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When we burn gas with ample oxygen, the by-products of combustion are carbon dioxide (CO2 ) and water (H2O). These pass through the kiln without much effect. If we fire the kiln with insufficient oxygen by cutting down the air to the burners or by sliding in the damper to restrict the flow of air through the kiln, there isn't enough oxygen in the kiln atmosphere to make all the combustion by-products be H2O or CO2.. Instead we get some hydrogen (H2 ) and some carbon monoxide (CO)--this is called a reducing atmosphere. Hydrogen and carbon monoxide would very much prefer to be water and carbon dioxide, so they attack the oxides in the clay and glazes to get the oxygen they need. Thus the metallic oxides are reduced--they're left with less oxygen and this alters the color of the clay and glazes. In reduction firing copper often tends toward red, and iron bearing clays will turn grey. Most metals can exist in many states of oxidation, and several of those can exist at the same time in a glaze, so in practice the results of reduction firing can be quite unpredictable. Much depends on the degree of reduction--how starved for the oxygen the kiln was when the glazes were melting.