Oxidation

clean flame or radiant heat from an electric kiln
no excess unburned carbon, and therefore efficient use of fuel
glazes are generally brighter, especially in the earthenware range.

Reduction

oxygen is reduced: "dirty" smoke and soot results
unburned carbon changes the color mainly of copper and iron for beautiful effects in certain glazes.

A REDUCTION ATMOSPHERE OCCURS WHEN excess heated carbon (monoxide) steals oxygen atoms from any iron or copper present. This changes the molecular structure of both. Carbon monoxide becomes carbon dioxide, and the number of atoms present in the copper and iron effects the network of interlinked atoms that reflects light, thus changing their color.

Reduction Glaze Types

In the Earthenware Range

Terra Sigillata

Terra sigillata is a fine grain earthenware slip glaze (very soft).
An oxidizing atmosphere causes a soft iron terra cotta red, while a reduction atmosphere creates black, when iron-bearing clay is used.
To create the red and black designs, the Greeks first reduced and then reoxidized. The terra sigillata coating remains black because it is sealed by the heat, but the area where the terra sig has be scraped away is more porous and will return to the soft red color, resulting in a black on red design.

In Raku Firing with Post-firing Reduction

Metallic looking surfaces are created in the cooling stage in copper-bearing glazes that have been fired to cone 06-04 (up to 1900° F).
The copper begins to react with oxygen to form a thin layer of copper oxide on the surface of the glaze.
As the temperature falls from 700-400° F, the color range shifts from magenta - violet - purple - blue - yellow green - gray green.

In the Stoneware - Porcelain Range

Copper red or sang-de-boeuf - blood red instead of oxidation green, with copper.
Celadon green, soft transluscent jade green instead of yellow-brown with iron.
Chun blue - thick, opalescent pale blue from small percents of iron and copper.
Tenmoku, or iron saturated glaze, can turn a lush rusty red to red-black instead of flat dark brown.
Iron spots can be drawn from the clay body into the glaze causing a decorative effect. The warm toasty color of reduced iron-bearing clay results from the iron reoxidizing as the kiln cools; the clay may remain gray under a clear glaze if the reduction is sealed in by the glaze.
Salt Glaze is formed by introducing salt into a kiln at the time stoneware glazes would be melting. As the salt vaporizes, sodium is released and combines with silica in the clay body. Toxic fumes are also released.
Saggar Firing is ware placed in a refractory container. This either protects the ware from uneven kiln fire or reduction, or is stuffed with combustible material to introduce local reduction.