Fining of Glass Melts

The degassing of silicate melts plays an important role in both the Earth and glass sciences. Volcanic eruptions are driven by the expansion of exsolved volatiles. These volatiles are initially dissolved in magmas at high pressures and later released as one or more fluid phases during ascent of the magma to the Earth’s surface due to decompression (Webster and Botcharnikov 2011, this volume; Oppenheimer et al. 2011, this volume). During fining of glass melts the oversaturation of the silicate liquid with respect to dissolved volatiles is induced by heating. Although the process of volatile oversaturation occurring in geologic and industrial melts differs, the fundamental mechanisms controlling melt degassing are similar in both cases, particularly with respect to the nucleation and growth of gas bubbles in the melt and their upward migration driven by buoyancy. Another similarity of natural and technical melt degassing is the role of sulfur. Tremendous amounts of sulfur in form of SO2 and H2S are produced by volcanic eruptions at the surface or by the cryptic degassing of magmas ascending through the earth’s crust (see Oppenheimer et al. 2011, this volume). During the fining of industrial glass melts, one takes advantage of the solubility behavior of sulfur in silicate melts. Sulfate added to the melt batch as a raw material, decomposes to release gaseous compounds during fining that enhance the expansion of existing bubbles which can, as a result, ascend more readily to the melt surface. Hence, the release of gaseous sulfur dioxide is a major issue in both volcanic eruptions and industrial glass production. In this chapter we will have a detailed look at the process of fining in industrial mass glass production. Glassproductsarefoundinwidefieldsofdailylife,suchasautomotiveglazing,architecture, food and pharmaceutical packaging, and optical devices or opto-electronic components in computers and consumer electronics. The …