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Possibilities of progress in glass melting
Journal of Non-Crystalline Solids 73 (1985) 451-46l North-Holland, Amsterdam
451
P O S S I B I L I T I E S OF P R O G R E S S IN G L A S S M E L T I N G Michael CABLE Department of Ceramics, Glasses and Polymers, Sheffield University, UK
The application of scientific ideas to progress in glass melting since about 1750 AD is briefly reviewed with more attention to the past 25 years than more distant times. Some of the present gaps in scientific background and glass melting practice are identified. Some topics in which important progress may be hoped for in the next twenty years are considered.
1. An incomplete history of glass melting The wise person who wishes to predict what is likely to happen in the future will first study development of the subject up to the present. This, of course, is no guarantee of success but the alternatives, such as astrology, are not likely to gain greater support. Glass making has a long history and obviously evolved largely by trial and error before scientific understanding was possible. However, the literature from the past two hundred years that demonstrates clear insight into glass melting, in terms of the best possible when it was written. remains disappointingly meagre. This makes it very difficult to produce a disinterested review of how understanding improved in the relatively recent past. The oldest important contribution to understanding of glass making that displays a critical attitude and attempts to explain observed phenomena in terms of scientific principles appears to be the collected works of Bosc D'Antic [1] (written between 1758 and 1780). Bosc made some notable experiments, such as proving that gall was rich in Na2SO 4 and that white fumes evolved as gall was dissipated were also largely N a z S O 4, but was also obviously limited by the alchemical attitude still prevalent about heat and many aspects of chemistry. For example, the decotorizing action of manganese, by which its purple colour was bleached as it was reduced and iron oxidized from ferrous to ferric, was still assumed to be due to its loss by evaporation. Bosc was very scathing about conventional wisdom that he realized to be unsound and also very insistent that large scale manufacture could not be properly understood only by small scale experiments in a gentleman's study. Another early author whose work is now forgotten but was well regarded in the earlier part of the nineteenth century was another Frenchman, Loysel [2]. whose strong point seems to have been physics rather than chemistry; his book (which appeared in both French and German editions) is the earliest on glass 0022-3093/85/$03.30 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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making to mention temperature scales, making comparisons between Wedgwood's and R6aumur's methods of estimating degrees of heat. Other notable scientific pioneers of the early nineteenth century either left only scanty records or did not publish much of their work. Joseph Fraunhofer (1787-1826) was orphaned at an early age and apprenticed to a glazier. He developed his scientific talents from virtually nothing almost entirely by his own efforts and began to be noticed as a teenager. This led to him becoming a professional glass technologist, first assisting then supplanting Guinand in the development of optical glass manufacture in Bavaria [3]. Although Fraunhofer left some writings about the properties of glasses, especially testing of chemical durability [4], he seems not to have left a significant contribution to the chemistry of glass making; his greatest gifts were in physics. Faraday, surely one of the greatest experimental scientists, spent several years (1825-33) successfully developing methods of producing homogeneous pieces of glass sufficiently large to make good quality lenses for telescopes and other components for optical instruments. His innovations included the use of platinum for melting crucibles and stirrers as well as an effective way of homogenizing molten glass in a shallow tray. Faraday [5] left only one fascinating account of his methods and observations but these seem to have fallen into disuse when he abandoned glass technology for electromagnetism. The archives of the Royal Society or the Royal Institution may contain much more of value to glass making recorded by Faraday but this remains unexplored, see however Usselman [6]. Another elusive pioneer was the Reverend William Vernon Harcourt (1789-1871) who may be considered Yorkshire's earliest glass scientist. He was born a Vernon at Sudbury in Derbyshire but his father inherited the Harcourt estates and also became, in due course, Archbishop of York. William was one of the founder members of the British Association for the Advancement of Science and became its first Secretary [7]. Whilst Rector of Wheldrake (near York) and Canon at York Minister he spent many years (approximately 1834-61) making small scale glass melts in a quest to extend the range of optical glasses available. This work continued until a few days before his death. The range of elements that he included in glasses, melted in a small hydrogen furnace of his own design [8], is very surprising. The glasses that he made included phosphates and borates as well as silicates and he experimented with oxides of Li, Be, B, Na, Mg, A1, Si, P, K, Ca, Ti, V, Cr, As, Mn, Ni, Zn, Sr, Mo, Sn, Sb, Cd, Ba, W, T1, Pb, Bi and U as well as some fluorides and sulphides [9]. His work likewise was never properly recorded. The problems of obtaining homogeneous glass early in the 19th century are demonstrated by the statement of Dollond in 1828 that, in the five preceding years he had been unable to obtain a single disk of flint glass good enough to make an objective of 4½ inch (11.4 cm) diameter [10]. The last third of the nineteenth century saw further notable publications on glass making, the best of which is Bontemps's Guide du Verrier [11] but Peligot [12] and Appert and Henrivaux [13] also deserve to be mentioned. However,
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none of these demonstrates much progress in the chemistry and physics of glass making. The next notable German pioneer was Otto Schott (1851-1935) whose PhD thesis was on the chemistry of glass melting [14]. During his long life he maintained a very keen interest in applying scientific knowledge to glass making and left about two dozen publications but also (and more important in practice) established the firm whose name and repute we all know so well. The impressive early achievements of that firm were documented long ago by Hovestadt [15] but he recorded little about improved understanding of glass melting. It may be appropriate to note here that von Wittorf [16] published a study of reactions between silica and Li2CO 3, Na2CO 3, K2CO 3, Rb2CO 3 and Cs2CO 3 in 1904. During the early part of the twentieth century when Norbert Kreidl was still too young to be able to influence things, say up to 1930 (his first paper appeared in 1929), a few workers such as Cobb [17] Niggli [18] and Hedvall [19] did useful work on reactions involving silica and Bezborodov [20] began similar studies involving boric oxide but no leap forward could be claimed. A notable review by Turner [21] in 1930 shows that, although studies of reactions between alkali carbonates and silica could be traced back nearly eighty years, little progress had yet been made in understanding glass melting. However, Turner did note the beginning of work by differential thermal analysis, the earliest notable work being by Tammann and Oelsen [22], and recognized its importance. On the other hand Turner says nothing about homogenizing and his only comment on refining is, in an aside, to pose one question. Progress in the next twenty years or so was much greater. First of all, 1930 saw the publication of the first important investigation of refining by Gehlhoff, Kalsing and Thomas [23]. Notable largely empirical but very valuable works on batch-free time and the influence of important factors were done by Preston and Turner [24] and Potts and his colleagues [25] but homogenizing and refining remained relatively neglected. There was little recognition at that time of the need to consider both chemical and physical phenomena in glass melting. One of the earliest notable attempts to bring together all aspects of melting was the first edition of Jebsen-Marwedel's popular book [26]. During this period the spirit of pioneers like Vernon Harcourt and Schott had a resurgence and many new compositions were investigated, many for optical purposes; Kreidl had a part in this as is shown by 25 entries under his name in the index to Mazurin's [27] compendium on glass properties. Thermogravimetric analysis was exploited much more widely, admittedly using very crude theoretical models. The most notable work being a series of papers by Kr6ger and his colleagues [28-37]; other interesting studies being by Harrington et al. [38] and by Matveev and Frenkel [39]. Differential thermal analysis was slower to gain extensive use but particularly valuable work was done by Wilburn and his colleagues [40-42]. Around 1950 the possibilities of studying flow in furnaces by using physical models were taken up in many companies and produced some interesting
