Another form of firing that potters should know is reduction firing which means incomplete combustion due to insufficient oxygen in the kiln to react with the fuel. Therefore, there is carbon monoxide left in the kiln after firing and when there is less oxygen in the combustion, carbon from the fuel extracts oxygen from the metal oxides in the clay and glaze which make the color of the workpiece to be the color of the oxide that has changed, for example, copper coating will get the red color of copper oxide, etc.
This reduction firing is often used for high-fire sintering of stoneware and porcelain vessels because using low fire will cause negative effects on the lead-containing glaze. In addition to requiring special paints for lead-free coatings, reduction firing is very smoky and may produce toxic fumes. The kiln should therefore be located outside of the building and far from the molding room and should have good ventilation. After firing, the kiln holes should be completely sealed to prevent cold air from entering the kiln.
However, the reduction firing range is a continuous sintering from oxidation sintering. Atmospheric adjustment was initiated from approximately 950°C until the end of the firing. The incomplete combustion of the kiln leads to insufficient oxygen in the kiln therefore there will be carbon monoxide left after firing. (CO)