This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
These furnaces may be so fired and the air supply to the fire so regulated as to make the furnace action oxidizing or reducing. Oxidation demands (1) an excess of air beyond that needed for complete oxidation of every combustible part of the fuel and (2) that the excess air be at the required heat to combine with (burn) the material to be oxidized.
Reduction demands (1) that the supply of air be deficient for the complete burning of the fuel and (2) that the unburned gases from the fuel be kept at or above their igniting temperature, in which case they will extract oxygen from oxides in the charge having a less affinity for oxygen. An oxidizing atmosphere necessitates a thin fire and a full supply of air over and under the fuel, while a reducing atmosphere necessitates a thick fire with small air supply, particularly above the fuel. In both operations it is imperative that the fire shall burn vigorously enough to maintain the degree of heat required. Reverberatory furnaces are not economical in fuel.