Gas and bubbles in molten glasses

In this presentation some procedures are shown how one can calculate the release of gases after increasing temperatures. Also called fining reactions in the glass melt. Results are given for antimony oxide and sulfate fining, The increase of the internal pressure of gases in the melt, which is caused by the release of oxygen or sulphur dioxide, is derived from thermodynamic calculations and can: be presented as a function of the redox state of the melt. The temperatures for initialization of the antimony oxide (refining process in soda-lime glass melts, depending on redox state are given for some cases. The temperature range for sulfate refining in flint or green container glasses is determined, assuming O-2 and SO2 as the major (refining gases. The growth rate and the change in composition of bubbles due to diffusion of these refining gases, is given for different redox states, temperatures and polyvalent ions in soda-lime glass melts. The calculated optimum additions of (refining agent, are for antimony about 0.6 wt.-% (Sb) and for sulphate fining 0.5 wt.-% sulphur (1.25 wt-% SO3 addition, to the batch) in oxidized soda-lime glassmelts. It has been shown that a further increase of these concentrations hardly changes the bubble growth rates and even might increase foaming problems. The ascension rate of the growing bubbles to the surface of the melt during the fining stage is calculated and depends strongly but on the redox rate of the melt for both sulphate as antimony refining. The influence of the redox state on the fining efficiency ill an antimony containing melt is completely different from a melt using sulfate as a fining agent.