A ceramic glaze is a layer of glass fused to a clay body through heat. At firing temperature, the raw glaze materials — powders suspended in water — melt together into a liquid that flows, bubbles, and flattens before cooling into the surface of the finished piece. The colour, texture, and opacity of that surface are not intrinsic to the colourant alone: they are products of the chemical composition of the glaze, the temperature it reaches, and the atmosphere in which it fires.
The Three Structural Components
Ceramic glaze chemistry is usually described using a triaxial model: silica, alumina, and flux. Understanding how these three interact is the basis for formulating or adjusting glazes from raw materials.
Silica (SiO₂)
Silica is the glass former. It melts at around 1720°C — well above any ceramic firing temperature — and requires fluxes to bring its melting point down into a practical range. Increasing silica in a glaze generally increases gloss, hardness, and durability. Decreasing it produces a softer, more matte surface. Most studio glazes contain silica in the range of 0.3 to 0.5 mol in the Seger formula (also called Unity Molecular Formula or UMF).
Alumina (Al₂O₃)
Alumina is the stabiliser. It raises the melting point and increases viscosity — a glaze with insufficient alumina will run off the pot during firing and damage the kiln shelf. Alumina also affects the surface quality: higher alumina tends toward satin or matte finishes; lower alumina allows the silica to produce a glossier surface. Ratios between silica and alumina (the Si:Al ratio) are one of the primary levers in glaze reformulation.
Fluxes (RO/R₂O)
Fluxes lower the melting point of the silica and alumina mixture, making the glaze workable at kiln temperatures. Different fluxes become active at different temperature ranges. At low fire (cone 06–04), lead was historically the dominant flux, but it has been largely abandoned for health and safety reasons; commercial low-fire glazes use fritted fluxes containing boron, sodium, potassium, and lithium. At mid-fire (cone 6), the most common fluxes are calcium (from whiting), barium, strontium, magnesium, and zinc. At high fire (cone 10+), calcium dominates, supplemented by magnesium, potassium, and sodium from feldspar.
Colourants and Metal Oxides
Colour in a ceramic glaze comes from transition metal oxides added in small percentages to the base glaze formula. The same oxide can produce markedly different results depending on the base chemistry, the firing temperature, and the kiln atmosphere.
- Iron oxide (Fe₂O₃ / FeO): The most versatile. In oxidation, it fires amber, honey, brown, or rust depending on concentration (0.5–10%). In reduction, it shifts toward grey-green celadons, dark tenmoku (iron-saturate), or amber-to-black breaking glazes.
- Cobalt carbonate (CoCO₃): Blue in both oxidation and reduction at concentrations of 0.25–1%. Remarkably stable across temperature ranges. Higher concentrations produce darker, violet-tinged blues.
- Copper carbonate (CuCO₃): Green in oxidation (1–3%); oxblood red in heavy reduction. Copper volatilises at high temperatures and can contaminate adjacent pieces in a kiln load if used heavily.
- Manganese dioxide (MnO₂): Brown to purple depending on base glaze (1–5%). Can cause blistering if concentration is too high or the glaze is too stiff.
- Rutile (TiO₂ with iron impurities): Produces streaked, variegated surfaces rather than flat colour; used in small quantities (1–4%) to break up glaze uniformity.
- Chrome oxide (Cr₂O₃): Produces opaque, dry greens in most applications, but turns pink when fired adjacent to or with tin oxide. Chrome-bearing glazes require careful loading in shared kilns.
Surface Texture: Matte vs. Gloss
A matte glaze is not simply an underfired gloss glaze. True matting is achieved by either increasing alumina, introducing calcium or magnesium in quantities that produce micro-crystalline surface structures, or formulating a glaze with a high silica-to-flux ratio that does not fully resolve into a smooth glass. Pseudo-mattes — glazes that appear matte because they are fired below their maturation temperature — are often soluble, porous, and unsuitable for functional ware.
Crystalline glazes are a specialised category in which large zinc silicate crystals (2–8 cm) are grown on the glaze surface during a controlled cooling cycle. This requires a very low alumina content (which would otherwise prevent crystal growth), a slow, programmed cool in the kiln, and a catching cup to collect the highly fluid glaze that runs from the piece during firing.
Food Safety
Not all glazes are safe for contact with food or drink. Glazes containing lead or barium as primary fluxes, or high concentrations of manganese, can leach into food particularly in the presence of acids. The standard test is ASTM C738/C927, which measures cadmium and lead release from fired ware under acetic acid conditions.
In Canadian studio practice, lead glazes are rarely used. Barium-bearing glazes are used on exterior surfaces only. For functional ware intended for domestic use, most studio potters work with glaze recipes that use calcium, magnesium, and zinc as primary fluxes, which are considered food-safe at full maturation temperature.
The American Ceramic Society and Digitalfire's food-safe reference provide the most accessible technical discussions of this topic for studio potters.
Formulating Glazes from Raw Materials
Many Canadian studio potters mix glazes from dry raw materials rather than purchasing pre-mixed compounds. This allows precise control over chemistry and cost, and enables reformulation when a material becomes unavailable. The standard process involves writing a recipe as a percentage batch by weight, calculating the resulting UMF (Unity Molecular Formula), and comparing the result against established target ranges for the intended temperature and surface type.
Glaze calculation software — particularly Glazy (an open-source community database of studio glaze recipes) and Digitalfire — are the primary digital resources used by Canadian studio potters for this work. Both allow comparison of a glaze's chemistry against documented high-fire or mid-fire reference ranges.
Related Articles
Understanding Kiln Types and Firing Methods — how firing atmosphere changes the result of the glaze chemistry.
Hand-Building Techniques for Beginner Potters — preparing clay forms for bisque and glaze firing.