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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, classified by chemical composition.

The industrial synthesis of glasses can also be based on a large systematic collection of experimental data of the properties of glass‐forming systems [10–14]. Because at a microscopic level, atomic interactions are primarily pairwise (Chapter 2.7), one can in particular make use of empirical composition–property relations [15–20] of the type

(1)

where P denotes a macroscopic property, p_j is the mol or weight fraction of component j, and the a_j and b_j coefficients are sets of empirical parameters representing the contribution of component j to the property P of the glass.