Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 76

The fusion of a glass of a given composition requires a suitable set of raw materials (Chapter 1.2). These are selected according to their availability and quality, which, in turn, determine their price. The issue of availability may be complex since it includes geological occurrence (for natural materials), production capacity (for manufactured materials), infrastructure for recycling and upgrading (for cullet) as well as transport distance, number of tenders, political stability at the source, etc. Quality is likewise a manifold issue as it concerns chemical composition (from impurities to the main component), mineralogical composition (special attention being paid to side minerals difficult to melt), and grain size distribution (with a particular concern to under‐ and oversized grain fractions). Among chemical impurities, iron is a critical factor. It is present in virtually every natural raw material but is generally tolerated in glass only at very low levels (Table 1) except, of course, when it is itself a major component of the product as in fire‐resistant glass fibers (Chapter 9.3). Here, the iron content is given in terms of stoichiometric Fe₂O₃ irrespective of its actual valence state. Yet, iron is generally present as Fe²⁺ and Fe³⁺ whose relative abundances depend on the redox state of the melt. Owing to the strong absorption bands of both cations, iron has a strong impact on the color of the glass even at low concentrations (Chapter 6.2). And because of its strong absorption in the 600–4000 μm wavelength range, which is that of the heat radiation in the furnace, Fe²⁺ acts as a blinding agent to limit tightly the transparency of the melt to IR radiation (Figure 1). In Figure 1, the furnace radiation is illustrated by a back‐body‐type curve. This is an oversimplification. The actual flame radiation spectrum in a furnace is characterized by strong emission lines of the H₂O and CO₂ molecules in the flame (H₂O: 0.9–1.1, 1.8–2.0, 2.5–3.2 μm; CO₂: 2.7–3.0, 4.2–4.7 μm) and by black‐body radiation from soot particles. The radiation enters the melt directly to a certain extent; however, chiefly via emission (emission coefficient ε ≈ 0.5) and diffuse reflection from the top lining (the crown) of the furnace. The curve shown in Figure 1 is an envelope of the actual radiation only. Irrespective of the above details, furnaces in which glasses with high Fe²⁺ contents are melted thus exhibit large vertical temperature gradients and low bottom temperatures; heat transfer from the combustion space has then to be brought about by the convective motion of the melt. By contrast, melts with very low amounts of Fe²⁺ weakly absorb energy from the combustion space since Fe³⁺ does not influence IR absorption. As a consequence, they display high temperatures at the bottom of furnaces. Controlling the redox state of the melt thus is important not only for color generation but also for furnace operation, a general conclusion that also applies for instance to green glasses colored by Cr³⁺.

Table 1 Maximum iron contents in various types of glasses, given in ppm of stoichiometric ferric iron (Fe₂O₃).

Glass type	ppm Fe2O3
Optical glass	10
Ultra‐white glass	100
Continuous fibers	200
Flint container glass	250
Standard float glass	300
Amber container glass	2 500
Cr‐green container glass	10 000

Figure 1 Absorption bands of Fe²⁺, Fe³⁺, and Cr³⁺ in a glass melt, and radiation intensity in the combustion space of a furnace illustrated in a simplified way by black‐body radiation emitted at 1600 °C from the upper lining (the crown) of the furnace; relative intensities.