All kilns for ceramic work heat ware to high temperatures. Fuel-fired kilns can do something more—they can fire the work in reduction. Here are the fundamentals of reduction firing.

Defining the Terms 

Neutral Firing: When the ratio of oxygen to fuel fed to the kiln is perfectly balanced, with no shortage or excess of oxygen needed to burn the fuel completely.

Oxidation Firing: When the ratio of oxygen to fuel fed to the kiln is greater than required to burn the fuel completely.

Reduction Firing: When the ratio of oxygen to fuel fed to the kiln is less than required to burn the fuel completely.

Kiln Atmosphere Science 

The most convenient fuels for reduction firing are natural gas or propane. However, wood, sawdust, coal, and oil are also effective fuels. Reduction is achieved by burning any of these organic materials with an insufficient quantity of air to achieve complete combustion. By starving the kiln of air, the burning fuel needs another source of oxygen and finds it in the ceramic oxides, which are part of the work loaded into the kiln. This process changes the work.

To visualize reduction, view the chemical reaction that occurs when natural gas burns. Natural gas is almost entirely methane, with the chemical formula CH4. It has one carbon (C) atom with four hydrogen (H) atoms attached to it. For any fire to burn, there also needs to be a source of oxygen (O). In kilns, the oxygen comes from air. Oxygen molecules in air consist of two oxygen atoms linked together. They have the chemical formula O2. The chemical reaction of natural gas burning with oxygen from air in a neutral firing is CH4 + 2O2 = CO2 + 2H2O. This is chemical shorthand for saying one methane molecule plus two oxygen molecules burn to produce one carbon dioxide molecule and two molecules of water. The water will, of course, be water vapor because the burner flame is hotter than 212°F (100°C).

The ratio of oxygen to methane entering the kiln’s burners is less than 2:1 in a reduction firing. A slightly smaller ratio than 2:1 oxygen to fuel produces light reduction. In this type of kiln atmosphere, some carbon atoms will not have two oxygen atoms to combine with but rather only one, and will become carbon monoxide, CO. A much smaller oxygen-to-fuel ratio, one approaching 1:1, produces very heavy reduction, with the methane being converted to carbon and hydrogen (H2). Oxidation occurs at an O2:CH4 ratio of 2:1 or greater. Whether CO is present, and how much is present, after combustion depends on the oxygen:methane ratio, in other words, on how much O2 is available. The further the kiln is placed into reduction, the more CO and less CO2 will form, until there is only CO. Then, continuing into heavier reduction, the amount of CO diminishes and more carbon (soot) forms.

If heavy reduction occurs before the ceramic body vitrifies or the glaze melts, some of the soot may be trapped in the clay body. That causes a weakness called black coring. Soot on the surface of a form that is then covered by melting glaze becomes darker in color. This is known as carbon trapping and is often used intentionally, particularly when firing American shino glazes.

An example of carbon trapped beneath an American Shino glaze on porcelain. The piece is by the late Malcolm Davis. The particular Shino glaze recipe shown is one of literally dozens used or published by Davis. He was constantly adjusting the proportions and amounts of flux and clay ingredients in the recipe, looking for the elusive “perfect” Shino.

Carbon Monoxide

Carbon monoxide (CO), which is produced during reduction firings, is toxic to humans. It’s also colorless, odorless, and tasteless. That makes it impossible to detect in the air without a CO detector. CO is slightly lighter than air, so it will accumulate nearer the ceiling than the floor. Because of the danger of CO poisoning, all ceramic kilns should be fired either outside or in a kiln room equipped with ventilation fans.

Testing for Reduction

An oxidizing kiln atmosphere is necessary early in a firing to burn off organic materials and sulfur in a clay body and glaze, and because it saves on fuel. Fire in oxidation to about 1652°F (900°C). The degree of oxidation has no effect on the color of fired clay or glaze. The degree of reduction, however, does. Excessive oxidation early in the firing wastes fuel because it’s heating extra air that isn’t contributing anything to the finished product. Judge the best air-to-fuel mixture for oxidation by first setting the amount of fuel for the desired heating rate. Then, adjust the kiln damper to maximize the heating rate for the amount of fuel being burned.

Reduction is difficult to judge and not everyone can afford an oxygen analyzer in the kiln. It can be observed visually though. Open peep holes carefully, while wearing PPE (gloves and goggles, at a minimum). Check whether the kiln is under positive pressure. If it is, hot kiln vapor will be coming out of the peep hole. Carefully passing a sheet of paper by the open peep hole will reveal positive pressure—the paper will ignite. Carefully insert a long, slender sliver of dry wood into the kiln through the peep hole. If the kiln atmosphere is oxidizing, the wood will burst into flame. If the kiln is in reduction the wood will char and smoke, but won’t begin to burn until it’s pulled back out of the kiln. In addition to observing flame at the peep holes, looking at the burners can help gauge reduction as well. A clean, blue flame at the burners indicates oxidation, while a sooty, yellow flame reveals heavy reduction.

Seeking a Sooty Surface

Carbon trapping is just what it says. Pure carbon (soot) is produced from fuel in a kiln and deposits on glazed ware. When the glaze melts, it traps the carbon.

To make soot, a hot, fuel-fired kiln is starved of air so it makes a smoky, sooty flame. The kiln will typically be around 1600°F (870°C) to make soot. However, the temperature must not be so much hotter than this that the glaze has already melted. That would entirely prevent trapping carbon. How hot is too hot? It depends on the glaze.

Once heavy reduction is achieved and soot is produced, it is critical to the process that the kiln remain in at least some level of reduction until it reaches the temperature at which the glazes on the ware melt. If the kiln is allowed to go into oxidation at any time between making soot and when the glaze melts, the soot will almost instantly burn off and no carbon will be trapped.

However, as soon as the glazes melt enough to seal over, the carbon is permanently trapped under the glaze. Once the glaze melts, the kiln can be safely placed back into oxidation to finish the firing and any oxygen in the kiln can no longer reach the carbon to burn it away. Knowing when the glaze has melted requires experience and testing. 

An accurate indication of kiln temperature is very helpful to successful carbon-trap firing. Glazes for carbon trapping typically have high amounts of low-melting-point fluxes—most often sodium, but sometimes lithium, or a combination of the two. These two fluxes lower the seal point of these glazes. A soluble sodium source, usually soda ash, is also commonly used so that as the water evaporates from the glaze, it leaves a concentration of the dissolved sodium at the glaze surface, even further lowering the melting point there.

Because sodium and lithium melt at relatively low temperatures, as low as 1800°F (980°C), and most carbon trapping is done with porcelain or sometimes stoneware that is ultimately fired to cone 5–10, any glaze used must be stiff (viscous) enough that it will not run. For this reason most carbon-trap glazes have high amounts of alumina, sourced from a significant portion of clay in the recipe.

the author
Dave Finkelnburg is a studio potter, practicing engineer, and a regular contributor to Ceramics Monthly. He earned his master’s degree in ceramic engineering from Alfred University.