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Fluorine release from glasses containing up to 1 per cent fluorine was studied in the temperature range from 350 to 500 C. Experimental evidence is presented to show that fluorine is released as hydrogen and alkali fluorides, not as fluorine gas or silicon tetrafluoride.
Temperature-dependence of fluorine release can be expressed by a general equation of the following type: log F = (-A/T) + B. The time-dependence of fluorine release can be expressed by the following relations: F = m t1/2 + b. The constants A, B, m, and b were determined for several Kimble glasses. At the annealing points (= temperature of same viscosity - log n = 13.4), glasses of the same composition group release fluorine in proportion to the fluorine content. Glasses containing lead or boric oxide evolve more fluorine than similar glasses without these oxides.
Senior Research Engineer, Kimble Glass Co., Toledo, Ohio