GUESTAUTHOR David Dingledy The Ohio Stote University
Columbus
Textbook Errors, 36
Flow of Glass Under Its Own Weight
In the days of the Venetian republic the pr11dty for rvrcnling t l w rurnposirion rind pn,pt.rrics of zl:~qawas denth. In more rrwnt times this itnformntion was carefully guarded by codes and symbols rivalling those of the alchemist. At the present time, however, there are good sources of information on glass in libraries for ceramics and glass technology (1-4). Unfortunately these standard reference works are not usually available to the beginning chemistry student in his chemistry department library. A common misconception of the nature of glass found in general chemistry texts' is that ordinary glass will flow under its own weight at room temperature. Such a notion is contrary to common experience with the many glass objects used in everyday life; we do not expect to see our picture windows end up as puddles on the window sill! The most striking refutation lies in the use of glass as the material of construction for large telescope lenses and mirrors, such as the 200 inch diameter Palomar mirror. Even very slight flow in these accurately ground and polished volumes of glass would make the instruments worthless. The conclusion drawn from these practical observations-that glass is dimensionally stable-is reinforced by a consideration of the viscosity involved. Although it is not possible to measure the viscosity of ordinary glass directly a t room temperature, vahes have been obtained by the extrapolation to lower temperatures of viscosity curves obtained empirically a t higher temperatures, from 500' on up. A direct extrapolation gives values above 10loOpoises a t room temperature; however, this assumes that viscous equilibrium has been maintained in the cooling. Actually a somewhat lower viscosity must result from the freezing-in of higher temperature equilibria. One value that has been suggested is 10Z1poises (5). If it is decided that the values obtained by extrapolation should be accepted, then the flow can be calculated using the lower estimates of the viscosity of ordinary glass a t room temperature and setting the load a t the maximum, i.e., the breaking tension. I n
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practice, this stress can only be approached in very careful laboratory work. The result of the calculation is that the fractional increase of the length of the glass under maximum tensile loading amounts to only 10-"er year (6). This would result in the extension of a glass rod one meter long by one-tenth of a millimeter in ten years. It is apparent that flow of this magnitude would not be seen by the casual observer. The classical experiments designed to measure the flow of glass at room temperature were reported over 30 years ago by Lord Rayleigh and C. D. Spencer in separate articles having the same title: "Do glass tubes and rods bend under their own weight?" (7, 8). Both arrived at the same conclusion: glass rods and tubes of mature age do not bend in the manner suspected. Rayleigh had suspended a relatively heavy load from the center of a glass rod on horizontal supports one meter apart for seven years. Spencer had done the same with a glass tube for six years. In addition, Spencer commented that the tube, which would roll on a flat surface before the test, showed a "permanent" deformation a t the center of nine millimeters at the conclusion of the experiment. It is of interest to note that this comment has been used to support the notion that glass flows under its own weight at room temperature (9),despite Spencer's conclusion to the contrary. Further work by Spencer then elucidated at least partly the nature of the deformation (10). Glass fibers were stressed by winding them about tubes of 2-cm diameter and then stored at somewhat above room temperature. Upon release from the tubes after storage the fibers were bowed to a radius of about sixty centimeters. However, upon floating the bent fibers on mercury they straightened out, rapidly a t first and more slowly lat,er. Therefore, the deformation was not the result of viscous flow, which is not reversed upon release of loading. I n view of the misinterpretation of this reversible deformation as viscous flow, it is desirable to know the true basis for this phenomenon. Fortunately, the explanation also serves for a number of other peculiarities in the behavior of glass which until recently had not been considered the results of a common structural mechanism. Among these are the specification of "mature" glass by Rayleigh and Spencer and the related observation of the secular ice point rise in thermometers. The latter phenomenon, which in earlier days required the storage of thermometer tubing to "mature" it in order to prevent small dimensional changes in the glass from affecting the calibration of the
