《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_glass fiber-17 Perspectives on the History of Glass Composition

ournal J Am Ceram Soc. s -813(1998) Perspectives on the History of Glass Composition Charles r. Kurkiian Bell Communications Research(Bellcore), Morristown, New Jersey 07960 William R. Prindle,f Santa Barbara, California 93105 The 100th anniversary of The American Ceramic s (see Fig. 1).Up to this time, very little real glass science had corresponds approximately with the 100th anniversary of been done, although, with the limited tools at their disposal what might be considered the start of the age of glass sci- earlier workers did quite remarkable things. Most work was nce, i.e the publication, in germany, in 1886, of the cata- done in an attempt to understand what soda-lime-silica glasse. og of Schott und Genossen, containing 44 optical glass were and to improve their quality. Schott- conducted detailee compositions. The American Ceramic Society centennial studies of the effects of various additions and substitutions to seems, accordingly, to be an appropriate occasion to exam- the basic soda-lime glass composition. He and winkelmann, ine the history of glass composition that both preceded and were the first to attempt to model glass behavior development followed the seminal work of Schott and to survey some of by means of a set of factors with which properties could be the major discoveries and changes in glass composition as ted to odest task ust described. The history of glass structure theories is it has continued to be so with only slight changes. Variations in considered, particularly with regard to the effects of com- roduction techniques and specific use requi osition on structure, and how these relate to glass pro to the deliberate addition of a variety of other oxides, so that erties. The article then continues with a discussion of recent most commercial"soda-lime"glasses now contain six or special glasses and concludes with a description of light more constituents. Over the years, the bulk of commercial guide glasses, the discovery of which has changed the na lasses for most purposes has continued to be based on silica as ture of glass science and the glass industry the primary glass former Research(by X-ray technologies, optical spectroscopy L. Introduction physical property measurements, etc. )during the 20th century wl corresponds approximately with the 100th anniversary of stand their structure and properties well enough to predip.o has been conducted on simple glass compositions to attempt to E 100th anniversary of The American Ceramic Society understand glasses as materials as well as to attempt to t might be considered the start of the age of glass science erties f the disclosure. in 1886. of the work of otto schott and ernst requirements. Although glasses with rather remarkable prope Abbe in Germany. This was the publication of the catalog of ties ranging from infrared transmission and superionic conduc- the"Glastechnisches Laboratorium Schott und genossen tivity to biological activity have been discovered, it is probably not entirely accurate to say that we can design a glass for a iven purpose. Available commercial silicate glasses do their job quite admirably, but they are rather complicated glasses H. A. Anderson--contributing editor that fulfill rather simple tasks. In 1970, the di a simple(titania-silica-silica compound glass fiber could con duct light over rather long distances without requiring ampli- fication has resulted in a"new glass industry"-the"light script No. 190479. Received December 30, 1997; approved February 16, guide industry. Since then, in somewhat of a turnabout, ber, American Ceramic Society scientists have discovered that these simple glasses display a ed from Corning Incorporated, Corning, NY wide range of unexpected, new, complicated, and often incom centennialfeature

Perspectives on the History of Glass Composition Charles R. Kurkjian* Bell Communications Research (Bellcore), Morristown, New Jersey 07960 William R. Prindle*,† Santa Barbara, California 93105 The 100th anniversary of The American Ceramic Society corresponds approximately with the 100th anniversary of what might be considered the start of the age of glass sci￾ence, i.e., the publication, in Germany, in 1886, of the cata￾log of Schott und Genossen, containing 44 optical glass compositions. The American Ceramic Society centennial seems, accordingly, to be an appropriate occasion to exam￾ine the history of glass composition that both preceded and followed the seminal work of Schott and to survey some of the major discoveries and changes in glass composition as well as the reasons that led to them. Although it is certainly of interest to consider a more complete history of the glass industry, we have opted to attempt the more modest task just described. The history of glass structure theories is considered, particularly with regard to the effects of com￾position on structure, and how these relate to glass prop￾erties. The article then continues with a discussion of recent special glasses and concludes with a description of light￾guide glasses, the discovery of which has changed the na￾ture of glass science and the glass industry. I. Introduction THE 100th anniversary of The American Ceramic Society corresponds approximately with the 100th anniversary of what might be considered the start of the age of glass science— the disclosure, in 1886, of the work of Otto Schott and Ernst Abbe in Germany. This was the publication of the catalog of the ‘‘Glastechnisches Laboratorium, Schott und Genossen’’ (see Fig. 1).1 Up to this time, very little real glass science had been done, although, with the limited tools at their disposal, earlier workers did quite remarkable things. Most work was done in an attempt to understand what soda–lime–silica glasses were and to improve their quality. Schott2 conducted detailed studies of the effects of various additions and substitutions to the basic soda–lime glass composition. He and Winkelmann3,4 were the first to attempt to model glass behavior development by means of a set of factors with which properties could be calculated. As a result of the coincidental natural occurrence of alkali, alkaline-earth ‘‘impurities,’’ and sand, soda–lime–silica glass became the ‘‘staple’’ glass composition very early in time, and it has continued to be so with only slight changes. Variations in production techniques and specific use requirements have led to the deliberate addition of a variety of other oxides, so that most commercial ‘‘soda–lime’’ glasses now contain six or more constituents. Over the years, the bulk of commercial glasses for most purposes has continued to be based on silica as the primary glass former. Research (by X-ray technologies, optical spectroscopy, physical property measurements, etc.) during the 20th century has been conducted on simple glass compositions to attempt to understand glasses as materials as well as to attempt to under￾stand their structure and properties well enough to predict prop￾erties from composition and to design a glass from a list of requirements. Although glasses with rather remarkable proper￾ties ranging from infrared transmission and superionic conduc￾tivity to biological activity have been discovered, it is probably not entirely accurate to say that we can design a glass for a given purpose. Available commercial silicate glasses do their job quite admirably, but they are rather complicated glasses that fulfill rather simple tasks. In 1970, the discovery that a simple (titania–silica)–silica compound glass fiber could con￾duct light over rather long distances without requiring ampli￾fication has resulted in a ‘‘new glass industry’’—the ‘‘light￾guide industry.’’ Since then, in somewhat of a turnabout, scientists have discovered that these simple glasses display a wide range of unexpected, new, complicated, and often incom￾H. A. Anderson—contributing editor Manuscript No. 190479. Received December 30, 1997; approved February 16, 1998. *Member, American Ceramic Society. † Retired from Corning Incorporated, Corning, NY. J. Am. Ceram. Soc., 81 [4] 795–813 (1998) Journal centennialfeature 795

Journal of the American Ceramic SocietyKurkjian and Prindle Vol 8I. No 4 Glasschmelzerei for optische und andere wisseaschaftliche Zwocke ATDwr 13解00t78 Glastechnisches Laboratorium 6.1 0o4|,g Schott d Gen JENA 8.7Borwf-Ftind 180b443 D a IM Ielts saliad-Flat a e. sat- Flat it watis J111s 1404s m o, 1sa Lee samt-Flix 810 hitra Wml.plant.,19p自 Fig 1. Photograph of Otto Schott and pages from glass catalog pletely explained properties. We attempt in this review to Provide a brief review of the history of glass structure and lustrate some of the interesting events in this long property relations In this article we Bring the history up-to-date by discussing some new speci Provide a brief review of the early history of glass: glasses and the new era of optical fibers. Review the work of Abbe and Schott, i. e the start of glass Our purpose here is to provide an overview of the very large science, field of inorganic glasses for the benefit, perhaps, of a re- Review the development of more-modern glass compo searcher new to glass. We attempt to provide a sense of the

pletely explained properties. We attempt in this review to il￾lustrate some of the interesting events in this long history. In this article we ● Provide a brief review of the early history of glass; ● Review the work of Abbe and Schott, i.e., the start of glass science; ● Review the development of more-modern glass composi￾tions; ● Provide a brief review of the history of glass structure and property relations; ● Bring the history up-to-date by discussing some new special glasses and the new era of optical fibers. Our purpose here is to provide an overview of the very large field of inorganic glasses for the benefit, perhaps, of a re￾searcher new to glass. We attempt to provide a sense of the present and future of glasses–properties–understanding as well Fig. 1. Photograph of Otto Schott and pages from glass catalog. 796 Journal of the American Ceramic Society—Kurkjian and Prindle Vol. 81, No. 4

