Reduction firing is a ceramic firing technique in which the oxygen supply inside the kiln is deliberately restricted, creating a fuel-rich atmosphere that changes the chemistry of clays and glazes in ways an oxygen-rich environment cannot. Instead of combustion gases having all the oxygen they need, they pull oxygen atoms from the materials being fired, particularly from metal oxides like iron and copper compounds embedded in the clay body and glaze. The result is a distinct palette of colors, surface qualities, and mineral structures that potters have pursued for thousands of years, from ancient Chinese celadon to classical Greek black-figure ware.
What Happens Inside a Reduction Kiln
In a normal, oxygen-rich firing (called oxidation firing), the kiln has more than enough air for the fuel to burn completely. Metal oxides in the clay and glaze stay fully oxidized. Iron oxide remains in its ferric (Fe³⁺) state, producing earthy reds, browns, and tans. Copper oxide stays as cupric oxide, yielding greens and turquoises. Everything proceeds predictably.
Reduction firing flips that equation. The potter partially closes the kiln’s air intake or damper so the fuel cannot get enough oxygen from the incoming air. The hungry combustion gases then scavenge oxygen from wherever they can find it, including from metal oxides in the ceramic ware itself. Iron atoms lose an oxygen partner and shift from the Fe³⁺ state to the Fe²⁺ state. Copper compounds can be stripped all the way down to metallic copper. These chemical shifts change how light interacts with the glaze and body, producing colors that are simply impossible under oxidation conditions.
The process is not instantaneous or uniform. Research on clay rich in organic matter has shown that even in a kiln with an oxidizing atmosphere, the interior of a thick ceramic wall can develop reducing conditions because oxygen diffuses sluggishly through the pottery. This creates what ceramicists call a “sandwich structure,” where the outer margins of the piece are oxidized (often reddish from hematite) while the core remains dark and reduced, containing different mineral assemblages and a different iron oxidation state than the exterior.1ScienceDirect (Applied Clay Science). Influence of firing conditions on ceramic products: Experimental study on clay rich in organic matter Potters working with reduction firing exploit this oxygen-starved chemistry on purpose, turning the entire kiln atmosphere into a reducing environment so the effect reaches the surfaces and glazes, not just the buried core.
How Iron Changes Color Under Reduction
Iron is the most common colorant in ceramic materials, and its behavior under reduction is the single biggest reason potters use the technique. In an oxidized glaze, iron tends to produce warm yellows, ambers, and rust-brown tones because ferric iron (Fe³⁺) absorbs light in a way that emphasizes those wavelengths. Shift that iron to its ferrous state (Fe²⁺) through reduction, and the color palette moves toward cool greens, blue-greens, and grays.
This is exactly how celadon glazes get their famous jade-like color. Analysis of celadon glazes from the Southern Song Guan Kiln found that the iron in the glaze layer was predominantly Fe²⁺, with Fe²⁺/Fe³⁺ ratios ranging from about 1.6 to 2.2 across different samples, confirming that these pieces were fired in a reducing atmosphere. The samples with higher ratios of Fe²⁺ to Fe³⁺ displayed deeper blue and green tones, while those with relatively more Fe³⁺ trended toward yellow.2npj heritage science. The mechanisms of color, opacifying, and cracks in celadon glazes from the Southern Song Guan Kiln Researchers studying celadon glaze chemistry more broadly have noted that the equilibrium between Fe²⁺ and Fe³⁺ in the glaze melt is what governs the final color, with higher Fe²⁺ proportions producing deeper blue tones.3Elsevier / ScienceDirect. Analysis of the influence of iron source and its occurrence state on the color of celadon glaze
The practical takeaway for potters: the same iron-bearing glaze recipe can yield a warm amber in an oxidation kiln and a cool green in a reduction kiln. The chemistry of the colorant is identical; the atmosphere determines which version of iron ends up dominating the finished surface. Even small variations in how strongly reducing the atmosphere is, or where the piece sits inside the kiln, can shift the color noticeably. Dragon kilns used during the Southern Song Dynasty, for instance, had different temperatures and atmospheric conditions at different positions inside the kiln chamber, producing measurable differences in the Fe²⁺/Fe³⁺ ratio and therefore different colors on pieces fired in the same load.4npj heritage science. The mechanisms of color, opacifying, and cracks in celadon glazes from the Southern Song Guan Kiln
Copper Reds and Oxblood Glazes
If iron is the workhorse of reduction firing, copper is the showpiece. Under oxidation, copper oxide produces greens. Under reduction, something more dramatic happens: the copper compounds in the glaze are reduced all the way to metallic copper in colloidal form, meaning particles so small they are suspended within the glass matrix of the glaze. These tiny metallic copper particles scatter and absorb light in a way that produces vivid reds, ranging from a soft blush to the deep, saturated hue known as “oxblood” or sang de boeuf.5Journal of the American Ceramic Society. Constitution of Copper‐Red Glazes
Copper reds have a well-earned reputation as some of the trickiest glazes in ceramics. The window of reduction needed is narrow. Too little reduction and the copper stays oxidized, giving you green. Too much reduction, or reduction at the wrong temperature, and the copper can volatilize out of the glaze entirely or form larger metallic aggregates that turn the surface muddy brown or gray instead of red. The colloidal particles that produce the red need to be extremely fine and evenly distributed, which demands precise control over the kiln atmosphere, temperature, and cooling cycle. This difficulty is why historical copper-red pieces, particularly those from Chinese kilns of the Song, Yuan, and Ming dynasties, are among the most prized ceramics in existence. Each successful piece represented a kiln load where everything went right in a process with very little margin for error.
