Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 40

Figure 1 The initial melting step in the making of float glass: the 1‐m deep bath of raw materials melted by the flames of a cross‐fired furnace (Chapter 9.7). Pulls ranging from 500 to 1000 tons/day and mean residence times of at least 24 hours. Electro‐fused refractory materials made up of alumina‐zirconia‐silica in contact with the melt, and of alumina and alumina‐silica elsewhere (cf. Chapter 9.8).

Source: Photo courtesy Simonpietro Di Pierro, Saint‐Gobain Research Paris.

Compared with crude steel (1700 million tons/year worldwide) and especially with cement (4300 Mtons), glass (about 120 Mtons) is produced in relatively small quantities. In terms of product value or volume, however, the imbalance is significantly reduced since the cost of cement is about one sixth of that of window glass and steel about three times as dense. But what differentiates glass most from these other two inorganic pillars of modern civilization is the remarkable diversity of its uses illustrated throughout the Encyclopedia.

In Europe, for which the data are the most readily available, the 35 Mtons produced in 2017 were split into container (21.4), flat (10.1), domestic (1.3), reinforcement (0.7), and other (1.1) glass. For both container and flat glass, the world market is estimated to be in the 60–80 billion $ range and is expected to keep growing in the years to come at yearly rates higher than 5% on average, with large geographical differences (cf. Chapter 9.6). And growth rates should be higher still for new products such as the smart glass used in a variety of electronic devices (cf. Chapter 6.10), whose market should increase by a factor of 3 from 2017 to 2023 from the current few billion $ per year.

Like cement and steel producers, glassmakers sell more than 90% of their production to other industries. Most uses of glass are nonetheless familiar to anyone. These are summarized in the first chapter of this section where R. Conradt points out their strong dependence on chemical composition of the glasses and on their ensuing physical properties, explaining that the reason why the still‐dominant soda‐lime silicates were empirically found so early in the history of glassmaking is simply because they lie close to the eutectic of the Na₂O–CaO–SiO₂ system.

Even though glass is now made in many different ways for different applications, the traditional procedure of making it by cooling of a batch melted at high temperatures remains by far prevailing. As one readily realizes when looking at the original glazing of late‐nineteenth‐century buildings, the long‐standing problem faced by glassmakers was to achieve chemical homogeneity. The mass production of defect‐free glass is a relatively recent achievement. It has resulted from better furnaces (Figure 1; Chapters 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).