A reducing atmosphere is an atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidizing gases or vapours, and which may contain actively reducing gases such as hydrogen, carbon monoxide and gases that would oxidize in the presence of oxygen, such as hydrogen sulfide.
In metal processing, a reducing atmosphere is used in annealing ovens for relaxation of metal stresses without corroding the metal. An inert gas, usually nitrogen, is used, or for more extreme conditions, hydrogen gas.
A reducing atmosphere is also used in order to produce specific effects on ceramic wares being fired. A reduction atmosphere is produced in a fuel fired kiln by reducing the draft and depriving the kiln of oxygen. This reduced level of oxygen causes incomplete combustion of the fuel and raises the level of carbon inside the kiln. At high temperatures the carbon will bond with and remove the oxygen in the metal oxides used as colorants in the glazes. This loss of oxygen results in a change in the color of the glazes because it allows the metals in the glaze to be seen in an unoxidized form. A reduction atmosphere can also affect the color of the clay body. If iron is present in the clay body, as it is in most stoneware, then it will be affected by the reduction atmosphere as well.
In most commercial incinerators, exactly the same conditions are created to encourage the release of carbon bearing fumes. These fumes are then oxidized in reburn tunnels where oxygen is injected progressively. The exothermic oxidation reaction maintains the temperature of the reburn tunnels. This system allows lower temperatures to be employed in the incinerator section, where the solids are volumetrically reduced.
The same principle applies to planets. Many scientists think the early Earth had a reducing atmosphere, along with Mars and Venus. This would have proven to be a good environment for to evolve the first photosynthetic metabolic pathways which gradually increased the oxygen portion of the atmosphere, changing it to what is known as an oxidizing atmosphere. With increased levels of oxygen, the evolution of the more efficient aerobic respiration might have been enabled, allowing animal life to evolve and thrive.
Though most scientists conceive of the early atmosphere as reducing, a 2011 article in Nature found that cerium oxidation in zircon—which comprises the oldest rocks on Earth at roughly 4.4 billion years of age—was comparable to that of present-day lava. This observation implies that Hadean atmospheric oxygen levels were similar to those of today.
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