Major Oxides in Glazes

When you mix a glaze, you’re combining several powdered materials, each of which contributes certain oxides to the final melt. These oxides determine how easily the glaze melts, how fluid or matte it becomes, and how it ultimately looks and behaves on your fired pieces.

The Three Essential Oxide Groups

In the previous section What is Glaze?, we introduced three primary oxide groups:

1. Glass Former (Silica / SiO₂) Essentially the backbone of all glazes.
2. Fluxes (e.g., Na₂O, K₂O, CaO, MgO, B₂O₃) Fluxes lower silica’s melting point and can radically change color, surface finish, and thermal expansion.
3. Stabilizer (Alumina / Al₂O₃) Alumina raises viscosity, prevents the glaze from running off your pot, and adds durability.

Yet in practice, most raw materials supply more than one oxide. For example, feldspar contributes silica, alumina, and fluxes all at once. Let’s look at each major oxide (or oxide family) to see how it shapes a glaze and which raw materials commonly provide it.

Silica (SiO₂): The Glass-Former

Silica is the primary glass-former. Without it, you wouldn’t get that hard, glassy surface. Pure silica melts only at extremely high temperatures (above 3100 °F / 1700 °C). To make it melt in typical ceramic kilns (cone 06–10), potters rely on fluxes (network modifiers).

A glaze high in silica is generally more scratch-resistant and durable. It also tends to have a lower thermal expansion, which can help reduce crazing.

Common Sources of Silica

Many glaze materials can provide SiO₂. Some sources are almost pure silica, while others are silicate minerals that contain silica combined with other oxides.

• Quartz (aka Flint or Silica): Ground silica that’s nearly pure SiO₂.
• Feldspar: Though often considered a flux source, feldspar contains a significant portion of silica as well as alumina.
• Clay (Kaolin , Ball Clay ): Clay itself is an alumino-silicate, meaning it provides both silica and alumina.
• Wollastonite (CaSiO₃): Primarily used for its calcium (flux) content, but also adds silica.

Alumina (Al₂O₃): The Stabilizer

Alumina boosts a glaze’s viscosity, helps it stay put on the pot, and improves durability. Though it’s often called a stabilizer by potters, in ceramic science terms it’s typically an “intermediate” that can participate in the glass network. If you add too little alumina, your glaze may run; too much can stiffen the melt into a matte.

Common Sources of Alumina

• Kaolin (China Clay): A very pure clay (Al₂O₃·2SiO₂·2H₂O) that supplies alumina and silica. Kaolin is an ideal source of alumina for glazes and is often added specifically to increase Al₂O₃ content.
• Ball Clay: Similar to kaolin but with more impurities and slightly higher plasticity. It can also help with suspension of the glaze slurry.
• Feldspar: Contains moderate alumina along with silica and alkali fluxes. For example, potash feldspar is roughly 18% Al₂O₃.
• Alumina Hydrate (Al(OH)₃): A direct, refined source of Al₂O₃, though it’s refractory and doesn’t melt easily, so it’s not as readily dissolved into the glaze as clay or feldspar and thus rarely used as a primary source of alumina.

Fluxes: Lowering the Melting Point

Flux is the collective term for oxides that reduce the melting point of silica, making it molten at normal firing ranges. Without fluxes, we would need extremely high temperatures to melt a glaze. Fluxes are also known as “network modifiers.”

Temperature Thresholds for Flux Behavior

A key idea from ceramic science is that not all fluxes work equally well at all temperatures. For instance:

• Alkali metal oxides (Na₂O, K₂O, Li₂O) help from low-fire up through cone 10.
• Alkaline earths (CaO, MgO, BaO, SrO) need more heat to truly flux—often around cone 1–2 or higher. At low-fire, they may do very little unless paired with boron or alkalis.
• Boron (B₂O₃) fluxes over a broad range but is actually a network former in technical terms.
• Zinc (ZnO) can help from mid-fire onward; at lower temps or in reduction, it’s either ineffective or volatile.

