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Fusion is of course the high‐temperature process through which a glass is synthesized from the relevant raw materials. In this chapter, fusion and melting will be used synonymously as no preference for either term obtains in glass manufacturing. Nevertheless, the matter deserves a few comments because both are used to describe different processes under conditions of constant pressure. They may denote:

1 A first‐order phase transition of a single‐component system (such as pure H2O, SiO2, or CaAl2Si2O8) from the solid to the liquid state. This transition occurs at a unique melting (or fusion) temperature Tm where the solid and liquid coexist; between them, however, there exist discontinuities in enthalpy and entropy, which are the enthalpy (∆Hm) and entropy (∆Sm = ∆Hm/Tm) of melting (or of fusion).

2 The transition of a thermodynamically stable assemblage of different crystalline phases to the liquid state. Upon heating, such a system passes through a temperature range at which the solid and liquid phases coexist; the solidus (Tsol) and liquidus (Tliq) temperatures are the lower and upper bounds, respectively, of this interval.

3 The transition of any mixture of crystalline phases to the liquid state upon heating. Since such phases are not in thermodynamic equilibrium, they begin to react mutually in the solid state so that the actual path of fusion may be unpredictably complicated.

4 A special technique, often used by artists, to join pieces of glass together to form an object. It makes use of the fact that a glass, upon heating, undergoes gradual softening from a rigid condition below the glass transition temperature Tg to the liquid state at T > Tg. At sufficiently high temperatures, glass pieces may then be joined together by viscous flow. The transition from a crystalline state is here nonexistent, which distinguishes clearly this special meaning of fusion from the three others.

In glassmaking, it is of course case (3) that matters, which is why it will be exclusively dealt with in this chapter. It begins with the heating of a mixture of granular solids, the batch, and is completed when a homogeneous liquid state is reached. Regardless of the complexity of its chemical composition, any glass is associated with a liquidus and a solidus temperature between which crystals and melt can coexist in thermodynamic equilibrium. Upon not too fast heating, a liquid for instance begins to form at the liquidus temperature of the system as determined by its overall chemical composition.

At the industrial scale, the fusion of glass is a most complex energy‐intensive, high‐temperature process. The transformations involved in are multiphase, multicomponent chemical reactions, which are quite different from a student's simple concept of chemical reaction. The goal of the fusion process consists in delivering a workable glass melt of high quality at high production rates and low specific‐energy consumption, thereby abiding with environmental legislation. It brings together the issues of reactor technology, bulk solid melting, particle dissolution, as well as redox and acid–base chemistry of the melt. Fusion takes place in specifically designed glass furnaces. Small amounts of specialty glass are melted discontinuously in crucible, pot, or day tank furnaces; the melting compartments may be considered as crucibles of different size, ranging from a few kg to a few 100 kg. For continuous melting of small amounts of some specialty glasses, rotary kilns are used. But the vast majority of glasses is continuously melted in large glass furnaces whose production capacities range from a few tons to 1000 t per day.

Regardless of this diversity, fusion involves the same steps that will be successively described in this chapter. The first is the careful preparation of the batch from the appropriate raw materials. The second takes place at high temperatures through the various reactions that lead to a melt through complete dissolution of even the most refractory starting materials. The third step aims at producing a homogenous, bubble‐free product by physical and chemical fining. Finally, this chapter will briefly discuss the economically and environmentally important energetics of the fusion process. A review of earlier work dealing with these issues is the feature article by Cable [1].