Glossary

Links

Acronyms

CoTE – Coefficient of Thermal Expansion. See also CTE.
CTE – Calculated Thermal Expansion . Refers to a glaze or clay’s expansion/contraction rate with temperature changes.
EPS – In some ceramics contexts, can refer to Expanded Polystyrene used for certain molds or forms. Less common, always clarify by context.
EPK – Edgar Plastic Kaolin, a common, relatively pure kaolin clay.
LOI – Loss on Ignition. The percentage of mass lost due to release of water vapor and gases when a material is heated under specified conditions.
MSH – Mohs Scale of Hardness .
M.S.D.S. (Material Safety Data Sheet) – Document detailing toxicity information for ceramic materials, often supplied by vendors.
NZK – New Zealand Halloysite .
OM4 – “Old Mine #4” ball clay, widely used in North America.
RIO – Red Iron Oxide .
SG – Specific Gravity. In ceramics, the ratio of a glaze’s weight to the weight of an equal volume of water, used to control glaze consistency.
WCI – Water Column Inches. A measurement of gas pressure (e.g., for gas kilns).
YIO – Yellow Iron Oxide .

Terms

Absorbency - The ability of a material (like bisque-fired clay) to soak up water. In glazing, absorbency helps the glaze slurry adhere to bisqueware.
Absorption - The measured amount of water that a fired clay can take in, expressed as a percentage of its dry weight. Lower absorption typically indicates a higher firing temperature or a more vitreous clay body.
Acids - In glaze chemistry, acids are typically silica-rich or boron-based materials that combine with bases (fluxes) and neutrals (stabilizers) to form a glass.
Alkaline / Alkaline Fluxes - Refers to substances (often containing sodium, potassium, lithium, or boron) that lower the melting temperature of silica in glaze chemistry. Alkaline fluxes typically lead to bright, fluid glazes. Example: soda feldspar is an alkaline flux source.
Alkaline Earths - Fluxes in high-temperature glazes that include calcium, magnesium, barium, and strontium. They promote durable surfaces and influence color responses in high-fire glazes.
Alumina - Aluminum oxide (Al₂O₃). A key stabilizer in clay bodies and glazes, making them hard and durable. Sourced from clays like kaolin or ball clay.
Aluminum Silicate - A major component of many clay minerals (e.g., feldspar, kaolin). It contributes both alumina and silica to ceramic bodies and glazes.
Amorphous - Describes materials lacking a crystalline structure, such as glass.
Annealing - Controlled cooling of glass or glaze to reduce internal stress. In ceramics, “annealing” is most relevant for glass-forming processes or specialized glazes with large crystalline phases.
Atmosphere - The atmosphere inside a kiln during firing—oxidizing, reducing, neutral, or specialty (salt, soda, wood). It affects clay body color and glaze results.
Atmospheric Firing - Any firing method where the kiln atmosphere itself deposits or modifies the glaze—e.g., salt, soda, or wood-firing.
Back Pressure - In fuel-fired kilns, the internal pressure balancing combustion airflow and damper settings.
Ball Clay - A secondary clay with very fine particle size, high plasticity, and relatively high organic content. Used in clay bodies and glazes to improve workability, suspension, and dry strength.
Ball Mill - A rotating drum that grinds ceramic raw materials into fine powders. Commonly used for feldspar, quartz, etc.
Basalt Body (Basalt Ware) - A clay body that fires to a dark brown or black color, reminiscent of basalt rock.
Bases - In glaze chemistry, these are flux oxides (e.g., Na₂O, K₂O, CaO) that react with silica (acids) and alumina (stabilizers).
Bentonite - An extremely plastic clay (rich in montmorillonite). Added in small percentages (1–3%) to clay bodies or glazes to increase plasticity and help suspend glaze ingredients.
Bisque (Bisque Firing) - Clay that has been fired once at a lower temperature to make it porous yet structurally stronger than bone-dry clay. Bisque ware readily accepts glaze. Tip: It’s crucial that pieces reach bone dry before bisque firing to avoid moisture-related explosions.
Blistering - A glaze defect creating bubbles or craters, often from trapped gases or overly rapid firing.
Bloating - Clay defect causing blisters or bulges on a pot’s surface, typically from trapped carbon or sulfur during firing or overfiring a body.
Body Reduction - In reduction firing, the early introduction of a reduced atmosphere affects the clay body’s color (e.g., removing oxygen from iron).
Bone Ash - Calcined animal bone (or synthetic equivalent), used in bone china or as a glaze ingredient.
Bone China - Translucent porcelain containing bone ash; typically very white and strong at lower porcelain-range temperatures.
Bone Dry - A completely dried state of unfired clay. No physical water remains, and the piece is extremely fragile. Necessary before firing to avoid steam explosions.