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 in

as 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

Journal of the American Ceramic SocietyKurkjian and Prindle Vol 81. No 4 Table 1. Glass Compositions Oxide content(wt%) Glasst SiO, BO3 Na,O ,O Cao Mgo AlO Fe2O3 (1) Egypt, 1500 BC 678 3.8 3.22 (2)Palestine, 4th Centur ()Sudan, 3rd century (4)Italy, 9th-10th centurie 77.8 8.7 0.7 ()Container glass, 1980 (6)1: 1: 6 soda-lime-silica 15 (9)Schott thermometer glass 12.0 II msil glass I 1)Schott Welsbach chimney 47000602 0.3 1.8 7.0 3.3 12)Nonex' 0.4 (14)E-glass, typical 54.0 17.5 high sodium); (3)Brill, calculate materials.Glass contains other oxides:(I (x708902:1m place of sand if high clarity was desired. The stones were light by small gold or copper crystals(-50 nm in diameter) that reduced to fine particles by pounding in a mortar, and the silica are formed by the precipitation of the metals in their atomic owder then often was fritted with the alkali salts. Neri gave state. The formation of the metal crystals is enhanced by re- onsiderable attention to alkali preparation, discussing in some heating the glass ("striking")and by the presence of reducing detail the washing of various plant ashes to prepare alkali salts agents, e.g., stannous chloride. The red glasses found in old for clear crystal glass. The purification process consisted of church windows are most likely copper reds, either coppe repeated sieving of the raw salt, dissolving it in boiling water, rubies, suspensions of cuprous oxide, or copper stains, because filtering, and evaporating. Thus the impurities causing color, gold rubies do not seem to have been made with any certainty such as iron compounds, were left behind Unfortunately, much of the alkaline-earth and alumina of the Opaque glasses colored by suspensions of relatively large shes were left behind as well; therefore, many clear glasses crystals(with diameters in the micrometer range), where the prepared from the purified raw materials had relatively poor crystals behave essentially as color pigments, have been known resistance to attack by moisture. The much-admired clear since antiquity. The pigments are generally insoluble or of cristallo"glass ed in Venice-Murano in the early limited solubility in the matrix glass. Some of the opaque 1500s suffered from low lime and magnesia content. As a ors formed in this way are white glasses containing suspensions of tin oxide, arsenic pentoxide, or calcium antimonate, and lass of that period now in museums have developed surface yellow glasses colored by lead antimonate. Opaque blue crizzling(a multitude of fine surface fractures) because of their glasses colored by copper calcium silicate or cobalt alumi- poor chemical durability; some extreme examples are sticky to nate, green glasses colored by chromic oxide, and brown or the touch and appear to sweat. 26 These cristallo glasses provide red-browns from iron or iron-manganese oxide mixtures also an example of an unintended consequence of the desire to re used optimize one glass property, colorless clarity in this case, caus- The most dramatic examples of colored glass are probably ng a deterioration in another property, durability the church windows of the middle with the greatest ( Colored Glasses created during the 10th through 14th centuries. Most of these windows also contain much stained glass, wherein a colorant is Although the preceding section described colorless glasses diffused into the glass surface at temperatures well below that purposely made free of unwanted color, other glasses were of molten glass. Copper reds and silver yellows are perhaps the colored purposely for decoration since the earliest days of best-known examples of surface stains glassmaking. Glassworkers in Egypt, the Middle East, and the Roman Empire knew that small amounts of certain salts could (4) Lead Glasses be incorporated in the melt to produce strongly colored glasses, The first major departure from alkali-lime-silica glasses probably the first example ue. This addition of colorants was came during the 17th century with the commercial introduction some transparent, some opac of the use of minor ingredients to of lead flint glasses. Lead had long been a minor constituent in change glass properties to produce a desired effect glazes, mosaics, and artificial gems. It was introduced as cal- The earliest and most widely used solution colorants were cined lead or lead oxide, primarily for its fluxing effect. Neri salts of copper(blue-green from the presence of Cu), iron discussed lead glasses at some length in L'Arte Vetraria and (blue to green from Fe2*, yellow to brown from Fe+), and emphasized that great care must be taken to thoroughly calcine manganese(amethyst or purple from Mn+).27 The use of small the lead to avoid the formation of molten lead because"the quantities of manganese as a decolorizer to compensate for iron least lead remaining breaks out the bottom of the pots and lets colors, also known in the Middle Ages, was referred to by all the metall run into the fire. "25 Agricola and Neri and was used by the Venetians in the pro- Shortly after the publication of Merrett's English translation duction of cristallo. Cobalt was first used in the 14th century of Neri's work in 1662, George Ravenscroft, an English glass BC (deep blue from Co). The use of chromium as a solution merchant, turned glassmaker to develop a clear glass based colorant probably began early in the 19th century English ingredients.29, 30 This latter requirement was motivated Copper and gold ruby glasses were prized highly for their by the difficulty English glassmakers were experiencing beauty and for their scarcity, the latter a result of the difficulty obtaining raw materials at acceptable cost, because a monopoly of producing these colloidal colors. Both glasses presented se- controlled the import of plant ashes for soda. 31 The glass mer rious challenges to the glassmaker because of their sensitivity chants also were struggling with unresponsive to composition, melting conditions, and subsequent thermal ers, much breakage in transit, and oppresduced history. The ruby color is caused by the selective absorption of series of experiments, Ravenscroft int

place of sand if high clarity was desired. The stones were reduced to fine particles by pounding in a mortar, and the silica powder then often was fritted with the alkali salts. Neri gave considerable attention to alkali preparation, discussing in some detail the washing of various plant ashes to prepare alkali salts for clear crystal glass. The purification process consisted of repeated sieving of the raw salt, dissolving it in boiling water, filtering, and evaporating. Thus the impurities causing color, such as iron compounds, were left behind. Unfortunately, much of the alkaline-earth and alumina of the ashes were left behind as well; therefore, many clear glasses prepared from the purified raw materials had relatively poor resistance to attack by moisture. The much-admired clear ‘‘cristallo’’ glass produced in Venice–Murano in the early 1500s suffered from low lime and magnesia content. As a result, many of the elegant examples of the elaborate Venetian glass of that period now in museums have developed surface crizzling (a multitude of fine surface fractures) because of their poor chemical durability; some extreme examples are sticky to the touch and appear to sweat.26 These cristallo glasses provide an example of an unintended consequence of the desire to optimize one glass property, colorless clarity in this case, caus￾ing a deterioration in another property, durability. (3) Colored Glasses Although the preceding section described colorless glasses purposely made free of unwanted color, other glasses were colored purposely for decoration since the earliest days of glassmaking. Glassworkers in Egypt, the Middle East, and the Roman Empire knew that small amounts of certain salts could be incorporated in the melt to produce strongly colored glasses, some transparent, some opaque. This addition of colorants was probably the first example of the use of minor ingredients to change glass properties to produce a desired effect. The earliest and most widely used solution colorants were salts of copper (blue-green from the presence of Cu2+), iron (blue to green from Fe2+, yellow to brown from Fe3+), and manganese (amethyst or purple from Mn3+).27 The use of small quantities of manganese as a decolorizer to compensate for iron colors, also known in the Middle Ages, was referred to by Agricola and Neri and was used by the Venetians in the pro￾duction of cristallo. Cobalt was first used in the 14th century BC (deep blue from Co2+). The use of chromium as a solution colorant probably began early in the 19th century.27 Copper and gold ruby glasses were prized highly for their beauty and for their scarcity, the latter a result of the difficulty of producing these colloidal colors. Both glasses presented se￾rious challenges to the glassmaker because of their sensitivity to composition, melting conditions, and subsequent thermal history. The ruby color is caused by the selective absorption of light by small gold or copper crystals (∼50 nm in diameter) that are formed by the precipitation of the metals in their atomic state. The formation of the metal crystals is enhanced by re￾heating the glass (‘‘striking’’) and by the presence of reducing agents, e.g., stannous chloride. The red glasses found in old church windows are most likely copper reds, either copper rubies, suspensions of cuprous oxide, or copper stains, because gold rubies do not seem to have been made with any certainty until the 17th century.25,28 Opaque glasses colored by suspensions of relatively large crystals (with diameters in the micrometer range), where the crystals behave essentially as color pigments, have been known since antiquity. The pigments are generally insoluble or of limited solubility in the matrix glass. Some of the opaque col￾ors formed in this way are white glasses containing suspensions of tin oxide, arsenic pentoxide, or calcium antimonate, and yellow glasses colored by lead antimonate. Opaque blue glasses colored by copper calcium silicate or cobalt alumi￾nate, green glasses colored by chromic oxide, and brown or red-browns from iron or iron-manganese oxide mixtures also are used. The most dramatic examples of colored glass are probably the church windows of the Middle Ages, with the greatest created during the 10th through 14th centuries. Most of these windows also contain much stained glass, wherein a colorant is diffused into the glass surface at temperatures well below that of molten glass. Copper reds and silver yellows are perhaps the best-known examples of surface stains. (4) Lead Glasses The first major departure from alkali–lime–silica glasses came during the 17th century with the commercial introduction of lead flint glasses. Lead had long been a minor constituent in glazes, mosaics, and artificial gems. It was introduced as cal￾cined lead or lead oxide, primarily for its fluxing effect. Neri discussed lead glasses at some length in L’Arte Vetraria and emphasized that great care must be taken to thoroughly calcine the lead to avoid the formation of molten lead because ‘‘the least lead remaining breaks out the bottom of the pots and lets all the metall run into the fire.’’25 Shortly after the publication of Merrett’s English translation of Neri’s work in 1662, George Ravenscroft, an English glass merchant, turned glassmaker to develop a clear glass based on English ingredients.29,30 This latter requirement was motivated by the difficulty English glassmakers were experiencing in obtaining raw materials at acceptable cost, because a monopoly controlled the import of plant ashes for soda.31 The glass mer￾chants also were struggling with unresponsive foreign suppli￾ers, much breakage in transit, and oppressive tariffs.32 After a series of experiments, Ravenscroft introduced a clear potash Table I. Glass Compositions Glass† Oxide content (wt%) SiO2 B2O3 Na2O K2O CaO MgO Al2O3 Fe2O3 (1) Egypt, 1500 BC‡ 67.8 16.08 2.08 3.8 2.89 3.22 0.92 (2) Palestine, 4th Century 70.5 15.7 0.8 8.7 0.6 2.7 0.4 (3) Sudan, 3rd century 64.2 15.9 2.65 10.2 2.73 2.06 2.3 (4) Italy, 9th–10th centuries 77.8 6.4 8.7 2.1 0.7 2.2 0.8 (5) Container glass, 1980 73.0 13.7 0.4 10.6 0.3 1.8 (6) 1:1:6 soda–lime–silica 75.3 13.0 11.7 (7) Faraday ‘‘heavy glass’’‡ 10.6 15.6 (8) ‘‘Jena Standard Glass’’‡ 67.2 2.0 14.0 7.0 2.5 (9) Schott thermometer glass 72.0 12.0 11.0 5.0 (10) Schott utensil glass 73.7 6.2 6.6 5.5 3.3 (11) Schott Welsbach chimney‡ 75.8 15.2 4.0 (12) Nonex‡ 73.0 16.5 4.25 (13) Pyrex 80.5 12.9 3.8 0.4 2.2 (14) E-glass, typical 54.0 10.0 17.5 4.5 14.0 † (1) Morey,5 Table I-1 (10); (2) Brill,20 Jalame glass, (low potassium, high sodium); (3) Brill,21a Sedeinga tomb glass, (high potassium, high magnesium); (4) Brill,21b Frattesina glass, (mixed alkali); (5) Ryder and Poole43; (6) by calculation; (7) Faraday;37 (8) Hovestadt,40 Jena glass 16III, 1884; (9) Hovestadt,40 p. 246, Jena glass 59III, 1889, ‘‘ideal thermometer glass’’; (10) Steiner,42 p. 172, Jena glass 202III, 1893, recalculated from batch; (11) Steiner,42 p. 172, Auer von Welsbach gas light chimney, Jena glass 276III, 1895, recalculated from batch; (12) Corning code 7720; (13) Corning code 7740, Morey;5 (14) Aubourg and Wolf,46 typical composition, can vary, depending upon manufacturer and materials. ‡ Glass contains other oxides: (1) 0.54% Mn2O3, 1.51% CuO, and 1.0% SO3; (7) 70% PbO; (8) 7.0% ZnO; (11) 4.0% Sb2O3 and 0.9% As2O3; (12) 6.25% PbO. 798 Journal of the American Ceramic Society—Kurkjian and Prindle Vol. 81, No. 4

