A firing where the atmosphere inside the kiln has sufficient supplies of oxygen to react with the glaze and clay body surfaces (and thus produce the colors characteristic of this). Electric kilns are synonymous with oxidation firing. However these kilns lack the air flow of their gas counterparts. Stagnant air inside can be saturated with carbon and other products of decomposition from bodies and glazes and these can use up oxygen in the chamber. This produces more neutral atmosphere than intended or realized (especially if glaze and body materials have high LOIs). Directly-connected kiln vents improve this situation but still do not approach the ventilation in gas kilns.
The same glaze in reduction (left) and oxidation at cone 10
It is not just iron oxide that changes character from oxidation to reduction. Of course, cobalt can fire to a bright blue in oxidation also, but this will only happen if its host glaze is glossy and transparent. In this case the shift to reduction has altered the character of the glaze enough so that its matte character subdues the blue.
Alberta slip fired in reduction (left) is much darker than in oxidation at cone 10.
This is 100% of the pure material. Notice how the iron is fluxing it more on the left, it is beginning to run. And how the reduction atmosphere amplifies the color of the iron (by changing it to the metallic state).
How reduction firing can affect glaze color
An example of how the same dolomite cobalt blue glaze fires much darker in oxidation than reduction. But the surface character is the same. A different base glaze having the same colorant might fire much more similar. The percentage of colorant can also be a factor in how similar they will appear. The identity of the colorant is important, some are less prone to differences in kiln atmosphere. Color interactions are also a factor. The rule? There is none, it depends on the chemistry of the host glaze, which color and how much there is.
Gleason ball clay fired test bars from cone 7-11 oxidation and cone 10 reduction
The oxidation bars (lower) are typical of most ball clays. They burn off-white and darken slightly in color as they approach maturity (above cone 10). Ball clays are fairly refractory, often as much more more than common fireclays. Ball clays almost always have some soluble salts and these reveal themselves in reduction firing (top bar).
Cone 5 reduction mug (left) and cone 6 oxidation mug (right)
These are made from Plainsman M340 and have transparent glazes. Notice the grey color of reduction vs. the yellowish of oxidation.
Difference between oxidation and reduction! GR10-C matte on Plainsman H443
Same body, same glaze. Left is cone 10 oxidation, right is cone 10 reduction. What a difference! This is a Ravenscrag Slip based glaze on a high-fire iron stoneware. In reduction, the iron oxide in the body and glaze darkens (especially the body) and melts much more. The behavior of the tin oxide opacifier is also much different (having very little opacifying effect in reduction).
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