Chemistry for Everyone
Glass Doesn’t Flow and Doesn’t Crystallize and It Isn’t a Liquid Stephen J. Hawkes Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003;
[email protected] Glass is widely believed to flow in historic time, and there are science teachers who believe it. As a beginning teacher I taught it myself. Mea culpa. It has been asserted in the popular press (1, 2) and even in scientific literature and introductory texts (3–5). This paper reviews the evidence and reaches the conclusions in the title. Historic Glass One approach to this enquiry is to ask conservators who routinely work with antique glass whether they have observed it overlapping the fixtures at the bottom of the glass, or whether the glass is thinner at the center of glass windows where the flow would be greatest than at the edges where it would be slowest, and whether there is thicker glass at the bottom. They have denied repeatedly and consistently that they have observed such phenomena (6–8; personal communication with Moore, D., Colonial Williamsburg Foundation). Gibson states that in a lifetime of dismantling medieval glass he has seen hundreds of pieces that were thicker at the top (7). So it is unnecessary to invoke the “explanation” that artists setting the glass pieces would be likely to set the thicker part lower. No statistical study of thickness of old window glass has ever been found. In the light of their experience, glass scientists are unlikely to undertake such research. A report on colonial glass at Williamsburg describes how the method of manufacture necessarily produced uneven glass, which would be thicker in some places than in others (9). Flow in Prolonged Time If glass showed perceptible flow in a few centuries, then some volcanic glasses would show substantial flow in geologic time. It would penetrate crevices in other rocks and form blobs of flattened glass. The discovery of such formations would be clear evidence of flow (albeit very slow flow) but such phenomena have not been reported.1 Similarly, astronomical mirrors show no deformity after standing for more than a century, although it is asserted that expected deformation from the alleged flow would be observable and ruinous (4 ). Flow under High Pressure? Bridgman provided evidence on this question by his experiments on glass under extreme pressure (10, 11). He found that there was a short period of flow as the glass was compressed but then no further flow. When flow was prevented, the glass could not be compressed. So there is no flow in the normal sense of the word but the phenomenon is better interpreted as a molecular rearrangement.
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Surface Flow A curious type of flow has been reported in the surface layer of glass that has been rapidly cooled. When glass is scratched with a fine diamond point, several surface effects have been observed, one of which—on glass that has cooled quickly—is a form of surface flow. Peychès explains that “There exists a surface skin of more or less appreciable thickness in which the molecules are less strongly bonded than in the rest of the mass of the glass, which has been chilled less rapidly. In these surface layers viscous flow takes place when the glass is subject to stress at low temperature. Only fine annealing can cause such mobility to disappear” (12). If this really is regular viscous flow, it follows that there is a form of glass which, if it could be created in bulk, would flow under pressure. However, it could not apply to ancient window glass or any other glass that presently exists. Glass workers know that in order to cut glass it is necessary to break it at a scratch that has been made no more than two minutes earlier. Otherwise it will heal, appearing the same but losing its ability to guide the crack (13). It has been suggested that this is the result of glass flow in the freshly made groove, but it seems more likely that it is related to the formation of the hydrated layer, which is known to occur within five minutes (14). Measurement of Viscosity of Cold Glass There is a rumor that the viscosity of cold glass has actually been measured. I have been unable to find any literature reference to such measurement and none of the people who suggested it to me has been able to supply a clue that leads me to one. Literature values that I have found have in every case been the result of extrapolation from high temperatures. If any reader provides a clue that leads me to a viscosity measurement on cold glass, I will offer the editor a retraction. Such a measurement would have to measure actual flow against a much greater background of anelastic deformation and would also have to allow for changes in the microstructure of the glass during the experiment. I believe this is not possible with present technology. A review of various methods of determining glass viscosity (15) shows none that is useful above 1015 Pa s, so the much higher values reported for cold glass must presumably be obtained by theoretical extrapolation. No evidence for the flow of cold glass can be found from viscosity measurement using present technology. There is no evidence to support a belief that viscosity could be measured with sufficiently sensitive technology at some future time.
