A reducing environment is one chacterized by little or no free oxygen (dissolved or as a gas). In chemistry, reduction is the reverse of oxidation. That is, the oxidation state of an atom (independent or within a molecule) is reduced by the addition of electrons. A reduced gas is thus hydrogen-rich.

Hydric soils

Reducing conditions can develop in soils that are saturated with water long enough for microbial activity to use up all available dissolved oxygen. The result is an alteration of the chemistry of the soil, to a type known as hydric; (see wetlands).

Reduction firing in ceramics

A reducing atmosphere (or reduction atmosphere) is a condition created in a kiln in order to produce specific effects on the 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.

References

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Soil chemistry Pottery