Understanding Glaze Chemistry: Oxides, Fluxes and Silica

Ceramic glazes are glassy coatings fused to clay surfaces during firing. Their visual properties — colour, surface texture, opacity, and shine — are determined by their chemical composition and the temperature at which they are fired. Understanding the basic chemistry of glazes allows a potter to move beyond following recipes toward diagnosing problems, making intentional adjustments, and formulating new surfaces.

The Three-Part Glaze System

Every functional ceramic glaze is built from three component types, regardless of the specific materials used to provide them:

Silica (Glass Former)

Silica (SiO₂) is the primary glass-forming oxide. It melts at approximately 1710°C on its own — far above typical pottery firing temperatures — so it cannot form a glaze without the assistance of fluxes. The amount of silica in a glaze directly affects durability: too little silica produces a soft, easily scratched surface prone to leaching; too much raises the melt temperature beyond the target cone.

Silica is introduced through materials such as silica sand, quartz, flint, or from feldspars which contain silica alongside other oxides.

Alumina (Stabiliser)

Alumina (Al₂O₃) stiffens molten glaze, preventing it from running off vertical surfaces during firing. It increases durability and hardness in the fired surface. At high levels, alumina produces matte surfaces; at lower levels, glossy surfaces are possible when sufficient silica and flux are present.

Alumina typically enters a glaze through kaolin (china clay) or calcined alumina, and is also present in feldspar.

Flux (Melting Agent)

Fluxes lower the melting point of silica, making glaze formation possible at kiln temperatures. Different fluxes produce different surface qualities and are active at different temperature ranges:

• Calcium (from whiting or wollastonite) — the most common high-fire flux. Produces durable, glossy surfaces at cone 9–10 and matte surfaces in higher concentrations. Less active below cone 6.
• Potassium and sodium (from feldspar or soda ash) — active across a wide range. Increase surface gloss and contribute to fluid melts. High-sodium glazes show stronger response to oxide colorants.
• Zinc oxide — active at mid-fire (cone 6) and contributes to crystalline surfaces under specific cooling conditions.
• Lithium (from spodumene or lithium carbonate) — a powerful low-temperature flux, used in small quantities at cone 6 to improve melt fluidity.
• Barium carbonate — produces distinctive matte surfaces at cone 6. Barium is toxic in its raw state and requires handling precautions.

Oxide Colorants

Ceramic colorants are metal oxides added to glaze bases in small percentages to produce colour. The same oxide can produce markedly different colours depending on the glaze chemistry, firing temperature, and kiln atmosphere.

Common Oxide Colorants

The following oxides are among the most widely used in studio pottery:

• Cobalt carbonate / cobalt oxide — produces blue in most glaze bases and atmospheres. One of the most consistent colorants, active at low percentages (0.5–2%). At high percentages produces very dark, near-black surfaces.
• Iron oxide — the most versatile colorant, producing yellows, tans, rusty browns, and deep blacks depending on percentage and atmosphere. In reduction firing, iron glazes can produce celadons (soft grey-green at low percentages) and tenmoku (dark brown-black at high percentages).
• Copper carbonate / copper oxide — produces greens in oxidation and reds in strong reduction. Copper is volatile at high temperatures and can travel as vapour to affect neighbouring pots in a kiln.
• Manganese dioxide — produces purples, browns, and tans. Often used in combination with other colorants. At high percentages, manganese can cause blistering during firing.
• Chrome oxide — produces green in most contexts but shifts dramatically in the presence of zinc (producing brown), or tin (producing pink to red through chrome-tin reactions).
• Rutile (titanium with iron) — produces mottled, variegated surfaces with blue and tan flashing. Commonly used in combination glazes where visual movement is desired.

Oxidation vs. Reduction

Kiln atmosphere — the presence or absence of oxygen during firing — profoundly affects glaze colour when iron or copper are present. Electric kilns fire in oxidation: sufficient oxygen is always present, and metals remain in their oxidised state. Gas and wood kilns can be controlled to fire in reduction: a fuel-rich atmosphere draws oxygen from metal oxides in the clay and glaze, shifting their colours.

Iron-bearing glazes in reduction shift from warm yellows and browns (oxidation) to grey-green celadons or black tenmoku surfaces. Copper glazes, which are green in oxidation, can shift to deep red (sang de boeuf or oxblood glaze) under sustained reduction — though achieving this reliably requires careful temperature and atmosphere control throughout the firing.

In Canadian studios, access to reduction firing depends on facility type. Many urban and educational studios use electric kilns and fire in oxidation. Community studios with gas kilns — including some craft centres in British Columbia, Ontario, and Nova Scotia — allow reduction work. Wood firing facilities are available at select centres and are often operated as communal multi-day firings.

Glaze Fit and Cone Temperature

A glaze designed for cone 10 applied to a cone 6 firing will almost certainly under-fire: the flux oxides will not melt sufficiently, producing a rough, crawled, or underdeveloped surface. Conversely, applying a cone 06 glaze at cone 10 will over-fire it — the glaze may run off the pot entirely and damage the kiln shelf.

Thermal expansion compatibility between glaze and clay body — expressed as Coefficient of Thermal Expansion (COE) — determines whether a glaze fits without crazing (fine cracks in the glaze from the glaze contracting more than the clay) or shivering (glaze flaking off because it contracts less). Formulating glazes with a COE slightly lower than the clay body creates compressive fit, which is generally considered favourable for functional ware durability.

Testing Practices

Reliable glaze development depends on systematic testing. Line blends — a series of test tiles varying a single variable (such as flux percentage or colorant amount) in increments — are more informative than adjusting multiple variables simultaneously. Keeping a documented record of each test, including the recipe, application thickness, firing cone, and kiln atmosphere, allows patterns to be identified over multiple firings.

Glaze calculation software significantly reduces the arithmetic burden of reformulating glazes and translating between raw material recipes and molecular chemistry. Glazy.org provides a free, open database of glaze recipes and built-in calculation tools used widely in studio pottery education.