Burnishing - Polishing the surface of unfired clay or terra sigillata to a sheen using a smooth tool or cloth.
Calcine - Heating a raw material to drive off chemically bound water, burn out impurities, or change phases (e.g., calcining kaolin, bone, or talc).
Calcium Carbonate (Whiting) – Supplies CaO flux in high-fire glazes, can encourage a matte or stable surface.
Candling - A very slow, low-temperature preheat to remove residual water. Reduces risk of cracking or steam explosions.
Carbon-Coring (Black-Coring) - A firing defect in dense clay bodies trapped in early reduction or from too-rapid firing. Carbon remains inside the clay, causing bloating or black coloration.
Carbon-Trapping - An intentional or unintentional effect in certain reduction or soda firings, where carbon is trapped in the glaze, creating smoky or streaky patterns.
Chemically-Combined Water - Molecular water bonded within clay or glaze materials, released during firing.
Chamois (Chammy, Shammy) - Soft leather used to smooth rims or other surfaces on wheel-thrown pots.
China Clay - Another term for kaolin (a pure, white-firing primary clay).
China Stone (Cornish Stone) - A traditional British pottery stone mined in Cornwall, used historically in bone china and certain porcelain formulations.
Clay - A group of hydrous aluminum phyllosilicate minerals that become plastic when wet and harden permanently when fired. In practice, “clay” often refers to both the mineral and any workable mixture containing it.
Clay Body - A formulated mixture of clays and other materials (grogs, fluxes, fillers) chosen for specific firing ranges or working properties (e.g., earthenware, stoneware, porcelain).
Clay-Glaze Interface - Zone where glaze and clay body merge during firing. Good interface helps glaze adhesion and durability.
CMC Liquid - Carboxymethyl Cellulose gum solution added to glazes or slips for better brushability and suspension.
Cobalt Oxide/Carbonate – Powerful blue colorant (very little required).
Coefficient of Expansion - See CTE. A measure of how much a material expands with temperature changes—critical for matching glaze to clay body.
Combustion - Oxidation reaction of fuel releasing heat. In ceramics, controlling combustion affects kiln atmosphere and temperature rise.
Cone (e.g., Cone 6, Cone 10) - Pyrometric cones used to measure “heatwork” in a kiln. Each cone number corresponds to a specific softening/bending temperature range when factoring time + temperature. Also used as shorthand for firing ranges (e.g., “fire to Cone 6”).
Copper Oxide/Carbonate – Produces green in oxidation, red in reduction.
Crawling - A glaze defect where the glaze retracts and leaves bare patches. Often from a too-thick application, dust/grease on the bisque, or high clay content in the glaze.
Crazing (Crackle) - A fine network of cracks in the fired glaze, usually due to a mismatch of thermal expansion (glaze shrinks more than the clay). Sometimes done intentionally as a decorative “crackle glaze.”
Cristobalite - A high-temperature polymorph of silica (forming above ~2200 °F). It has a high expansion rate and can cause fit issues or cracking upon cooling.
Crystalline - Solid matter with a repeating, ordered molecular structure (e.g., quartz). Contrasts with amorphous solids like glass.
Cullet - Crushed or recycled glass used in glazes as a source of silica.
Deairing - Removing entrapped air from clay or slips, often via vacuum pugmills to improve consistency and reduce bloating.
Deflocculate (Deflocculation) - Adding small amounts of chemicals (e.g., sodium silicate, soda ash) to break up agglomerated particles, lowering viscosity (common in casting slips).
Devitrification - Unwanted crystallization in a cooling glaze, often creating a dull or foggy surface. Intentional devitrification can produce crystalline glazes.
Dispersion / Dispersoids - In molten glazes, dispersoids are undissolved particles (e.g., titanium inclusions). Uniform dispersion helps achieve consistent color/texture.
Draw-Rings (Draw Trials) - Test rings used in vapor glazing (salt/soda/wood) to observe deposit levels in real-time.
Dunting - Serious cracking from thermal shock in the cooling cycle or from mismatch stresses.
Earthenware - A low-fire clay body (~Cone 06–02) that remains somewhat porous without a glaze. Often red or buff due to iron content. Common for terracotta flower pots, tiles, or folk pottery.
Element - An electric kiln’s heating coil made of high-temperature wire alloys. Also generically refers to an essential chemical “element” in the periodic table.
Enamels (China Paints) - Low-fire overglaze decorations that fuse at relatively low temperatures onto an already-glazed piece.
Engobe - A slip-like coating—often pigmented—that’s applied to leather-hard ware to alter color or surface quality. Engobes typically have more non-clay materials than a simple slip.
