A glaze is a thin layer of glass fused to a ceramic body during firing. The chemistry that determines whether that glass is matte or glossy, fluid or stable, opaque or translucent, is governed by a set of relationships between its constituent oxides. Understanding those relationships — even at a basic level — gives a studio potter far more control over outcomes than working from recipes alone, and makes the difference between understanding why a glaze behaved unexpectedly and simply accepting the result.
What a Glaze Actually Contains
Every glaze, regardless of its visual character, contains three functional groups of oxides:
- Glass formers: Silica (SiO₂) is the primary glass former. It creates the glassy matrix that holds everything else in place. Without enough silica, a glaze will be chemically unstable and may leach over time.
- Fluxes: Fluxes lower the melting point of silica by disrupting its crystalline structure. Common fluxes include calcium oxide (from whiting), potassium oxide (from feldspars), sodium oxide, and magnesium oxide. The choice and ratio of fluxes determine the melt temperature and surface character.
- Stabilisers: Alumina (Al₂O₃) is the primary stabiliser. It increases viscosity in the melt, preventing the glaze from running off vertical surfaces. A glaze with very low alumina will be fluid and likely crawl or run; too much alumina produces a dry, underfired surface.
Colourants — iron oxide, cobalt carbonate, copper carbonate, rutile — sit outside this functional framework. They operate by introducing transition metal ions into the glass matrix, which absorb certain wavelengths of light and produce colour. Adding a colourant changes a glaze's colour but does not directly affect its melt behaviour, although some colourants act as secondary fluxes in quantity (iron oxide at 8–10%, for example, noticeably increases melt fluidity).
The Unity Molecular Formula and Seger Formula
The unity molecular formula (UMF) is the standard framework for comparing glazes. It expresses the molar ratio of each oxide in a glaze normalised so that the total molar quantities of the flux oxides always equal 1.0. This normalisation allows direct comparison between glazes that use different raw materials but arrive at similar chemical compositions.
The Seger formula — named after the nineteenth-century German ceramicist Hermann Seger — is the traditional form of the UMF. It groups oxides into three columns:
- RO / R₂O (fluxes): CaO, MgO, K₂O, Na₂O, Li₂O, etc.
- R₂O₃ (stabilisers): Al₂O₃, Fe₂O₃, B₂O₃ (when used as a flux at low temperatures, B₂O₃ moves columns)
- RO₂ (glass formers): SiO₂, TiO₂, ZrO₂
For a standard cone 6 oxidation glaze, typical ranges run approximately 0.2–0.4 mol Al₂O₃ and 3.0–5.0 mol SiO₂ per 1.0 mol total flux. A glaze with SiO₂ below 2.5 is considered chemically unstable; one above 5.5 may not melt fully at cone 6 without a high-flux compensating. These are guidelines, not absolute rules — fired temperature, kiln atmosphere, and clay body interaction all shift the effective behaviour of a given formula.
Why Recipes Alone Are Not Enough
A recipe that works reliably in one studio may behave completely differently in another if the raw materials are sourced from different suppliers. Feldspars vary considerably in their sodium/potassium ratio between batches and brands. Whiting purity varies. The only way to understand whether two "identical" recipes will behave the same way is to compare their unity molecular formulas, not their batch weights.
Line Blends: The Practical Testing Method
A line blend is a systematic test in which two materials — or two complete glazes — are combined in a graduated sequence to observe the effect of progressively shifting the ratio. A standard line blend might test 10 tiles: tile 1 is 100% of material A, tile 10 is 100% of material B, and tiles 2–9 contain A:B ratios of 90:10, 80:20, 70:30, and so on.
Line blends are used routinely in Canadian studio practice to accomplish several tasks:
- Finding the effective flux level in a base glaze (by blending in increasing amounts of a fluxing material such as whiting or wood ash)
- Locating the transition point between a matte and gloss surface in a calcium-silica ratio test
- Determining the maximum colourant addition before opacity or crawling occurs
- Comparing two feldspar sources to understand their relative fluxing strength
A line blend is fired on a test tile with a vertical face rather than a horizontal surface, so that glaze movement — running or crawling — is visible in the result. Applying the test glaze to the same body used in production work is critical: a test fired on a dense porcelain tile and then applied to a porous stoneware body in production will yield different results because glaze absorption during application depends on body porosity.
Tri-Axial Blends and Grid Tests
Where a line blend tests variation along a single axis (A vs. B), a tri-axial blend tests variation across three materials simultaneously. A standard tri-axial blend of three base glazes produces 15 test tiles arranged in a triangular grid, each representing a different ratio of the three components. This format is particularly useful when exploring a new base glaze family or testing ash combinations, because it reveals not only the range but also unexpected interactions at specific ratios.
Grid tests — sometimes called biaxial blends — are a two-dimensional extension of the line blend. A 5×5 grid varies one material along the horizontal axis and another along the vertical, producing 25 test tiles that cover a wide range of compositions in a single firing. This is an efficient format for colourant testing, where both the base glaze silica:alumina ratio and the iron content, for example, need to be varied independently.
Reading Test Results
A glaze test tile tells a limited story if read in isolation, but within a line or grid sequence, it reveals patterns. Key observations for each tile include: surface texture (gloss, satin, matte, dry), surface stability (even coverage vs. crawling, pinholing, or blistering), colour and colour consistency, and melt behaviour on the vertical face (running, static, or underfired).
Pinholes are surface defects in which gas trapped in the glaze melt escapes during cooling but the glaze surface has stiffened too quickly to heal. They are typically a sign of either too-rapid cooling through the glaze maturation temperature, insufficient alumina to increase melt viscosity, or volatile compounds in the clay body that off-gas later than the glaze can accommodate.
The Digitalfire Ceramic Reference Library by Tony Hansen is the most comprehensive freely accessible technical resource on glaze chemistry in studio ceramics and is widely used by Canadian studio potters for both formulation and troubleshooting.
