| http://www.w3.org/ns/prov#value | - ent propagation of any cracks and to provide flexibility for resisting thermal and mechanical shock, the insulating material including the oxides of lead, sodium and magnanese, the amount of lead oxide in the polycrystalline insulating material being greater than the amount of any other oxide in the material and the oxides of lead, sodium and manganese comprising at least forty-seven percent (47%)
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