A method of melting raw materials by a cross-fired melting furnace which has: - a melting tank for receiving raw materials to be melted and for accommodating a melted materials bath; - a melting chamber located above said melting tank and comprising a first side wall, a second side wall opposite said first side wall, a back wall located at an upstream area of said melting tank, a front wall located at a downstream area of said melting tank, and a roof; - N first ports (31,..., 36) being provided in the first side wall in horizontally spaced locations between said back wall and front wall, each of said at least one series of N first ports (31,... 36) being associated with a corresponding first burner of a series of N first burners (11,..., 16); - N second ports (41,... 46) being located in the second side wall in horizontally spaced locations between said back wall and front wall, each of said N second ports being located opposite a first port to define N couples of first and second ports (31, 41,..., 36, 46); wherein re-circulating combustion products flow in a substantially vertical loop above a flame; the method comprising: introducing a first fraction X1 of fuel into said melting chamber via said first burners, introducing a second fraction X2 of auxiliary fuel, with X2 + X1 being equal to 1, using at least one auxiliary fuel injector, the at least one auxiliary fuel injector being arranged in the cross-fired melting furnace in said roof or in the side wall not comprising burners introducing fuel so that the at least one auxiliary fuel injector introduces the second fraction X2 of auxiliary fuel, in the direction of the flow of said re-circulating combustion products, without additional oxidiser, into said re-circulating combustion products, the auxiliary fuel injector being located at a point where said second fraction X2 of auxiliary fuel will mix with the recirculating combustion products, before reaching incoming oxidiser introduced by a port, the velocities of the jets introducing the fraction X1 of fuel and the fraction X2 of auxiliary fuel being adapted so that the sum of their corresponding jet momenta is comprised between plus or minus 30% of a value corresponding to the jet momentum of the fuel when X2 equals zero, and the energy provided by the quantity of the sum of the first fraction of fuel X1 and the second fraction of fuel X2 being adapted to produce a given required energy for melting said materials without over-fuelling the furnace. - a melting tank for receiving raw materials to be melted and for accommodating a melted materials bath; - a melting chamber located above said melting tank and comprising a first side wall, a second side wall opposite said first side wall, a back wall located at an upstream area of said melting tank, a front wall located at a downstream area of said melting tank, and a roof; - N first ports (31,..., 36) being provided in the first side wall in horizontally spaced locations between said back wall and front wall, each of said at least one series of N first ports (31,... 36) being associated with a corresponding first burner of a series of N first burners (11,..., 16); - N second ports (41,... 46) being located in the second side wall in horizontally spaced locations between said back wall and front wall, each of said N second ports being located opposite a first port to define N couples of first and second ports (31, 41,..., 36, 46); introducing a first fraction X1 of fuel into said melting chamber via said first burners, introducing a second fraction X2 of auxiliary fuel, with X2 + X1 being equal to 1, using at least one auxiliary fuel injector, the at least one auxiliary fuel injector being arranged in the cross-fired melting furnace in said roof or in the side wall not comprising burners introducing fuel so that the at least one auxiliary fuel injector introduces the second fraction X2 of auxiliary fuel, in the direction of the flow of said re-circulating combustion products, without additional oxidiser, into said re-circulating combustion products, the auxiliary fuel injector being located at a point where said second fraction X2 of auxiliary fuel will mix with the recirculating combustion products, before reaching incoming oxidiser introduced by a port, the velocities of the jets introducing the fraction X1 of fuel and the fraction X2 of auxiliary fuel being adapted so that the sum of their corresponding jet momenta is comprised between plus or minus 30% of a value corresponding to the jet momentum of the fuel when X2 equals zero, and the energy provided by the quantity of the sum of the first fraction of fuel X1 and the second fraction of fuel X2 being adapted to produce a given required energy for melting said materials without over-fuelling the furnace. in the direction of the flow of said re-circulating combustion products, without additional oxidiser, into said re-circulating combustion products, the auxiliary fuel injector being located at a point where said second fraction X2 of auxiliary fuel will mix with the recirculating combustion products, before reaching incoming oxidiser introduced by a port, the velocities of the jets introducing the fraction X1 of fuel and the fraction X2 of auxiliary fuel being adapted so that the sum of their corresponding jet momenta is comprised between plus or minus 30% of a value corresponding to the jet momentum of the fuel when X2 equals zero, and the energy provided by the quantity of the sum of the first fraction of fuel X1 and the second fraction of fuel X2 being adapted to produce a given required energy for melting said materials without over-fuelling the furnace.