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Reduction Firing

A kiln atmosphere which is deficient in free oxygen. In traditional ceramics, reduction firing requires a specially designed fuel fired kiln that restricts the flow of incoming air so there is enough to burn the fuel and no more (in some cases it is restricted so that is actually less than enough to introduce carbon into the atmosphere). This condition is accomplished in gas kilns by increasing back-pressure or reducing the amount of primary air available to each burner. The result is an increase in gases like carbon, hydrogen and CO. These are very aggressive in wanting to combine with oxygen, they steal it from within bodies and glazes. Hydrogen is small and particularly oxygen-hungry.

Reduction firing produces different colors and visual effects because metallic oxides willing to give up oxygen convert to their reduced or more metallic form. The associated visual effects include some impossible or difficult to achieve in oxidation. Good examples are copper which burns red (it fires green in oxidation), iron which becomes a powerful flux and produces a wide range of intense and earthtone browns (it is refractory in oxidation), celadon greens and blue and dolomite mattes. A particularly interesting effect is iron speckling in clay bodies. In fact, because almost all natural clays contain iron, reduction firing normally gives completely different clay surface effects than oxidation.

Many people fire their gas kilns up in oxidation but at two places in the ramp (e.g. cone 06, 10) they reduce the kiln for a period (for body and glaze reductions). Others begin reduction firing around cone 06 and continue it to the end. Many people do a period of oxidation at the end of a reduction firing to clean the atmosphere and soak the glaze to heal bubbles that result from the active volatilization (an accompanying bubble formation and surface disruption) that reduction induces. In many cases color breaks in glazes are a result of localized reoxidation of the melt surface (the effect depends on glaze thickness and evenness of coverage). Tenmoku glazes are an example of this, the brown thinner areas are oxidized.

An oxygen probe provides a direct measurement of the amount of reduction and enables one to more easily maintain the critical balance between oxidation and incomplete combustion. While these devices are quite expensive there are very few people employing this process that are not at least planning to get one (rather than just eyeballing the flame).

Reduction firings are not without hazard. Any form of incomplete combustion can generate smoke and deadly gases (CO for example, is colorless and odorless). It is important that gas kilns be vented well, and if possible, that a CO alarm be installed.

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.

FeO (iron oxide) is a very powerful flux

This cone 10R glaze, a tenmoku with about 12% iron oxide, demonstrates how iron turns to a flux in reduction firing and produces a glaze melt that is much more fluid. In oxidation, iron is refractory and does not melt well (this glaze would be completely stable on the ware in an oxidation firing at the same temperature, and much lighter in color).

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.

Copper red reduction glaze at cone 9 reduction

Courtesy of Angela Walford.

A gas kiln built by Luke Lindoe in the 1960s is still used at Plainsman Clays today

It fires very evenly from top to bottom and front to back. We have used it for quality control to fire thousands of porosity and shrinkage test bars to monitor the maturity of the clay bodies. Oh, we also fire pottery in this!

Example of a modern automatic firing reduction gas kiln for use by studio potters

Courtesy of Bailey Kilns.

Cone 10 Reduction firing is the home of the most magic oxide in ceramics: iron

It is a powerful glaze flux, variegator and crystalizer, a colorant of many characters in bodies and glazes and a specking agent like no other. And it is safe and cheap!

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).

Messing up the firing of a copper red glaze

Copper red glazes require tight control of the reduction firing. The mug on the left is grey and brown by the foot, the other has developed no color at all on some parts. These were fired to cone 10R with reduction starting at cone 010 and going all the way up. There was no clearing or soaking period at the end of the firing. This is the Red Celadon recipe.

The multitude of things iron oxide can do in reduction

Iron oxide is an amazing glaze addition in reduction. It produces celadons at low percentages, then progresses to a clear amber glass by 5%, then to an opaque brown at 7%, a tenmoku by 9% and finally metallic crystalline with increasingly large crystals past 13%. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.

Iron oxide goes crazy in reduction

Cone 6 iron bodies that fire non-vitreous and burn tan or brown in oxidation can easily go dark or vitreous chocolate brown (or even melting and bloated in reduction). On the right is Plainsman M350, a body that fires light tan in oxidation, notice how it burns deep brown in reduction at the same temperature. This occurs because the iron converts to a flux and the glass development that occurs brings out the dark color. On the left is Plainsman M2, a raw high iron clay that is quite vitreous in oxidation, but in reduction it is bloating badly. When reduction bodies are this vitreous there is a much great danger of black coring.