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papers, for example, P6ych6s [43], Sawai et al. [44], Kruszewski [45], which explored some of the points raised in a characteristic way by Preston [46]. The serious analysis of the information obtainable from tracer experiments in operating furnaces, initiated by Hampton [41], likewise began to develop. Experimental studies of refining also began to revive as evidenced by Sch6nborn [48], Schilling and Franck [49], Dubrul [50] and Bastick [51] but the first attempt to test quantitatively the ideas about refining by rise to the surface implicit in Jebsen-Marwedel [26] appears to have been that of Cable [52]. About this time a number of authors began to analyse the way in which simple laminar flow can attenuate and align layers on inclusions in a liquid of uniform viscosity, see Mohr [53], McKelvey [54] and Cooper [55], and attempts to assess cordiness in regular production received attention (Ghering [56]). Continuous production of optical glasses in small tanks also began during this period.
2. The recent past
One ought to be able to speak of the recent past with confidence but it is not always easy to recognize the most important advances from close range and significant advances of commercial advantage may also remain unpublicized for considerable periods. The uncertainties of predicting the future thus begin here. Studies of melting have continued largely by established techniques (batchfree time, TGA, DTA and hot stage microscopy) but it is difficult to claim any major advance in recent years. It has been realized that, for normal silicate glasses, the dissolution of the silica is more important than the decomposition of the carbonates so that the usefulness of thermogravimetric analysis (TGA) is necessarily distinctly limited. There is a need for more specific methods of identifying individual high temperature processes in systems where several complex processes may be occurring at the same time. Cable and Martlew [57-59] have tried to extend knowledge of the earliest stages of glass making by studying the dissolution of silica rods in liquids such as might be present in the early stages of reaction (Na2CO3-NazSiO3, Na2CO3-NazSO4, NazCO3-CaCO 3, etc). It remains to be seen whether the interesting observations produce anything of great value. Electrochemical techniques might improve insight into the processes occurring at high temperatures as demonstrated by Papadopoulos [60], a very careful worker, but it is rather surprising that simple theories derived for homogeneous solutions seem to apply to inhomogeneous melts. Other electrochemical techniques such as voltammetry have also begun to provide interesting results about glass melts at high temperatures [61,62] but there must be problems in applying these to the kinetics of melting reactions and obviously inhomogeneous liquids. Progress in refining has been rather more rapid, perhaps because detailed studies started later. This renewed interest seems to have stemmed from the
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work of the author, see for example Cable [63-65], Cable and Naqvi [66], and from the experiments of Greene [67-70] on dissolution of bubbles. However, notable contributions have also been made by N~mec [71-73] and Mulfinger [74,75]. As a result it is now clear that there is an important discrepancy between observed rates of refining and what can be attributed to removal by rise to surface; also that mass transfer occurs between bubbles and the surrounding melt. This mass transfer usually involves at least three gases (CO 2, 02 and N2) and it is clear that both growth and dissolution are possible in different circumstances. Unfortunately neither the data nor the theoretical models available provide a complete understanding of all aspects of refining. Theoretical models have been incomplete in several ways and there is a very serious lack of reliable diffusivity and solubility data to confirm that theoretical models are correct. The recent work of Frade [76] puts theoretical predictions on a more secure formal basis but it still needs to be extended to deal with freely rising bubbles not only stationary spherically symmetrical ones. This work confirms the serious lack of sound basic data for gases dissolved in glasses. Some progress has also been made in studying the homogenizing of glasses. Contributions have been made to the theory of deformation of inclusions, methods of measuring the homogeneity of glasses and techniques for making small laboratory melts of high quality. Intuition suggests that the viscosity of an inclusion should affect its rate of deformation. This is indeed true and at the author's suggestion Eshelby [77] showed this could be analysed for an elliptical inclusion; this led to more detailed analysis by Bilby and his colleagues [78,79]. A review of all the theoretical work then recognized as relevant was undertaken by Cable [80]. At this conference Hopper has also indicated other reasons for studying the deformation of small scale inclusions. Although not yet extended to estimate the homogenizing action of various flow paths through furnaces, computation of flows in furnaces and forehearths is now making rapid progress. This may be exemplified by the work of Carling and his colleagues [81,82]. Such work still suffers from inadequate information about melting reactions and both flow and heat transfer in and around partially melted glass; it thus cannot yet contribute very much to understanding of refining or homogenizing in tank furnaces. Many glass companies find physical modelling of furnaces so useful that they give very little publicity to their efforts in this direction. A fascinating overview of progress in the melting of container glasses up to 1970 was provided by Garstang [83] whose paper implies that considerable progress has largely been the result of intelligently planned trial and occasional error conducted with minimal scientific input. Another less comprehensive recent paper by Barton [84] indicates similar slow but steady progress. Homogeneity is now recognized as a property which cannot be described accurately by only one parameter. A good discussion of the principles behind the most useful minimum description in terms of intensity and scale of segregation was supplied by Cooper [85]. For control purposes a simple
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instrumental method with simple data analysis is very desirable. Partly for these reasons the Christiansen filter method pioneered by Shelyubskii [86] has received considerable attention. The theory generally used, adapted from Raman [87], is not entirely appropriate as confirmed by Cable and Waiters [88]. Wang and Cable [89] have demonstrated that the type of spectrophotometer used has a very important effect on the result obtained as deduced by Aylward [90]. One of the most significant developments of recent years has been the considerable increase in the range of glasses used in microelectronic and fibre optic devices. This has been accompanied by a notable extension of methods of making glasses: particularly flame hydrolysis and other chemical vapour deposition techniques, also sol-gel techniques. These methods were all recently reviewed by Scherer and Schultz [91].
