Only possible in fuel-fired kilns (kilns heated by use of a combustible fuel), reduction does very interesting things to the glazes.
In addition to celadons, Oriental potters developed red glaze, which, like celadons, depends on a reduction atmosphere in the kiln to turn the right color. In simple terms, reduction is a phase during the firing cycle in which the oxygen level in the kiln is reduced, causing oxygen-starved flames to pull oxygen from the pots. Reduction begins at roughly 1500ºF and goes to about 1900ºF. Only possible in fuel-fired kilns (kilns heated by use of a combustible fuel), reduction does very interesting things to the glazes. Red glazes turn red only under properly balanced reduction conditions. Too little or too much reduction turns the red glaze white, or in some cases, pale green. Celadons are nearly as touchy as reds, in terms of their dependence on the kiln atmosphere, turning pale brown or tan in the wrong atmosphere. For the Chinese to consistently produce their wares they had to be masters of their craft. They developed pottery from its functional but rugged beginnings to a high level of craftmanship, consistently creating beautiful vessels of the finest quality.