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Glaze firing Reduction: Reduction firings are normally carried out in gas, wood or oil-fired kilns, as the burning of carbonaceous material is required to produce a reducing atmosphere. The carbon from the burning material combines with the oxygen present in the atmosphere; when carbon in the material being burnt exceeds the oxygen available in the atmosphere, incomplete burning occurs and carbon and carbon monoxide are formed. These then take oxygen from any available source, including the oxides in the clay and glazes present in the kiln. When this extraction of oxygen happens, clay and glazes become “reduced” and this can affect the colours obtained from the oxides. For example, copper oxide can produce red.