3. The future
Traditional glass melting techniques evolved over several thousand years and developed with some scientific insight for about two centuries-are hardly likely to collapse and disappear in another twenty years (now only four furnace campaigns) but changes in industrial practice are likely to accelerate. Competition from polymers must surely increase for many short term room temperature applications; increasing competition for glass containers seems likely to continue. Many medical and pharmaceutical products once always packed in glass are now routinely supplied in disposable polymer containers. Many of the challenges involved here remain outside the field of glass melting but any improvements in melting technology that improve glass quality and reduce melting costs must be beneficial. Flat glass seems less at risk from polymers because of its typical long life and needs for control of both ultra-violet and infra-red transmittance as well as abrasion resistance and freedom from ultra-violet degradation. Lighting, which sometimes involves operating temperatures of several hundred degrees Celsius, also seems a field in which glasses (and perhaps ceramics) will retain their pre-eminence for at least twenty years. Although sol-gel and vapor deposition methods of preparing glasses are rightly receiving much attention at present, it is difficult to see them replacing normal melting for large scale production of ordinary glasses. 3.1. Glass compositions
Glass compositions for most applications have been developed by trial and error over several decades and, except in optical glasses, economic factors seem often to reinforce understandable conservatism about considering changes. However, there are some signs of changes in attitude which may be exemplified by the mathematical and physical modelling of furnaces now so often under-
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taken. Although we still lack comprehensive and soundly based theoretical models for calculating glass properties from composition, a sufficiently large body of data exists to make this possible by empirical procedures in many important ranges of composition. This makes possible the converse and rather more useful exercise of specifying a set of properties then optimizing glass composition. Westerlund et al. [92] have shown how this may be done and have included batch costs so that the possibility of minimizing batch cost by (if necessary) slightly relaxing other constraints can be explored. Such models are, at present, of somewhat limited use; property composition relations need to be refined and extended, especially for liquidus temperature. Also we still have no satisfactory models for predicting ease of melting, refining or homogenizing, nor for scaling up.
4. Melting chemistry Ease of melting depends very much on choice of batch materials as well as on glass composition. Small proportions of sulphate and other refining agents can have very beneficial effects, however the quantity of quartz that has to be dissolved in almost certainly more important than the silica content of the melt. This is one of the reasons why cullet and Calumite can have desirable effects on glass melting efficiency. Following the same line of argument it should be worth investigating anew the possibilities of using minerals like felspars and nepheline syenite in larger proportions; this would require higher alumina contents than are now usually accepted. Although these minerals are considerably more expensive than sand they are much cheaper than soda ash or potash; if increased melting efficiency were achieved they could be economically effective. Further knowledge of these matters might assist large scale glass manufacturers. Although users naturally tend to specify glasses in terms of their room temperature properties glass manufacturers need to consider high temperature properties. The most important are viscosity-temperature relation and devitrification. Given the trend to faster output it is by no means clear that current preferred log ~/-T relations will remain the long term targets. Several aspects of melting chemistry deserve further study. Restrictions of several kinds are making manufacturers more interested than even before in matters like control of ferrous: ferric ratio and decolorizing. Unfortunately our understanding of these matters is primitive. Even the qualitative background is not clearly agreed and reliable quantitative data about the effects of temperature and glass composition on oxidation-reduction processes are surprisingly meagre. It is still common to find the behaviour of iron, for example, written implicitly using the Toop and Samis [93] model for the behaviour of oxygen, O- +O°~20
,
(1)
as if described by Fe2++ ~O 2 ~ Fe3++ ½0 -.
(2)
M. Cable / Possibilities of progress in glass melting
3,58
Such models correctly describe the effect of oxygen partial pressure for a particular composition but often fail completely for change of composition at constant oxygen fugacity. The suggestion of Budd [94] that appropriate ionic complexes should be used has been reinforced by Karlsson [95] who showed that Fe2++ ¼0 2 + ~ - - - ~ O - - ~
FeO~ 3-2~-
(0 < n < 4),
(3)
is a much more plausible model. Some of the deficiencies of attempting to deduce "oxygen ion activity" from measurements of CO 2, H 2 0 or SO 3 solubility (forming C O / - , 2 O H - and SO£-) were also pointed out by Cable [96]. We must hope for rapid progress here: such understanding of chemical equilibria would assist the development of laser and similar glasses.
