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The rate at which this carbon burns out is related to:
The amount of carbon present (that is, the amount of natural contaminants in the body). Some bodies have more contaminants than others, such as red clays. This needs to be considered when planning the firing.
Amount of air available (air provides oxygen for burnout) Air needs to be considered when planning the firing.
This is impacted by several factors. A load that is fired very quickly will not allow enough time for the oxygen to react with the carbon, form gases and leave the ware. If ware is stacked during bisque firing, oxygen may not be able to penetrate all surfaces of or inside all the pieces. Also, if gases are not removed from the kiln and replaces with fresh air, then there may not be sufficient oxygen to burn out the carbon.