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 thanCorning continued to explore borosilicate glass compositions 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. Therefore, 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. According 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 Corning for special applications that would benefit from low expansion and corrosion-resistant glasses. One noteworthy example 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 temperature necessary to survive the extremely slow cooling rate of the cast blank without devitrification or phase separation.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 occasionally result when some compositions were heat-treated. Further investigation revealed that these glasses were separating into two intermingled glassy phases, one of which was silicarich. 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 treatment 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 observation of a new phenomenon. Another important step in borosilicate glass composition development 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 compositions 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 increased 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 resistivity, superior resistance to attack by moisture, and good mechanical 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 patent issued to F. M. and F. J. Locke48 for a series of aluminosilicates that is broad enough to include these alkaline-earth aluminoborosilicates.) Also, most commercial fiberglass compositions 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 glassceramics. 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 crystalline 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. Although 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 precipitation of crystals from glass to create ruby or opal glasses, but the development of a process to control the massive crystallization 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, MacDowell, 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 silicate glass in a laboratory oven to perform the 600°C heat treatment. The temperature controller stuck in the ‘‘on’’ position 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. However, 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 tilecovered concrete floor, clanging like a piece of steel but remaining 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