5. Refining Refining, foaming and reboil of various types are other imperfectly understood phenomena needing greater chemical insight. This is particularly true when the traditional refining agents of arsenic and sulphate are steadily becoming less acceptable; arsenic because it is a poison, sulphate because of its likely contribution to acid rain. The traditional view of refining action is that oxygen is evolved at high temperatures but resorbed at lower temperatures. This is undoubtedly true as a qualitative statement of the trend but there is little evidence that melts become supersaturated with oxygen at the maximum temperatures used. The theoretical analyses of Frade [76] did not produce the typical observed CO 2 ~ 02 ~ N 2 change for any plausible sets of parameters. Although these predictions were for stationary bubbles the major effect of rise through the melt is likely to be a change in time scale rather than a crucial qualitative change in behaviour. The author believes that non-equilibrium kinetic effects may prove to be important. Progress here is badly needed but direct observation of bubbles during the early stages of melting and refining is difficult. There is also an important need for better methods of determining solubilities and actual dissolved concentrations of gases such as CO 2, 02, SO 3, H20 and N 2 in glass melts. Volatilization of alkali, boron, etc., and subsequent condensation of their products makes the most obvious methods unreliable. There is also poor understanding of the conditions that lead to nucleation and growth of bubbles when a liquid becomes supersaturated. A better theoretical background in this field would assist us considerably.
6. Homogenizing The idea, firmly implanted in scientists, that a glass is an isotropic material is likely to be greeted with derision by a glass technologist: this ideal equi-
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librium state is very rarely attained in practice. However, as products improve and the range of applications extends further and further, the need to come closer to this ideal in ordinary glass making becomes pressing. This is true in both laboratory and factory. Developments in techniques of homogenizing, techniques of measurement and underlying theory would all be beneficial. Problems of homogenizing, like those of refining, begin early in melting. In ordinary soda-lime-silica glasses lime can cause the major problems as shown in different ways by Cable and Bower [97] and Dietzel et al. [98]. Most of these problems can be solved by sol-gel methods of preparation but these require much more energy than normal melting and it is difficult to see them becoming an economic way of solving these problems. It is now known that high viscosity or low Reynolds number is not sufficient to ensure stability of liquid-liquid interfaces across which there are differences in viscosity or density (Yih [99]) and this aspect of theory deserves further study. The author and his colleagues (Jambor Sadeghi [100], Wang [101]) have shown that abandoning the engineer's first natural assumption of axial symmetry can produce great improvements in simple but very effective stirrers for small scale homogenizing. These ideas might find wider application. 7. Conclusion
On looking both back and forwards for about twenty years it is possible to see that exciting things may happen in glass melting. A twenty year retrospect in 2004 may well, whatever happens, make todays pundits seem very lacking in foresight but will predicting the future be any easier for the experts of those days? References [1] P. Bosc D'Antic, Collected Works, Containing many Memoirs on the Art of Glassmaking etc. (In French) 2 Vols. Paris (1780). [2] P. Loysel, Essai sur L'Art de la Verrerie, Paris (1800). [3] M. von Rohr, Joseph Fraunhofers Leben, Leistungen und Wirksamheit (Akademischc Verlag, Leipzig, 1929). [4] J. Fraunhofer, Bayer. Kunst u. Gerwerbeblatt, Sp. 1 19, (1866) (Reprint of publications from 1817 and 1819). [5] M. Faraday, Phil. Trans. 120. (1830) 1. [6] M.C. Usselman, Plat. Metals Rev. 27(4) (1983) 175. [7] J. Morrell and A. Thackray, Gentlemen of Science: early years of the British Association for the Advancement of Science (Clarendon, Oxford, 1981). [8] W. Vernon Harcourt, British Assoc.: Report of the 14th Meeting, York (1844) p. 82. [9] G.C. Stokes, British Association: Report of the 41st Meeting (1871) p. 38. [10] G. Bontemps, Guide du Verrier (Librairie du Dictionnaire des Arts et Manufactures, Paris. 1868) p. 654. [11] G. Bontemps, Guide du Verrier (Librairie du Dictionnaire des Arts et Manufactures, Paris, 1868) p. 776.
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J.M. McKelvey, Polymer Processing (Wiley, London, 1962) Ch 12. A.R. Cooper, Chem. Eng. Sci. 21 (1966) 1095. L.G. Ghering, J. Amer. Ceram. Soc. 27 (1944) 373. M. Cable and D. Martlew, Glass Technol. 12 (1971) 142. M. Cable and D. Martlew, Glass Technol. 25 (1984) 24. M. Cable and D. Martlew, Glass Technol. 25 (1984) 139. K. Papadopoulos, J. Amer. Ceram. Soc. 58 (1975) 364. K. Takahashi and Y. Miura, J. Non-Crystalline Solids 39 (1983) 527. K.H. Karlsson and M. Perander, Structure of Non-Crystalline Materials (Cambridge, 1982) (Taylor and Francis, London and New York, 1983) p. 92. [63] M. Cable, Symposium sur la fusion du Verre (Union Scientifique Continentale du Verre, Charleroi, 1958) p. 253. [64] M. Cable, Glass Technol. 1 (1960) 144. [65] M. Cable, Glass Technol. 2 (1961) 60; 151. [66] M. Cable and A.A. Naqvi, Glass Technol. 16 (1975) 2. [67] C.H. Greene and R. Gaffney, J. Amer. Ceram. Soc. 42 (1959) 271. [68] C.H. Greene and I. Kitano, Glastechn. Ber. (Proc. V Int. Glass Congress) 32K (1959) V/44-8. [69] C.H. Greene and H.A. Lee, J. Amer. Ceram. Soc. 48 (1965) 528. [70] C.H. Greene and D.R. Platts, J. Amer. Ceram. Soc. 52 (1969) 106. [71] L. N~mec, Glass Technol. 15 (1974) 153. [72] L. N~mec, J. Amer. Ceram. Soc. 69 (1977) 436. [73] L. N~mec, Glass Technol. 21(3) (1980) 134, 139. [74] H.O. Mulfinger, Glastekn. Tidskr. 