A lot of the literature about firing and glazes refers to oxidation and reduction firing. These terms sound scientific and off-putting. Because they are referring to chemical processes, they are complex. So, I decided to try and understand them better and write a description understandable to all us ‘non-chemists’.
Oxidation occurs when there is an excess of oxygen. As the kiln heats up compounds in the glaze break off and oxygen attaches itself to the glaze and clay. This causes metals in both to oxidize. Reduction occurs when there is a deficit of oxygen. This deficit creates gases in the kiln, which leach oxygen out of the glaze and clay.
Oxidation and reduction are processes that glazes and clay go through. These processes are caused by conditions in the kiln. Therefore, potters also refer to oxidation and reduction atmospheres when firing.
Oxidation and Reduction - A Look at Oxidation First.
Oxidation is a chemical reaction between oxygen and metal. Glazes and clays both contain metal components, for example, copper carbonate and iron oxide.
When materials are heated in the presence of lots of oxygen, the metals in the materials often oxidize.
If a substance oxidizes, it undergoes a chemical change. It gains oxygen molecules and it loses electrons.
This chemical change alters its appearance. Think about the Statue of Liberty. The green hue on the statue is oxidized copper. Similarly, rusted iron is the result of oxidation.
How Does Oxidation Work in a Kiln?
For example, the carbon element of copper carbonate will separate off. During the firing process, carbon elements are driven out of the glaze and clay.
When carbon compounds break away from metals and minerals, oxygen will move in and take their place. In this example, copper carbonate becomes copper oxide.
The addition of oxygen to metals and minerals changes their appearance. Oxygen will often give glazes bright colors.
One metaphor is the addition of oxygen to blood. Oxygenated blood is bright red. Although this parallel may not be chemically accurate it’s a helpful way to remember the effect of oxygen on pottery.
In addition to this, oxygen has high electronegativity1. This word can feel a bit off-putting and complicated. However, in simple terms, it means that oxygen attracts very small particles from other substances towards itself.
In fact, these small particles are electrons. When oxygen attaches itself to pottery in a kiln, it draws electrons from the glaze and clay. The transfer of electrons from a substance to oxygen is part of what is meant by oxidation.
An oxidation atmosphere in a kiln is one where there is excess oxygen. When oxygen combines with carbon it creates carbon dioxide. Carbon dioxide has two oxygen atoms for each carbon atom.
An oxygen-rich atmosphere is when there is more than oxygen atoms available than needed to create carbon dioxide.
Oxidation and Reduction - A Look at Reduction.
Oxidation firing creates bright clean colors. By contrast reduction firing tends to create rich, organic, earthy colors. It can also give pottery a speckled appearance, as flecks of iron in the clay peep through the glaze.
In oxidation, excess oxygen attaches itself to the surface of the glaze and clay. It then interacts with the materials to create the resulting pottery surface.
In direct contrast, in a reduction atmosphere, oxygen is actually drawn out of the glaze and clay. It is the drawing out of oxygen that creates contributes to the appearance of the glaze once fired.
How Does Reduction Work In The Kiln?
Reduction firing is generally used in fuel-burning kilns. The reason for this is that the process generates gases that can damage the elements in an electric kiln. Although some potters try reduction in electric kilns, it is generally associated with gas or fuel-burning kilns.
To create a reduction atmosphere, the relative amount of oxygen in the kiln is reduced. This can be done by increasing the amount of fuel present. Or restricting the amount of air going in. This depends on what kind of kiln you are using.
In combustion, fuel is turned into heat energy. When there are two atoms of oxygen for every one atom of carbon, all the fuel is converted during combustion. No fuel is wasted and all the oxygen is employed.
However, if the ratio of oxygen to carbon is less than 2:1, then the fuel is not burnt completely. There is not enough oxygen to completely convert the carbon to heat energy.
When this happens the flame becomes long, orange and smoky. Furthermore, the left-over carbon converts to gases. These gases include carbon monoxide and hydrogen.
Carbon monoxide seeks out oxygen to convert back to carbon dioxide. And hydrogen needs oxygen to burn. As a result, the gases in the reduction atmosphere, draw oxygen out of the glaze and clay.
Because oxygen is being leached out of the pottery, the metals in the glaze and clay are reduced further. This accounts for the intense, sometimes dense look of glazes from a reduction fire.
Taking the example of copper again, in an oxidizing atmosphere it turns green. Think again of the Statue of Liberty. By contrast, in a reducing atmosphere, it turns a rich red color.
What is a Neutral Atmosphere in Pottery?
Sometimes the atmosphere in a kiln is neither oxidizing or reducing. Rather, it is neutral. This means that there is enough oxygen in the kiln for the fuel to burn completely without waste. However, there is not so much that there is an excess of oxygen.
In a neutral atmosphere, the magical ratio of two atoms of oxygen to one atom of carbon is achieved.
Some potters2 suggest that actually achieving a neutral atmosphere is difficult. Particularly in a fuel-burning kiln. The reason for this is that it’s hard to balance out the presence of fuel and air perfectly. Particularly in the extreme environment of a kiln.
Other potters3 claim that firing may actually be taking place in a more neutral atmosphere than is intended.
For example, an electric kiln may be thought to be in oxidation. However, there is less airflow in an electric kiln than a gas kiln. Therefore, gases and carbon can build up in the atmosphere making the environment more neutral than intended.
As such, it’s thought that a neutral atmosphere is hard to achieve in a gas kiln. Whereas, it may be something that is stumbled upon unwittingly in an electric kiln.
Switching Between Oxidation and Reduction
Oxidation and reduction firing is not an either-or situation. Often potters will use a combination of both during a firing schedule. Some of the ways that potters will mix up their schedule are as follows:
- Creating a reduction atmosphere in the last stage of firing, before the kiln is shut down.
- Starting out in oxidation and at a couple of points during the fire, the kiln will be put into reduction.
- Putting the kiln into oxidation right at the end of a reduction fire. This can clear the atmosphere in the kiln and remove bubbles from the surface of the glaze caused by reduction.
Creating a reduction atmosphere in the last stage of firing, before the kiln is shut down.
Starting out in oxidation and at a couple of points during the fire, the kiln will be put into reduction.
Putting the kiln into oxidation right at the end of a reduction fire. This can clear the atmosphere in the kiln and remove bubbles from the surface of the glaze caused by reduction.
Final Thoughts About Oxidation and Reduction
It’s helpful to have an understanding of the difference between oxidation and reduction in firing. These different processes have a significant impact on how your pottery will look once it has been fired.
You may well only have access to one particular kind of kiln, electric or fuel burning. This will make a difference to how your ware is likely to be fired. As such it will impact on the kind of clay that you choose, and the glaze that you use.
I find it helpful to understand why clay and glaze go through their transformations in the kiln. Besides, looking into the chemistry of what is happening satisfies my inner nerd. I hope you have found it helpful too.
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