Patent ID: 7038363

Claim:
A spark plug comprising an insulator, on at least a part of a surface of the insulator being formed a marking layer and a glaze layer, wherein the marking layer is exposed on the surface of the insulator and contains: a Pb component of not more than 5% by mole in terms of PbO; an Si component of from 36% by mole to 72% by mole in terms of SiO 2 ; an Al component of from 0.1% by mole to 18% by mole in terms of Al 2 O 3 ; an alkali metal component containing at least one of an Li component, an Na component and a K component of from 0.1% by mole to 9% by mole in terms of Li 2 O, Na 2 O and K 2 O, respectively; a B component of from 3% by mole to 30% by mole in terms of B 2 O 3 ; and a color developing component containing at least one of a V component, a Cr component, an Mn component, an Fe component, a Co component, an Ni component, a Cu component, a Pr component, an Nd component, an Au component, a Cd component, an Sn component, an Sb component, an Se component, an S component and an La component of more than 0% by mole but not more than 50% by mole in terms of V 2 O 5 , Cr 2 O 3 , MnO 2 , Fe 2 O 3 , CoO, NiO, CuO, Pr 2 O 3 , Nd 2 O 3 , Au 2 O 3 , Cd 2 O 3 , SnO 2 , Sb 2 O 5 , SeO 2 , SO 3 and La 2 O 3 , respectively.