Alkaline Fluxes (Na₂O, K₂O, Li₂O)

The alkali metal oxides—potassium oxide (K₂O), sodium oxide (Na₂O), and lithium oxide (Li₂O)—are among the most active fluxes in ceramic glazes. They become effective at relatively low temperatures (from cone 06 up through mid/high fire) and drastically lower the melting point of the mix. In general, they’re known for producing very fluid melts and for creating glossy, transparent glazes. However, they also tend to raise glaze expansion, so high levels can cause crazing. Each alkali oxide behaves a bit differently, as outlined below.

Sodium Oxide (Na₂O)

Sodium is a strong flux especially effective at low to mid-fire temperatures. It produces very fluid melts, often resulting in glossy surfaces, but has relatively high thermal expansion—a glaze rich in Na₂O has a greater tendency to craze.

Common Sources

• Soda Feldspar (e.g., Minspar 200 ): Typically contains higher Na₂O relative to K₂O.
• Nepheline Syenite (e.g., A-270 ): A sodium–potassium rock with a higher overall flux content than feldspar, melting at lower temperatures.
• Soda Ash (Sodium Carbonate): Used in soda firing or occasionally added to glazes (though it’s highly soluble, so it’s not typical for stored glazes).
• Borate Frits: Many commercial frits incorporate Na₂O alongside boron to facilitate low-temperature melting.

Potassium Oxide (K₂O)

Potassium is another strong alkali flux that often yields bright, glossy glazes and can enhance color development. It also leads to high glaze expansion. It can promote crazing if used in excess, although to a lesser extent than sodium.

Common Sources

• Potash Feldspar (e.g., Custer Feldspar , G-200 Feldspar ): Real-world materials often contain a mix of K₂O and Na₂O, but with a higher proportion of K₂O.
• Nepheline Syenite (e.g., A-270 ): Also contains both Na₂O and K₂O but is favored for its lower melting range.
• Potassium Carbonate (Pearl Ash): Occasionally used in specialized or historical recipes and in salt or soda firing contexts.

Lithium Oxide (Li₂O)

Lithium is among the most powerful fluxes, capable of lowering the melting point even at small percentages. It has low thermal expansion compared to sodium and potassium and can help reduce crazing if used judiciously. Lithium can promote unique surface effects, including certain matte or crystalline textures, particularly at higher Li₂O levels. It can sometimes cause slight slurry-thickening issues in the bucket (especially lithium carbonate) and is more expensive than sodium/potassium sources.

Common Sources

• Lithium Carbonate: Dissolves easily in the glaze melt, though care is needed when mixing (it’s somewhat soluble in water).
• Spodumene and Petalite: Lithium aluminosilicate minerals, commonly used in stoneware and porcelain glazes for cone 6–10.
• Some Borate Frits: May incorporate lithium in addition to boron.

Alkaline Earth Fluxes (CaO, MgO, BaO, SrO) & ZnO

The alkaline earths—calcium (CaO), magnesium (MgO), barium (BaO), strontium (SrO)—plus zinc (ZnO) also act as fluxes but generally behave differently from the alkali metals. They typically melt at higher temperatures (most effective in cone 5–10 ranges) and form stiffer melts. Many alkaline earth–rich glazes develop satin or matte finishes if crystals grow upon cooling. Collectively, they contribute a lower thermal expansion than alkali oxides, which helps reduce crazing.

Calcium Oxide (CaO)

Calcium is a very common high-fire flux that promotes a durable, hard glass. Moderate amounts of CaO usually yield glossy surfaces; higher levels can promote matte or crystalline textures (calcium mattes). Calcium tends to lower glaze expansion relative to sodium/potassium fluxes, helping to reduce crazing.

Common Sources

• Whiting (Calcium Carbonate, CaCO₃): Decomposes during firing, releasing CaO.
• Wollastonite (CaSiO₃): Supplies both CaO and SiO₂, often helping glaze fit.
• Bone Ash (Calcium Phosphate): Also introduces some phosphate (P₂O₅).

Magnesium Oxide (MgO)

Magnesium is another high-temperature flux; melts less readily than CaO but imparts characteristic silky matte surfaces when present in sufficient quantity. It commonly yields buttery or waxy matte glazes (especially in slow-cool firings) and has an especially low thermal expansion, making it useful for combating crazing. (See “Magnesium Matte” .)

