Reduction Firing: How Oxygen Changes the Colors in Your Pottery
- Throw Clay LA
- 24 hours ago
- 7 min read
This week, we opened our 40-cubic-foot Geil gas kiln after Throw Clay LA’s first reduction firing.
For anyone unfamiliar with reduction firing, the basic idea sounds surprisingly simple: change the amount of oxygen available inside the kiln while the pottery is being fired.
What happens next is anything but simple.
By controlling the relationship between fuel and oxygen, a potter can change the chemistry taking place in clay and glaze. Colors can shift. Iron can behave differently. A glaze that looks one way in an electric kiln can emerge from a reduction firing looking dramatically different.
And although the person firing the kiln has considerable control over the process, there is always a delightful element of unpredictability. Two pots wearing the same glaze can emerge with noticeable differences depending on where they sat in the kiln and what happened in the atmosphere immediately around them.
That combination of control and uncertainty is part of what makes reduction firing so interesting.
First, what does “reduction” mean?
Most pottery at Throw Clay LA is fired in our electric kilns. Electric elements provide the heat, and there is generally plenty of oxygen available inside the kiln. We call this an oxidation firing.
A gas kiln gives us another variable to work with: the atmosphere inside the kiln. When we switch from electric elements to burning gas as the source of heat, we gain the ability to manipulate the amount of oxygen available during the firing. That gives us more flexibility in how glazes and clay bodies develop, but it also introduces more variables. Instead of controlling primarily temperature and heatwork, we are now also controlling the relationship between gas, oxygen and airflow. The results can be richer and more varied, but also less predictable from pot to pot and even across the surface of a single piece.
Gas needs oxygen to burn completely. When there is plenty of oxygen available relative to the amount of gas, combustion can occur efficiently. But a gas kiln can also be deliberately adjusted so there is more gas than the available oxygen can completely burn. That creates a reduction atmosphere. The resulting oxygen-poor environment changes the chemistry of the clay and glazes, affecting their final colors and surfaces. You will often hear this described as “starving the kiln of oxygen.” That's a useful shorthand, although the chemistry is a little more complicated.
With insufficient oxygen for complete combustion, carbon monoxide is produced. Carbon monoxide reacts with metallic oxides in the clay and glazes, taking oxygen from some of those compounds. This changes their chemical state and can alter the colors and surfaces that develop during the firing.
Same glaze. Different atmosphere. Different result.
Many of the colors we see in pottery come from metals and metallic compounds added to glazes. Copper, iron, cobalt and manganese are among the materials potters use to create color, and their chemistry can respond differently depending on the atmosphere inside the kiln.
Copper provides one of the classic examples. In an oxidation firing, copper is commonly associated with greens and blue-greens. Under the right reduction conditions, copper can instead produce brilliant reds.
Iron also responds strongly to atmosphere. Because iron is present not only in many glazes but also in many clay bodies, reduction can affect the color and surface of the entire pot. Depending on the clay, glaze and firing conditions, the results can range from subtle shifts in color to dramatic changes in the finished surface.
This is why reduction isn't simply another way of heating pottery to the same endpoint. The atmosphere is actively participating in the development of the clay and glaze, producing colors and surfaces that would not necessarily develop in an oxidation firing.
Temperature still matters enormously. So does heatwork, the combination of temperature and time that we measure with pyrometric cones. But in a reduction firing, we are also paying attention to the atmosphere the pots are experiencing: how much oxygen is available, when reduction begins, how strong it is and how long it lasts, while that heatwork accumulates.
So how do you control reduction?
This is where firing a large gas kiln becomes both science and craft.
The person firing the kiln controls the supply of gas and air, as well as how heat and combustion gases move through the kiln. One important tool is the damper, which affects the kiln's draft. Even relatively small damper adjustments can noticeably alter the atmosphere inside a gas kiln.
The goal isn't simply to shut down the oxygen supply as far as possible.
