Abstract: Conventional cupola apparatus and its operation are modified by suppressing the entrance of external air to the cupola and replacing the nitrogen component of the air injected into the conventional cupola with a recycled fraction of the cupola gas consisting substantially of CO and CO.sub.2 withdrawn from the cupola at a point above the melting zone and replacing the oxygen component of the air with externally supplied substantially pure oxygen. The remaining fraction of the cupola gas is exhausted from the cupola system. When the cupola is thusly operated on a steady state basis, a balance is maintained so that the atomic amount of the oxygen exhausted from the system is equal to the atomic amount of the injected oxygen.