a H°k,GL = standard enthalpy of component k in the glassy state; Hk,1300 = enthalpy of k in the liquid state at 1300 °C; cP,k,L = isobaric heat capacity of liquid k; m(k) = equilibrium amount of k in the multicomponent phase diagram; H°GLASS = standard enthalpy of the resulting glass; H1300,MELT = enthalpy of the melt at 1300 °C; ΔH1300 = heat content of the melt at 1300 °C relative to the glass at 25 °C.
b hm = FeO·Fe2O3, F = Fe2O3, M = MgO, C = CaO, N = Na2O, K = K2O, S = SiO2.
A better understanding of redox and acid base reactions in real furnaces is also desired. Although these reactions are well understood at the laboratory scale, the transfer to a real production situation is still set by experience rather than by scientific principles. In view of the large impact of these reactions on glass quality, progress in this area would be highly appreciated.
Finally, the glass industry is engaged in a quest to lower its overall energy consumption to decrease its operating costs and to satisfy increasingly stringent legislation imposed on high‐temperature industrial processes. The design of faster conversion batches is becoming important in this respect. Conventional glass formulae and batch recipes are no longer taken for granted. Efforts are in particular made to design batches that would melt along reaction pathways ensuring higher turnover rates than current randomly mixed batches. Progress may be achieved with selective batching, granulation processes bringing the reaction partners into close contact at the μm scale, preparation of core‐shell type pellets, or selective preheating of specific raw‐material combinations of the batch. In each case, of course, a prerequisite would be that the obtained energy savings are not offset by increased batch costs.
Appendix
The results plotted in Figure 7 for the dissolution of an ensemble of grains have been obtained from the analytical solution to the integral A(t) of Eq. (6) given by:
(18)
Here, erfc(z) denotes the complementary Gaussian error function of argument z, while y and s are used as abbreviations in the formula. It is true that sand dissolution does not proceed isothermally at a constant diffusion coefficient D in a real fusion process, but the utmost importance of the grain‐size distribution for a successful fusion process is nonetheless demonstrated clearly.
References
1 1 Cable, M. (1998). A century of development in glass melting. J. Am. Ceram. Soc. 81: 1083–1094.
2 2 Simpson, W. and Myers, D.D. (1978). The redox number concept and its use by the glass technologists. Glass Technol. 19: 82–85.
3 3 Nemeč, L. and Cincibusová, P. (2009). Glass melting and its innovation potentials: the potential role of glass flow in the sand dissolution process. Ceramics Silikaty 53: 145–155.
4 4 Nemeč, L., Jebavá, M., and Dyrčíková, P. (2013). Glass melting phemomena, their ordering and melting space utilization. Ceramics Silikaty 57: 275284.
5 5 Jebsen‐Marwedel, H. and Brückner, R. (1980). Glastechnische Fabrikationsfehler, “Pathologische” Ausnahmezustände des Werkstoffes Glas und ihre Behebung; Eine Brücke zwischen Wissenschaft, Technologie und Praxis, 229. Berlin: Springer.
6 6 Müller‐Simon, H. (1999). Sulfate fining in soda lime silicate glasses (in German). In: HVG Course 1999, 45–72. Offenbach: Deutsche Glastechnische Gesellschaft.
7 7 Conradt, R. (2008). The industrial glass melting process, Chapter II:24. In: The SGTE Casebook. Thermodynamics at Work (ed. K. Hack). Boca Raton: CRC Press.
8 8 Conradt, R. (2010). Thermodynamics of glass melting. In: Fiberglass and Glass Technology – Energy‐Friendly Compositions and Applications (eds. F.T. Wallenberger and P.A. Bingham), 385–412. Berlin: Springer.
9 9 Conradt, R. (2019). Prospects and physical limits of processes and technologies in glass melting. J. Asian Ceram. Soc. 7: 377–396.
Note
1 Reviewers: E. Muijsenberg, Glass Service a.s, Vsetin, Czech RepublicC. Rüssel, Friedrich Schiller University, Jena, Germany
1.4 Primary Fabrication of Flat Glass
Toru Kamihori
Production Technology Center, Asahi Glass Co., Ltd., Yokohama‐shi, Kanagawa, Japan
1 Introduction
Flat glass is ubiquitous in the modern world, from the facades of high‐rise buildings to the large windows of automobiles, the solar power generation systems, and the various kinds of displays that are now integral components of daily life. Not only did these new applications cause a tremendous increase of the world production from less than 7 105 in the early 1960s to 59 106 metric tons in 2014 (then with an annual growth rate of 7%), but they have also yielded dramatic improvements in glass quality and functionalities. For a glass material that had been manufactured for 2000 years with very little change, the industrial evolution observed during the last 50 years has been incredibly rapid indeed!
As experienced by ancient Roman glassmakers, flat glass made by pouring the melt on a solid substrate has a surface that is not smooth enough to ensure good transparency. Until the beginning of the twentieth century, flat glass had for this reason to be produced out of hollow glass to keep the defect‐free surface conferred by fire polish. As made in this way with either the crown or the cylinder process (Chapter 10.8), production of flat glass was very labor‐intensive, restricted to relatively small sheets and subject to wastage when cut into pieces for use. Besides, it did not yield high‐quality products as is obvious to anyone looking at an old window where objects are often seen distorted through the glass in which defects and streaks are also generally present as a result of the detrimental effects of temperature or composition heterogeneities that could not be avoided during the melting and forming processes.
Mechanization was pioneered from 1894 to 1916 by J. H. Lubbers at the American Window Glass company. Glass cylinders of constant diameter and wall thickness began in 1904 to be blown successfully with a well‐controlled low‐pressure air flow issued for 15–18 minutes from a machine that was dubbed iron lung [1]. Making much bigger cylinders, which eventually reached 1 m in diameter and more than 13 m in length (Figure 1), did reduce considerably cost and labor (whence the very strong Union opposition met by the new process), but did not result in consistently good quality because of the wavy surface and optical distortion induced by the flattening stage. Thanks to updraw processes designed in Belgium and in the United States, it then became possible in the following decade to bypass the hollow‐glass step. But these processes remained discontinuous because devitrified material had to be removed periodically from the production line.
A true revolution in glass