finished thermometer, is of course directly related to the dimensional stability of glass. Although glass is very near to being the perfect elastic solid, as shown by the use of "fused quartz" glass fibers in torsion balances for measuring extremely small weights as in the measurement of the adsorption of vavors on solids, anelastic effects are also exhibited by glass. The anelasticity in a soda-lime-silica glass (ordinary window and bottle glasses are soda-lime-silica glasses) has been shown (11) to be due to the diffusion of sodium ions in the structnre of the glass in response to the applied stress. If the stress is a continuing one, as in poorly annealed glass, the structure eventually accommodates itself to the stress inasfar as possible by the relocation of the sodium ions by diffusion, and the glass becomes "mature." On the other hand, if a stress is applied by external loading as in Rayleigh's and Spencer's experiments, and then later removed, there is a gradual disappearance of the time-dependent portion of the strain by the reverse diffusion. The difference between the above process and a viscous flow may be illustrated by reference to the formal picture of the structure of glass, in which the sodium ions are seen as occupying holes in a random silicon-oxygen network (18). Anelastic effects are the result of the motion of the sodium ions through openings in the network in response to the elastic distortion of the network under stress, thereby relieving to some small extent the induced strain. Upon removal of the stress the reverse motion takes place to restore the original structure, again not instantaneously since the diffusion is a time-dependent process. These effects can be displayed in a vibrational absorption spectrum (acoustic spectrum) in which the peaks correspond to frequencies of anelastic energy absorption due to diffusion. Experimental determination of the activation energy for diffusion of sodium ion measured in this way results in values of 16-20 kcal per mole, depending on the state of annealing of the glass. On the other hand, viscous effects are those resulting from the breaking of bonds within the silicon-oxygen structure, permitting portions of the network to move in relation to other portions when a stress is applied.
Since the flow is proportional to the stress there is on residual strain in the structure upon removal of the stress and thus no reversibility of flow. Finally, two minor related points may be mentioned. First, glasses under extremely high compressive stress become more dense (IS). However, the same phenomenon is noted also in crystalline substances. The change here is one of compression of the entire glass or crystal network with consequent shortening of the average interatomic distances. Second, the physico-chemical view of glass as a liquid has becn used as an argument for the flow of glass a t room temperature. While such a view is necessary to understand the continuity in the physical properties of glass in the region of transition between liquid and solid, it would be better to return to the fundamental distinction between liquid and solid to classify glass a t room temperature. If a liquid is defined as matter in a state which adjusts itself to the shape of the container while retaining a definite volume and a solid as matter in a state which retains it,s shape even when acted upon by outside forces, then there is no difficulty in classifying glass as a solid. We may therefore summarize: It is theoretically impossible for glass to flow under its own weight at room temperature. Experiments performed to measure this flow have shown that there is none. Distortion of glass in these experiments has been shown to be reversible and due to a diierent process than flow. Literature Cited (1) MOREY, GEORGE, "Properties of Glass" (ACS Monograph), Reinhold Publishing Corp., New York, 1938. (2) JONES,G. O., "Glass," Methuen and Co., London, 1956. (3) TURNER, W. E. S., Ed., "Constitution of Glass," Society of Class Technology, London, 1927. (4) TOOLEY, F. V., Ed., "Handbook of Glass Manufacture," Ogden Publishing Co., New York, 1953. (5) CONDON, E. U., AND ODISHAW, H., "Handbook of Physics," Vol. 8, McGraw-Hill Book Co., N. Y., 1958, p. 8-89. (6) JONES, C. O., op. eit., p. 71. (7) RAYLEIGH, LORD,Nature, 125, 311 (1930). (8)SPENCER, C. D., Nature, 125, 707 (1930). R., "Plasticity, Elasticity, and Structure of (9) HOUWINK, Matter," Cambridge University Press, London, 1937, p. 133. (10) PRESTON, FRANK, J. App. Phya., 13, 626 (1942). (11) FITZGERALD, J. V., J. Am. Ceram. Soc., 34, 314, 339, 388, 390 (1951). (12) ZACHARIA~EN, W. H., J . Am. Chem. Soe., 54, 3841 (1932). (13) BRIDGMAN, P. W., Ann. Report Smithsaian Inst., 1918, p. 185; 1925, p. 157; 1951, p. 199.
Volume 39, Number 2, Februory 1962
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