Abstract: Device for injecting non-oxidizing gas into a furnace with a graphite concentrator (12) as the heating element used to heat a primary preform in the enclosure of the furnace (14) and moving relative to the furnace in its axial direction, and comprising at the inlet orifice (24) and at the outlet orifice (26) of the enclosure a respective annular injector (40, 42) for injecting two conical curtains of non-oxidizing gas, the gas for each of the curtains being directed towards the apex of the cone and in the direction away from the enclosure (14) so as to prevent any entry into the enclosure of air that could cause combustion of the graphite.