Molly Ayala
Electric kilns are typically associated with oxidation firing, where oxygen is free to interact with glazes and clay bodies. However, it is possible to achieve a reduction firing in an electric kiln by using certain techniques, such as packing pots with carbon materials like sawdust or paper. This creates a temporary oxygen-deficient environment, allowing for unique surface effects and colour variations in the finished ceramics. Potters must decide whether to use oxidation or reduction firing based on the desired outcome of their ceramic pieces.
| Characteristics | Values |
|---|---|
| Firing type | Oxidation firing |
| Firing process | Oxygen is free to interact with glazes |
| Kiln type | Electric kiln |
| Temperature | Can be high or low |
| Colours | Bright, rich, vibrant |
| Glaze | Reliable, reproducible, bold chemistry |
| Corrosive effects | Wears out elements |
| Alternative | Reduction firing |
| Reduction firing kiln type | Gas or other fuel-burning kiln |
| Reduction firing process | Oxygen is prevented from interacting with glazes |
| Reduction firing temperature | High |
| Reduction firing colours | Mottled, earthy, speckled |
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What You'll Learn
Electric kilns are typically used for oxidation firing
Oxidation firing can be done at both high and low temperatures, and it produces very bright, rich colours. The glaze chemistry is built around oxidation, making the colours reliable and reproducible. Oxidation firing tends to mute earth tones while intensifying primary and secondary colours. For example, cobalt can fire to a bright blue in oxidation if its host glaze is glossy and transparent.
In contrast, reduction firing is typically done in a gas or other fuel-burning kiln. This is because the corrosive effects of reduction firing can wear out the elements in an electric kiln. Reduction firing involves partially restricting oxygen to create a reducing atmosphere, which affects clay and glaze chemistry. The oxygen molecules are not free to circulate and oxidize elements, resulting in unusual surface effects.
While it is challenging to create a reducing atmosphere in an electric kiln due to the constant airflow, it is not impossible. Potters can use techniques such as packing pots with carbon materials like sawdust or paper. As these carbon sources burn out, they temporarily lower the oxygen levels around the pots, creating a reducing atmosphere. However, this is a delicate balancing act that requires some trial and error to achieve good results.
Overall, electric kilns are typically used for oxidation firing due to the constant supply of air and the inability to fully control the oxygen levels. This results in vibrant and reproducible colours that show off bold glaze chemistry. While it is possible to create a reducing atmosphere in an electric kiln, it requires more effort and experimentation.
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Gas kilns are used for reduction firing
Electric kilns are typically used for oxidation firing. However, gas kilns are used for reduction firing.
Reduction firing is a method of firing stoneware that restricts or eliminates oxygen in the kiln. This is achieved by adjusting the fuel in a gas kiln or by adding organic material. As the oxygen is prevented from interacting with the glazes, the kiln achieves an oxygen-deficient environment. This process results in mottled, rich, and earthy colours, with the iron from the clay often showing through to create a speckled look.
Gas kilns are preferred for reduction firing because they offer better ventilation and airflow compared to electric kilns. The fuel in a gas kiln, such as natural gas or propane, can be manipulated to achieve the desired oxygen levels. The amount of gas entering the burner, the airflow that mixes with the gas, and the damper settings are crucial factors in achieving successful reduction firing.
Additionally, the size and weight of gas kilns contribute to more consistent firing schedules. The larger mass of the kiln relative to the product being fired allows for greater consistency in the firing process. This consistency is advantageous when aiming for specific glaze effects, as the firing schedule can be replicated as long as the glaze recipe, materials, and process remain unchanged.
It is important to note that carbon monoxide (CO) is produced during reduction firings, which is toxic to humans. Therefore, gas kilns used for reduction firing should be adequately vented, and a CO alarm should be installed for safety.
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Oxidation firing produces bright, rich colours
Oxidation firing produces bright and rich colours in ceramics. This is because, during oxidation firing, oxygen is free to interact with the glazes, creating bright and rich colours. The kiln's atmosphere has enough oxygen to react with the glaze and clay body surfaces, producing these characteristic colours. Oxidation firing can be done in an electric kiln or a gas kiln. However, electric kilns are synonymous with oxidation firing because, in a gas kiln, the vents must be adjusted to allow enough oxygen inside for full oxidation.
