Glossary

The primary intermediate oxide in glazes. Stiffens the melt, improves durability, and raises viscosity. Reported as the R₂O₃ component in UMF. Generally a stabilizing addition; deficiency is associated with running, crawling, and reduced durability.

A glaze formula expressed as parts by weight of raw materials before firing. Batch weight does not equal fired-weight chemistry because materials lose mass (LOI) on heating. See also: UMF, LOI.

Coefficient of Thermal Expansion. A material property describing how much a material expands or contracts per degree of temperature change. Measured values require dilatometry. Calculated COE from oxide sums is an approximation. Mismatch between glaze and clay body COE causes crazing or shivering.

A standardized measure of firing maturity defined by heatwork (temperature × time). Pyrometric cones deform at a defined maturity level. Common ceramic ranges: cone 06–04 (earthenware ~1000°C), cone 6 (stoneware ~1220°C), cone 10 (high-fire ~1300°C).

A network of fine cracks in the glaze surface. Caused by glaze having a higher COE than the clay body—the glaze contracts more than the body on cooling and goes into tension. Crazing is a food-contact risk factor on functional ware. Mitigated by lowering alkali flux or increasing silica.

The crystallization of glass during cooling. Produces a matte, rough, or cloudy surface that was not intended. Can indicate over-silica or calcium-heavy formulations at certain cooling rates.

An oxide that lowers glaze melt temperature. In UMF, fluxes occupy the RO and R₂O positions. Common fluxes: CaO, MgO, ZnO (RO group) and K₂O, Na₂O, Li₂O (R₂O group). Alkali fluxes increase thermal expansion; alkaline-earth fluxes are generally lower-expansion and higher durability.

Release of metal oxides from a fired glaze into food or liquid. Common concerns involve lead (Pb), barium (Ba), and lithium (Li) at food-contact surfaces. Measured by standardized leach tests (ASTM C738, EN 1388). Ceramic Stability flags risk factors but does not certify leach safety. Only laboratory testing can confirm.

Published ranges for UMF oxide values considered typical for stable glazes at a given cone. Useful as initial screening heuristics. Not a physical model; does not account for interactions, firing schedule, or clay body. Ceramic Stability uses limit formulas as one evidence source among several.

Loss on Ignition. The percentage of a raw material's mass lost as gas (CO₂, H₂O, SO₃, etc.) during firing. Must be removed before calculating fired-weight oxide contribution. High-LOI materials (whiting, carbonates, clays) require correction. Errors in LOI assumptions propagate into oxide calculations.

Oxide that forms the glass network. In glazes: silica (SiO₂) is the primary former. Boron (B₂O₃) acts as a network former at higher concentrations. Reported as RO₂ in UMF. Higher silica generally improves durability and lowers thermal expansion.

Small holes in the fired glaze surface caused by gas escaping from the body or glaze after the surface has sealed. Related to under-heatwork, fast firing, high clay content in glaze, or sulfur compounds. Distinct from crawling.

Oxide groupings in UMF notation. R₂O: monovalent alkali fluxes (K₂O, Na₂O, Li₂O). RO: divalent alkaline-earth fluxes (CaO, MgO, ZnO, BaO, SrO). R₂O₃: trivalent intermediates (Al₂O₃, Fe₂O₃, B₂O₃ in some systems). RO₂: network formers (SiO₂, TiO₂, ZrO₂).

Glaze spalling off the clay body in chips or flakes. The opposite failure mode to crazing. Caused by glaze having lower COE than the body—glaze goes into compression. Can produce sharp fragments. More dangerous than crazing on food-contact ware. Mitigated by increasing alkali flux or reducing silica carefully.

Silica-to-alumina molar ratio in UMF. A key proxy for glaze durability, hardness, and surface character. Ranges from ~5:1 (soft, matte-leaning) to ~10:1+ (glassy, durable). Low Si:Al associated with reduced chemical and mechanical durability.

The seven risk dimensions used in Ceramic Stability analysis: chemical (oxide durability), thermal (fit, expansion), mechanical (hardness, scratch), visual (color, surface), process (application, firing), supply (material sourcing), and regulatory (compliance context).

Unity Molecular Formula. A method of expressing glaze chemistry as molar ratios with fluxes normalized to 1.00. Enables comparison across recipes regardless of batch scale. Developed by Hermann Seger in the 19th century. Does not capture all relevant glaze properties (particle size, suspension, texture, application thickness).