Common Sources

• Dolomite (CaCO₃·MgCO₃): Provides both MgO and CaO upon decomposition.
• Talc (Mg₃Si₄O₁₀(OH)₂): Often used in low-fire glazes and clay bodies to supply MgO and silica.
• Light Magnesium Carbonate (MgCO₃): A pure source of MgO that decomposes during firing. Very light and fluffy in texture, often used in crawling glazes.

Barium Oxide (BaO)

Barium is valued for creating unique mattes and certain color responses. It’s considered somewhat refractory and can help produce subtle crystal growth upon cooling.

Common Sources

• Barium Carbonate (BaCO₃): The principal raw source for BaO. However, it is toxic, so many potters substitute strontium where possible.

Strontium Oxide (SrO)

Strontium behaves similarly to barium (matte finishes, interesting color modifications) but is much less toxic. Due to this, it is often used as a barium substitute for safer functional ware, though exact 1:1 swaps may require slight formula adjustments. Strontium produces smooth satin-to-matte surfaces in mid/high-fire ranges.

Common Sources

• Strontium Carbonate (SrCO₃): Decomposes during firing to release SrO.

Zinc Oxide (ZnO)

While not an alkaline earth chemically, zinc often gets grouped here because it acts as a high-fire flux with some similar behaviors. Zinc acts as a mid/high-fire flux that can promote glossy surfaces. It also encourages the growth of zinc-silicate crystals in specialized crystalline glazes, producing dramatic crystal patterns. At high temperatures in reduction firings, zinc is volatile and may be lost, although it may still be effective as an early flux in the glaze melt.

Common Sources

• Zinc Oxide (ZnO) powder is typically added directly.
• Some frits may incorporate zinc in their formula.

Related Glaze Categories

Bristol , Specialty → Crawling 

Lead Oxide (PbO)

Final Thoughts on Flux Balance

Balancing these alkaline and alkaline earth oxides (along with boron) is how potters fine-tune a glaze’s melting temperature, surface finish, color response, and thermal expansion. Too much of a powerful flux like Na₂O or K₂O may encourage crazing or runniness; adding alkaline earths such as CaO, MgO, or SrO often stiffens the melt, creates matte surfaces, and lowers expansion. Meanwhile, lithium can be a game-changer for special effects, and zinc can facilitate spectacular crystal growth under the right conditions.

All flux choices interact with colorants, so the journey of refining glazes often involves experimenting with various combinations of alkali/alkaline earth ratios to achieve the perfect melt, surface, and hue.

Boron (B₂O₃)

Boron is special: it’s actually a network former in ceramic science (like silica), but in ceramics we usually treat it as a flux, especially at earthenware and mid-fire. It reduces melting temperature drastically and widens the firing range (“forgiving glazes”). Nearly all low-fire and many mid-fire glazes rely on boron, usually from frits, colemanite, or Gerstley Borate.

Excessive B₂O₃ (above ~12% in some compositions) can reduce durability. Pinholes may occur if there’s too much boron and not enough heat work to smooth out the glaze.

Common Sources

• Borate Minerals: Traditionally, minerals like Colemanite (calcium borate) and ulexite (sodium calcium borate) were used in formulating glazes. These minerals release B₂O₃ (along with CaO/Na₂O and other oxides) when melted. Gerstley Borate is a popular raw material (a mix of colemanite and ulexite with clay) that was widely used in studio glazes as a source of B₂O₃ and CaO. However, raw borate minerals can be somewhat inconsistent and partially soluble in water.
• Boron Frits: Today, the most common source of boron in glazes is through frits – manufactured, pre-melted glasses that contain B₂O₃ and other oxides. For example, Ferro Frit 3134 , 3124 , 3195 , etc., are all boron frits commonly used in glazes. Frits have the advantage of being relatively insoluble and reliable. A frit labeled as a borate frit might contain B₂O₃ along with fluxes like CaO, Na₂O, etc., as well as other oxides.
• Boric Acid / Borax: In some cases, boric acid (H₃BO₃) or borax (sodium borate) is used, especially in specialty raku or low-fire glaze formulas. These are highly soluble in water, so they’re not ideal for stored glaze mixtures, but they do provide B₂O₃ and alkali flux if used fresh.

