A method of heating material in a furnace, comprising heating material comprising carbonaceous matter in a furnace having a flue, using heat generated by combustion in the furnace of fuel and gaseous oxidant being fed into the furnace, thereby producing carbon monoxide derived from the carbonaceous matter, wherein a flame (4, 15) is formed in the furnace which extends out of the furnace from the flue; characterizing the concentration of carbon monoxide in the flame from images of the flame taken outside the furnace by a digital camera that is located outside the furnace, by expressing electronically at least one parameter that corresponds to the intensity of the flame and that corresponds to the concentration of carbon monoxide in the flame, and determining the characterized concentration of carbon monoxide in the flame from predetermined correlations of the actual concentrations of carbon monoxide in a flame to expressed values of the at least one parameter; comparing the characterized concentration of carbon monoxide in the flame as characterized in accordance with step to a preestablished threshold concentration value for said concentration; when the characterized concentration of carbon monoxide in the flame exceeds said preestablished threshold concentration value, adjusting the amount of oxygen, the amount of fuel, or the amounts of both oxygen and fuel, being fed into the furnace that is available to react in the furnace to an amount or amounts thereof that is effective to lower the characterized concentration of carbon monoxide in the flame to be equal or less than the preestablished threshold concentration value for a predetermined length of time, while continuing to characterize the concentration of carbon monoxide in the flame from images of the flame taken by the digital camera outside the furnace, characterised in that images are taken by the digital camera of the flame extending out of the furnace from the flue. heating material comprising carbonaceous matter in a furnace having a flue, using heat generated by combustion in the furnace of fuel and gaseous oxidant being fed into the furnace, thereby producing carbon monoxide derived from the carbonaceous matter, wherein a flame (4, 15) is formed in the furnace which extends out of the furnace from the flue; characterizing the concentration of carbon monoxide in the flame from images of the flame taken outside the furnace by a digital camera that is located outside the furnace, by expressing electronically at least one parameter that corresponds to the intensity of the flame and that corresponds to the concentration of carbon monoxide in the flame, and determining the characterized concentration of carbon monoxide in the flame from predetermined correlations of the actual concentrations of carbon monoxide in a flame to expressed values of the at least one parameter; comparing the characterized concentration of carbon monoxide in the flame as characterized in accordance with step to a preestablished threshold concentration value for said concentration; when the characterized concentration of carbon monoxide in the flame exceeds said preestablished threshold concentration value, adjusting the amount of oxygen, the amount of fuel, or the amounts of both oxygen and fuel, being fed into the furnace that is available to react in the furnace to an amount or amounts thereof that is effective to lower the characterized concentration of carbon monoxide in the flame to be equal or less than the preestablished threshold concentration value for a predetermined length of time, while continuing to characterize the concentration of carbon monoxide in the flame from images of the flame taken by the digital camera outside the furnace,