A completely automatic reduction gas kiln. This is heaven!

Blaauw kiln at the Potter's in Residence program at Medalta Historic site in Medicine Hat, Alberta. This type of kiln has an automatic controller that controls the firing schedule and atmosphere inside the kiln. These kilns are expensive, but bring the same degree of precision and convenience to reduction firing that electric kiln users have been enjoying for some time.

Reduction and oxidation color difference in a cone 10 red fireclay

Plainsman FireRed fireclay fired to cone 10R. This shows the effect of reduction where the body is exposed to the kiln atmosphere (very dark burning) and where it is not (inner foot ring).

Reduction speckle: a product of iron particles in the body

In reduction firing, where insufficient oxygen is present to oxidize the iron, natural iron pyrite particles in the clay convert to their metallic form and melt. The nature of the decorative speckled effect depends on the size of the particles, the distribution of sizes, their abundance, the color of the clay and the degree to which they melt. The characteristics of the glaze on the ware (e.g. degree of matteness, color, thickness of application, the way it interacts with the iron) also have a big effect on the appearance.

The strange vitrification profile of a talc body

This body is made from approximately 50:35:15 ball clay:talc:silica:silica sand. These test bars are fired from cone 2 to 9 oxidation (bottom to top) and 10 Reduction and from them the porosity and fired shrinkage can be measured (shown for each bar). Notice that the fired shrinkage is pretty stable from cone 2 to 8, but accelerates at cone 9 oxidation. But in reduction this stage has not been reached yet. The same thing happens with porosity, the cone 9 bar is dramatically more dense than the cone 8 one. But in reduction, it is still porous.

A cone 10 Reduction bowl has cracked after continuous use

This was made by a potter who carefully designed his own body and glaze recipe and obtained a high quality kiln in which to fire a line this line of ware. These pieces are being used in a restaurant and this one has failed. Why? It could be pushing up against the limits of this type of clay body (even at its best). Compared to what typical restaurant ware is made from it has more porosity (demonstrated by the color change around the crack). It likely has entrapped carbon inside. It has lower strength (a big issue restaurant ware which is handled so much). Larger particles of high expansion minerals create vulnerability to sudden temperature changes. If this is all true then all of the bowls will eventually crack. That being said, what if they do not all crack? Maybe this crack appeared to relieve stresses within the ware from uneven drying or uneven firing (due to technique or uneven thickness). Some bodies expand upon absorbing water, if that is the case with this then the bare clay surfaces (which permit water entry) could be an issue also.

Reduction Polar Ice vs. Oxidation Polar Ice

Polar Ice (Plainsman Clays) has been fired to cone 10R (left). This is beyond the recommended cone 6 range, but it worked well in this instance. The result is even more translucency and a translucency of a different character: blue! This looks much more like real blue polar ice.

Laguna B-Mix on Steroids! I have wedged in 10% and 20% Plainsman P.E.S.

Both pieces have a transparent glaze, G1947U. The bar in the front is PES (Performance Enhancing Substance)! PES is made from 50:50 Plainsman A1 and St. Rose Red, it behaves like a red fireclay. BMix has some specks anyway, so why not concentrate them into some awesome aesthetics? The addition does not affect the working properties of BMix. Well, actually it does. Pieces dry better. Fired strength and maturity are minimally affected (porosity increases from about 1% to about 1.3%). With 20% addition the surface of the unglazed clay is almost metallic. Silky matte glazes are stunning on a body like this.

An iron stone concretion found in a quarry in southern Saskatchewan

These are very hard, high in iron and can be as large as volkswagens. Tiny iron concretion particles cause specking in fired ware, especially in reduction.

Iron speckled cone 10 reduction stoneware with dolomite glaze

Plainsman H443. By Tony Hansen. The silky matte yet functional surface of this type of glaze, combined with the iron speckle from the body bleeding through it, has been a key reason why many have sought the cone 10R temperature range for pottery.

Out Bound Links

  • (Glossary) Oxidation

    A firing where the atmosphere inside the kiln has ...

  • (Glossary) Reduction Speckle

    An effect created by firing a clay containing high...

In Bound Links

By Tony Hansen

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