Glass melting silica

The practice of heating and forming glass-like figures from melted silica (sand) was known at least three or four thousand years ago. In the eighteenth century early chemists were aware of some kind of link between sand (silica) and quartz but were unaware that a new, unidentified element was involved. 

When mixed with sodium carbonate (soda ash) and calcium carbonate (powdered limestone) and heated until the mixture melts, silica (sand) forms glass when cooled. Glass of all types has near limitless uses. One example is Pyrex, which is a special heat-resistant glass that is manufactured by adding boron oxide to the standard mixture of silica, soda ash, and limestone. Special glass used to make eyewear adds potassium oxide to the above standard mixture. 

Figure 2.9 Arrhenius plot showing temperature variation of nohle gas diffusion coefficients. Samples 1-3 are glass melts 4, 5, and 14 are vitreous silica 6 is commercial glass 7 and 14 are B203 8-10 are mixtures of alkali oxides with B203, Si02, and A1203 11 and 12 are obsidians 13 and 15 are Si02. Reproduced from Hiyagon (1981).

FIG. 144. Spectral transmission of soda-lime-silica glass melted with an addition of 2%Fe203 (curve (7)— oxidation melting) and with 2%FeO (curve (2) — reduction, after Kocik and Nebfensky, 1962). 

Large quantities of refractory materials are required for the construction of melting furnaces. The suifaces of the tank furnaces in contact with the glass melt are lined with melt cast or isostatically pressed bricks (e.g. of zirconia) and the cover over the glass tanks is manufactured from mullite or silica materials. 

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