Glass melting defects

Figure 2.6 Photochromic glass (a) glass melt containing dissolved CuCl and AgCl (b) melt is cast into a homogeneous glass blank (c) heat treatment precipitates crystallites (much exaggerated in size here) in the blank and (d) sodium chloride structure of AgCl containing copper impurities and Frenkel defects.

Glass devitrification Formation of crystals (seeds) in a glass melt, usually occurring when the melt is too cold. These crystals can appear as defects in glass fibers. 

Whereas fused-cast alpha-beta alumina refiactories have been used in the feeder chaimel application for many decades, the use vibro-cast alumina in feeder channels is more recent. In this application, the key performance criteria include glass melt contact corrosion resistance and glass defect potential. Service temperatures are expected to be in the range of 12S0 C - I3S0°C. 

Melter-Created Glass Defects. Despite best efforts to generate a perfectly uniform glass melt, within any melter there are naturally occurring processes that oppose those efforts. These include refractory corrosion (dissolution), electrode corrosion, and preferential volatilization of some species from the melt surface. These produce localized and sometimes more global deviations from the desired glass composition. Localized composition deviations lead to inhomogeneities in the product, called cord and striae. 

The development of optical fiber is a fascinating story, especially with regard to the introduction of CVD as the primary method of fabrication, but the details are beyond the scope of this text. Suffice to say that traditional glass-forming technologies, which were based on melt processing, did not provide fibers free from defects such as bubbles or with sufficient purity to prevent enormous attenuation losses. The lowest losses of optical quality glasses were on the order of 1000 dB/km, whereas optical fibers today... 

The common feature of the internal reactions discussed so far is the participation of electronic defects. In other words, we have been dealing with either oxidation or reduction. We now show that reactions of the type A+B = AB can take place in a solvent crystal matrix as, for example, the formation of double oxides (CaO +Ti02 = CaTi03) in which atomic (ionic) but no electronic point defects are involved. Although many different solvent crystal matrices can be thought of (e.g., metals, semiconductors, glasses, and even viscous melts and surfaces), we will deal here mainly with ionic crystal matrices in order to illustrate the basic features of this type of solid state reaction. 

The risk of devitrification rises the longer a glass is kept in a softened or melted state, and it is also linked to how dirty the glass is. Devitrification typically begins as a surface phenomenon, using either dirt or some other surface defect as a nucle-ation point. The devitrification process may be assisted by variations in the exte-... 

As discussed in the section on particulate pollution, the rate of volatilization of the more volatile glass species is reduced for oxy/fuel. Without this preferential loss of some components from the surface of the melt, the concentration in the melt is more likely to be homogeneous. Concentration nonhomogeneity can result in cord defects or other visual distortions in the glass product. 

Fast-mixing oxy/fuel burners can overheat the glass locally, leading to numerous glass defects such as cord and seeds. On the melt surface, foam formation or reboil is... 

---