Spark plug

A spark plug, comprising:

• a cylindrical metallic shell having aground electrode;
• an insulator fixed within said metallic shell, said insulator having a coating of glaze on an outer surface extending from a head portion to a trunk portion of said insulator;
• and
• a center electrode fixed in an axial bore formed in said insulator,
• wherein the Pb content of said glaze is not more than 10 wt. % in terms of PbO and said glaze contains 49.1-64.5 wt. % of SiO.sub.2, 5.0-15.0 wt. % of B.sub.2 O.sub.3, 0-10.1 wt. % Al.sub.2 O.sub.3, 0-7.4 wt. % Na.sub.2 O, 0-5.5 wt. % CaO, and 0-10.2 wt. % ZnO.

A spark plug, comprising:

• a cylindrical metallic shell having a ground electrode
• an insulator fixed within said metallic shell, said insulator having a coating of glaze on an outer surface extending from a head portion to a trunk portion of said insulator;
• and
• a center electrode fixed in an axial bore formed in said insulator,
• wherein the Pb content of said glaze is not more than 10 wt. % in terms of PbO and said glaze contains 55-64.5 wt. % of SiO.sub.2, 5.0-25.0 wt. % of B.sub.2 O.sub.3, 10.5-55.9 total wt. % of two or more oxides selected from a group consisting of Al.sub.2 O.sub.3, Na.sub.2 O, CaO, ZnO, BaO, Li.sub.2 O, and Bi.sub.2 O.sub.3.

A spark plug, comprising:

• a cylindrical metallic shell having aground electrode;
• an insulator fixed within said metallic shell, said insulator having a coating of glaze on an outer surface extending from a head portion to a trunk portion of said insulator;
• and
• a center electrode fixed in an axial bore formed in said insulator,
• wherein the Pb content of said glaze is not more than 10 wt. % in terms of PbO and said glaze contains 55.0-64.5 wt. % of SiO.sub.2, 5.0-25.0 wt. % of B.sub.2 O.sub.3, 0-10.1 wt. % Al.sub.2 O.sub.3, 0-7.4 wt. % Na.sub.2 O, 0-5.5 wt. % CaO, and 0-10.2 wt. % ZnO.

A spark plug, comprising:

• a cylindrical metallic shell having aground electrode;
• an insulator fixed within said metallic shell, said insulator having a coating of glaze on an outer surface extending from a head portion to a trunk portion of said insulator;
• and
• a center electrode fixed in an axial bore formed in said insulator,
• wherein the Pb content of said glaze is not more than 10 wt. % in terms of PbO and said glaze contains 49.1-64.5 wt. % of SiO.sub.2, 5.0-25.0 wt. % of B.sub.2 O.sub.3, 0-6.0 wt. % Al.sub.2 O.sub.3, 0-7.4 wt. % Na.sub.2 O, 0-5.5 wt. % CaO, and 0-10.2 wt. % ZnO.