Chemistry for Everyone
846 Journal of Chemical Education Vol. 77 No. 7 July 2000 JChemEd.chem.wisc.edu
Glass Doesnt Flow and Doesnt Crystallizeand It Isnt a Liquid
Stephen J. HawkesDepartment of Chemistry, Oregon State University, Corvallis, OR 97331-4003; firstname.lastname@example.org
Glass is widely believed to flow in historic time, and thereare science teachers who believe it. As a beginning teacher Itaught it myself. Mea culpa. It has been asserted in the popularpress (1, 2) and even in scientific literature and introductorytexts (35). This paper reviews the evidence and reaches theconclusions in the title.
One approach to this enquiry is to ask conservators whoroutinely work with antique glass whether they have observedit overlapping the fixtures at the bottom of the glass, orwhether the glass is thinner at the center of glass windowswhere the flow would be greatest than at the edges where itwould be slowest, and whether there is thicker glass at thebottom. They have denied repeatedly and consistently that theyhave observed such phenomena (68; personal communicationwith Moore, D., Colonial Williamsburg Foundation). Gibsonstates that in a lifetime of dismantling medieval glass he has seenhundreds of pieces that were thicker at the top (7 ). So it isunnecessary to invoke the explanation that artists settingthe glass pieces would be likely to set the thicker part lower.
No statistical study of thickness of old window glass hasever been found. In the light of their experience, glass scientistsare unlikely to undertake such research. A report on colonialglass at Williamsburg describes how the method of manu-facture necessarily produced uneven glass, which would bethicker in some places than in others (9).
Flow in Prolonged Time
If glass showed perceptible flow in a few centuries, thensome volcanic glasses would show substantial flow in geo-logic time. It would penetrate crevices in other rocks and formblobs of flattened glass. The discovery of such formationswould be clear evidence of flow (albeit very slow flow) but suchphenomena have not been reported.1 Similarly, astronomicalmirrors show no deformity after standing for more than acentury, although it is asserted that expected deformationfrom the alleged flow would be observable and ruinous (4 ).
Flow under High Pressure?
Bridgman provided evidence on this question by hisexperiments on glass under extreme pressure (10, 11). Hefound that there was a short period of flow as the glass wascompressed but then no further flow. When flow was pre-vented, the glass could not be compressed. So there is noflow in the normal sense of the word but the phenomenon isbetter interpreted as a molecular rearrangement.
A curious type of flow has been reported in the surfacelayer of glass that has been rapidly cooled. When glass isscratched with a fine diamond point, several surface effectshave been observed, one of whichon glass that has cooledquicklyis a form of surface flow. Peychs explains thatThere exists a surface skin of more or less appreciablethickness in which the molecules are less strongly bondedthan in the rest of the mass of the glass, which has been chilledless rapidly. In these surface layers viscous flow takes placewhen the glass is subject to stress at low temperature. Onlyfine annealing can cause such mobility to disappear (12). Ifthis really is regular viscous flow, it follows that there is aform of glass which, if it could be created in bulk, wouldflow under pressure. However, it could not apply to ancientwindow glass or any other glass that presently exists.
Glass workers know that in order to cut glass it is necessaryto break it at a scratch that has been made no more than twominutes earlier. Otherwise it will heal, appearing the samebut losing its ability to guide the crack (13). It has beensuggested that this is the result of glass flow in the freshlymade groove, but it seems more likely that it is related to theformation of the hydrated layer, which is known to occurwithin five minutes (14).
Measurement of Viscosity of Cold Glass
There is a rumor that the viscosity of cold glass hasactually been measured. I have been unable to find anyliterature reference to such measurement and none of thepeople who suggested it to me has been able to supply a cluethat leads me to one. Literature values that I have found havein every case been the result of extrapolation from hightemperatures. If any reader provides a clue that leads me to aviscosity measurement on cold glass, I will offer the editor aretraction.
Such a measurement would have to measure actual flowagainst a much greater background of anelastic deformationand would also have to allow for changes in themicrostructure of the glass during the experiment. I believethis is not possible with present technology. A review ofvarious methods of determining glass viscosity (15) showsnone that is useful above 1015 Pa s, so the much higher valuesreported for cold glass must presumably be obtained bytheoretical extrapolation.
No evidence for the flow of cold glass can be found fromviscosity measurement using present technology. There is noevidence to support a belief that viscosity could be measuredwith sufficiently sensitive technology at some future time.
Chemistry for Everyone
JChemEd.chem.wisc.edu Vol. 77 No. 7 July 2000 Journal of Chemical Education 847
It is often asserted that glass is a supercooled liquid. Whenmolten glass is cooled it eventually adopts the glassy state atthe glass transition temperature. This temperature varieswith the rate of cooling so that the material may be a liquidat the same temperature at which, under other circumstances,it would be a glass. Such a liquid is said to be supercooledand eventually undergoes the glass transition at a lowertemperature than if it had not supercooled. The propertiesof the supercooled liquid are a simple extrapolation of theproperties of the melt until it reaches the glass transition. At theglass transition the rate of change of most physical propertieswith temperature undergo a sharp transition: specifically, thegraphs of length and of heat capacity and especially of entropyagainst temperature have marked changes in slope (16 ). Coolglass is substantially different from the supercooled liquid(even at the same temperatures) in these and other measurableproperties, as well as in the obvious properties that are knownto artisan glass workers.