29 (1974) 81. [75] H.O. Mulfinger, Glastechn. Ber. 49 (1976) 232. [76] J.R. Frade, Ph D thesis, Sheffield University (1983). [77] J.D. Eshelby, Private communication (1968) derived from Proc. Roy. Soc. London (1957) A241 376:A252 (1959) 561. [78] B.A. Bilby, J.D. Eshelby and A.K. Kundu, Tectonophysics 28 (1975) 265. [79] B.A. Bilby and M.L. Kolbuszewski, Proc. Roy. Soc. London. Ser. A355 (1977) 335. [80] M. Cable, The theory of homogenizing glass melts (Dept. of Ceramics, Glasses and Polymers, Sheffield University, 1977). [81] J.C. Carling, Glass Technol. 23 (1982) 201. [82] C.J.M. McSheehy, J.C. Carling, and H. Rawson, Glass Technol. 22 (1981) 34. [83] A. Garstang, Glass Technol. 12 (1971) 1. [84] J.L. Barton, Riv. Staz. Sper. Vetro 12 (1982) 19. [85] A.R. Cooper, Glass Technol. 7(1) (1966) 2. [86] V.I. Shelyubskii, Steklo Keram. 17(8) (1960) 17. [87] C.V. Raman, Proc. Indian Acad. Sci. A29 (1949) 381. [88] M. Cable and S.D. Waiters, Glass Technol. 21 (1980) 279. [89] S.S. Wang and M. Cable, Paper in preparation (1984). [90] N.H. Aylward, PhD Thesis, Sheffield University (1980). [91] G.W. Scherer and P.C. Schultz, in: Glass: Science and Technology, Vol. 1: Glass-Forming Systems, eds., DR. Uhlmann and N.J. Kreidl (Academic Press. New York, 1983) Ch. 2, p. 49. [92] K.H. Karlsson, T. Westerlund and L. Hatakka, J. Amer. Ceram. Soc. 66(8) (1983) 574. [93] G.W. Toop and C.S. Samis, Trans. Met. Soc., AIME 224 (1962) 878. [94] S.M. Budd, Phys. Chem. Glasses 7 (1966) 210. [95] K. Karlsson, Glastekn. Tidskr. 32 (1977) 6. [96] M. Cable, in: Molten Salt Technology, ed., D.G. Lovering (Plenum Press, New York, 1982) p. 223. [97] M. Cable and C. Bower, Glass Technol. 6 (1965) 197. [98] A. Dietzel, O.W. Fl6rke and H.C. Williams, Glastechn. Ber. 38 (1965) 322. [99] C.S. Yih, Stratified Flows (Academic Press, New York, 1980) p. 418. [100] J. Jambor Sadeghi, PhD thesis, Sheffield University (1980). [101] S.S. Wang and M. Cable, paper in preparation (1985).
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P O S S I B I L I T I E S OF P R O G R E S S IN G L A S S M E L T I N G Michael CABLE Department of Ceramics, Glasses and Polymers, Sheffield University, UK
The application of scientific ideas to progress in glass melting since about 1750 AD is briefly reviewed with more attention to the past 25 years than more distant times. Some of the present gaps in scientific background and glass melting practice are identified. Some topics in which important progress may be hoped for in the next twenty years are considered.
1. An incomplete history of glass melting The wise person who wishes to predict what is likely to happen in the future will first study development of the subject up to the present. This, of course, is no guarantee of success but the alternatives, such as astrology, are not likely to gain greater support. Glass making has a long history and obviously evolved largely by trial and error before scientific understanding was possible. However, the literature from the past two hundred years that demonstrates clear insight into glass melting, in terms of the best possible when it was written. remains disappointingly meagre. This makes it very difficult to produce a disinterested review of how understanding improved in the relatively recent past. The oldest important contribution to understanding of glass making that displays a critical attitude and attempts to explain observed phenomena in terms of scientific principles appears to be the collected works of Bosc D'Antic [1] (written between 1758 and 1780). Bosc made some notable experiments, such as proving that gall was rich in Na2SO 4 and that white fumes evolved as gall was dissipated were also largely N a z S O 4, but was also obviously limited by the alchemical attitude still prevalent about heat and many aspects of chemistry. For example, the decotorizing action of manganese, by which its purple colour was bleached as it was reduced and iron oxidized from ferrous to ferric, was still assumed to be due to its loss by evaporation. Bosc was very scathing about conventional wisdom that he realized to be unsound and also very insistent that large scale manufacture could not be properly understood only by small scale experiments in a gentleman's study. Another early author whose work is now forgotten but was well regarded in the earlier part of the nineteenth century was another Frenchman, Loysel [2]. whose strong point seems to have been physics rather than chemistry; his book (which appeared in both French and German editions) is the earliest on glass 0022-3093/85/$03.30 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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making to mention temperature scales, making comparisons between Wedgwood's and R6aumur's methods of estimating degrees of heat. Other notable scientific pioneers of the early nineteenth century either left only scanty records or did not publish much of their work. Joseph Fraunhofer (1787-1826) was orphaned at an early age and apprenticed to a glazier. He developed his scientific talents from virtually nothing almost entirely by his own efforts and began to be noticed as a teenager. This led to him becoming a professional glass technologist, first assisting then supplanting Guinand in the development of optical glass manufacture in Bavaria [3]. Although Fraunhofer left some writings about the properties of glasses, especially testing of chemical durability [4], he seems not to have left a significant contribution to the chemistry of glass making; his greatest gifts were in physics. Faraday, surely one of the greatest experimental scientists, spent several years (1825-33) successfully developing methods of producing homogeneous pieces of glass sufficiently large to make good quality lenses for telescopes and other components for optical instruments. His innovations included the use of platinum for melting crucibles and stirrers as well as an effective way of homogenizing molten glass in a shallow tray. Faraday [5] left only one fascinating account of his methods and observations but these seem to have fallen into disuse when he abandoned glass technology for electromagnetism. The archives of