Too much reduction isn't necessarily better reduction. Incomplete combustion is less efficient, and excessive or poorly timed reduction can contribute to unwanted results. The kiln also still needs to climb in temperature.
Instead, firing involves balancing several things at once: temperature rise, gas, airflow, pressure, draft and the desired degree of reduction.
And those relationships change as the firing progresses.
But isn't the kiln either reducing or not reducing?
Not really.
Reduction isn't an on/off switch. Think of the kiln atmosphere as a continuum. At one end is oxidation, where there is enough oxygen for the gas to burn completely, with oxygen left available in the kiln atmosphere. A neutral atmosphere is closer to the point where the available oxygen and gas are balanced for complete combustion. Move farther in the other direction, with increasingly more gas than the available oxygen can completely burn, and you move from light to increasingly heavy reduction.
The person firing the kiln can move back and forth along that continuum during different stages of the firing.
And then there's the kiln itself
This is where another layer of variability comes in.
Our Geil is a 40-cubic-foot kiln, a large three-dimensional chamber filled with shelves, posts and pottery. Flames and hot gases move around and between all of those objects as they travel through the kiln and eventually exit through the flue.
The way the kiln is loaded adds another layer of variability. The size and shape of each pot, where it is placed, how close it is to neighboring pieces, and the height and spacing of the shelves can all influence how flame, heat and atmosphere move through the kiln. A densely packed shelf can redirect the flow. A large pot can create a sheltered area behind it. One part of the kiln may run hotter than another, while another area may experience stronger or weaker reduction.
That means loading a gas kiln isn't simply a matter of fitting as many pots as possible onto the shelves. Placement matters, and understanding where to place pots with particular clays and glazes.
Every gas kiln has its own characteristics. With repeated firings, potters learn where it tends to run hotter or cooler, where reduction tends to be stronger, and how different loads change the movement of heat and atmosphere. Raising a shelf, changing the spacing between pots or loading the kiln more densely can change what happens inside.
Some of this can be measured and deliberately controlled. Some can be anticipated through experience. And some can only be discovered by firing the kiln, opening it, studying the results and doing it again.
Controlled unpredictability
That doesn't mean reduction firing is random.
We can control the firing schedule. We can monitor heatwork with cones. We can adjust fuel, air and draft. We can decide when to introduce reduction and how strongly to reduce. We can record what happened during the firing and compare those records with the finished work.
But we can't specify the exact appearance of every square inch of every pot and then order the kiln to reproduce it.
There are simply too many interacting variables: glaze thickness, clay body, placement, neighboring pots, flame path, temperature, heatwork, atmosphere and cooling history among them.
That variability is one of the reasons reduction-fired pottery can have such depth. A glaze surface may break differently over an edge. Iron may respond differently across a form. Subtle changes in color can record differences in the atmosphere that moved around the pot while the kiln was firing.
The finished piece becomes, in part, a record of what happened inside the kiln.
Our first reduction firing
We have fired our Geil gas kiln many times, but this was our first time firing it in reduction. We already know a great deal about how this kiln heats, draws and responds to different loads. Now we're adding another dimension to what it can do.
And, of course, the best part came when we opened the kiln.
This load gave us a chance to see familiar clays and glazes transformed by a reduction atmosphere. Some results were what we expected. Others surprised us. And there were plenty of pots that made us want to look more closely, figure out exactly what happened, and see if we can make it happen again.
That's part of the appeal of reduction firing. We can understand the chemistry, carefully control the firing and make informed choices about how and where we place the work. But there is still room for discovery every time the kiln opens.
We can't wait to fire the next one.
About Throw Clay LA
Whether you’re curious to try the wheel for the first time or ready to dive deeper,
Throw Clay LA offers one-time pottery classes, 6-week Build Skills introductory courses, 6-week Refine & Explore courses, both designed as weekly studio communities where students develop skills side by side.
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