Other Oxides

While colorants aren’t usually considered part of the main glaze skeleton, they can significantly alter how a glaze looks and behaves, sometimes acting as secondary fluxes.

Iron Oxide (Fe₂O₃)

Iron oxide is both a colorant and, under certain conditions, a flux. In oxidation (electric kiln), it yields browns, tans, and reds; it remains relatively refractory but provides classic earthy hues. In reduction (gas/wood firing with limited oxygen), it shifts to ferrous oxide (FeO), which acts as a strong flux—leading to fluid, deep-colored glazes such as tenmoku (black/brown) and celadon (blue-green). High iron levels can also foster crystal development, especially in iron-red and matte glazes.

Common Sources

• Red Iron Oxide (Ferric Oxide): ~95%+ Fe₂O₃, very consistent and pure, especially synthetic varieties, the most common way to source iron.
• Spanish Red Iron Oxide: a brown red natural earth based on hematite deposits in Spain containing 85% iron oxide.
• Yellow Iron Oxide: A hydrated form that calcines to red iron when fired. This is not the same as yellow ochre, which is a naturally-occurring impure mixture of yellow iron oxide, clay, etc.
• Yellow Ochre: Soft and crumbly, often contaminated with clay, limestone, and other matter.
• Black Iron Oxide (Magnetite): Contains iron in a more reduced state; can melt more easily and is often used for speckling or strong coloration.
• Iron-Bearing Clays/Slips: Natural clays (like terra cotta) or high-iron slips (e.g.,Alberta Slip , Redart ) that supply iron (and other oxides) for unique glaze effects.
• Misc. Iron Compounds: Materials like Crocus Martis  or Iron Chromate , though less commonly used in standard recipes.

Related Glaze Categories

Iron , Iron → Celadon , Iron → Celadon → Blue , Iron → Celadon → Green , Iron → Celadon → Yellow , Iron → Celadon → Chun , Iron → Amber , Iron → Tenmoku , Iron → Tea Dust , Iron → Tessha, Hare’s Fur , Iron → Kaki, Tomato Red , Iron → Oil Spot , Iron → Slip-Based , Yellow → Iron 

Cobalt (CoO / Co₃O₄)

A potent, reliable blue colorant for both oxidation and reduction. Even tiny additions (0.1–1%) can yield intense blues. Excess cobalt can result in dark blue-black glazes or surface defects.

Common Sources

• Cobalt Oxide: A strong black powder.
• Cobalt Carbonate: A lavender powder that decomposes to CoO in firing.

Related Glaze Categories

Blue , Blue → Cobalt 

Copper (CuO / Cu₂O)

Produces greens, turquoise and blues in oxidation and the famous copper reds (like oxblood) in reduction. Can also yield turquoise or metallic effects in certain base glazes or with special firing. Higher amounts may increase glaze fluidity and risk leaching if not well-formulated.

Common Sources

• Copper Oxide (Black): CuO (black powder).
• Copper Oxide (Red): Cu₂O (red powder), reduced form of the black copper oxide (CuO).
• Copper Carbonate: A greenish powder that’s easier to disperse.

Related Glaze Categories

Red → Copper , Red → Copper → Oxblood , Red → Copper → Flambe , Red → Copper → Peach Bloom , Green → Copper , Green → Oribe , Turquoise , Blue (containing Copper) 

Chromium (Cr₂O₃)

Typically yields green (often olive/forest tones). In certain bases, it can produce pink (chrome-tin pink), yellows, or browns. Chromium can fume at high temperatures, sometimes flashing nearby tin glazes pink.

Common Sources

• Chrome Oxide: A dull green powder used sparingly (1–2% or less).

Related Glaze Categories

Green → Chrome , Pink (including Chrome) 

Manganese (MnO₂ / MnCO₃)

Contributes browns, purples, or blackish tones, depending on glaze composition. Common in speckled effects (coarse manganese granules). Can fume at high fire, creating halos on nearby ware.

Common Sources

• Manganese Dioxide: Dark brown-black powder, can cause speckling if used in coarse form.
• Manganese Carbonate: Lighter tan powder, disperses more evenly for uniform color.

