the possibility of predicting what remains possible. Such predictions were attempted at a meeting to celebrate Kreidl’s 80th birthday.14 The sections that follow immediately have to do with the early history of glass. The reader is directed to the papers of Cable15–17 and symposia arranged by Kingery18,19 for other interesting insights into this history. II. Early Glasses (1) Middle Eastern Origins and Roman Growth The earliest known synthetic glasses were created in Asia Minor several millennia ago. Some isolated examples may be as early as 7000 BC, but it is clear that, by 2500 BC, there were many sources, probably first in Mesopotamia, then in Egypt. The first glassmakers were motivated to create decorative ob￾jects, possibly to simulate gems and semiprecious stones, using sintered bodies of silica and desert soda (natron) with appro￾priate colorants, such as copper, manganese, and iron salts. There was no demonstrated interest in transparency at this time. Beads also were made, and, later, small vessels were constructed by coating sand cores with a glassy skin—the cores were removed after forming. The earlier and sometimes parallel development of ceramic and metallurgical processes undoubtedly influenced and aided the growth of early glass technology; some furnace improve￾ments and raw materials were applicable to glassmaking. The extensive Egyptian tradition of faience making also must have had an effect, contributing knowledge of raw materials and sintering techniques. About 2000 years ago, the blowing of glass articles with a pipe was invented, probably in Syria, and this advance in tech￾nology was followed by a rapid increase in the use of glass￾ware. Glassblowing spread quickly through the Roman Empire, and soon glass bowls and drinking vessels were in use through￾out society, in both ordinary households and among the ruling classes. A desire for clear and transparent vessels came with this remarkable growth of blown glass production. Accord￾ingly, strong efforts were directed toward the elimination of iron and other contaminants, particularly for the higher-quality glassware.5 (2) Raw-Material Preparation—The Search for Transparency Early glasses in the western world were almost all soda– lime–silica compositions that varied depending upon the avail￾ability of raw materials, but generally differed little from pres￾ent-day commercial glasses (Table I). Beach sand and a crude source of alkali were typical ingredients, with both the sand and the alkali containing enough lime or magnesia to give chemical durability adequate for the purposes of the time. In the case of the sand, fragments of shells provided some lime, and plant ash generally brought some magnesia along with the soda and potash. Two different sources of alkali affected the composition of early glasses. On the Eastern Mediterranean littoral natron (hy￾drated Na2CO3) was usually the favored alkali, because it was available from northern Egypt (see, e.g., glass 2 in Table I).20 Further east, in Mesopotamia and Persia, the alkali was usually provided by plant ash that contained more K2O (2%–4%) and MgO (2%–6%) (see, e.g., glass 3 in Table I).21a The alkali content of the ash was influenced by the soil in which the plants grew: plants that grew in salty soil or near the sea were high in soda, whereas those that grew inland had higher potash con￾tents.22 Agricola (1556)23 refers to the use of salts made from the ashes of salty herbs as well as to natron and ‘‘rock-salt.’’ When these were not available, he suggested the ashes of oak could be used, or, as a last resort, the ashes of beech or pine. The practice of using natron to produce higher-soda glasses continued in the Mediterranean region through early and me￾dieval times. However, there was a surge in the use of potash in glassmaking during the 9th through 13th centuries, before soda again became the predominant alkali.24 Much glass made in the Middle Ages was dark green, dark brown, or almost black as a result of the impurities present. This ‘‘waldglas,’’ or forest glass, often was used for bottles and drinking vessels, but interest grew in preparing clearer, more￾transparent glass. Although little is known about glass technol￾ogy in the middle ages, we do know that some attention was given to the purification of raw materials. One of the major sources of glass technology information in this period comes from L’Arte Vetraria,25 written by Antonio Neri, an Italian priest and glassworker, in 1612, and translated to English in 1662 by Christopher Merrett, an English physician and one of the founders of the Royal Society. (It also was translated by Johann Kunckel in 1679; both Merrett and Kunckel added valuable personal observations on glassmaking.) Agricola and Neri devoted considerable space to raw-material preparation, discussing the careful selection of crystals (quartz) and clean ‘‘white stones free of black or yellow veins’’ to be used in April 1998 Perspectives on the History of Glass Composition 797