A term for pottery firing conditions in which the supply of air is limited or the fuel damp. The fuel does not totally burn under these circumstances and the gases contain carbon monoxide rather than oxygen. This generally results in black-surfaced pottery as opposed to the red produced in an oxidizing atmosphere, though shades of color may vary if the conditions during firing are not stable. Native Americans and Mesoamericans understood the effect of a reducing atmosphere, so that gray and black pots are found as well as the red and brown ones fired in an oxidizing flame.