April 1998 Perspectives on the History of Glass Composition lead glass that was the ancestor of English lead crystal. The "Deceptively like a Solid ald Hoffman ognized the potential of Ravenscr ofts invention and negotiated to buy his entire production. The first glasses suf- The conference is on Glass, in Montreal. Wintry light declines fered from poor chemical durability and crizzling, and it was a to penetrate windows, and soon will be lit glass-enclosed glows that we m lk into the night(fortified by bot few years before a truly moisture-resistant lead crystal was mineral waters metric of order trespassing on prevailing produced. The glass was called"crystal, " and the fact that lead chaos that giv was the key ingredient was kept secret by Ravenscroft and his its viscious. tra immediate successors. These glasses also were called flint The beginning was, is silica, this peon stuff glasses, because they were based on high-purity silica from the of the earth, in quartz, cristobalite, coesite flint nodules found commonly in the Cretaceous chalk deposits stishovite. Pristine marching bands of atoms of southeast England, plus calcined lead oxide, niter(potassium (surpassing the names we give them) nitrate), and potash from wood ashes (good quality potash had build crystalline lattices from chains, rings, of St become more readily available in the latter part of the 17th alternating with oxygen, each silicon tetrhedrally century). A substantial business grew in the manufacture of coordinated by Os, each oxygen lead crystal articles that took advantage of the higher refractive ion, so different from the life-giving, inflaming index and the ease of cutting and polishing of the lead flint to tomic gas, joining two silicons; on to rings create sparkling goblets, bowls, and vases in diamondoid perfection in cristobalite The 17th century also was a period of growing interest in helical O-Si-O chains in quartz, handed ing, mirror images of each other, hard, ionic SiO science, and glass improvements became driven by scientists seeking better optical instruments, particularly telesc lileo and Kepler made a number of discoveries in optics that time lent to the earth: then lava flowed, the air blew thicker, still no compound or simple eye to made possible considerable improvement in telescopes, using fret defect into the unliquid from which silica the soda-lime-silica crown glasses of the time. Crown glas crystallized. But in time we did come, handy was the name given to window glass of the period that was set to garner sand, limestone, soda ash, to break made by the crown process, wherein a large blown bubble of the still witness of silica. Heat disrupts. Not the glass was transferred to a pontil, opened, and spun into a cir warmth of Alabama midsummer evenings. not cular disk by centrifugal force. )However, later optical physi our hand but formless wonder of prolonged fire, cists and astronomers found themselves increasingly frustrated he blast of air drawn in, controlled fire storms. Sand, which is silica, melts. To a liquid, where by poor glass quality and by the difficulty imposed by chro- matic aberration in obtaining a clear, sharp focus. After New order is local but not long-range. Atoms wander from their places, bonds break, tetrahedra in a ton explained the refraction of light by prisms, he examined tizzy, juxtapose, chains tilt, bump and stretch- many glasses and studied their dispersion( the variation in re- Jaggerwalky ractive index with wavelength ). Because the glasses The restive structures in microscopic turmoil probably all reasonably similar in composition, given the lim- meld to gross flow, bubbling eddies of the melt. ited variety of glasses available, he concluded, incorrectly, that all glasses had the same dispersion, and, therefore, that chro- Peace in crystal meshes matic aberration was an uncorrectable fault in lenses. Accord- n hot yellow flux. But the gloved gly, Newton then decided that it was useless to attempt to men who hold the ladies get nervy volca anoes on their minds. So-tilt, pour .. douse, build a better refracting telescope and switched his energies to o quench, freeze in that micro lurch reflecting telescopes. Others did the same, and refracting tele- Glass forms and who would have thought it clear? scopes went into ecl During the early 1730s, Chester Moor Hall, an English law- We posit that the chanced, in its innards so upset, er with an amateur interest in telescopes, recognized that lead ught not be transparent. Light scattered from entangled polymer blocks, adventitious dirt, flint glasses had higher dispersion than soda-lime crown owes it to us-oh, we see it so clearly--to lasses. He reasoned that chromatic aberration could be cor- sh in black or at least rected by an objective lens with two elements: a convex crown in the muddy browns of spring run-off, another element and a concave flint element (Crown"'and"flint But light's submicroscopic tap dance is done in became the terms used to describe, respectively, low refractive The crossed fields shimmer, resonant, they plink index(low dispersion) and high refractive index(high disper- sion)optical glasses, respectively. This doublet worked, and toms matter, their neighbors less, the tangle of the locked-in some telescopes were built using this first achromatic lens. The iquid irrelevant in the birthing of color, or lack of it. invention was not patented or publicized, however, and was Optical fibers Crystal Palace rediscovered by John Dollond, who patented it in 1758. Dol- The Worshipful Company of Glass Sellers lond and his son Peter were skillful, well-known opticians, and recycled Millefiori they were quite successful in marketing the achromats. These Prince Ruperts drops doublets were largely successful in bringing the red light and Chartres. Rouen. Amiens blue light to focus in a common image, although a secondary spectrum remained. This improvement should have encouraged network modifiers the palomar mirror moked for viewing e nvestigations of the effects of composition on the optical properties of glass, but progress was slow because of the prob- etched with hydrofluor lems of making homogeneous glass. Poor-quality optical glass annealed persisted until stirring of the melt was introduced by Pierre softening point Louis Guinand and his successors in the beginning of the 19th y Joseph Fraunhofer entered optical physics from the practical e, working for a time in an optical institute where Guinand was employed both as a glassmaker and as an optician grinding and polishing lenses. Fraunhofer made some excellent achro- From The Metamict State, pp. 44-48 University of Central Florida Press, Orlando, FL, 1987 mats, which helped revive refracting telescopes In the proces he experimented with glass compositions and recogn re choices were needed in refractive index and dispersion

lead glass that was the ancestor of English lead crystal. The Worshipful Company of Glass Sellers of London, a trade guild, quickly recognized the potential of Ravenscroft’s invention and negotiated to buy his entire production. The first glasses suf￾fered from poor chemical durability and crizzling, and it was a few years before a truly moisture-resistant lead crystal was produced. The glass was called ‘‘crystal,’’ and the fact that lead was the key ingredient was kept secret by Ravenscroft and his immediate successors. These glasses also were called flint glasses, because they were based on high-purity silica from the flint nodules found commonly in the Cretaceous chalk deposits of southeast England, plus calcined lead oxide, niter (potassium nitrate), and potash from wood ashes (good quality potash had become more readily available in the latter part of the 17th century). A substantial business grew in the manufacture of lead crystal articles that took advantage of the higher refractive index and the ease of cutting and polishing of the lead flint to create sparkling goblets, bowls, and vases. The 17th century also was a period of growing interest in science, and glass improvements became driven by scientists seeking better optical instruments, particularly telescopes. Ga￾lileo and Kepler made a number of discoveries in optics that made possible considerable improvement in telescopes, using the soda–lime–silica crown glasses of the time. (Crown glass was the name given to window glass of the period that was made by the crown process, wherein a large blown bubble of glass was transferred to a pontil, opened, and spun into a cir￾cular disk by centrifugal force.) However, later optical physi￾cists and astronomers found themselves increasingly frustrated by poor glass quality and by the difficulty imposed by chro￾matic aberration in obtaining a clear, sharp focus. After New￾ton explained the refraction of light by prisms, he examined many glasses and studied their dispersion (the variation in re￾fractive index with wavelength). Because the glasses were probably all reasonably similar in composition, given the lim￾ited variety of glasses available, he concluded, incorrectly, that all glasses had the same dispersion, and, therefore, that chro￾matic aberration was an uncorrectable fault in lenses. Accord￾ingly, Newton then decided that it was useless to attempt to build a better refracting telescope and switched his energies to reflecting telescopes. Others did the same, and refracting tele￾scopes went into eclipse until well into the 18th century.33 During the early 1730s, Chester Moor Hall, an English law￾yer with an amateur interest in telescopes, recognized that lead flint glasses had higher dispersion than soda–lime crown glasses. He reasoned that chromatic aberration could be cor￾rected by an objective lens with two elements: a convex crown element and a concave flint element. (‘‘Crown’’ and ‘‘flint’’ became the terms used to describe, respectively, low refractive index (low dispersion) and high refractive index (high disper￾sion) optical glasses, respectively.) This doublet worked, and some telescopes were built using this first achromatic lens. The invention was not patented or publicized, however, and was rediscovered by John Dollond, who patented it in 1758. Dol￾lond and his son Peter were skillful, well-known opticians, and they were quite successful in marketing the achromats.34 These doublets were largely successful in bringing the red light and blue light to focus in a common image, although a secondary spectrum remained. This improvement should have encouraged investigations of the effects of composition on the optical properties of glass, but progress was slow because of the prob￾lems of making homogeneous glass. Poor-quality optical glass persisted until stirring of the melt was introduced by Pierre Louis Guinand and his successors in the beginning of the 19th century. Joseph Fraunhofer entered optical physics from the practical side, working for a time in an optical institute where Guinand was employed both as a glassmaker and as an optician grinding and polishing lenses. Fraunhofer made some excellent achro￾mats, which helped revive refracting telescopes. In the process, he experimented with glass compositions and recognized that more choices were needed in refractive index and dispersion ‘‘Deceptively like a Solid’’ Roald Hoffman The conference is on Glass, in Montreal. Wintry light declines to penetrate windows, and soon will be lit glass-enclosed glows so that we may talk, talk into the night (fortified by bottled mineral waters), of the metric of order trespassing on prevailing chaos that gives this warder of our warmed up air, clinker, its viscious, transparent strength. The beginning was, is silica, this peon stuff of the earth, in quartz, cristobalite, coesite, stishovite. Pristine marching bands of atoms (surpassing the names we give them) build crystalline lattices from chains, rings, of Si alternating with oxygen, each silicon tetrhedrally coordinated by O’s, each oxygen ion, so different from the life-giving, inflaming diatomic gas, joining two silicons; on to rings in diamondoid perfection in cristobalite; helical O-Si-O chains in quartz, handed in coiling, mirror images of each other, hard, ionic SiO2. There must be reasons for such perfection— time lent to the earth: then lava flowed, the air blew thicker, still no compound or simple eye to fret defect into the unliquid from which silica crystallized. But in time we did come, handy, set to garner sand, limestone, soda ash, to break the still witness of silica. Heat disrupts. Not the warmth of Alabama midsummer evenings, not your hand but formless wonder of prolonged fire, the blast of air drawn in, controlled fire storms. Sand, which is silica, melts. To a liquid, where order is local but not long-range. Atoms wander from their places, bonds break, tetrahedra in a tizzy, juxtapose, chains tilt, bump and stretch— Jaggerwalky. The restive structures in microscopic turmoil meld to gross flow, bubbling eddies of the melt. Peace in crystal meshes, peace in hot yellow flux. But the gloved men who hold the ladies get nervy volcanoes on their minds. So—tilt, pour . . . douse, so quench, freeze in that micro lurch. Glass forms, and who would have thought it clear? We posit that the chanced, in its innards so upset, ought not be transparent. Light scattered from entangled polymer blocks, adventitious dirt, owes it to us— oh, we see it so clearly—to lose its way, come awash in black or at least in the muddy browns of spring run-off, another flux. But light’s submicroscopic tap dance is done in place. The crossed fields shimmer, resonant, they plink electron orbits of O and Si. Atoms matter, their neighbors less, the tangle of the locked-in liquid irrelevant in the birthing of color, or lack of it. Optical fibers Crystal Palace The Worshipful Company of Glass Sellers recycled Millefiori prone to shattering Prince Rupert’s drops Chartres, Rouen, Amiens float Pyrex Vycor glass wool network modifiers the Palomar mirror smoked for viewing eclipses thermos lead glass microcrack etched with hydrofluoric acid spun frustration bull’s eyes annealed borosilicate softening point High winds on Etna or Kilauea spin off the surface of a lava lake thin fibers. Pele’s hair. The Goddesses’ hair, here black. From The Metamict State; pp. 44–48. University of Central Florida Press, Orlando, FL, 1987. April 1998 Perspectives on the History of Glass Composition 799

