| 摘要 |
PROBLEM TO BE SOLVED: To improve production yield by suppressing the occurrence of cracks in fused glass at the time of cutting during production. SOLUTION: A glass compsn. which consists, by weight % of 15 to 30% SiO2, 60 to 65% PbO, less than 10% A2O (A: an alkaline metal) and the balance consists at least of one kind from among B2O3, TeO2, Bi2O3, Al2O3, ZnO and BaO with Vickers hardness such that 300<=Hv<400 is used as the fusion glass 5 for forming the gaps 8, 9 of a head core 1.
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