A solid oxide fuel cell generator is provided for electrochemically reacting a fuel gas with a flowing oxidant gas at an elevated temperature to produce power. The generator includes a generator section receiving a fuel gas and a plurality of elongated fuel cells extending through the generator section and having opposing open fuel cell ends for directing an oxidant gas between opposing plena in the generator. A sealant defines a seal on the fuel cells adjacent at least one of the fuel cell ends. The sealant is a modified lanthanum borate aluminosilicate glass composition having a minimal amount of boron oxide and silica, and in which the sealant maintains substantially constant physical characteristics throughout multiple thermal cycles.