An atmosphere in the kiln during firing, in which the amount of oxygen entering the kiln is severely restricted. If there is insufficient oxygen in the kiln, one of the products of the combustion of fuels is carbon monoxide. At high temperatures carbon monoxide will combine with oxygen from oxides in ceramic materials. When these oxides have some of their oxygen taken by the carbon monoxide (which becomes carbon dioxide in the process), then they are said to have been reduced. This process is essential in order to produce some glaze colours, most notably the greens and blues of celadon glazes.