I reduction fire in a natural gas fired kiln. Any type of firing that burns fuel (propane, wood, and coal), is considered a reduction firing. The pots are covered in glazes with different powdered metals for the colorants. The kiln is heated slowly. Around 1,750 degrees I decrease the amount of available oxygen inside of the kiln. I do this by decreasing the air going into the kiln, which happens by pushing in the damper in the chimney, and by increasing the gas pressure. This creates an oxygen starved atmosphere inside the kiln, and the gas, which wants to burn really bad, draws oxygen out of the clay body, and out of the glazes/metal oxides. This helps to develop the colors you see on the final product.
This is an example of how the kiln is not just heated but "fired." By watching the length and color of the flames that come through the cracks around the door, I am able to gauge the amount of oxygen reduction inside of the kiln. The final temperature of about 2,360 degrees both melts the glazes, and vitrifies the clay, making it waterproof and functional.
This firing process takes about 12 hours to heat, and 2 days to cool.
I use natural gas as fuel in this kiln because I feel reduction firing gives a visual depth to the colors of the glaze, and also creates more of a relationship between impurities (mostly iron) in the clay, and glaze. With natural gas I have more control of the atmosphere in the kiln, as opposed to the wood kiln where variables such as wood type and kiln style play large roles in the firing process, which is why this process is probably the preferred firing method for most high-fired pottery, though some potters will use electricity to fire in an oxidizing atmosphere.