Journal of the American Ceramic SocietyKurkjian and Prindle Vol 8I. No 4 Glass is much significant advances in the discovery and isolation of new el more gentile, graceful ements, and, in the 1830s, Harcourt began investigations of the effects of many of these new elements on the optical properties long the elem and noble than any cadmium, fluorine, lithium, magnesium, molybdenum, tungsten, uranium, and vanadium. He also studied the Metall It is more effects of other elements, including antimony, arsenic, barium, boron, phosphorus, tin, and zinc, first introduced into glass by delightful, polite, and but also melted some phosphates, borates, and titanates, in part sightly than any other neous glass. He did not widely publicize his findings, but Sir George Stokes, the noted mathematician and physicist, learned of his work, collaborated with him, and helped to bring the material at this day results to the attention of the scientific community in 1871, the ear of Harcourts death. In 1874. Stokes made a small. three- known to the world omponent lens that was largely free of the secondary spectrum from some of Harcourts glasses. Therefore, even though Har Antonio neri. 1612 courts glasses were not completely homogeneous, his work demonstrated that different glassmaking ingredients did bring changes in dispersion and refractive index that could yield glasses that began to solve the optical problems of the time.16,31,39 beyond those of ordinary crown and flint glasses if better lenses were to be made that corrected the secondary spectrum. Fraun- British glass industry to investigate further the effects of dif- ferent glass constituents, it did little beyond the production of was probably the discoverer of the mixed-alkali effect, noting some standard optical crowns and flints by Chance Brothers in that glasses with mixed alkalis had superior durability. (The irmingham. Experimentation to develop new glas served in some properties when one alkali ion is gradually tions was severely constrained at that time in England by ex- substituted for another alkali ion. This phenomenon is observed orbitant taxes on all glassmelting. Therefore, Chance Brothers in properties affected by transport mechanisms, such as ele concentrated instead on improving the quality of the standare trical conductivity, dielectric loss, internal friction, and self- glasses by stirring the I nelt. Accordingly, the initiative in glass diffusion. )Later, Fraunhofer's spectral studies enabled him to composition research passed to German glassmakers, who built on the work of fraunhofer and harcourt 31 make observations on dispersion for the principal glass com- ponents of the day 17, 22,25,35,36 As is evident from the foregoing, until the late 19th century Impressed by Fraunhofer's results, the Royal Society estab- he development of new glasses was largely a matter of ar lished, in 1824, a commission consisting of Michael Faraday occasional fortuitous discovery. These early investigations, al John Herschel( the astronomer), and George Dollond(another though often motivated by a need, were not pursued of the famed clan of opticians), to study the possibility of atically, had difficulty in yielding a homogeneous prod exce on of Harcourts work, usually used the same making superior glasses for telescope objectives. Faraday be w dients hovestadt wrote. in 1900 el came interested in glassmelting and made some prolonged in ment of the art of glassmaking in response to optical require of melting glasses in platinum containers and the importance of fluxes broke the monotony of a uniform series of crowns and tiring melts to improve homogeneity. His experiments, un- flints. 740 fortunately, did not contribute much to the knowledge of glass omposition, although he did demonstrate that boron could be Ised in glassmaking to make a passable lead borosilicate flint llL Abbe and Schott lass. Faraday later(1845)conducted some experiments of which he demonstrated the Faraday effect(rotation of the plane a microscope maker at the university. Similar to the telescope of polarization of light in a magnetic field ). 15,33,35, 37,38 makers, Abbe soon realized that a wider variation in dispersion It may seem surprising today that eminent scientists and for a given refractive index was needed to remove completely intellectuals of the time were deeply interested in finding so- states,Goethe, then Prime Minister of a German duchy. in of Otto Schott, a young German chemist who had been explor- University of Jena, it would be most important to determine ing glassmelting phenomena in connection with his family' the relation of refraction and dispersion in your (barium and glassworks in Westphalia. Schott contacted Abbe and sent him strontium glasses 6 should be pleased to contribute the samples might aid Abbe in his research for glasses with dif- of a wide range of elements on the properties of glass was the laborating and thus was born one of the greatest and most Schott moved to Jena in 1882 to be closer to abbe and Zeiss Abbe(the scientist), Schott(the glassmaker), and Zeiss(the instrument builder) worked together in a synergistic manner that bore dramatic results. abbe and Schott d discuss the tThe poem on the 就运 composition changes to be made, Schott would then prepare homogeneous glass melts, and Abbe would measure the results If the properties appeared to be an improvement, Zeiss would grind and polish lenses and observe the performance of the

beyond those of ordinary crown and flint glasses if better lenses were to be made that corrected the secondary spectrum. Fraun￾hofer also wrote about the chemical durability of glasses and was probably the discoverer of the mixed-alkali effect, noting that glasses with mixed alkalis had superior durability. (The mixed-alkali effect is the distinctly nonlinear behavior ob￾served in some properties when one alkali ion is gradually substituted for another alkali ion. This phenomenon is observed in properties affected by transport mechanisms, such as elec￾trical conductivity, dielectric loss, internal friction, and self￾diffusion.) Later, Fraunhofer’s spectral studies enabled him to make observations on dispersion for the principal glass com￾ponents of the day.17,22,25,35,36 Impressed by Fraunhofer’s results, the Royal Society estab￾lished, in 1824, a commission consisting of Michael Faraday, John Herschel (the astronomer), and George Dollond (another of the famed clan of opticians), to study the possibility of making superior glasses for telescope objectives. Faraday be￾came interested in glassmelting and made some prolonged in￾vestigations during 1825–1830 that demonstrated the benefits of melting glasses in platinum containers and the importance of stirring melts to improve homogeneity. His experiments, un￾fortunately, did not contribute much to the knowledge of glass composition, although he did demonstrate that boron could be used in glassmaking to make a passable lead borosilicate flint glass. Faraday later (1845) conducted some experiments of significance with his ‘‘heavy glass’’ (see glass 7 in Table I), in which he demonstrated the Faraday effect (rotation of the plane of polarization of light in a magnetic field).15,33,35,37,38 It may seem surprising today that eminent scientists and intellectuals of the time were deeply interested in finding so￾lutions to glass composition problems. For example, Vogel states, ‘‘Goethe, then Prime Minister of a German duchy . . . in 1829 wrote to his friend, the noted chemist Do¨breiner at the University of Jena, ‘it would be most important to determine the relation of refraction and dispersion in your [barium and strontium] glasses . . . I should be pleased to contribute the modest funding . . .’.’’36,‡ The first, however, to make an extensive study of the effects of a wide range of elements on the properties of glass was the Rev. William Vernon Harcourt, an English clergyman. The late 18th century and the early 19th century was a period of highly significant advances in the discovery and isolation of new el￾ements, and, in the 1830s, Harcourt began investigations of the effects of many of these new elements on the optical properties of glass. Among the elements he first used in glass were be￾ryllium, cadmium, fluorine, lithium, magnesium, molybdenum, nickel, tungsten, uranium, and vanadium. He also studied the effects of other elements, including antimony, arsenic, barium, boron, phosphorus, tin, and zinc, first introduced into glass by others. Harcourt did not confine his studies to silicate glasses, but also melted some phosphates, borates, and titanates, in part because he found it difficult to fuse the silicates to a homoge￾neous glass. He did not widely publicize his findings, but Sir George Stokes, the noted mathematician and physicist, learned of his work, collaborated with him, and helped to bring the results to the attention of the scientific community in 1871, the year of Harcourt’s death. In 1874, Stokes made a small, three￾component lens that was largely free of the secondary spectrum from some of Harcourt’s glasses. Therefore, even though Har￾court’s glasses were not completely homogeneous, his work demonstrated that different glassmaking ingredients did bring changes in dispersion and refractive index that could yield glasses that began to solve the optical problems of the time.16,31,39 Although the work of Harcourt should have encouraged the British glass industry to investigate further the effects of dif￾ferent glass constituents, it did little beyond the production of some standard optical crowns and flints by Chance Brothers in Birmingham. Experimentation to develop new glass composi￾tions was severely constrained at that time in England by ex￾orbitant taxes on all glassmelting. Therefore, Chance Brothers concentrated instead on improving the quality of the standard glasses by stirring the melt. Accordingly, the initiative in glass composition research passed to German glassmakers, who built on the work of Fraunhofer and Harcourt.31 As is evident from the foregoing, until the late 19th century, the development of new glasses was largely a matter of an occasional fortuitous discovery. These early investigations, al￾though often motivated by a need, were not pursued system￾atically, had difficulty in yielding a homogeneous product, and, with the exception of Harcourt’s work, usually used the same few ingredients. Hovestadt wrote, in 1900, ‘‘. . . the develop￾ment of the art of glassmaking in response to optical require￾ments kept, for a long time, to one narrow groove, and no new fluxes broke the monotony of a uniform series of crowns and flints.’’40 III. Abbe and Schott Ernst Abbe, professor of physics at Jena University, became interested in optical glasses through his work with Carl Zeiss, a microscope maker at the university. Similar to the telescope makers, Abbe soon realized that a wider variation in dispersion for a given refractive index was needed to remove completely the secondary spectrum from optical images. He wrote on the subject in the late 1870s, and his remarks attracted the interest of Otto Schott, a young German chemist who had been explor￾ing glassmelting phenomena in connection with his family’s glassworks in Westphalia. Schott contacted Abbe and sent him some lithium glasses he had prepared with the thought the samples might aid Abbe in his research for glasses with dif￾ferent optical properties. By 1881 Abbe and Schott were col￾laborating and thus was born one of the greatest and most productive associations in the history of glass composition.31,40 Schott moved to Jena in 1882 to be closer to Abbe and Zeiss. Abbe (the scientist), Schott (the glassmaker), and Zeiss (the instrument builder) worked together in a synergistic manner that bore dramatic results. Abbe and Schott would discuss the composition changes to be made, Schott would then prepare homogeneous glass melts, and Abbe would measure the results. If the properties appeared to be an improvement, Zeiss would grind and polish lenses and observe the performance of the ‡ The poem on the previous page was authored by Roald Hoffman, the Nobel Laureate in chemistry in 1981 with Kenichi Fukui for ‘‘His application of molecular orbital theory to chemical reactions.’’ Also, the 1977 Nobel Prize in physics was awarded to P. W. Anderson, Sir N. F. Mott, and J. H. van Vleck for ‘‘Their funda￾mental theoretical investigations of the electronic structure of magnetic and disor￾dered systems.’’ The eminent scientists continue to find glassy systems of interest. ‘‘Glass . . . is much more gentile, graceful, and noble than any Metall, . . . it is more delightful, polite, and sightly than any other material at this day known to the world,’’ Antonio Neri, 1612 800 Journal of the American Ceramic Society—Kurkjian and Prindle Vol. 81, No. 4

