Basics of Oxidation
When heated sufficiently, many substances oxidize if there is free oxygen available. Volatile portions of compounds and molecules break free and the free oxygen then can attach to the remaining material, forming oxides. This process is called oxidation.
In firing a pottery kiln, the materials will normally convert to their oxide forms. For example, when copper carbonate is fired, the carbon will detach and burn off. As soon as the copper-carbon bond is broken, available oxygen will rush in and attach to the copper, forming copper oxide.
When a kiln is not in reduction, and there is enough oxygen for efficient fuel consumption but not an abundance of oxygen, the kiln can be considered to be in a neutral atmosphere. Electric kiln firings are generally considered to have either a neutral or slightly oxidizing atmosphere.
Many potters question if there is any such thing as a truly neutral atmosphere. Their main point is that there is enough oxygen in the kiln so that the glaze and clay body materials do oxidize.
Fire requires oxygen to burn. When there is a lack of oxygen, the fuel does not burn completely and the kiln atmosphere becomes filled with free carbon. The free carbon atoms will aggressively grab up any oxygen atoms they can find. In fact, carbon atoms are so oxygen-hungry that they are able to break molecular bonds. The carbon literally robs the clay and glaze materials of their oxygen.
When the carbon reduces the amount of oxygen in the clay and glaze molecules, the colors and textures of the clays and glazes can change. These changes can sometimes be quite dramatic. For example, glazes with a high iron content may be rather stiff at cone 9 in an oxidation atmosphere, and quite fluid and runny at cone 9 in a reduction atmosphere.
Oxidation and Reduction in Terms of Firing Schedules
Electric kilns are naturally in an oxidation or neutral atmosphere. With fuel-burning kilns, however, care must be taken to ensure that the kiln does not go into reduction until the latter part of the firing, usually the last half hour to the last hour and a half.
A kiln entering reduction at too low a temperature can result in clay and glaze defects, including bloating and carbon coring.