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 glassmakingfinished 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