At the end of step M4, the melt is finally conveyed to the forming area where it is transformed into hollow ware (Chapter 1.5), or flat glass (Chapter 1.4), or any other type of product. To maintain a high glass quality, the filling level of the melting compartment must be kept constant. Therefore, the sequence of process steps P1 to M4 must be well balanced logistically. This constraint puts a stringent time interval to act for online corrections to steps P4–P6; the buffer silo between steps P5 and P6 thus serves the sole purpose of widening this interval. Along the path from P6 to M4, no action for correction is possible at all.
3 Batch Preparation
3.1 Raw Materials
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.
3.2 Calculation of Batch Composition
An accurate chemical analysis of every raw material is a prerequisite for batch preparation (Chapter 1.2). On this basis, one swiftly determines the batch composition by solving a system of linear equations where data are arranged in a specific way (Table 2). First, the total number of different oxides in the raw material basis is determined (6 in the example shown). The target glass composition is entered as an oxide column vector YTARGET. The large matrix shaded in gray contains the results of raw‐material analyses. It is arranged in the order of carriers of the respective glass oxides. For each oxide that is not represented by a specific raw material, the entry 1 is filled in the matrix. If an oxide has more than one carrier raw material (in the example, Al2O3 has two carriers, namely feldspar and Calumite®), then their ratio has to be specified, and the respective column entries are merged to a single column in proportion of this ratio. Through these operations, the gray area takes the form of a square matrix M. Next, a preliminary vector RPRE of the batch composition is obtained from the product RPRE = M −1·YTARGET. It may contain negative figures since it is impossible to make for example an iron‐free glass from iron‐containing raw materials. To derive the actual batch‐composition vector R, these negative figures are thus set to zero. The real glass composition is then given by the product YREAL = M·R before YREAL is normalized to 100 wt % and R to 1000 kg of resulting glass, or 2000 kg of sand (see following paragraph), or to any other convenient reference mass.