the Royal Society or the Royal Institution may contain much more of value to glass making recorded by Faraday but this remains unexplored, see however Usselman [6]. Another elusive pioneer was the Reverend William Vernon Harcourt (1789-1871) who may be considered Yorkshire's earliest glass scientist. He was born a Vernon at Sudbury in Derbyshire but his father inherited the Harcourt estates and also became, in due course, Archbishop of York. William was one of the founder members of the British Association for the Advancement of Science and became its first Secretary [7]. Whilst Rector of Wheldrake (near York) and Canon at York Minister he spent many years (approximately 1834-61) making small scale glass melts in a quest to extend the range of optical glasses available. This work continued until a few days before his death. The range of elements that he included in glasses, melted in a small hydrogen furnace of his own design [8], is very surprising. The glasses that he made included phosphates and borates as well as silicates and he experimented with oxides of Li, Be, B, Na, Mg, A1, Si, P, K, Ca, Ti, V, Cr, As, Mn, Ni, Zn, Sr, Mo, Sn, Sb, Cd, Ba, W, T1, Pb, Bi and U as well as some fluorides and sulphides [9]. His work likewise was never properly recorded. The problems of obtaining homogeneous glass early in the 19th century are demonstrated by the statement of Dollond in 1828 that, in the five preceding years he had been unable to obtain a single disk of flint glass good enough to make an objective of 4½ inch (11.4 cm) diameter [10]. The last third of the nineteenth century saw further notable publications on glass making, the best of which is Bontemps's Guide du Verrier [11] but Peligot [12] and Appert and Henrivaux [13] also deserve to be mentioned. However,
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none of these demonstrates much progress in the chemistry and physics of glass making. The next notable German pioneer was Otto Schott (1851-1935) whose PhD thesis was on the chemistry of glass melting [14]. During his long life he maintained a very keen interest in applying scientific knowledge to glass making and left about two dozen publications but also (and more important in practice) established the firm whose name and repute we all know so well. The impressive early achievements of that firm were documented long ago by Hovestadt [15] but he recorded little about improved understanding of glass melting. It may be appropriate to note here that von Wittorf [16] published a study of reactions between silica and Li2CO 3, Na2CO 3, K2CO 3, Rb2CO 3 and Cs2CO 3 in 1904. During the early part of the twentieth century when Norbert Kreidl was still too young to be able to influence things, say up to 1930 (his first paper appeared in 1929), a few workers such as Cobb [17] Niggli [18] and Hedvall [19] did useful work on reactions involving silica and Bezborodov [20] began similar studies involving boric oxide but no leap forward could be claimed. A notable review by Turner [21] in 1930 shows that, although studies of reactions between alkali carbonates and silica could be traced back nearly eighty years, little progress had yet been made in understanding glass melting. However, Turner did note the beginning of work by differential thermal analysis, the earliest notable work being by Tammann and Oelsen [22], and recognized its importance. On the other hand Turner says nothing about homogenizing and his only comment on refining is, in an aside, to pose one question. Progress in the next twenty years or so was much greater. First of all, 1930 saw the publication of the first important investigation of refining by Gehlhoff, Kalsing and Thomas [23]. Notable largely empirical but very valuable works on batch-free time and the influence of important factors were done by Preston and Turner [24] and Potts and his colleagues [25] but homogenizing and refining remained relatively neglected. There was little recognition at that time of the need to consider both chemical and physical phenomena in glass melting. One of the earliest notable attempts to bring together all aspects of melting was the first edition of Jebsen-Marwedel's popular book [26]. During this period the spirit of pioneers like Vernon Harcourt and Schott had a resurgence and many new compositions were investigated, many for optical purposes; Kreidl had a part in this as is shown by 25 entries under his name in the index to Mazurin's [27] compendium on glass properties. Thermogravimetric analysis was exploited much more widely, admittedly using very crude theoretical models. The most notable work being a series of papers by Kr6ger and his colleagues [28-37]; other interesting studies being by Harrington et al. [38] and by Matveev and Frenkel [39]. Differential thermal analysis was slower to gain extensive use but particularly valuable work was done by Wilburn and his colleagues [40-42]. Around 1950 the possibilities of studying flow in furnaces by using physical models were taken up in many companies and produced some interesting
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papers, for example, P6ych6s [43], Sawai et al. [44], Kruszewski [45], which explored some of the points raised in a characteristic way by Preston [46]. The serious analysis of the information obtainable from tracer experiments in operating furnaces, initiated by Hampton [41], likewise began to develop. Experimental studies of refining also began to revive as evidenced by Sch6nborn [48], Schilling and Franck [49], Dubrul [50] and Bastick [51] but the first attempt to test quantitatively the ideas about refining by rise to the surface implicit in Jebsen-Marwedel [26] appears to have been that of Cable [52]. About this time a number of authors began to analyse the way in which simple laminar flow can attenuate and align layers on inclusions in a liquid of uniform viscosity, see Mohr [53], McKelvey [54] and Cooper [55], and attempts to assess cordiness in regular production received attention (Ghering [56]). Continuous production of optical glasses in small tanks also began during this period.
