9.7 and 9.8), higher melting temperatures, and more carefully selected raw materials. In the Chapter 1.2, S. Di Pierro thus discusses the importance of the specifications, sources, and management of raw materials needed to avoid high rejection costs after melting operations that must be as fast as possible for economic reasons (Chapter 1.2). Being common to most glassmaking processes, fusion itself is then reviewed by R. Conradt from a dual thermodynamic and kinetic standpoint; the account includes not only the fundamental reaction and dissolution steps of the batch ingredients but also the fining and homogenization of the melt produced (Chapter 1.3).
The second part of the section is devoted to the making of three basic products. Flat glass is dealt with by T. Kamihori. He begins with the first mechanical methods devised at the turn of the nineteenth and twentieth centuries, turns to the famous float process, which revolutionized the flat‐glass industry in the 1960s, and ends with the recent downdraw processes widely used to produce new glasses for electronic applications with ever stricter quality specifications (Chapter 1.4). Container glass is considered by C. Roos who briefly presents the first forming devices designed at the beginning of the twentieth century before describing the various ways in which a bottle is now shaped with Individual Section machines at extremely high rates and may then be protected by treatments such as coating to enhance resistance to breakage (Chapter 1.5). In the next chapter, the drawing of continuous glass fibers for the relatively small but important reinforcement market is considered by H. Li and J. Watson in terms of both processes and composition evolutions driven by the need to improve chemical and physical properties (Chapter 1.6). That computer modeling of glassmaking has become an important tool to save time and money in the design or improvements of plants is explained by P. Prescott and B. Purnode in the final chapter of this section, which shows that, in industry too, fundamental insights and an accurate knowledge of the physical properties of melts have become badly needed (Chapter 1.7).
Other processes and their products are too diverse to be gathered into a common chapter. Hence, they are described along with some of their important applications: the secondary fabrication of flat glass in Chapter 9.2, the making of thermal insulation fibers in Chapter 9.3, of sol–gel products in Chapter 8.2, of glass tubes in Chapter 7.7, and of light bulbs in Chapter 6.9. Other fabrication issues are dealt with in chapters devoted to modern furnaces (Chapters 9.7 and 9.8), cullet recycling (Chapter 9.9), and the history of glassmaking processes (Chapters 10.5, 10.7, and 10.8).
1.1 Glass Production: An Overview
Reinhard Conradt
RWTH Aachen University, Aachen, Germany
1 Introduction
The term “glass” may either refer to a special state of matter in general or to a group of industrially manufactured materials. The chart in Figure 1 presents, from a chemical point of view, an overview of a large number of systems that can be easily transferred into the glassy state. In this chart, special emphasis is given to the industrially relevant group of silicate glasses because, by volume or mass, the vast majority of the glasses produced belong to it. Nonsilicate oxide glasses and other inorganic nonmetallic glasses, nevertheless, play an essential role in the production of highly specialized functional materials such as optical fibers (Chapter 6.4). The group of “other glasses” comprises materials of very different nature. Within this group, metallic glasses (Chapter 7.11) are finding a variety of practical applications whereas organic glasses (Chapters 8.7 and 8.8) have long played a major role at the industrial scale.
No attempt is made here to present a concise definition of the glassy state in general. From a practical point of view, however, glasses comprise a group of noncrystalline homogeneous and isotropic materials characterized by the absence of any microstructure. Thus, in contrast to (poly)crystalline materials, the bulk properties of which are essentially tailored via their microstructure, those of glasses are chiefly designed via their chemical composition; by contrast, thermal treatment has a comparatively small “fine‐tuning” effect, which may, nevertheless, become crucial for specific products (e.g. optical or strong glasses).
At the atomic scale, the very same bonding interactions are present in isochemical condensed phases, i.e. in liquids, glasses, and crystalline polymorphs. Therefore, the chemical and electronic properties of glasses resemble those of their crystalline counterparts – with the reservation that glasses typically possess larger molar volumes, higher entropies, and higher (less negative) enthalpies of formation. In other words, they are thermodynamically less stable than crystals. Nevertheless, their macroscopic properties reflect in essence the same dependences on chemical composition as their crystalline counterparts. Without mentioning a host of other polymorphs, SiO2 may, for example, exist under ambient conditions as quartz, cristobalite, or vitreous silica; thermodynamic stability decreases in the given order. The same applies to hydrolytic stability, a macroscopic property for which all SiO2 polymorphs nonetheless stand out by comparison with other oxides.
In general, information on atomic bond strengths, compound formation energies, and phase equilibria in a system of a given chemical composition may serve as reliable guidelines to explore the relation between the chemical composition of a glass and its macroscopic properties. It would go too far to draw the same conclusion for the relation between the chemical composition and the short‐range order structure. Although there is ample experimental proof for such a relation in many systems [1], the general claim may be misleading, even erroneous is specific instances. Yet, in any case, the energetics pertaining to a specific glass structure is in general very close to that of an isochemical crystalline system. Energetics, in turn, is the key factor governing the relation between the chemical composition of a glass and its macroscopic properties. For this reason, equilibrium phase diagrams ([2, 3], Chapter 5.2) and thermochemical databases [4–9] are most helpful tools in the design of glass compositions with desired properties.
Figure 1 Glass‐forming systems,