When glass is stressed it undergoes an immediate elasticdeformation, 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 beenmistaken for a permanent deformation and used as evidencefor flow or that glass is a liquid. Spencers classic experimentshows that this claim is a misinterpretation (17, 18).
The earliest reference to this kind of phenomenon thathas come to light was the statement by Ostwald in an 1893text that glass tubing must be kept lying flat, otherwise itbecomes permanently curved (19). The later work of Rayleighand others shows that this is untrue (20).
It is also widely believed that glass crystallizes spontane-ously in historic time. One recent introductory science text (nowreplaced by a later edition) had a picture of an Egyptian glassbowl with some white patches, which were said to be incipientcrystallization. However, deterioration of glass is the resultof attack by water or water vapor (14 ). This causes a crust ofhydrated silica, which appears as a white surface on the glass.This may be followed by further chemical action such asleaching of the metal ions or attack by carbon dioxide, causingfurther deterioration, which appears as further encrustationor pits. Glass displayed in museums has been known to dete-riorate visibly in as little as a few months when the humidityis not carefully controlled (21). In geologic time this processconverts volcanic glass to perlite, an opalescent hydrate ofthe original glass (22).
A better test of crystallization over geologic time wouldbe to examine glass from extraterrestrial bodies that have noatmosphere to corrode it. Particles of moon glass have micro-scopic pits and grooves that appear to be the result of micro-meteorites striking the glass surfaces (23), but no examplehas been reported of moon glass that has visibly crystallizedafter it has cooled. Future crystallographic examination
may show microcrystallites, and this would be evidence ofgeologically slow crystallization.
Such a possibility is addressed theoretically by Kny andNauer (24 ). They calculated that crystallization could notreach a volume fraction of 106 in less than 1000 years underthe most favorable conditions, and 106 to 1017 years underrealistic conditions. Newton quotes them as believing thatthey have found microcrystallites about 20 nm in size in earlymedieval glass, but comments However, if so, it is so rare asto be negligible (25). There has been no subsequent reportof such microcrystallites in antique glass. Kny and Nauer alsoquote Besborodov (26 ) as saying that obsidian has manycrystalline inclusions, whereas tektite samples do not. Theyaccount for this by the larger concentration of OH groupsin the obsidian, which has 100 times more water than thetektites. If crystallization depends on the presence of water,the crystals may be perlite rather than crystallized glass. Suchmicrocrystallites would be significant in glass science, but wouldnot be relevant to the visible deterioration of antique glass.
Students should not be taught that crystallization canoccur in historic time. Deterioration in ancient glass is explained,by those best qualified to discuss the matter, as the result ofchemical attack, a phenomenon that has been the subject ofmuch research (14 ).
Structure of Glass
The belief that glass is a liquid is often supported by theassertion that its molecular structure is random like a liquids.Glass actually has a number of states with entropy minima,which are therefore nonrandom (27). They can be interconvertedby heat, pressure or strain. A well-known example of suchinterconversion is annealing. If liquid glass is cooled quicklyit solidifies in a molecular arrangement that is unstable andmay shatter spontaneously. Glassblowers routinely transformthis into a more stable state by annealing it at a temperaturebelow the melting point.
Angell gives the following theoretical denial in an authori-tative paper on the physics of glass (16 ):
The fact that glasses are brittle solids at temperaturesbelow their glass transition temperatures implies that thearrangement of particles taken up as a liquid cools belowTg can be described by a point in configuration spacenear the bottom of a potential energy minimum in thisspace. If this were not so, the system would move in thedirection dictated by the collective unbalanced forceacting on it, and some sort of flow would occur. Not-withstanding the legend about medieval cathedral windows,this does not occur in glassy systems held at temperaturesless than half their glass transition temperatures, even ongeological time scales.
Rather than having an amorphous arrangement, glassesform as a weak reflection of a three-dimensional crystalstructure, which is so energetically incompetent that it canbarely compete with the disordered form (16 ). Texts routinelyshow diagrams contrasting the regular hexagonal arrangementof crystalline silica with a less ordered arrangement of contigu-ous polygons of varying size in glass. Such an arrangement is asrigid as the silica, but perhaps less stable thermodynamically.
Chemistry for Everyone
848 Journal of Chemical Education Vol. 77 No. 7 July 2000 JChemEd.chem.wisc.edu
Whether glass is a liquid must depend on its ability toflow, or to spontaneously adopt a new conformation of equalenergy. This has not been demonstrated either theoreticallyor experimentally, so an assertion that cold glass is a liquidmust be regarded as incorrect.
Students should not be taught that glass is a liquid becausesuch teaching creates a mental concept that is divorced fromreality. It leads inevitably to the fallacies that glass flows orcrystallizes after a long enough time. Even if a sensible argu-ment could be discovered that glass is a liquid in some esotericsense, it would create more confusion than enlightenment.
Glass is a rigid solid with a lower degree of molecularorder (higher entropy) than a crystal but with greater molecularorder (lower entropy) than a liquid.
1. It has been argued that volcanic glass has a higher silica con-tent than historic window glass and would therefore have greaterviscosity, flow more slowly, and perhaps not be deformed even ingeologic time. Actual data do not support this. Tables of composi-tion of geological glasses give silica content varying from 35 to 76%,whereas the SiO2 content of antique glass varies from 50 to 75%.
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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, 2030.21. Brill, R. H. IIC Congress on Conservation in Archeology and
the Applied Arts; Stockholm, 1975; pp 121134. Quoted fromConservation of Glass; Newton, R.; Davison, S. Eds.; Op. cit.;p 142.
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