2. The recent past
One ought to be able to speak of the recent past with confidence but it is not always easy to recognize the most important advances from close range and significant advances of commercial advantage may also remain unpublicized for considerable periods. The uncertainties of predicting the future thus begin here. Studies of melting have continued largely by established techniques (batchfree time, TGA, DTA and hot stage microscopy) but it is difficult to claim any major advance in recent years. It has been realized that, for normal silicate glasses, the dissolution of the silica is more important than the decomposition of the carbonates so that the usefulness of thermogravimetric analysis (TGA) is necessarily distinctly limited. There is a need for more specific methods of identifying individual high temperature processes in systems where several complex processes may be occurring at the same time. Cable and Martlew [57-59] have tried to extend knowledge of the earliest stages of glass making by studying the dissolution of silica rods in liquids such as might be present in the early stages of reaction (Na2CO3-NazSiO3, Na2CO3-NazSO4, NazCO3-CaCO 3, etc). It remains to be seen whether the interesting observations produce anything of great value. Electrochemical techniques might improve insight into the processes occurring at high temperatures as demonstrated by Papadopoulos [60], a very careful worker, but it is rather surprising that simple theories derived for homogeneous solutions seem to apply to inhomogeneous melts. Other electrochemical techniques such as voltammetry have also begun to provide interesting results about glass melts at high temperatures [61,62] but there must be problems in applying these to the kinetics of melting reactions and obviously inhomogeneous liquids. Progress in refining has been rather more rapid, perhaps because detailed studies started later. This renewed interest seems to have stemmed from the
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work of the author, see for example Cable [63-65], Cable and Naqvi [66], and from the experiments of Greene [67-70] on dissolution of bubbles. However, notable contributions have also been made by N~mec [71-73] and Mulfinger [74,75]. As a result it is now clear that there is an important discrepancy between observed rates of refining and what can be attributed to removal by rise to surface; also that mass transfer occurs between bubbles and the surrounding melt. This mass transfer usually involves at least three gases (CO 2, 02 and N2) and it is clear that both growth and dissolution are possible in different circumstances. Unfortunately neither the data nor the theoretical models available provide a complete understanding of all aspects of refining. Theoretical models have been incomplete in several ways and there is a very serious lack of reliable diffusivity and solubility data to confirm that theoretical models are correct. The recent work of Frade [76] puts theoretical predictions on a more secure formal basis but it still needs to be extended to deal with freely rising bubbles not only stationary spherically symmetrical ones. This work confirms the serious lack of sound basic data for gases dissolved in glasses. Some progress has also been made in studying the homogenizing of glasses. Contributions have been made to the theory of deformation of inclusions, methods of measuring the homogeneity of glasses and techniques for making small laboratory melts of high quality. Intuition suggests that the viscosity of an inclusion should affect its rate of deformation. This is indeed true and at the author's suggestion Eshelby [77] showed this could be analysed for an elliptical inclusion; this led to more detailed analysis by Bilby and his colleagues [78,79]. A review of all the theoretical work then recognized as relevant was undertaken by Cable [80]. At this conference Hopper has also indicated other reasons for studying the deformation of small scale inclusions. Although not yet extended to estimate the homogenizing action of various flow paths through furnaces, computation of flows in furnaces and forehearths is now making rapid progress. This may be exemplified by the work of Carling and his colleagues [81,82]. Such work still suffers from inadequate information about melting reactions and both flow and heat transfer in and around partially melted glass; it thus cannot yet contribute very much to understanding of refining or homogenizing in tank furnaces. Many glass companies find physical modelling of furnaces so useful that they give very little publicity to their efforts in this direction. A fascinating overview of progress in the melting of container glasses up to 1970 was provided by Garstang [83] whose paper implies that considerable progress has largely been the result of intelligently planned trial and occasional error conducted with minimal scientific input. Another less comprehensive recent paper by Barton [84] indicates similar slow but steady progress. Homogeneity is now recognized as a property which cannot be described accurately by only one parameter. A good discussion of the principles behind the most useful minimum description in terms of intensity and scale of segregation was supplied by Cooper [85]. For control purposes a simple
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instrumental method with simple data analysis is very desirable. Partly for these reasons the Christiansen filter method pioneered by Shelyubskii [86] has received considerable attention. The theory generally used, adapted from Raman [87], is not entirely appropriate as confirmed by Cable and Waiters [88]. Wang and Cable [89] have demonstrated that the type of spectrophotometer used has a very important effect on the result obtained as deduced by Aylward [90]. One of the most significant developments of recent years has been the considerable increase in the range of glasses used in microelectronic and fibre optic devices. This has been accompanied by a notable extension of methods of making glasses: particularly flame hydrolysis and other chemical vapour deposition techniques, also sol-gel techniques. These methods were all recently reviewed by Scherer and Schultz [91].
3. The future
Traditional glass melting techniques evolved over several thousand years and developed with some scientific insight for about two centuries-are hardly likely to collapse and disappear in another twenty years (now only four furnace campaigns) but changes in industrial practice are likely to accelerate. Competition from polymers must surely increase for many short term room temperature applications; increasing competition for glass containers seems likely to continue. Many medical and pharmaceutical products once always packed in glass are now routinely supplied in disposable polymer containers. Many of the challenges involved here remain outside the field of glass melting but any improvements in melting technology that improve glass quality and reduce melting costs must be beneficial. Flat glass seems less at risk from polymers because of its typical long life and needs for control of both ultra-violet and infra-red transmittance as well as abrasion resistance and freedom from ultra-violet degradation. Lighting, which sometimes involves operating temperatures of several hundred degrees Celsius, also seems a field in which glasses (and perhaps ceramics) will retain their pre-eminence for at least twenty years. Although sol-gel and vapor deposition methods of preparing glasses are rightly receiving much attention at present, it is difficult to see them replacing normal melting for large scale production of ordinary glasses. 3.1. Glass compositions
Glass compositions for most applications have been developed by trial and error over several decades and, except in optical glasses, economic factors seem often to reinforce understandable conservatism about considering changes. However, there are some signs of changes in attitude which may be exemplified by the mathematical and physical modelling of furnaces now so often under-
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taken. Although we still lack comprehensive and soundly based theoretical models for calculating glass properties from composition, a sufficiently large body of data exists to make this possible by empirical procedures in many important ranges of composition. This makes possible the converse and rather more useful exercise of specifying a set of properties then optimizing glass composition. Westerlund et al. [92] have shown how this may be done and have included batch costs so that the possibility of minimizing batch cost by (if necessary) slightly relaxing other constraints can be explored. Such models are, at present, of somewhat limited use; property composition relations need to be refined and extended, especially for liquidus temperature. Also we still have no satisfactory models for predicting ease of melting, refining or homogenizing, nor for scaling up.
