It is important to understand something about the chemical process of combustion in a ceramic pottery kiln in order to understand how different mixtures of oxygen and fuel affect the appearance of the final product (clays and glazes). Perfect combustion occurs when two atoms of oxygen combine with one atom of carbon to produce one molecule of carbon dioxide, plus heat. However, perfect combustion is not often achieved in practice, since that implies that exact proportions of oxygen and fuel combine in a perfect mix, with nothing left over. The fuels most commonly used in kilns are propane and natural gas. These gases are hydrocarbons, and when they are mixed properly with oxygen and then ignited, they produce water vapor, carbon dioxide, and heat. The oxygen used in a kiln comes, of course, from the air; but air only contains about 23% oxygen by weight; the other 77% is mostly nitrogen. Nitrogen plays no part in the combustion process, but it does absorb heat from this process and so can be considered to be a drag on the efficiency of the combustion, resulting in a cooler flame.
When ceramic kilns are fired, there are three possible states which the interior of the kiln can attain; and each of these states affects the appearance of the finished product ceramic pottery. A neutral atmosphere is the ideal, since it is only in a neutral atmosphere that perfect combustion, and with it the most efficient use of fuel, can obtain. If the amount of fuel is decreased – or if the amount of oxygen is increased – compared to the neutral firing state, then the mix becomes fuel-lean, which makes for a shorter and clearer flame. This is called an oxidation atmosphere, and results in a higher rate of temperature increase. In the early stage of firing, excess oxygen in the atmosphere aids in decomposing inorganic and organic sulfates and carbonates. In the later stage of firing, excess oxygen can enhance the glaze color or finish.
On the other hand, if the amount of fuel is increased – or if the amount of oxygen is decreased – compared to the neutral state, then the mix becomes fuel-rich, which makes for a long, smoky flame signaling incomplete combustion. This is called a reduction atmosphere. The excess carbon from the fuel combines with a single oxygen atom to make carbon monoxide. To return to its normal state of carbon dioxide, the carbon collects oxygen atoms from the metal oxides contained in the glaze, which changes the final color of the glaze. Therefore, striving to obtain a neutral atmosphere in the kiln is necessary to be able to control the appearance of the product. Also, reduction results in a slower rate of temperature increase. With the huge increases in fuel costs which have occurred over the past few years – and which will undoubtedly increase in the future – it is economically as well as artistically important to achieve as close to a neutral atmosphere in the kiln as possible. It might be a good idea, therefore, to look into the purchase of an oxygen probe to help in maintaining a neutral atmosphere in the ceramic kiln.