In oxidation firing, the low combustible-to-oxygen ratio means that the flame does not steal oxygen from the pot and glaze materials. This results in glaze coloration and the transformation of oxide structure. For example, cobalt can fire to a bright blue in oxidation if its host glaze is glossy and transparent. The percentage of colourants can also be a factor in the final appearance.
Oxidation firing tends to mute earth tones while intensifying primary and secondary colours. This process allows potters to achieve vibrant and reproducible results, showcasing bold glaze chemistry. The colour interactions depend on the chemistry of the host glaze, the colour, and the amount used.
While oxidation firing produces bright and rich colours, reduction firing creates more unusual surface effects by limiting the available oxygen. This technique is typically done in a gas or other fuel-burning kiln, as the corrosive effects can wear out the elements on an electric kiln. In reduction firing, oxygen is prevented from interacting with the glazes, resulting in mottled, rich, and earthy colours.
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Reduction firing produces earthy colours
Reduction firing is a specialised ceramic firing technique that occurs when there is insufficient oxygen in the kiln atmosphere for complete fuel combustion. This oxygen-deprived environment creates carbon monoxide at high temperatures, which then extracts loosely bonded oxygen from materials in clay, particularly iron and copper compounds. This chemical alteration process results in distinctive colour changes and effects that are often valued by potters and collectors. The unique interplay between the kiln atmosphere and the ceramic material produces these results.
Reduction firing typically produces earthy colours, and the iron from the clay often shows through, giving a speckled look. The degree of atmospheric consistency inside the kiln determines the range of colours produced. For example, the clay must have enough iron to stain it and act as a flux in the reduction kiln atmosphere. Additions of iron oxide will enhance the effect, however, thorough testing is needed to achieve the right balance.
The array of colours produced by reduction firing may be unexpected. For example, copper glaze should be red in reduction firing and green in oxidation firing. In oxidation firing, the kiln atmosphere has sufficient supplies of oxygen to react with the glaze and clay body surfaces, producing bright, rich colours. In reduction firing, oxygen is prevented from interacting with the glazes during glaze maturation, resulting in more mottled and earthy colours.
The colour changes are due to the chemical changes that affect the finished ceramics. The excess fuel molecules "steal" oxygen from the clay and glazes, causing these changes. Common methods to create a reduction atmosphere include partially closing the damper to restrict airflow, increasing the fuel supply, or introducing additional combustible materials like wood or oil into the kiln.
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Oxidation firing is simple and offers reproducible results
Electric kilns are typically used for oxidation firing. This is because oxidation requires an atmosphere with sufficient oxygen to react with the glaze and clay body surfaces during firing. In an electric kiln, oxidation firing is simple—no steps need to be taken to limit airflow, and oxygen is free to interact with the glazes.
In contrast, reduction firing requires specific steps to restrict airflow and limit oxygen availability. For example, in a gas-fired kiln, vents must be adjusted to control the amount of oxygen inside. In an electric kiln, it is challenging to achieve true reduction firing due to the constant oxygen supply. However, it is possible to mimic a reducing effect by packing pots with carbon materials like sawdust or paper, which temporarily burn out and lower oxygen levels.
Oxidation firing offers vibrant and reproducible results, showcasing bold glaze chemistry. The colours produced are typically bright and rich, with intensified primaries and secondaries, and muted earth tones. The oxidation process also allows for the use of high or low temperatures, providing flexibility in the firing process. Glaze chemistry is built around oxidation, making the colours reliable and consistent.
The predictability of oxidation firing makes it a popular choice among potters and hobbyists. It is essential to understand the chemistry of the host glaze and the colour interactions to achieve the desired results. The identity of the colourant and its percentage can also impact the final outcome. Overall, oxidation firing is a straightforward and controllable process that yields consistent and vibrant results, making it a valuable technique in ceramics.
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Frequently asked questions
Oxidation firing is a kiln firing technique where the kiln has a sufficient supply of oxygen to react with the glaze and clay body surfaces. This produces bright, rich, and vibrant colours.
Reduction firing is a kiln firing technique where oxygen is partially restricted, creating a reducing atmosphere that affects clay and glaze chemistry. This technique is typically done in a gas or other fuel-burning kiln. It produces mottled, rich, earthy colours.
Electric kilns are typically used for oxidation firing as they lack airflow control. However, it is possible to achieve a reducing effect by packing pots with carbon materials like sawdust or paper.
