Oxidation and Reduction Firing Schedules

Linda Arbuckle Course Notes


John Baymore Thoughts

From a thread in Ceramic Arts Daily:
There are many variables involved in developing specific cycles for specific effects with specific bodies and glazes..... but in general, the concept of "body reduction" as somehow being totally separate from "glaze reduction" is not quite accurate to what is actually happening.

It is all a "blur" that runs together seamlessly.

If you want to cause reduction effects on compounds in the body, you need to start reduction before the surface of the clay body or the surface of the overlying glaze layer becomes gas impermeable to the two prime reducing agents in firings; carbon monoxide and hydrogen. Carbon monoxide is the main reducing agent in most of our firings because hydrogen at elevated temperatures is so reactive it usually "finds" available oxygen first in the kiln's gases.

Note that carbon particles are SO large that they are quite non-useful for "reduction" in a kiln. They don't easily get into the clay or glaze surface. SO all that smoke some people get on gas kilns..... wasted effort. (Unless you are doing carbon trap shinos! ;) )

So the molecular size of the reducing agent getting into the clay is the first question in developing the firing cycle. As the body tightens, it does not allow the gases to get to the compounds below the immediate surface. The main compound that we use to get "reduction" colorations in our work are iron compounds, reducing the valiancy to the black state from the oxidized red state.

FeO is a powerful flux on silica (SiO2) and which also happens to color the resultant glass at the same time. So as the reduced iron 'bleeds' into the glassy phase of the body it also colors it (grey). This is particularly effective on small hematite nodules....giving lots of reduced iron in a small physical location (iron spotting).

When the SURFACE of the tight body that is NOT under the glaze is then allowed to get in contact again with oxygen as the kiln is shut off and cooling, the surface turns to the reddish-browns we associate with "reduction" firing. The body, if it is vitrified, is gas impermeable to oxygen ... and the inside it remains a greyish coloration.

Importantly, the diffuse reduced iron compounds in the glassy phase of the body start to also work their way into the melting glaze layer over the already reduced body. If the glaze surface starts to melt and become gas impermeable to the reducing agents before reduction had occurred, the body under the glaze does not get reduced. So the interaction of the body with the glaze is less. Plus the coloration of the body will be base on the red state (or the original state) of the iron compounds.

Once the glaze surface is totally gas impermeable to the reducing agents then no amount of reduction will reduce stuff own inside the glaze layers or the underlying body (except maybe because of the bubbling of the glaze allowing some interior matter to be reduced on the surface). And once reduced inside, when the kiln is shut off and cooling and exposed to oxygen, the glaze surface also, just like the clay surface is oxidized. Unless you fire down in reduction, or pump the kiln full of an inert gas (industry does this kind of stuff) the whole outside of "reduced" pieces is re-oxidized.

Different glazes and different clay bodies have different firing characteristics. There is no "one size fits all" firing cycle. You have to find what gets the best out of your clay and glazes. And different glazes and bodies often are not getting optimum effects when fired together in the same cycle. What is great for one may be just OK for others. We are back to that stock phrase of mine.... "test, test, test".

At one point I did some research on one particular celadon glaze on one particular body. Over a LOT of firings it was determined that reduction at a certain intensity had to occur before cone 04 and after the kiln reached cone 4 it made absolutely no difference how the kiln was fired as to oxidation or reduction levels. Reduction too low below 04 (cone 012 and down), and the body exhibited a tendency to carbon core and bloat and bleb. Optimum turned out to be slightly oxidizing or neutral fire up to about cone 06 to 04 to start a light reduction, and that level was maintained at the same level until cone 4. Then fired in slight oxidation to neutral to the cone 10 end point, then cooled in oxidation.

Some thoughts for ya' there.

Born To Fire, I’m Inspired I want one!


Reduction - smooth repeatable results in about eight hours!


Raku, lasers and reduction - fun!


Other References

Reduction fired low-temperature ceramics; D.Dawson, O.Kent; Post-Medieval Archaeology 33 ( 1999), 164-178