The Three-Stage Firing of Ancient Greek Pottery
One of the most sophisticated historical uses of reduction firing had nothing to do with glazes at all. The distinctive black-and-red decoration on ancient Athenian pottery was achieved through a three-stage firing cycle that alternated between oxidizing and reducing atmospheres. The sequence, known as oxidation-reduction-oxidation (ORO), exploited the different behavior of a fine clay slip and the coarser clay body under atmospheric changes.6PubMed. Confocal XANES and the Attic black glaze: the three-stage firing process through modern reproduction
The process worked like this. First, the kiln was fired in an oxidizing atmosphere, turning everything red as iron in both the slip and the body oxidized. Then the potter sealed the kiln vents to create a reducing atmosphere, and both the slip and the body turned black as iron reduced. In the final stage, the vents were reopened to re-oxidize the kiln. Because the fine-grained slip had vitrified (fused into a glassy state) during the reduction phase, oxygen could no longer penetrate it, so it stayed black. The coarser clay body, being more porous, allowed oxygen back in and returned to red. The result was the high-contrast decoration that has survived on thousands of museum pieces.7Journal of Raman Spectroscopy. Investigating the firing protocol of Athenian pottery production: A Raman study of replicate and ancient sherds
This technique is a reminder that reduction firing is not just about color chemistry in glazes. It can alter the physical structure of clay materials, changing how porous or vitrified they become, and those structural differences can be locked in or reversed depending on what atmospheric conditions follow. Greek potters understood this empirically long before anyone could explain the iron chemistry behind it.
Carbon Trapping and Surface Effects
Reduction firing also introduces carbon into the equation. In a fuel-rich atmosphere, incomplete combustion produces carbon monoxide and free carbon particles. Some of this carbon gets trapped in the clay body or glaze, contributing to the dark tones in reduced stoneware and porcelain. Raman spectroscopy of Southern Song Guan celadon ware, for example, detected carbon peaks in the glaze, further confirming the reducing atmosphere used during firing.8npj heritage science. The mechanisms of color, opacifying, and cracks in celadon glazes from the Southern Song Guan Kiln
Carbon trapping is particularly prominent in certain Japanese-influenced pottery traditions. Shino glazes, for instance, are traditionally fired in reduction atmospheres where carbon can become embedded in the thick, often crawling glaze surface, producing warm orange “flashing” where the glaze is thin and white or gray where it is thick. Research on crawling Shino glazes fired at 1250°C in a reduction atmosphere has explored how additives like alumina, calcium carbonate, and magnesium carbonate affect the physical properties and appearance of these glazes, with magnesium carbonate at around 15% by weight producing the best visual results.9Scientific.Net. Effect of Al2O3, CaCO3 and MgCO3 on Physical Properties of Crawling Shino Glaze in Reduction Atmosphere
The sandwich structure mentioned earlier, with a dark reduced core and lighter oxidized margins, is partly a carbon-trapping effect. Organic matter in the clay burns off easily at the surface where oxygen is available, but in the interior it carbonizes rather than combusting, contributing to the black core. Potters who fire very thick-walled pieces or use clay with high organic content sometimes see this sandwich effect even in supposedly oxidation firings.10ScienceDirect (Applied Clay Science). Influence of firing conditions on ceramic products: Experimental study on clay rich in organic matter
Reduction Firing in Practice
Unlike oxidation firing, which can be done in an electric kiln simply by letting the heating elements do their work in the ambient atmosphere, reduction firing requires a fuel source that consumes oxygen. Most reduction kilns burn natural gas, propane, or wood. The combustion itself creates the oxygen-hungry atmosphere when the potter restricts the air supply. Electric kilns do not combust anything, so achieving reduction in them requires workarounds like introducing combustible material directly into the kiln chamber, which is uncommon and hard to control.
