The present invention relates to a glass/ceramic sealing system. A sealing glass having a coefficient of thermal expansion in excess of 160x10-7in/in/ DEG C. is provided. The glass is useful for matched sealing of copper and copper based alloys. The glass is capable of ceramization, greatly increasing the resistance of the glass to moisture penetration and fracture due to mechanical damage. The temperature of ceramization has been designed so that a solutionized copper alloy leadframe is age hardened during ceramization. The glass has particular utility in the manufacture of electronic packages and multi-layer circuitry.