April 1998 Perspectives on the History of Glass Composition Schott und genossen As stated by Douglas and Frank, 31The success of the were added to the list every few years, and the effect on the firm [ Schott und Genossen] was spectacular. Its first price manufacture of optical systems was so great that Germany list of 1886 contained forty-four optical gla asses of which which had previously imported ninety percent of its optical nineteen were essentially new compositions. The first systems from England and France, started to export to these supplement of 1888 added twenty-four glasses, including countries. Thus, an industrial development which was ac- ew barium light flints which were remarkable for complished in less than ten years virtually eliminated exist mall dispersion pared with refractive index. They ng manufacturers and, for about 30 years, until the outbreak ed so little lead oxide that the usual light absorption of the World War I, Jena held an effective world monopoly by flint glasses was greatly reduced. New glasses in the manufacture of optical glass lime, and lead oxide and eventually added 28 othere/a potash, optical glasses. In the 1890s, the group at Jena analyzed the rIse of ass meters. It was noted in one of the early observations of quantities of at least 10% to produce glasses of refractive in- the mixed alkali effect that the zero rise was particularly pro- dexes and dispersions substantially different from those made nounced(more than one celsius degree) in glasses with ap- proximately equal quantities of soda and potash Glasses made The techniques used by Abbe and Schott in their studies with either only soda or only potash as the alkali suffered only were based on careful observation and measurement, although one tenth or less the secular rise as the mixed alkali glasses almost totally empirical, because no reasonable theories existed The most stable glass was found to be a borosilicate(see glass to guide their work. Additions of new minor ingredients were 9 in Table 1). 40 made to correct or offset faults in the original compositions Improved laboratory glassware also resulted from Schott's For example, Schott found that, in borate and phosphate further pursuit of boron in glass with the discovery that boro- glasses, alkalis had to be used very sparingly, if at all; other silicate glasses had exceptional resistance to attack by boiling vise surface staining resulted on exposure to air. However water. Accordingly, these glasses also made excellent boiler when alumina, zinc oxide, and barium oxide were added the gauge glasses. It also was noted that boric oxide was the most surface durability could be improved enough to make the effective addition to silicate glasses in reducing the coefficient glasses serviceable. Schott soon learned that the addition of of thermal ex and this discot some elements would have no effect on optical properties but glassware with oved resistance to thermal shock 40 would have a favorable effect on other properties In the remarkably short period from 1879 to 1886, Otto In an effort to at least make their results usable, Schott, and Schott, with the assistance of Abbe and Zeiss, created and Winkelmann developed what probably was the first composi- offered commercially a surprising array of optical glasses. Be- tion-property model. They produced a series of oxide factors sides using a systematic approach to glass composition re that allowed them to calculate the value of a property knowing search, Schott had mastered the small-scale melt-stirring pro the composition. Today, many such models are available be- cess so as to be able to make a homogeneous product. The cause of computers(see Cable) glasses also had been carefully characterized, so they were sold Early useful results were obtained with boron, barium, and with exact measured values of refractive index and dispersion fluorine, leading to families of borosilicate crowns, barium This work was a watershed in the history of glass composition flints, and fluor crowns. (The demarcation between crowns and in that it demonstrated for the first time the ability to tailor the flints is arbitrarily defined by their dispersion and is shown properties of a glass by judicious adjustments in composition in Fig. 2.)The government was quite impressed by the pI based on a composition-property model ress and made some large grants to support the work of the laboratory that became, in 1884, the Jena firm of Schott und Genossen IV. Modern Glasses The discoveries of abbe and Schott were not confined to (1) Soda-Lime-Silica Glasses Although sand and alkali were known from the earliest days of glass to be necessary ingredients, the role of lime was not Flints apparent until much later times. Lime was not recognized as an important glass constituent by early glassmakers, because ad- equate amounts of lime were generally added unknowingly as an impurity in the sand and alkali. Lime appears to have been added consciously to glass batches in Roman times, but Neri mentioned lime only casually in suggesting that small quanti ties could be added to make a very fair and beautiful Crystall. 2 Only in the 17th, 18th, and 19th centuries did the increase in chemical durability brought about by the addition of lime to alkali silicate glasses become understood. Bohemian glassmakers added lime to their fine crystal in the 17th century and, during the late 1700s, P D. Deslandes added up to 6% lime to increase the resistance of Saint-Gobain's plate glass to attack by moisture. Guinand and Fraunhofer observed that it was necessary to add lime to increase glass durability, and, in historical development of optical glasses, 34(White area within curve 1830, J.B. Dumas, a French glass technologist, noted that the presents modern glasses(Morey et al.), hatched area represents ear. chemical durability of glass was oved by adding one part lier glasses, i. e, 1880-1934( Schott et al ); and black area represents of lime to one part of soda and six parts of silica. The addition glasses before 1880.) of lime to the batch became essential in practical glassmaking

finished pieces, then feed his observations back to Abbe and Schott. In this manner they started with silica, soda, potash, lime, and lead oxide and eventually added 28 other elements in quantities of at least 10% to produce glasses of refractive in￾dexes and dispersions substantially different from those made previously. The techniques used by Abbe and Schott in their studies were based on careful observation and measurement, although almost totally empirical, because no reasonable theories existed to guide their work. Additions of new minor ingredients were made to correct or offset faults in the original compositions. For example, Schott found that, in borate and phosphate glasses, alkalis had to be used very sparingly, if at all; other￾wise surface staining resulted on exposure to air. However, when alumina, zinc oxide, and barium oxide were added, the surface durability could be improved enough to make the glasses serviceable. Schott soon learned that the addition of some elements would have no effect on optical properties but would have a favorable effect on other properties. In an effort to at least make their results usable, Schott, and Winkelmann developed what probably was the first composi￾tion–property model.4 They produced a series of oxide factors that allowed them to calculate the value of a property knowing the composition. Today, many such models are available be￾cause of computers (see Cable41). Early useful results were obtained with boron, barium, and fluorine, leading to families of borosilicate crowns, barium flints, and fluor crowns. (The demarcation between crowns and flints is arbitrarily defined by their dispersion and is shown in Fig. 2.) The government was quite impressed by the prog￾ress and made some large grants to support the work of the laboratory that became, in 1884, the Jena firm of Schott und Genossen. The discoveries of Abbe and Schott were not confined to optical glasses. In the 1890s, the group at Jena analyzed the problem of the secular rise of the zero in the aging of glass thermometers. It was noted in one of the early observations of the mixed alkali effect that the zero rise was particularly pro￾nounced (more than one celsius degree) in glasses with ap￾proximately equal quantities of soda and potash. Glasses made with either only soda or only potash as the alkali suffered only one tenth or less the secular rise as the mixed alkali glasses. The most stable glass was found to be a borosilicate (see glass 9 in Table I).40 Improved laboratory glassware also resulted from Schott’s further pursuit of boron in glass with the discovery that boro￾silicate glasses had exceptional resistance to attack by boiling water. Accordingly, these glasses also made excellent boiler gauge glasses. It also was noted that boric oxide was the most effective addition to silicate glasses in reducing the coefficient of thermal expansion, and this discovery led to laboratory glassware with improved resistance to thermal shock.40 In the remarkably short period from 1879 to 1886, Otto Schott, with the assistance of Abbe and Zeiss, created and offered commercially a surprising array of optical glasses. Be￾sides using a systematic approach to glass composition re￾search, Schott had mastered the small-scale melt-stirring pro￾cess so as to be able to make a homogeneous product. The glasses also had been carefully characterized, so they were sold with exact measured values of refractive index and dispersion. This work was a watershed in the history of glass composition in that it demonstrated for the first time the ability to tailor the properties of a glass by judicious adjustments in composition based on a composition–property model.42 IV. Modern Glasses (1) Soda–Lime–Silica Glasses Although sand and alkali were known from the earliest days of glass to be necessary ingredients, the role of lime was not apparent until much later times. Lime was not recognized as an important glass constituent by early glassmakers, because ad￾equate amounts of lime were generally added unknowingly as an impurity in the sand and alkali. Lime appears to have been added consciously to glass batches in Roman times, but Neri mentioned lime only casually in suggesting that small quanti￾ties could be added ‘‘. . . to make a very fair and beautiful Crystall.’’12 Only in the 17th, 18th, and 19th centuries did the increase in chemical durability brought about by the addition of lime to alkali silicate glasses become understood. Bohemian glassmakers added lime to their fine crystal in the 17th century, and, during the late 1700s, P. D. Deslandes added up to 6% lime to increase the resistance of Saint-Gobain’s plate glass to attack by moisture.31 Guinand and Fraunhofer observed that it was necessary to add lime to increase glass durability, and, in 1830, J. B. Dumas, a French glass technologist, noted that the chemical durability of glass was improved by adding one part of lime to one part of soda and six parts of silica. The addition of lime to the batch became essential in practical glassmaking Schott und Genossen As stated by Douglas and Frank,31 ‘‘The success of the firm [Schott und Genossen] was spectacular. Its first price list of 1886 contained forty-four optical glasses of which nineteen were essentially new compositions. The first supplement of 1888 added twenty-four glasses, including eight new barium light flints which were remarkable for their small dispersion compared with refractive index. They contained so little lead oxide that the usual light absorption shown by flint glasses was greatly reduced. New glasses were added to the list every few years, and the effect on the manufacture of optical systems was so great that Germany, which had previously imported ninety percent of its optical systems from England and France, started to export to these countries. Thus, an industrial development which was ac￾complished in less than ten years virtually eliminated exist￾ing manufacturers and, for about 30 years, until the outbreak of the World War I, Jena held an effective world monopoly in the manufacture of optical glass.’’ Fig. 2. Refractive index, n, versus reciprocal dispersion, n, showing historical development of optical glasses.34 (White area within curve represents modern glasses (Morey et al.); hatched area represents ear￾lier glasses, i.e., 1880–1934 (Schott et al.); and black area represents glasses before 1880.) April 1998 Perspectives on the History of Glass Composition 801

