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 workregular 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