Timing matters as much as atmosphere. Potters typically do not reduce throughout the entire firing. A common approach in stoneware and porcelain firing is to begin reduction once the kiln reaches a certain temperature (often in the range of 1000°C to 1100°C) and maintain it through the peak temperature and sometimes into the cooling cycle. Starting reduction too early can cause problems with clay body maturation. Starting it too late may not allow enough time for metal oxides in the glaze to convert. The degree of reduction also varies: “light reduction,” where the atmosphere is mildly fuel-rich, produces subtler effects than “heavy reduction,” where thick smoke rolls from the kiln’s chimney and the atmosphere is aggressively starved of oxygen.
Wood-fired kilns add another layer of complexity. The ash from the wood fuel itself lands on the ceramic surfaces and melts into a natural glaze at high temperatures. In a reducing atmosphere, this ash glaze takes on characteristics impossible to replicate in a gas kiln, since the composition and volume of the ash varies with the type of wood, the draft of the kiln, and where the piece sits relative to the firebox. Pit firing, one of the oldest firing methods, also tends to produce uniformly reduced ceramics because the ware is buried in the fuel and has virtually no access to fresh air.11ScienceDirect (Applied Clay Science). Influence of firing conditions on ceramic products: Experimental study on clay rich in organic matter
Why Some Potters Prefer Oxidation
Given the rich effects reduction can produce, you might wonder why anyone would choose oxidation firing. The answer comes down to predictability and practicality. Oxidation firing in an electric kiln is clean, consistent, programmable, and requires no combustion expertise. You load the kiln, set a controller, and get repeatable results. Reduction firing demands an experienced hand at the damper, a supply of combustion fuel, a kiln built to handle flame and gas flow, and a willingness to accept variability from firing to firing.
Modern ceramic artists working in oxidation have also developed glaze chemistries that approximate some reduction effects without the atmosphere manipulation. Certain colorants and opacifiers can produce blues, greens, and reds in an electric kiln. These glazes are useful, but experienced potters will tell you the results are not the same. The subtle variations, the “depth” in a reduction-fired celadon, the way carbon trapping creates unique marks on each piece: these emerge from the interaction between atmosphere and material that oxidation glazes simulate but do not replicate. It is a real aesthetic difference, not just snobbery, though reasonable people can prefer either approach.
Safety and Ventilation Concerns
Reduction firing introduces health and environmental considerations that oxidation firing largely avoids. The incomplete combustion that defines reduction produces carbon monoxide, which is toxic even in small concentrations. Kiln rooms where reduction firing takes place need serious ventilation. A study of kiln emissions and potters’ exposures found that concentrations of contaminants in small, ventilated kiln rooms still ranked among the highest measured, raising concern that unventilated spaces could easily reach unacceptable levels. Custom-designed exhaust hoods and industrial-grade ventilation systems were the most effective strategies, while passive diffusion and simple wall or window fans were the least effective.12PubMed. Kiln emissions and potters’ exposures
Beyond carbon monoxide, reduction atmospheres can volatilize certain glaze materials more readily than oxidation atmospheres. Metals like copper and some flux compounds can become airborne at high temperatures in a reducing environment. For potters firing in shared studio spaces or attached to living quarters, the ventilation question is not optional. An outdoor kiln or a kiln room with purpose-built exhaust is the minimum responsible setup for regular reduction work.
Raku and Other Specialized Reduction Techniques
Raku firing is probably the most widely practiced form of post-firing reduction among studio potters. In Western-style raku, the piece is pulled from the kiln while still glowing hot and placed in a container of combustible material like newspaper or sawdust. The material ignites, and the container is sealed, creating a small reduction chamber around the piece. The carbon-rich atmosphere blackens any exposed (unglazed) clay, while glazed areas often develop a crackled, metallic, or iridescent surface depending on their composition. Copper glazes treated this way can shift from green to copper-metallic to deep red within seconds.
Saggar firing is another technique that creates localized reduction. Pieces are placed inside a lidded container (the saggar) along with combustible materials like sawdust, salt, or organic matter. During firing, the burning material inside the saggar consumes oxygen and creates a miniature reducing atmosphere. This approach lets potters achieve reduction effects inside an otherwise oxidation kiln, though the results are less controlled and more about surface decoration than deep glaze chemistry.
Obvara firing, a technique with roots in Eastern European pottery traditions, involves dipping a hot piece into a fermented liquid (often a flour-water-yeast mixture), where the organic material instantly carbonizes on the surface. While not a kiln-atmosphere reduction technique in the strict sense, it relies on the same principle of carbon deposition under oxygen-poor conditions at the ceramic surface. These specialized approaches share a common thread: they all manipulate the availability of oxygen around the ceramic ware to achieve effects that a simple, oxygen-rich environment cannot produce.