Journal of the American Ceramic Sociery-Kurkjian and prindle Vol 81. No 4 Cao 30 2::3 a20s0 Fig 3. Soda-lime-silica phase diagram(R S. Roth, T Negas, and L P Cook; Fig 5321 in Phase Diagrams for Ceramists. Vol IV. Edited by G. Smith. American Ceramic Society, Columbus, OH, 1981) to preserve durability as the use of synthetic soda ash(pure possibility with the discovery of extensive deposits in Turkey Na2CO3, with no CaCO3)became wide-spread. This practice Chile, and, in particular, Califomia, in the latter part of the 19th an early in the 19th century because soda ash from the century Leblanc process became available and its use was common noted in an earlier section. Abbe an Schott. d ractice after the 1860s when the Solvay process became the 1880s, were the first to use boron compounds in glass in sig nificant amounts, first in optical glasses, then in glasses for Most of the current commercial glasses are soda-lime-silica laboratory apparatus. Both Faraday and Harcourt had made glasses. It is significant that the compositions of these glasses, some use of boron in glass, but Abbe and Schott clearly estab- used typically for containers and flat glass, has changed little shed that borosilicate glasses had superior resistance than ranging from 10%to 200s from 65% to 75% silica, with alkali soda-lime-silica glasses to chemical attack and had better ther- over the centuries. ra and lime as the balance. althou mal shock resistance because of their lower coefficient of thel nost older soda-lime-silica glasses contained a few percent mal expansion. The introduction of the Auer von Welsbach alumina from raw-material impurities and from refractories, a mantle in gas lamps in 1887 created a need for a lamp cylinder similar amount has been added customarily since Schott, based or chimney with improved resistance to thermal shock. Schott on his observations of Thuringian glasses, demonstrated, in the met this need with a glass containing 15% boric oxide(see late 1880s, that it benefited durability and resistance to devit- glass ll in Table I)having a very low coefficient of expan- rification Glasses with these compositions are relatively easy sion.3640,42 to melt and form, do not devitrify easily, and generally have At about this time in the United States, cracking and break reasonable resistance to attack by moisture. They can be made age of the globes of railroad brakemen's lanterns was a prob- quite color-free and nontoxic with pure raw materials that ar lem when rain showers struck the hot glass. Why Schott heat available worldwide at acceptable cost. 5. 40 resistant glasses were not used to solve this problem is not Improvements in melting technology-e. g, more-resistant known; perhaps it was due to the poor international technical refractories and higher temperatures--have increased chemical communications of the times. Corning Glass Works, the lead- durability through lower alkali and higher lime contents. This ing U.S. manufacturer of lamp chimneys and bulbs for electric trend also has been encouraged by the comparative costs of lamps, was asked to investigate the matter, and it became the soda and lime, and, currently, economic factors are the princi- first assignment of the newly formed research laboratory in pal determinant for soda-lime-silica compositions. Enough is 1908. In 1909, Corning introduced a borosilicate glass that now known about the effects of composition on properties to solved the lantern globe thermal shock problem, but had poor permit major glass constituents to be adjusted several percent- chemical durability. Corning 's first research director, Eugene age points to compensate for differences in raw-material prices C. Sullivan, a chemist hired from the U.S. Geological Survey to reach the lowest-cost composi and William C. Taylor, a young chemist colleague recently from Massachusetts Institute of Technology, worked further on (2) Borosilicate Glasses the problem. By 1912, they had perfected a chemically du- The first new major glass system to be explored beyond the rable, shock-resistant, lead borosilicate glass marketed under soda-lime-silica glasses utilized the other great glass-former the name Nonex (for nonexpanding) that reduced lantern boric oxide, important for its many commercial applications. globe breakage by 60%(see 12 in Table 1). Nonex also Although borax was known and used in the Middle ages as an proved to double the life of the battery jars used by the rail- in practical glassmaking became a realistic roads in their new electrically powered signal systems

to preserve durability as the use of synthetic soda ash (pure Na2CO3, with no CaCO3) became wide-spread. This practice began early in the 19th century, because soda ash from the Leblanc process became available and its use was common practice after the 1860s when the Solvay process became the principal source of soda. Most of the current commercial glasses are soda–lime–silica glasses. It is significant that the compositions of these glasses, used typically for containers and flat glass, has changed little over the centuries, ranging from 65% to 75% silica, with alkali ranging from 10% to 20%, and lime as the balance. Although most older soda–lime–silica glasses contained a few percent alumina from raw-material impurities and from refractories, a similar amount has been added customarily since Schott, based on his observations of Thuringian glasses, demonstrated, in the late 1880s, that it benefited durability and resistance to devit￾rification. Glasses with these compositions are relatively easy to melt and form, do not devitrify easily, and generally have reasonable resistance to attack by moisture. They can be made quite color-free and nontoxic with pure raw materials that are available worldwide at acceptable cost.5,40 Improvements in melting technology—e.g., more-resistant refractories and higher temperatures—have increased chemical durability through lower alkali and higher lime contents. This trend also has been encouraged by the comparative costs of soda and lime, and, currently, economic factors are the princi￾pal determinant for soda–lime–silica compositions. Enough is now known about the effects of composition on properties to permit major glass constituents to be adjusted several percent￾age points to compensate for differences in raw-material prices to reach the lowest-cost composition.43 (2) Borosilicate Glasses The first new major glass system to be explored beyond the soda–lime–silica glasses utilized the other great glass-former, boric oxide, important for its many commercial applications. Although borax was known and used in the Middle Ages as an exotic flux, its use in practical glassmaking became a realistic possibility with the discovery of extensive deposits in Turkey, Chile, and, in particular, California, in the latter part of the 19th century.42 As noted in an earlier section, Abbe and Schott, during the 1880s, were the first to use boron compounds in glass in sig￾nificant amounts, first in optical glasses, then in glasses for laboratory apparatus. Both Faraday and Harcourt had made some use of boron in glass, but Abbe and Schott clearly estab￾lished that borosilicate glasses had superior resistance than soda–lime–silica glasses to chemical attack and had better ther￾mal shock resistance because of their lower coefficient of ther￾mal expansion. The introduction of the Auer von Welsbach mantle in gas lamps in 1887 created a need for a lamp cylinder or chimney with improved resistance to thermal shock. Schott met this need with a glass containing 15% boric oxide (see glass 11 in Table I) having a very low coefficient of expan￾sion.36,40,42 At about this time in the United States, cracking and break￾age of the globes of railroad brakemen’s lanterns was a prob￾lem when rain showers struck the hot glass. Why Schott heat￾resistant glasses were not used to solve this problem is not known; perhaps it was due to the poor international technical communications of the times. Corning Glass Works, the lead￾ing U.S. manufacturer of lamp chimneys and bulbs for electric lamps, was asked to investigate the matter, and it became the first assignment of the newly formed research laboratory in 1908. In 1909, Corning introduced a borosilicate glass that solved the lantern globe thermal shock problem, but had poor chemical durability. Corning’s first research director, Eugene C. Sullivan, a chemist hired from the U.S. Geological Survey, and William C. Taylor, a young chemist colleague recently from Massachusetts Institute of Technology, worked further on the problem.44 By 1912, they had perfected a chemically du￾rable, shock-resistant, lead borosilicate glass marketed under the name Nonext (for nonexpanding) that reduced lantern globe breakage by 60% (see glass 12 in Table I). Nonex also proved to double the life of the battery jars used by the rail￾roads in their new electrically powered signal systems. Fig. 3. Soda–lime–silica phase diagram (R. S. Roth, T. Negas, and L. P. Cook; Fig. 5321 in Phase Diagrams for Ceramists, Vol. IV. Edited by G. Smith. American Ceramic Society, Columbus, OH, 1981). 802 Journal of the American Ceramic Society—Kurkjian and Prindle Vol. 81, No. 4

