Glass composition for an electric lamp, stem and bulb for an electric lamp using the same, and electric lamp using the same

摘要

To suppress breakage of glass for an electric lamp in a process in which the glass that has already been formed is processed further, a glass composition for an electric lamp is provided. The glass composition contains, expressed in mol %, 70 to 85% of SiO₂, 12 to 17% of R₂O, and 2 to 8.5% of MO (where R represents at least one selected from Li, Na and K, and M represents at least one selected from Mg, Ca, Sr, Ba, Zn and Pb). In the glass composition, the respective contents of CaO, MgO, BaO and SrO satisfy the relationship, CaO+MgOBaO+SrO. The glass composition has a brittleness index value B determined by the Vickers hardness test of 7,000 m^−1/2. Preferably, the contents of SrO and BaO are 0 to 0.5% and 0.1 to 1%, respectively. More preferably, the respective contents of K₂O and Na₂O satisfy the relationship, K₂ONa₂O.