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
Chemistry for Everyone
Supercooling It is often asserted that glass is a supercooled liquid. When molten glass is cooled it eventually adopts the glassy state at the “glass transition temperature”. This temperature varies with the rate of cooling so that the material may be a liquid at the same temperature at which, under other circumstances, it would be a glass. Such a liquid is said to be “supercooled” and eventually undergoes the glass transition at a lower temperature than if it had not supercooled. The properties of the supercooled liquid are a simple extrapolation of the properties of the melt until it reaches the glass transition. At the glass transition the rate of change of most physical properties with temperature undergo a sharp transition: specifically, the graphs of length and of heat capacity and especially of entropy against temperature have marked changes in slope (16 ). Cool glass is substantially different from the supercooled liquid (even at the same temperatures) in these and other measurable properties, as well as in the obvious properties that are known to artisan glass workers. Deformation When glass is stressed it undergoes an immediate elastic deformation, which is followed by a slow inelastic deformation. When the stress is removed, the glass reverts to its original state. Since the inelastic reversion is slow, this has sometimes been mistaken for a permanent deformation and used as evidence for flow or that glass is a liquid. Spencer’s classic experiment shows that this claim is a misinterpretation (17, 18). The earliest reference to this kind of phenomenon that has come to light was the statement by Ostwald in an 1893 text that glass tubing “must be kept lying flat, otherwise it becomes permanently curved” (19). The later work of Rayleigh and others shows that this is untrue (20). Crystallization (Devitrification) It is also widely believed that glass crystallizes spontaneously in historic time. One recent introductory science text (now replaced by a later edition) had a picture of an Egyptian glass bowl with some white patches, which were said to be incipient crystallization. However, deterioration of glass is the result of attack by water or water vapor (14 ). This causes a crust of hydrated silica, which appears as a white surface on the glass. This may be followed by further chemical action such as leaching of the metal ions or attack by carbon dioxide, causing further deterioration, which appears as further encrustation or pits. Glass displayed in museums has been known to deteriorate visibly in as little as a few months when the humidity is not carefully controlled (21). In geologic time this process converts volcanic glass to perlite, an opalescent hydrate of the original glass (22). A better test of crystallization over geologic time would be to examine glass from extraterrestrial bodies that have no atmosphere to corrode it. Particles of moon glass have microscopic pits and grooves that appear to be the result of micrometeorites striking the glass surfaces (23), but no example has been reported of moon glass that has visibly crystallized after it has cooled. Future crystallographic examination
may show microcrystallites, and this would be evidence of geologically slow crystallization. Such a possibility is addressed theoretically by Kny and Nauer (24 ). They calculated that crystallization could not reach a volume fraction of 10᎑6 in less than 1000 years under the most favorable conditions, and 106 to 1017 years under realistic conditions. Newton quotes them as believing that they have found microcrystallites about 20 nm in size in early medieval glass, but comments “However, if so, it is so rare as to be negligible” (25). There has been no subsequent report of such microcrystallites in antique glass. Kny and Nauer also quote Besborodov (26 ) as saying that obsidian has many crystalline inclusions, whereas tektite samples do not. They account for this by the larger concentration of OH groups in the obsidian, which has 100 times more water than the tektites. If crystallization depends on the presence of water, the crystals may be perlite rather than crystallized glass. Such microcrystallites would be significant in glass science, but would not be relevant to the visible deterioration of antique glass. Students should not be taught that crystallization can occur in historic time. Deterioration in ancient glass is explained, by those best qualified to discuss the matter, as the result of chemical attack, a phenomenon that has been the subject of much research (14 ). Structure of Glass The belief that glass is a liquid is often supported by the assertion that its molecular structure is random like a liquid’s. Glass actually has a number of states with entropy minima, which are therefore nonrandom (27). They can be interconverted by heat, pressure or strain. A well-known example of such interconversion is “annealing”. If liquid glass is cooled quickly it solidifies in a molecular arrangement that is unstable and may shatter spontaneously. Glassblowers routinely transform this into a more stable state by annealing it at a temperature below the melting point. Angell gives the following theoretical denial in an authoritative paper on the physics of glass (16 ): The fact that glasses are brittle solids at temperatures below their glass transition temperatures implies that the arrangement of particles taken up as a liquid cools below Tg can be described by a point in configuration space near the bottom of a potential energy minimum in this space. If this were not so, the system would move in the direction dictated by the collective unbalanced force acting on it, and some sort of flow would occur. Notwithstanding the legend about medieval cathedral windows, this does not occur in glassy systems held at temperatures less than half their glass transition temperatures, even on geological time scales.
Rather than having an amorphous arrangement, glasses form as a weak reflection of a three-dimensional crystal structure, which is so energetically incompetent that it can barely compete with the disordered form (16 ). Texts routinely show diagrams contrasting the regular hexagonal arrangement of crystalline silica with a less ordered arrangement of contiguous polygons of varying size in glass. Such an arrangement is as rigid as the silica, but perhaps less stable thermodynamically.