April 1998 Perspectives on the History of Glass Composition Corning continued to explore borosilicate glass composi publication, Dumbaugh and Danielson7reference a ons and their applications. In 1913, Corning physicist Jesse T. ent issued to F. M. and F J. Locke48 for a series of Ittleton suggested that Nonex glass vessels might be used for silicates that is broad enough to include these alka baking pans, and a cake baked by his wife in the bottom of a aluminoborosilicate. ) Also, most commercial fiberglass com- battery jar demonstrated that the idea was sound. However, positions are based on ternary or quaternary eutectics, thus Nonex contained too much lead for food preparation taking advantage of the well-known fact that the most stable ore, a lead-free borosilicate composition was developed by Sullivan and Taylor which was named Pyrex"(see glass 13 in Table I). A pressed Pyrex ovenware line was introduced in ( Photosensitive Glasses and Glass-Ceramics 1915. and it became an immediate sales success. At about this No account of the history of glass compositions would be me, the supply of glassware to U.S. laboratories from Schott complete without some reference to the discovery of glass- nd it soon became apparent that blown Pyrex borosilicate glass-ceramics is presented hee scription of the early work on glass was an extremely The natural tendency of glasses to devitrify must have led because of its low expansion and its chemical durability. Ac assmakers occasionally over the ages to consider pursuing cording to morey, 5 this] glass has the lowest liquidus the phenomenon to its logical conclusion--a completely crys temperature of any known mixture having so high a silica talline product. The French chemist Reaumur is known to have content and in this fact doubtless is to be found a clue to its attempted to produce crystalline vessels by holding glass exceptional ability to withstand devitrification bottles packed in gypsum at a red heat for several days. Al- Other variations of borosilicate glass were explored by Corn- though these did devitrify to a crystalline form, he was unable ing for special applications that would benefit from low ex- to control the process, and the bottles were deformed and of pansion and corrosion-resistant glasses. One noteworthy ex- ow strength. Others experimented with the limited prec ample was the development of a glass with an expansion 25% tation of crystals from glass to create ruby or opal glasses, but r than that of Pyrex ovenware that was used for casting the the development of a process to control the massive crystal n. mirror for Mt. Palomar's Hale telescope in 1934. The zation of bulk glass did not occur until the middle of the 20th omers had sought the lowest possible expansion in their century uest for a mirror that would show the least distortion with There was a series of glass composition inventions during temperature changes. This glass also had the low liquidus tem the 1940s and 1950s based upon nucleation and controlled perature necessary to survive the extremely slow cooling rate rowth of crystals experiments conducted at Corning Glass of the cast blank without devitrification or phase separa Works. The work was led by S. Donald Stookey, but ultimately involved several others, including Armistead, Beall, Mac Glass chemists Harrison P. Hood and Martin E. nordberg observed during experimentation with borosilicates at Corning Stookey had been investigating nucleation and precipitation the 1930s that very large changes in properties would occa- of crystals in ruby and opal glasses and found, in 1942, tl onally result when some com copper, gold, and silver could be deposited as tiny particles of her investigation revealed that these glasses were separating metal through photo- rogeneous nucleation. The nto two intermingled glassy phases, one of which was silica- process was aided by the presence of a"sensitizer, " such as rich. A composition was developed that separated upon heat cerium or tin. Taking this a step further in 1951, he learned he treatment into a very-high-silica phase ar could photo-induce a sodium fluoride opal in a silicate glas hase with the latter being easily dissolved and leached out by nucleated with silver(Fotaliteo lot nitric acid. The remaining porous, silica-rich skeleton cou Stookey made an unanticipated discovery, in 1954, of the then be consolidated into a solid, pore-free form by heating controlled crystallization of glass ceramics. He had invented, in This glass, given the name Vycor, in 1939, could be melted the late 1940s, a process for preparing lithium silicate crystal nd formed easily before the leach consolidation steps images in glass with a photosensitive technique( Fotoform The 96% silica composition of the finished glass meant that it that it Very small quantities of silver(-006 mol%)were introduced had physical and chemical properties closely approaching those in the lithium silicate glass as a nucleating agent, and, after of pure-silica glass, yet it could be fashioned before heat treat- selective exposure to ultraviolet radiation, a heat treatment at ment into shapes that were impossible to form with fused silica -600oC caused lithium metasilicate crystals to form that then because of the extremely high viscosity of silica glass at even could be leached out of the unexposed glass very high temperatures. 45 This is an example of a new glass One day, Stookey placed a plate of preexposed lithium sili- composition being created by carefully exploiting the observa- cate glass in a laboratory oven to perform the 600oC heat tion of a new phenomenon treatment. The temperature controller stuck in the"on""posi- Another important step in borosilicate glass composition de- tion and the glass was heated to 900C, where it normally elopes rt alsoa ueredain the r 3p s wsth eed trdectior ot weld ati q uited hef t and ftuaid st ok eya was aela med y the insulation applications, and standard soda-lime-silica compo- knew that this glass melted and flowed below 700C, and he sitions were not suitable, because their conductivity was too believed that it would flow on to the floor of the oven. How- high. Soda-lime glasses also proved generally unsuitable for ever, in his own words, Imagine my astonishment on opening the door to see an undeformed, opaque solid plate! Snatching a them particularly vulnerable to attack and dissolution by water. pair of tongs, I immediately pulled the plate out of the hot Accordingly, Urban Bowes, R. A. Schoenlaub, and others at furnace, but it slipped from the tongs and fell to the tile- Owens-Corning Fiberglas reduced and ultimately removed the covered concrete floor, clanging like a piece of steel but re- alkalis and added boric oxide and alumina and increased the maining unbroken! It took no great imagination to realize that lime. 6 The boric oxide and lime reduced viscosity and in- this piece of Fotoform was not glass, but something new and creased durability, and the alumina also helped durability and different. It must have crystallized so completely that it could lowered the liquidus. The resulting lime-alumina-borosilicate not flow, even though the temperature was more than 200C lass(see glass 14 in Table I) had excellent electrical resistin above the softening temperature of the glass. And obviously it ity, superior resistance to attack by moisture, and good me was much stronger than ordinary glass. 50 An examination of hanical properties. Therefore, it is not surprising that glasses he fine-grained glass-ceramic that had been formed revealed of this general composition make up 90% of the continuous that it was composed of lithium disilicate and quartz crystals, glass fiber tly produced. (Historical note: In this same nd was much harder and higher in electrical resistivity than

Corning continued to explore borosilicate glass composi￾tions and their applications. In 1913, Corning physicist Jesse T. Littleton suggested that Nonex glass vessels might be used for baking pans, and a cake baked by his wife in the bottom of a battery jar demonstrated that the idea was sound. However, Nonex contained too much lead for food preparation. There￾fore, a lead-free borosilicate composition was developed by Sullivan and Taylor which was named Pyrext (see glass 13 in Table I). A pressed Pyrex ovenware line was introduced in 1915, and it became an immediate sales success. At about this time, the supply of glassware to U.S. laboratories from Schott and other European suppliers was interrupted by World War I, and it soon became apparent that blown Pyrex borosilicate glass was an extremely good glass for laboratory apparatus because of its low expansion and its chemical durability. Ac￾cording to Morey,5 ‘‘. . . [this] glass has the lowest liquidus temperature of any known mixture having so high a silica content and in this fact doubtless is to be found a clue to its exceptional ability to withstand devitrification.’’ Other variations of borosilicate glass were explored by Corn￾ing for special applications that would benefit from low ex￾pansion and corrosion-resistant glasses. One noteworthy ex￾ample was the development of a glass with an expansion 25% lower than that of Pyrex ovenware that was used for casting the 200 in. mirror for Mt. Palomar’s Hale telescope in 1934. The astronomers had sought the lowest possible expansion in their quest for a mirror that would show the least distortion with temperature changes. This glass also had the low liquidus tem￾perature necessary to survive the extremely slow cooling rate of the cast blank without devitrification or phase separa￾tion.22,45 Glass chemists Harrison P. Hood and Martin E. Nordberg observed during experimentation with borosilicates at Corning in the 1930s that very large changes in properties would occa￾sionally result when some compositions were heat-treated. Fur￾ther investigation revealed that these glasses were separating into two intermingled glassy phases, one of which was silica￾rich. A composition was developed that separated upon heat treatment into a very-high-silica phase and a very-alkali-rich phase, with the latter being easily dissolved and leached out by hot nitric acid. The remaining porous, silica-rich skeleton could then be consolidated into a solid, pore-free form by heating. This glass, given the name Vycort, in 1939, could be melted and formed easily before the leaching and consolidation steps. The 96% silica composition of the finished glass meant that it had physical and chemical properties closely approaching those of pure-silica glass, yet it could be fashioned before heat treat￾ment into shapes that were impossible to form with fused silica because of the extremely high viscosity of silica glass at even very high temperatures.45 This is an example of a new glass composition being created by carefully exploiting the observa￾tion of a new phenomenon. Another important step in borosilicate glass composition de￾velopment also occurred in the 1930s with the introduction of E-glass fibers. A fiberglass material was needed for electrical insulation applications, and standard soda–lime–silica compo￾sitions were not suitable, because their conductivity was too high. Soda–lime glasses also proved generally unsuitable for fiberglass, because the great surface area to volume ratio made them particularly vulnerable to attack and dissolution by water. Accordingly, Urban Bowes, R. A. Schoenlaub, and others at Owens–Corning Fiberglas reduced and ultimately removed the alkalis and added boric oxide and alumina and increased the lime.46 The boric oxide and lime reduced viscosity and in￾creased durability, and the alumina also helped durability and lowered the liquidus. The resulting lime–alumina–borosilicate glass (see glass 14 in Table I) had excellent electrical resistiv￾ity, superior resistance to attack by moisture, and good me￾chanical properties. Therefore, it is not surprising that glasses of this general composition make up 90% of the continuous glass fiber currently produced.46 (Historical note: In this same publication, Dumbaugh and Danielson47 reference a 1925 pat￾ent issued to F. M. and F. J. Locke48 for a series of alumino￾silicates that is broad enough to include these alkaline-earth aluminoborosilicates.) Also, most commercial fiberglass com￾positions are based on ternary or quaternary eutectics, thus taking advantage of the well-known fact that the most stable glasses are often close to eutectic compositions. (3) Photosensitive Glasses and Glass-Ceramics No account of the history of glass compositions would be complete without some reference to the discovery of glass￾ceramics. Accordingly, a brief description of the early work on glass-ceramics is presented here. The natural tendency of glasses to devitrify must have led glassmakers occasionally over the ages to consider pursuing the phenomenon to its logical conclusion—a completely crys￾talline product. The French chemist Re´aumur is known to have attempted to produce crystalline vessels by holding glass bottles packed in gypsum at a red heat for several days. Al￾though these did devitrify to a crystalline form, he was unable to control the process, and the bottles were deformed and of low strength.49 Others experimented with the limited precipi￾tation of crystals from glass to create ruby or opal glasses, but the development of a process to control the massive crystalli￾zation of bulk glass did not occur until the middle of the 20th century. There was a series of glass composition inventions during the 1940s and 1950s based upon nucleation and controlled growth of crystals experiments conducted at Corning Glass Works. The work was led by S. Donald Stookey, but ultimately involved several others, including Armistead, Beall, Mac￾Dowell, Araujo, Rittler, and Grossman. Stookey had been investigating nucleation and precipitation of crystals in ruby and opal glasses and found, in 1942, that copper, gold, and silver could be deposited as tiny particles of metal through photo-induced heterogeneous nucleation. The process was aided by the presence of a ‘‘sensitizer,’’ such as cerium or tin. Taking this a step further in 1951, he learned he could photo-induce a sodium fluoride opal in a silicate glass nucleated with silver (Fotalitet).50 Stookey made an unanticipated discovery, in 1954, of the controlled crystallization of glass ceramics. He had invented, in the late 1940s, a process for preparing lithium silicate crystal images in glass with a photosensitive technique (Fotoformt). Very small quantities of silver (∼0.06 mol%) were introduced in the lithium silicate glass as a nucleating agent, and, after selective exposure to ultraviolet radiation, a heat treatment at ∼600°C caused lithium metasilicate crystals to form that then could be leached out of the unexposed glass. One day, Stookey placed a plate of preexposed lithium sili￾cate glass in a laboratory oven to perform the 600°C heat treatment. The temperature controller stuck in the ‘‘on’’ posi￾tion and the glass was heated to 900°C, where it normally would be quite soft and fluid. Stookey was alarmed by the overheating, and he was certain that he had ruined the oven. He knew that this glass melted and flowed below 700°C, and he believed that it would flow on to the floor of the oven. How￾ever, in his own words, ‘‘Imagine my astonishment on opening the door to see an undeformed, opaque solid plate! Snatching a pair of tongs, I immediately pulled the plate out of the hot furnace, but it slipped from the tongs and fell to the tile￾covered concrete floor, clanging like a piece of steel but re￾maining unbroken! It took no great imagination to realize that this piece of Fotoform was not glass, but something new and different. It must have crystallized so completely that it could not flow, even though the temperature was more than 200°C above the softening temperature of the glass. And obviously it was much stronger than ordinary glass.’’50 An examination of the fine-grained glass-ceramic that had been formed revealed that it was composed of lithium disilicate and quartz crystals, and was much harder and higher in electrical resistivity than April 1998 Perspectives on the History of Glass Composition 803

