recovery zone whereas the port 39 is located in the flue 31 according to the invention the concentration of the oxidizing species is measured by the ports 36 37 38 39 each port receiving a laser beam via an optical fiber or a laser beam emitter a receiver being provided in the opposite wall of the furnace or else a mirror which sends the beam back parallel to the incident beam the receiver being placed beside the emitter in the zone 32 temperature below t decarb the fuel and oxidizer flow rates for the burners in the zone 32 must be adjusted according to the invention so as to create an oxygen content in the atmosphere in this zone 32 measured by the corresponding diode laser of between 0 5 and 4 vol and preferably between 2 and 3 vol if the equalizing zone 32 is not fitted with burners this correction may be made by the addition of oxidizer via lances for example oxygen lances the amount injected being controlled by the measurement of the oxygen content by the diode laser the measurement is preferably carried out as close as possible to the product either via the port 38 in this zone 32 or via the port 39 that is to say in the smoke extraction duct where the same oxygen content is monitored if the measurement shows a lack of oxygen this lack must be corrected by regulating the burners hence increasing the rate of flow of oxidizer oxygen to the burners of the zone 32 or of the preceding zone in zone 32 a protective layer of fe 2 o 3 and fe 3 o 4 will be formed and reinforced by the presence.