Abstract: In the manufacture of clay or refractory ware containing oxidizable material, stacks of the ware are passed through a tunnel kiln and a flow of air passed over the stacks. Fuel burners are operated to provide the necessary stages for firing the ware. These stages include an oxidation stage in which residual oxidizable material is oxidized. Gas containing more oxygen than air (i.e. commercially pure oxygen or oxygen-enriched air) is introduced into the region of the kiln where the oxidation takes place. The oxygen-rich gas is caused to penetrate at least one stack for the time being in the oxidation region so as to increase the concentration of oxygen throughout the individual items of ware within said stack. It is thus possible to oxidize all the residual oxidizable material. At least 50% of such oxidation is typically confined to a temperature range of 100.degree. C. The heat thereby released assists in the firing of the ware.