JChemEd.chem.wisc.edu • Vol. 77 No. 7 July 2000 • Journal of Chemical Education
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Chemistry for Everyone
So? Whether glass is a liquid must depend on its ability to flow, or to spontaneously adopt a new conformation of equal energy. This has not been demonstrated either theoretically or experimentally, so an assertion that cold glass is a liquid must be regarded as incorrect. Students should not be taught that glass is a liquid because such teaching creates a mental concept that is divorced from reality. It leads inevitably to the fallacies that glass flows or crystallizes after a long enough time. Even if a sensible argument could be discovered that glass is a liquid in some esoteric sense, it would create more confusion than enlightenment. Glass is a rigid solid with a lower degree of molecular order (higher entropy) than a crystal but with greater molecular order (lower entropy) than a liquid. Note 1. It has been argued that volcanic glass has a higher silica content than historic window glass and would therefore have greater viscosity, flow more slowly, and perhaps not be deformed even in geologic time. Actual data do not support this. Tables of composition of geological glasses give silica content varying from 35 to 76%, whereas the SiO2 content of antique glass varies from 50 to 75%.
Literature Cited 1. Vos Savant, M. Ask Marilyn; Parade Magazine, Nov. 19, 1995, p 19. 2. Conroy, H. In Glasgow Herald; 11 March 1996. 3. Tolman, C. A.; Jackson, N. B. In Essays in Physical Chemistry; Lippincott, W. D., Ed.; American Chemical Society: Washington, DC, 1988; p 22. 4. Plumb, R. C. J. Chem. Educ. 1989, 66, 994–996. 5. Resnick, R.; Halliday, D.; Krane, K. S. Physics, 4th ed.; Wiley: New York, 1992; p 377. 6. Newton, R., Davison, S. In Conservation of Glass; Newton, R.; Davison, S. Eds.; Butterworth-Heinemann: Woburn, MA, 1996; p 13.
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7. Reese, K. M. Chem. Eng. News 1990, 69(Feb 26), 168. 8. Gibbs, P. Is Glass Liquid or Solid? http://math.ucr.edu/home/baez/ physics/glass.html (accessed Jan 2000). 9. Davies, I. Window Glass in Eighteenth Century Williamsburg; Report AR46; Colonial Williamsburg Foundation: Williamsburg, PA, 1970. 10. Bridgman, P. W. Sˇ imon, I. J. Appl. Phys. 1953, 24, 405–413. 11. Bridgman, P. W. Proc. Am. Acad. Arts Sci. 1952, 81, 170. 12. Peychès, I. J. Soc. Glass Technol. 1952, 36, 178. 13. Berlye, M. K. The Encyclopedia of Working with Glass; Oceana: New York, 1968; p 17. 14. Newton, R. G. In Conservation of Glass; Newton, R.; Davison, S. Eds.; Butterworth-Heinemann: Woburn, MA, 1996; Chapter 4. 15. Scholze, H.; Kreidl, N. J. In Glass Science and Technology, Vol. 3; Uhlmann, D. R.; Kreidl, N. J., Eds.; Academic: New York, 1986; pp 234–236. 16. Angell, C. A. Science 1995, 267, 1925. 17. Preston, F. W. J. Am. Ceramic Soc. 1935, 18, 220. 18. Preston, F. W. J. Appl. Phys. 1942, 13, 626. 19. Ostwald, W. Manual of Physico-chemical Measurements; Walker, J., Translator; Macmillan: London, 1894; p 66. 20. Preston, F. W. Glass Technol. 1973, 14, 20–30. 21. Brill, R. H. IIC Congress on Conservation in Archeology and the Applied Arts; Stockholm, 1975; pp 121–134. Quoted from Conservation of Glass; Newton, R.; Davison, S. Eds.; Op. cit.; p 142. 22. Cas, R. A. F.; Wright, J. V. Volcanic Successions, Modern and Ancient; Chapman and Hall: Boston–London, 1987; p 84. 23. Hamblin, W. K.; Christiansen, E. H. Exploring the Planets; Macmillan: New York, 1990; pp 85–86. 24. Kny, E.; Nauer, G. J. Non-Cryst. Solids 1978, 29, 207–214. 25. Newton, R. G. The Deterioration and Conservation of Painted Glass. A Critical Bibliography; Occasional Papers II; British Academy and Oxford University Press: Oxford, 1982; pp v, 58. 26. Bezborodov, M. A. Chemie und Technologie der antiken und mittelalterlichen Gläser; Philipp von Zabern: Mainz, 1975. 27. Angell, C. A. J. Phys. Chem. Solids 1988, 49, 863–871.
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