4. Melting chemistry Ease of melting depends very much on choice of batch materials as well as on glass composition. Small proportions of sulphate and other refining agents can have very beneficial effects, however the quantity of quartz that has to be dissolved in almost certainly more important than the silica content of the melt. This is one of the reasons why cullet and Calumite can have desirable effects on glass melting efficiency. Following the same line of argument it should be worth investigating anew the possibilities of using minerals like felspars and nepheline syenite in larger proportions; this would require higher alumina contents than are now usually accepted. Although these minerals are considerably more expensive than sand they are much cheaper than soda ash or potash; if increased melting efficiency were achieved they could be economically effective. Further knowledge of these matters might assist large scale glass manufacturers. Although users naturally tend to specify glasses in terms of their room temperature properties glass manufacturers need to consider high temperature properties. The most important are viscosity-temperature relation and devitrification. Given the trend to faster output it is by no means clear that current preferred log ~/-T relations will remain the long term targets. Several aspects of melting chemistry deserve further study. Restrictions of several kinds are making manufacturers more interested than even before in matters like control of ferrous: ferric ratio and decolorizing. Unfortunately our understanding of these matters is primitive. Even the qualitative background is not clearly agreed and reliable quantitative data about the effects of temperature and glass composition on oxidation-reduction processes are surprisingly meagre. It is still common to find the behaviour of iron, for example, written implicitly using the Toop and Samis [93] model for the behaviour of oxygen, O- +O°~20
,
(1)
as if described by Fe2++ ~O 2 ~ Fe3++ ½0 -.
(2)
M. Cable / Possibilities of progress in glass melting
3,58
Such models correctly describe the effect of oxygen partial pressure for a particular composition but often fail completely for change of composition at constant oxygen fugacity. The suggestion of Budd [94] that appropriate ionic complexes should be used has been reinforced by Karlsson [95] who showed that Fe2++ ¼0 2 + ~ - - - ~ O - - ~
FeO~ 3-2~-
(0 < n < 4),
(3)
is a much more plausible model. Some of the deficiencies of attempting to deduce "oxygen ion activity" from measurements of CO 2, H 2 0 or SO 3 solubility (forming C O / - , 2 O H - and SO£-) were also pointed out by Cable [96]. We must hope for rapid progress here: such understanding of chemical equilibria would assist the development of laser and similar glasses.
5. Refining Refining, foaming and reboil of various types are other imperfectly understood phenomena needing greater chemical insight. This is particularly true when the traditional refining agents of arsenic and sulphate are steadily becoming less acceptable; arsenic because it is a poison, sulphate because of its likely contribution to acid rain. The traditional view of refining action is that oxygen is evolved at high temperatures but resorbed at lower temperatures. This is undoubtedly true as a qualitative statement of the trend but there is little evidence that melts become supersaturated with oxygen at the maximum temperatures used. The theoretical analyses of Frade [76] did not produce the typical observed CO 2 ~ 02 ~ N 2 change for any plausible sets of parameters. Although these predictions were for stationary bubbles the major effect of rise through the melt is likely to be a change in time scale rather than a crucial qualitative change in behaviour. The author believes that non-equilibrium kinetic effects may prove to be important. Progress here is badly needed but direct observation of bubbles during the early stages of melting and refining is difficult. There is also an important need for better methods of determining solubilities and actual dissolved concentrations of gases such as CO 2, 02, SO 3, H20 and N 2 in glass melts. Volatilization of alkali, boron, etc., and subsequent condensation of their products makes the most obvious methods unreliable. There is also poor understanding of the conditions that lead to nucleation and growth of bubbles when a liquid becomes supersaturated. A better theoretical background in this field would assist us considerably.
6. Homogenizing The idea, firmly implanted in scientists, that a glass is an isotropic material is likely to be greeted with derision by a glass technologist: this ideal equi-
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librium state is very rarely attained in practice. However, as products improve and the range of applications extends further and further, the need to come closer to this ideal in ordinary glass making becomes pressing. This is true in both laboratory and factory. Developments in techniques of homogenizing, techniques of measurement and underlying theory would all be beneficial. Problems of homogenizing, like those of refining, begin early in melting. In ordinary soda-lime-silica glasses lime can cause the major problems as shown in different ways by Cable and Bower [97] and Dietzel et al. [98]. Most of these problems can be solved by sol-gel methods of preparation but these require much more energy than normal melting and it is difficult to see them becoming an economic way of solving these problems. It is now known that high viscosity or low Reynolds number is not sufficient to ensure stability of liquid-liquid interfaces across which there are differences in viscosity or density (Yih [99]) and this aspect of theory deserves further study. The author and his colleagues (Jambor Sadeghi [100], Wang [101]) have shown that abandoning the engineer's first natural assumption of axial symmetry can produce great improvements in simple but very effective stirrers for small scale homogenizing. These ideas might find wider application. 7. Conclusion
On looking both back and forwards for about twenty years it is possible to see that exciting things may happen in glass melting. A twenty year retrospect in 2004 may well, whatever happens, make todays pundits seem very lacking in foresight but will predicting the future be any easier for the experts of those days? References [1] P. Bosc D'Antic, Collected Works, Containing many Memoirs on the Art of Glassmaking etc. (In French) 2 Vols. Paris (1780). [2] P. Loysel, Essai sur L'Art de la Verrerie, Paris (1800). [3] M. von Rohr, Joseph Fraunhofers Leben, Leistungen und Wirksamheit (Akademischc Verlag, Leipzig, 1929). [4] J. Fraunhofer, Bayer. Kunst u. Gerwerbeblatt, Sp. 1 19, (1866) (Reprint of publications from 1817 and 1819). [5] M. Faraday, Phil. Trans. 120. (1830) 1. [6] M.C. Usselman, Plat. Metals Rev. 27(4) (1983) 175. [7] J. Morrell and A. Thackray, Gentlemen of Science: early years of the British Association for the Advancement of Science (Clarendon, Oxford, 1981). [8] W. Vernon Harcourt, British Assoc.: Report of the 14th Meeting, York (1844) p. 82. [9] G.C. Stokes, British Association: Report of the 41st Meeting (1871) p. 38. [10] G. Bontemps, Guide du Verrier (Librairie du Dictionnaire des Arts et Manufactures, Paris. 1868) p. 654. [11] G. Bontemps, Guide du Verrier (Librairie du Dictionnaire des Arts et Manufactures, Paris, 1868) p. 776.
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