Abstract: A blackening process of producing an oxide layer consisting predominantly of Fe.sub.3 O.sub.4 on an iron surface in an oxidizing atmosphere and at a temperature of from 500.degree. to 650.degree. C, whereby the oxidizing atmosphere is produced by understoichiometrically burnt fuel gas without removal of water from the combustion gases. In the combustion chamber, the apparent reaction temperature is controlled by cooling to provide an apparent reaction temperature of from 500.degree. to 800.degree. C. The combustion products are conducted into a heated blackening chamber at a temperature substantially above the dew point of the combustion products. The CO characteristic of the combustion gases is monitored and is used to control the air and fuel supplies.