Combustion and Reduction Firing in Pottery Kilns

Introduction

One reason to build a fuel-burning kiln is to produce beautiful reduction glaze effects that can’t be achieved in an electric kilns without damaging the elements. Whether kilns are wood-fired, gas-fired or oil-fired, fuels are mainly compounds of carbon and hydrogen and can be used to produce reduction effects.

• During complete combustion, oxygen from the air combines with the carbon in the fuel, forming carbon dioxide and with the hydrogen to form water vapour.
• The reaction releases energy in the form of heat.
• In reduction, there’s not enough oxygen in the kiln atmosphere for complete combustion, so some oxygen is drawn from metal oxides in glazes — altering their chemistry and colour.
• If reduction is attempted in an electric kiln, the elements are damaged by oxygen being taken from the oxide coating that protects them.

This article outlines the chemical changes involved in combustion, oxidation and reduction, explains the important differences between types of fuel and the practical implications for design and operation of a fuel-fired ceramic kiln.

Principles of Combustion in a Kiln

In basic terms, combustion is a chemical reaction where a fuel combines with oxygen from the air releasing energy in the form of heat and light. The hot products of combustion may carry residues such as ash or other compounds through the kiln as they return to the atmosphere through the chimney. Fuels are generally compounds of carbon and hydrogen, and the main products from burning are carbon dioxide and water.

Perfect combustion

Neutral or Oxidising Atmosphere?

This ‘perfect’ combustion is sometimes described as providing a neutral atmosphere in the kiln, but in reality, complete combustion can never be achieved without a surplus of oxygen, and this is known as an oxidising atmosphere.

The atmospheric air only contains about 22% oxygen. The remaining 78% of air is mainly nitrogen which plays no part in combustion but it must still be heated as it passes through the kiln. Too much excess air therefore wastes energy, and if not well managed, the excess airflow can stall the temperature rise, making it difficult to reach high glaze temperatures.

Reducing Atmosphere

Free carbon can also be trapped in the clay body or within glazes at various stages of the firing. A reducing atmosphere in the early stages can prevent organic carbon burning off from the clay body, which is normally undesirable. At higher temperatures, the effect of carbon-trapping under glazes can be deliberately used for decorative effect.

Proper management of the reducing atmosphere through the firing cycle is therefore essential for repeatable effects, and there are a two very important implications:

• Since the fuel doesn’t burn completely, more of it is needed to sustain the rise in temperature – if there’s not enough fuel, temperature rise stalls, and once again, the target temperature may not be reached.
• Carbon monoxide is being produced. This is a very poisonous gas that is colourless and odourless, so very hard to detect. It must be safely vented to prevent accumulation and prevent exposure to anyone near the kiln.

Fuel Types

The physical form of the fuel is a result of the underlying chemical structure which has a big impact on how predictable the results of a firing will be.

Gas Fuels

Gases such as natural gas or propane are composed of relatively simple molecules and are produced under controlled industrial conditions, making them clean‑burning and highly predictable.

They are supplied under pressure, and ignite instantly from a spark when mixed with air. Simple burners with appropriate controls make it easy to manage heat output, flame length and the gas-air mixture to produce oxidising or reducing conditions.

Safety is a very important consideration for such flammable materials, so supply and usage is subject to strict regulation that varies in different jurisdictions.

Liquid Fuels

Liquid fuels like kerosene, heating oil and waste oils must be vaporised to mix with air to burn. They require sustained ignition heat rather than a brief spark and the design of the burner must provide for this. It may be as simple as the wick of an oil lamp, or a heated plate in a kiln, or it may involve a sophisticated pressurised atomiser for the purpose.

Oils often contain long complex hydrocarbon chains that break apart and combine with oxygen during combustion. As they break up, some of the chains may react with each other, forming carbon rings and lattices resulting in soot and smoke. Waste oils invariably contain other impurities that may also affect the firing. Because of this, oil‑fired kilns may have a less predictable atmosphere than gas‑fired ones.

Solid Fuels

Solid fuels like wood or coal are much more complicated than gas or oil, because they are derived from a mixture of complex natural organic compounds that were incorporated into the wood as it grew. This may include a variety of minerals from the soil as well as a subtantial proportion of combined water.

Combustion Stages for Wood

Combustion of wood proceeds through several temperature‑dependent stages, and as the fire is stoked with fresh wood, the firebox contains fuel at different stages simultaneously.

• Below about 260°C (500°F), the wood mainly dries out and starts to char but doesn’t flame yet.
• At about 260°C, pyrolysis begins to release combustible gases that mix with oxygen and burn with long, visible flames.
• After most of the gases are gone, charcoal remains— mostly solid carbon. This doesn’t melt or vaporize; instead, it burns on its surface where it contacts oxygen. This stage generates hot but short flames, usually only a few inches long. If air is blown through the charcoal, as in a blacksmith’s forge, it greatly increases the flame temperature and length.

Practical Implications for Wood-fired Kilns

• Managing kiln atmosphere in a wood-fired kiln is complex and is often described in almost mystical terms.
• The chimney doesn’t just provide an exhaust route for the products of combustion. It provides the essential draft to pull air through the firebox and kiln.
• The requirements for firebox, hearth, and chimney are very different for wood-fired kilns than for gas or oil kilns. Common internet advice and many widely referenced books confuse the requirements, often recommending unnecessarily large chimneys for gas kilns.
• The main products of burning wood are still carbon dioxide, carbon monoxide and water vapour, but the complex combustion stages and additional minerals together with flying ash particles all contribute to the kiln atmosphere and are responsible for some of the beautiful and often unpredictable effects unique to wood firing.

Colour and Appearance of Flames

Flame brightness and colour varies with the fuel type, temperature and soot content.

• Flames rich in soot appear bright yellow or orange, as glowing carbon particles emit visible light when heated.
• Clean‑burning fuels such as natural gas or propane exhibit blue or nearly invisible flames when supplied with sufficient oxygen because they produce few glowing particles.
• A reducing atmosphere is often recognisable by its hazy nature.
• The colours and behaviour of the flames give valuable information about the atmosphere in the kiln.

Conclusion

Taking account of the combustion process and the way different fuels behave is crucial for designing pottery kilns and for trouble-shooting any existing fuel-fired kiln.

Reduction firing opens additional creative opportunities, but does require careful control and additional safety considerations.

Very slow or stalling temperature rise in the kiln can be caused by either excess air, or by excess fuel and insufficient air.

Design requirements and dimensions for flue and chimney in gas-fired and oil-fired kilns are very different from the principles commonly applied for wood-fired kilns.