Journal of the American Ceramic SocietyKurkjian and Prindle Vol 81. No 4 regular glass. This episode demonstrates the power of the pre- in glass science to date. These rules describe what he consid- pared mind in recognizing a seminal event: Stookey quickly ered to be the necessary conditions for glass formation realized that, theoretically, all glasses can be converted to crys n oxygen is linked to not more than two central atoms talline bodies having new properties that depend on the nature The number of oxygen atoms surrounding a central atom of the particular crystals formed. I A vigorous research and must be small development program was then followed by Stookey, Beall, gen polyhedra share corners--not edges or faceswith nd others at Corning that continues to this day and has pro- duced many very useful glass-ceramics At least three corners in each oxygen polyhedra must be Stookey later(1959)made another discovery based on pho- tosensitive precipitation work with the invention of photochre Over the years, the validity of these rules has been debated nic glasses, i.e., glasses that darken when exposed to sunl Recently, many workers have argued for, b against, 62 and and regain their clarity when the ultraviolet radiation is re- about these rules and the"continuous random network"to moved. Stookey, acting on a suggestion from Armistead, in- The rules have been discussed and troduced silver halides(chlorides and bromides)in small quan- They also have been modernized in th tities(-0.5 wt%)to glasses that had been doped with a copper hat the topological basis of these rules has been extended ensitizer. The glasses were heat-treated at 600C to precipitate Gupta and Cooper and others 6s) saturated microcrystals of silver halide. when photons That such structures(as proposed by Zachariasen)were in- strike the microcrystals, some of the silver is reduced to the deed found in simple glasses was first demonstrated by Warrer metallic Ag state, with the electron being borrowed from a and co-workers(Fig 4(a)66, 67 using X-ray diffractometry,The chloride ion. The metallic silver particles color the glass gray or darken it. when the ultraviolet source is removed. the me- (a) tallic silver is oxidized back to Ag, and the glass clears. 52 v. Structure and Properties As stated earlier, the early soda-lime-silica glass composi- tions were very close to the standard soda-lime-silica glasses that are currently in use. The fact that they were first discov commercial composition, remains somewhat of a surprise However, the raw materials were relatively available, and com- positions that were very different would probably either cry tallize, dissolve or be unmeltable, and, therefore, in a sense this may be considered a Darwinian result. Shortly after the start of the glass science era, i.e., the early 1920s, after Schott's work, the opening of the Corning Research Laboratory in 1908 and the formation of the Department of Glass Technology(ini- tially named Department of Glass Manufacture)in 1915 under the direction of professor w.e. s. turner in Sheffield. En- gland, the new tool of X-ray diffractometry was first applied to the study of silicate crystals, and then to silicate glasses.53 This led to an overall attack by the scientific community on the blem of the understanding of this unusual material () History of Glass Structure Studies Although it was very early realized by Tammann that glass formation was a kinetic phenomenon, it also was clear that the kinetic processes involved were controlled by the details of the structure of the materials involved. Although, in 1926, Gold schmidt indicated that SiO2, GeO,, B,O3, P2Os, As,O As,S3, Sb,O3, as well as the(silica)model, BeF, were all able (b) to form glasses by themselves (i. e, they were glass-formers), the history of the discovery of the existence of these simple inorganic glass-formers was not clear. Even in the case of ilica, the first recorded instance of the recognition of its ability to form a glass on its own is not clear. Sosman points out that n 1813, Marcel formed glassy silica by heating small quartz crystals in an oxygen-injected alcohol lamp. Again, a check of the literature shows that, according to rawson, 7 in 1834, Ber- zelus melted glasses in several tellurite systems, while, in 1868, Roscoe investigated a series of Ba0-V2Os glasses. A indicated earlier, in the mid-1800s, Harcourtmelted mainly phosphate glasses, because he found that silicate glasses w The first detailed descriptions of the expected"crystal structures and the reasons that such structures formed glasses were proposed by Hagg, Goldschmidt, and Zacharia- discovery of X-rays had studied crystal structures with this tool and, therefore, quite understandably approached the problem of Fig 4.(a)Two-dimensional representation of a disordered sodium glass formation and structure from the results of those studies. silicate network. 66(b)Continuous random network model of vitreous Arguably, Zachariasen's rules constitute the most famous work

regular glass. This episode demonstrates the power of the pre￾pared mind in recognizing a seminal event: Stookey quickly realized that, theoretically, all glasses can be converted to crys￾talline bodies having new properties that depend on the nature of the particular crystals formed.51 A vigorous research and development program was then followed by Stookey, Beall, and others at Corning that continues to this day and has pro￾duced many very useful glass-ceramics. Stookey later (1959) made another discovery based on pho￾tosensitive precipitation work with the invention of photochro￾mic glasses, i.e., glasses that darken when exposed to sunlight and regain their clarity when the ultraviolet radiation is re￾moved. Stookey, acting on a suggestion from Armistead, in￾troduced silver halides (chlorides and bromides) in small quan￾tities (∼0.5 wt%) to glasses that had been doped with a copper sensitizer. The glasses were heat-treated at 600°C to precipitate supersaturated microcrystals of silver halide. When photons strike the microcrystals, some of the silver is reduced to the metallic Ag0 state, with the electron being borrowed from a chloride ion. The metallic silver particles color the glass gray, or darken it. When the ultraviolet source is removed, the me￾tallic silver is oxidized back to Ag+, and the glass clears.52 V. Structure and Properties As stated earlier, the early soda–lime–silica glass composi￾tions were very close to the standard soda–lime–silica glasses that are currently in use. The fact that they were first discov￾ered by accident, and yet are fortuitously close to the ideal commercial composition, remains somewhat of a surprise. However, the raw materials were relatively available, and com￾positions that were very different would probably either crys￾tallize, dissolve, or be unmeltable, and, therefore, in a sense, this may be considered a Darwinian result. Shortly after the start of the glass science era, i.e., the early 1920s, after Schott’s work, the opening of the Corning Research Laboratory in 1908, and the formation of the Department of Glass Technology (ini￾tially named Department of Glass Manufacture) in 1915 under the direction of Professor W. E. S. Turner in Sheffield, En￾gland, the new tool of X-ray diffractometry was first applied to the study of silicate crystals, and then to silicate glasses.53 This led to an overall attack by the scientific community on the problem of the understanding of this unusual material. (1) History of Glass Structure Studies Although it was very early realized by Tammann54 that glass formation was a kinetic phenomenon, it also was clear that the kinetic processes involved were controlled by the details of the structure of the materials involved. Although, in 1926, Gold￾schmidt55 indicated that SiO2, GeO2, B2O3, P2O5, As2O3, As2S3, Sb2O3, as well as the (silica) model, BeF2, were all able to form glasses by themselves (i.e., they were glass-formers), the history of the discovery of the existence of these simple inorganic glass-formers was not clear. Even in the case of silica, the first recorded instance of the recognition of its ability to form a glass on its own is not clear. Sosman56 points out that, in 1813, Marcel formed glassy silica by heating small quartz crystals in an oxygen-injected alcohol lamp. Again, a check of the literature shows that, according to Rawson,57 in 1834, Ber￾zelius melted glasses in several tellurite systems, while, in 1868, Roscoe investigated a series of BaO–V2O5 glasses. As indicated earlier, in the mid-1800s, Harcourt39 melted mainly phosphate glasses, because he found that silicate glasses were ‘‘pasty.’ The first detailed descriptions of the expected ‘‘crystal’’ structures and the reasons that such structures formed glasses were proposed by Ha¨gg,58 Goldschmidt,55 and Zacharia￾sen.59,60 They were crystallographers/chemists who, since the discovery of X-rays had studied crystal structures with this tool and, therefore, quite understandably approached the problem of glass formation and structure from the results of those studies. Arguably, Zachariasen’s rules constitute the most famous work in glass science to date. These rules describe what he consid￾ered to be the necessary conditions for glass formation: ● An oxygen is linked to not more than two central atoms; ● The number of oxygen atoms surrounding a central atom must be small; ● Oxygen polyhedra share corners—not edges or faces—with each other; ● At least three corners in each oxygen polyhedra must be shared. Over the years, the validity of these rules has been debated. Recently, many workers have argued for,61 against,62 and about these rules and the ‘‘continuous random network’’ to which they imply. The rules have been discussed and updated by Cooper.61,63 They also have been modernized in the sense that the topological basis of these rules has been extended (Gupta and Cooper64 and others65). That such structures (as proposed by Zachariasen) were in￾deed found in simple glasses was first demonstrated by Warren and co-workers (Fig. 4(a))66,67 using X-ray diffractometry. The Fig. 4. (a) Two-dimensional representation of a disordered sodium silicate network.66 (b) Continuous random network model of vitreous silica.74 804 Journal of the American Ceramic Society—Kurkjian and Prindle Vol. 81, No. 4