In pottery, oxides is a compound formed by the chemical union of oxygen with a different element. When combined with oxygen, these substances serve as vital constituents that establish the vitreous composition of glazes, govern their melting temperatures, and ultimately define the brilliant shades and tactile finishes of the finished ceramic pieces.
There are two distinct categories of oxides for potters: metal oxides and non-metal oxides.
The metal oxides are numerous and serve as fluxes, colouring agents, and opacifiers.
In contrast, the non-metal oxides are few and function as glass-forming substances and volatile components. Let’s learn a bit more.
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Why Metal and Non-Metal Oxides Matter
Metal Oxides (Colourants, Fluxes, and Opacifiers)
Metal oxides are crucial to the visual appearance of your pottery and instrumental in reducing melting temperatures.
Colourants
In Australia, pottery colourants are typically found in two principal categories: unprocessed metallic oxides/carbonates (for instance, cobalt, copper, and iron) and commercially produced ceramic stains (which are pre-fired pigment systems). Potters utilise these substances to add colour to unfired clay bodies, slips, underglazes, and glazes.
Fluxes
Potters frequently work with materials that possess melting points beyond their kilns’ maximum heat capacity. Fluxing agents lower these temperatures, allowing glazes to melt and clay bodies to sinter. Although flux itself often melts inefficiently, it reacts powerfully with other components. Low-fire glaze formulations can comprise a significant amount of flux, sometimes between 60% and 90%.
Opacifiers
An opacifier is a chemical element added into glazes to diminish or entirely remove their transparency, producing a visually solid and consistent surface. These elements scatter and refract light in various directions, which prevents the underlying clay’s colour and texture from influencing the glaze’s final appearance.
Among the most frequently employed opacifying agents are tin oxide, titanium oxide, and zirconium silicate. Each of these provides distinct characteristics concerning its whiteness, chemical stability, and the visual effect it imparts.
Metal Oxides: Mastering Colour, Flux, and Opacity in Glazes
Colour
The chromatic diversity in pottery is largely attributable to the judicious application of various metal oxides. For instance,
- cobalt oxide consistently yields vibrant blues,
- copper oxide can produce striking greens in oxidation or elusive reds in reduction atmospheres, demonstrating its sensitivity to firing conditions,
- iron oxide, a ubiquitous and versatile material, ranges from earthy yellows and browns to rich reds, depending on its concentration, the presence of other elements, and the kiln atmosphere,
- manganese dioxide contributes to purples and brown,
- chromium oxide is known for its greens.
The specific colour achieved is a complex interplay of the oxide’s chemical state, its interaction with other glaze components, and the thermal history of the firing process.
Fluxes
Beyond their chromatic contributions, many metal oxides function as critical fluxes, significantly influencing the melting behaviour of glazes. As mentioned before, fluxes lower the fusion temperature of ceramic materials, promoting the formation of a smooth, vitreous surface at achievable kiln temperatures. Oxides like sodium and potassium, along with calcium and magnesium, are common fluxes that enhance glaze flow and adhesion. Boron, often introduced as boric acid or borax, is a powerful flux that also contributes to glaze brilliance and durability. The precise balance of fluxes is crucial for achieving desired texture, preventing defects like crazing or shivering, and ensuring the glaze matures correctly without excessive run-off.
A full list of fluxes, as given by Frank Hamer and Janet Hammer, in their recommended and incredible book “The Potter’s Dictionary of Materials and Techniques” follows:
| Full List of Fluxes | ||
|---|---|---|
|
Lithium oxide |
Li₂O |
|
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Sodium oxide |
Na₂O |
|
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Potassium oxide |
K₂O |
|
|
Beryllium oxide |
BeO |
|
|
Magnesium oxide |
MgO |
|
|
Calcium oxide |
CaO |
|
|
Zinc Oxide |
ZnO |
|
|
Strontium oxide |
SrO |
|
|
Barium oxide |
BaO |
|
|
Lead oxide (*) |
PbO |
|
|
Boron oxide |
B₂O |
|
|
Bismuth oxide |
Bi₂O₃ |
|
|
Manganese oxide |
MnO |
|
|
Iron oxide |
FeO |
|
|
Cobalt oxide |
CoO |
|
|
Nickel oxide |
NiO |
|
|
Copper(I) oxide |
CuO |
|
|
Copper(II) oxide |
Cu₂O |
|
(*) highly toxic, requiring avoidance. All chemical elements pose risks and require the use of protective equipment such as respirators and gloves.
Opacity
Furthermore, the control over a glaze’s transparency or opacity is another vital aspect where metal oxides play a decisive role. Opacifiers, such as tin oxide, zirconium oxide, and titanium dioxide, work by scattering light within the glaze matrix, thereby obscuring the underlying clay body. Tin oxide is renowned for producing brilliant white opaque glazes, while zirconium oxide offers a more cost-effective alternative for achieving opacity. Titanium dioxide, in addition to its opacifying properties, can also introduce crystalline effects and subtle colour variations. The degree of opacity is directly related to the concentration, particle size, and dispersion of these oxides, allowing potters to achieve effects ranging from subtle translucence to complete visual impenetrability.
Non-Metal Oxides: The Structural Backbone and Performance Enhancers
Non-metal oxides in pottery involve primarily glass formers, stabilisers and low-temperature melt alternatives, as these non-metal oxides define your glaze and clay body’s backbone.
Silica is the primary glass former. This component is essential for achieving that firm, vitreous finish. Pure silica only liquefies at exceptionally high temperatures (above 3100°F / 1700°C). To facilitate its melting in standard ceramic kilns (cone 06–10), potters depend on fluxes, which are also known as network modifiers.
A glaze with a high silica concentration generally offers superior scratch resistance and increased durability. Moreover, it typically exhibits a lower thermal expansion, a property that can help diminish the occurrence of crazing. Common sources of silica are quartz, feldspar, kaolin, ball clay, wollastonite.
Alumina is the stabiliser. It increases the glaze’s thickness, helping it remain on the pot and improving its resilience. While potters often label it a stabiliser, in ceramic science it’s usually an “intermediate” that can join the glass network. Too little alumina can cause a glaze to flow excessively; too much may stiffen the molten material into a matte state. Main sources of alumina are kaolin, ball clay and feldspar.
As you might have noticed, in reality, the majority of raw materials offer multiple oxides. For instance, feldspar simultaneously provides silica, alumina and fluxes.
Conclusion
In conclusion, the influence of oxides extends far beyond their simple chemical definition; they matter, acting as critical determinants of both the aesthetic allure and the practical efficacy of materials. Given that various metal oxides respond significantly to the oxygen levels in your kiln, the specific firing conditions you select (for instance, electric oxidation versus gas reduction) is as important as the oxide itself.
From imparting specific hues and textures to conferring robust mechanical, electrical, thermal, and chemical properties, oxides are indispensable components in modern material science and engineering. Their multifaceted roles continually enable innovation across diverse industries, underscoring their profound and enduring significance in shaping the world around us.
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