Oxides are one of the most versatile coloring tools in ceramics. Mixed with water and brushed over an unfired glaze, they produce rich colors and patterns that range from deep cobalt blues to copper greens to iron-rich browns. You can also blend them directly into glaze recipes for uniform color. The key is knowing which oxides produce which colors, how to mix and apply them properly, and how firing conditions change the results.
What Oxide Washes Are and How They Work
An oxide wash is simply a metallic oxide powder mixed with water. You brush, sponge, stamp, or spray it onto your piece to add color. The most common approach is painting the wash over an unfired glaze, where the oxide melts into the glaze surface during firing and produces color. This technique has centuries of history: potters in China used cobalt oxide suspended in water to paint blue designs over transparent glazes, and Islamic potters brushed copper oxide patterns over unfired glaze surfaces.
One important rule: apply oxide washes over a glaze, not under it. Placing oxide beneath a glaze layer often causes the glaze to crawl and pull away from the surface, leaving bare patches on your piece. When painted on top, the oxide sinks into the molten glaze during firing and bonds properly.
Mixing an Oxide Wash
Start with a small amount of oxide powder and add water gradually until you reach a consistency similar to watercolor paint or thin cream. There’s no single fixed ratio because you’ll adjust thickness depending on how bold you want the color. A thinner wash gives a more translucent effect; a thicker application creates deeper, more saturated color.
Pure oxide and water mixtures tend to settle quickly and can feel gritty when brushing. Adding about half a teaspoon of a suspending agent like Vee-Gum-T to your batch helps keep the oxide particles suspended in the water and improves brushability. Some potters use CMC gum (a cellulose-based binder) for the same purpose, and combining the two works well for a smooth, paintable consistency. If you’re working with red copper oxide specifically, you’ll notice it floats on water due to a hydrophobic coating. A few drops of liquid dish soap breaks that surface tension and lets it disperse.
Common Oxides and the Colors They Produce
Three oxides form the backbone of most potters’ color palettes: cobalt, iron, and copper. Each behaves differently depending on how much you use and how you fire.
Cobalt oxide is the most reliable and potent colorant. At roughly 1.5% added to a glaze, it produces a strong blue to blue-violet in both oxidation and reduction firings. Because it’s so intense, even small amounts go a long way. A light wash gives a soft blue; too much quickly turns almost black.
Iron oxide is the most versatile. In oxidation firing, additions from 0.5% to 6% produce a range from pale amber to deep brown. In reduction firing, the same oxide behaves completely differently: 0.5% to 2% creates jade greens and celadons, while 5% to 6% produces the rich brown-black surface known as tenmoku. Iron is also unusual because reduction firing transforms it into a powerful flux, meaning it helps the glaze melt and flow much more readily. A tenmoku glaze with about 12% iron oxide will be extremely fluid in reduction but completely stable in oxidation at the same temperature.
Copper oxide gives turquoise blues and greens at 2% to 3% in oxidation. In reduction, those same percentages shift dramatically to reds and purples. Copper is an active flux, so adding it to a glaze can make the glaze run down the side of your pot. Keep test tiles handy and watch your application thickness.
Oxide vs. Carbonate Forms
Most coloring metals come in both oxide and carbonate forms, and the choice matters. Copper carbonate is the more commonly used form because it disperses evenly in a glaze slurry and melts uniformly, giving consistent color. It produces greens at 5% or less, blacks above 5%, and at very small additions (0.3% to 0.8%) it can yield blues in oxidation or copper reds in reduction.
Black copper oxide, by contrast, has larger particles that don’t melt as easily. This can create a speckled effect, which is sometimes desirable for visual texture but frustrating if you want even color. Copper oxide is also a stronger colorant, so you need less of it. One trade-off with carbonates: they release carbon dioxide gas during firing (around 572°F to 608°F), which can cause pinholes or blisters in the glaze surface if the timing of your firing doesn’t allow those gases to escape before the glaze seals over.
Application Techniques
Brushing is the most common method. Use a soft brush to paint oxide wash directly onto your glazed but unfired piece. You can blend colors, overlap different oxides, or stipple with a sponge for texture. Spraying gives a more even, atmospheric effect and works well for covering large surfaces or creating gradients.
For decorative line work, you can also scratch through a layer of oxide wash to reveal the glaze color beneath, a technique similar to sgraffito. This works especially well with darker oxides like cobalt or iron over a light-colored glaze.
Layering multiple glazes with oxide washes between or on top of them opens up more complex surfaces. Some potters dip a piece in a base glaze, brush cobalt or copper washes over specific areas, then add a second glaze layer on top. The interactions between layers during firing create colors and textures that neither glaze nor oxide would produce alone. Experimentation is essential here because results vary widely depending on your specific glazes, oxide thickness, and firing schedule.
How Firing Atmosphere Changes Everything
The atmosphere inside your kiln during firing is one of the biggest variables in oxide color. In oxidation firing (electric kilns, or gas kilns with plenty of airflow), metals stay fully oxidized and produce their “standard” colors. In reduction firing (restricting oxygen in a gas kiln), metallic oxides give up some of their oxygen and shift to a more metallic state, which changes both their color and their chemistry.
Copper is the most dramatic example. It fires green in oxidation but turns red in reduction. Iron shifts from ambers and browns in oxidation to the greens, celadons, and deep tenmoku surfaces that define much of traditional Asian ceramics. These effects are difficult or impossible to replicate in an oxidation kiln, which is one reason many potters specifically seek out reduction firing for oxide-heavy work.
Testing With Line Blends
Because small changes in oxide percentage produce visible color shifts, systematic testing saves time and materials. A line blend is the standard method. You prepare two batches of the same base glaze: one with no oxide added, and one with the maximum percentage you want to test (say, 10% red iron oxide). Then you use a syringe to mix the two in increments, creating a series of samples at 1%, 2%, 3%, and so on up to 10%.
Dip a small test tile into each mixture, coating half at a single thickness and the other half at double thickness (by dipping twice at an angle). Write the glaze name and oxide percentage on the back of each tile. After firing, you’ll have a clear visual map of how that oxide behaves in your glaze at every concentration. This approach works for any oxide and takes far less time than mixing each test batch individually from scratch.
Safety When Handling Oxides
Raw oxide powders are fine particles of metal compounds, and several of them pose real health risks when inhaled. Cobalt and manganese are highly toxic by inhalation, and long-term manganese exposure is linked to neurological damage. Wear a respirator rated for fine particulates whenever you’re weighing or mixing dry oxide powders. Gloves should be worn when handling both dry and wet forms, and it’s worth keeping separate work clothes for studio use. Basic hygiene matters: no eating, drinking, or smoking in the studio, and wash your hands thoroughly after handling any glaze materials.