The terms oxidation and reduction firing refer to the environment in the kiln, particularly during the final few hundred degrees of temperature rise. If there is more than enough oxygen present to burn the fuel completely (be that fuel gas or oil or wood), then you are said to fire in an oxidizing atmosphere. In this situation, the metal oxides that provide the color in the glaze will be converted to their most fully oxidized state and will have the color of that fully oxidized state. If less than enough oxygen is present to fully consume the fuel the firing is said to be a reduction firing. The metals in the glaze will be in mixed states. On an individual molecule basis, some of the metals will be in a fully oxidized state; others will be partially oxidized; others may be fully reduced and be present as the base metal instead of as an oxide. Oxidation firing, then, produces more uniform and predictable color, but some potters find them uninteresting. The most consistent oxidation firing is obtained in an electric kiln that has a small, constant stream of fresh air circulating through it. In fuel burning kilns there will almost always be some sections of the kiln that are in reduction even if, overall, there is an excess of oxygen present. Reduction firing produces more subtle variations in color and produces pots which are more difficult to reproduce because of the difficulty of controlling the reduction environment exactly the same way with each firing. That adds to the excitement, fun and mystery of opening the cooled kiln and, fortunately, seldom causes disappointment.