Related Glaze Categories

Purple → Manganese , Yellow → Manganese , Crystalline → Manganese 

Nickel (NiO)

Usually gives greys or soft browns and modifies other colors (e.g., toning down cobalt). It’s relatively weak, requiring higher percentages for noticeable color shifts.

Common Sources

• Nickel Oxide: A gray-green powder.
• Nickel Carbonate: A pale green powder, less common but easier to disperse.

Related Glaze Categories

Green → Nickel , Blue → Nickel , Purple → Nickel , Yellow → Nickel 

Titanium (TiO₂)

Titanium can opacify, crystallize and variegate glazes. Small additions of 1-2% dissolve with minimal effect, but above 2%, titanium creates translucent, silky opacity and mottling through micro-crystal formation. At 4-6%, it brightens colors while creating variegation, and at high amounts (10%+), produces matte, textured surfaces with visible crystallization.

Common Sources

• Rutile : A natural TiO₂ mineral containing iron, creates warm tones and variegated effects.
• Titanium Dioxide : Pure titanium dioxide is used when clean opacification is needed.
• Ilmenite : Ilmenite provides speckling effects.

Related Glaze Categories

Blue → Rutile  Green → Titanium 

Zirconium (ZrO₂)

Zirconium creates consistent, reliable opacity without variegation, producing dense, uniform whites and increasing both gloss and melt viscosity. It’s effective at both low and high temperatures, typically used at 8-12% for full opacity.

Common Sources

• Zirconium silicate (Zircopax , Superpax , Ultrox ) is the standard form, containing about 65% ZrO₂.

Tin Oxide (SnO₂)

A classic opacifier historically used in majolica and other white glazes. Tin oxide  produces a softer, milkier white than zircon. It also interacts with chrome to form pink hues. In recent years, tin oxide has become very expensive, and many opt for the cheaper zirconium silicates for opacifying effects.

Phosphorus (P₂O₅)

Phosphorus is a network former in ceramic chemistry, much like silica (SiO₂) and boron (B₂O₃). In practice, it usually enters a glaze through bone ash (calcium phosphate) or, to a lesser extent, natural wood ash, and most formulations keep it below about five percent. Though overshadowed by silica, even small amounts of phosphorus can influence a glaze’s overall character, encouraging subtle crystal growth that may yield soft mattes or milky, opalescent surfaces. Phosphates also tend to shift or soften colors, lending pastel or variegated effects that can be hard to achieve with other oxides.

Beyond aesthetics, phosphorus plays a notable role in specialized or “synthetic ash” glazes, where adding bone ash provides both a fluxing component (calcium) and phosphate in one material. This combination can mimic natural ash effects—like complex runs and variegation—without requiring a wood-kiln firing. While not a staple in every studio recipe, phosphorus offers potters a nuanced way to explore new textures, color breaks, and surface qualities when used thoughtfully.

Common Sources

1. Bone Ash (Calcium Phosphate) – The primary source, typically used at 1–5%.
2. Wood Ash – Contains smaller amounts of P₂O₅, varying by the wood species and burning process.

Related Glaze Categories

Iron → Celadon → Chun  Ash 

Putting It All Together

When you read a glaze recipe—something like:

• 40% Feldspar
• 30% Silica
• 20% Whiting
• 10% Kaolin
• +2% Red Iron Oxide (as colorant)

you’re essentially mixing up a balance of oxides:

• Feldspar brings sodium/potassium fluxes, plus some silica and alumina.
• Silica (Flint) is your main glass-former.
• Whiting supplies calcium flux.
• Kaolin adds alumina (and some silica) to stabilize the melt.
• Iron Oxide modifies color and, under reduction, can become a strong flux.

During firing, the fluxes break down and help silica melt, alumina keeps the melt from running, and the colorant, iron, changes the color of the glaze depending on atmosphere (oxidation vs. reduction). Each oxide’s proportion directly affects melt temperature, surface texture (matte vs. gloss), color, and thermal expansion.

Further Reading

• Chinese glazes : their origins, chemistry and re-creation by Nigel Wood 
• Clay and glazes for the potter by Daniel Rhodes 
• The ceramic spectrum by Robin Hopper