Glass melting oxidation

A desirable glass melts at a reasonable temperature, is easy to work with, and yet is chemically inert. Such a glass can be prepared by adding a third component that has bonding characteristics intermediate between those of purely ionic sodium oxide and those of purely covalent silicon dioxide. Several different components are used, depending on the properties desired in the glass. 

Potash (composed of potassium oxide), also a flux, was mainly used as a glass modifier. It was generally introduced into the glass melt in the form of either pearl ash, composed of potassium carbonate, vegetable ash, one of the main constituents of which is potassium carbonate, or saltpeter, a mineral composed of potassium nitrate. 

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).

Hundreds of thousands of different glass compositions have been devised, and they have been used in different ways. Much has been learned about which combination of chemicals will make the best glass for a particular purpose. For example, in 1664 an Englishman named Ravenscroff found that adding lead oxide (PbO) to a glass melt produced a brilliant glass... 

Fig. 4.4 Heating a glass melt the power (I2R) is introduced directly into the melt via tin oxide electrodes.

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