Eutectic - In ceramics, a mixture of oxides or materials whose combined melting point is lower than that of any individual constituent.
Feldspar - A major multi-oxide flux source in clay bodies and glazes. Potash and soda feldspars provide silica, alumina, and flux (K₂O or Na₂O).
Feldspathic - Anything derived from or containing feldspar.
Fire Clay - Coarse, highly refractory clay with minimal impurities. Often used in kiln furniture and high-fire bodies.
Firing - Heating ceramic ware in a kiln. Includes bisque firing (lower temp) and glaze firing (higher temp), or single firing.
Firing Cone (Target Cone) - A specific cone placed to indicate the desired “final” temperature or heatwork. Often placed in a kiln-shelf cone pack.
Firing Down - Control of kiln temperature decrease (cooling) to promote crystal growth or maintain atmosphere (used in certain glazes).
Firing Ramp - A schedule for how quickly the kiln heats (and sometimes cools), including any holds/soaks at particular temperatures.
Flameware - Ceramic ware formulated for direct-flame or stovetop cooking. Challenging to produce without thermal shock failures.
Flint - Historically, calcined and ground silica (quartz). “Flint” is often used interchangeably with silica in older UK pottery contexts.
Flocculate (Flocculation) - Opposite of deflocculation; causes particles in a slurry to clump, often used to thicken glazes or slips deliberately.
Flux - An oxide (or raw material) that lowers the melting temperature of silica. Common fluxes: sodium, potassium, calcium, lithium. Found in feldspar, whiting, frits, etc.
Frit - A manufactured, premelted combination of minerals, rapidly cooled and ground into a powder. Improves consistency and solubility in glaze formulas.
Fusion - In ceramics, the melting or flowing together of materials (e.g. silica, fluxes) to form glass.
Glass - An amorphous solid formed by cooling a molten mixture—silica-based in ceramics.
Glass-Former - Typically silica (SiO₂), the main structural component of most glazes.
Glassy-Phase - The molten, amorphous portion of a clay or glaze above certain temperatures that solidifies into glass as it cools.
Glaze - A vitreous coating applied to ceramics, melting during firing into a glass-like surface for decoration and function (e.g., waterproofing).
Glaze Compression - Occurs when the fired clay body shrinks more than the glaze, putting the glaze in compression. Moderate compression can enhance strength, but too much causes shivering.
Glaze-Firing - Firing at the temperature needed for the glaze to mature (melt). Can be oxidation, reduction, salt, etc., depending on the kiln and the glaze type.
Glaze-Fit - Compatibility between glaze and clay body expansions. Mismatch can cause crazing or shivering.
Glaze-Melt - The liquid phase of glaze at high temperature. Viscosity depends on the ratio of fluxes, silica, and alumina.
Glaze-Resist - Decoration technique using wax, latex, or other resist mediums to block glaze from selected areas.
Glaze Sgraffito - A decorative technique where the glaze is carved or scratched to reveal the clay body underneath.
Green / Greenware - Unfired clay objects, from wet to bone dry. Often subdivided into plastic, leather-hard, and bone-dry stages.
Green Glazing (Raw Glazing) - Applying glaze to unfired clay (greenware) and performing a single firing.
Green-Packing Density (GPD) - The particle packing density in a wet (green) clay, which influences workability, shrinkage, and strength.
Grog - Fired and crushed clay (or other ceramic material) added to a clay body to reduce shrinkage, improve structure, and increase thermal shock resistance.
Guard Cone - A cone with a higher rating placed to show if the kiln exceeds the intended temperature range.
Gum Additives (CMC Gum) - Binders that improve glaze adhesion to bisque, reduce dusting, and help with brushing.
Gum Arabic - A natural gum binder. Can increase green strength in fragile clays (like bone china) and enhance glaze adhesion.
Halloysite - A kaolin-like clay mineral. New Zealand Halloysite is known for its whiteness and translucency in porcelains and bone china.
Hand-Building - Forming clay without a wheel (e.g., pinching, coiling, slab-building). Allows shapes that might be difficult to throw.
Hard-Paste (Porcelain) - Typically European porcelain fired to very high temperatures (around 1400 °C) in a reducing atmosphere. Very dense and non-porous.
Heatwork - A combination of temperature and time. Pyrometric cones measure heatwork rather than just temperature.
HEPA - High-Efficiency Particulate Air filtration—used in studios to capture fine silica dust.
High-Fire - Firing ~Cone 8–10 (2300 °F and higher), typical for stoneware or true porcelain.
Hydrometer - Tool measuring specific gravity of liquids; can be used in glazes but is often less accurate than weighing out water vs. total volume.
Hygroscopic - Readily absorbing moisture from the air (e.g., plaster).
Inclusions - Undissolved particles in a glaze melt that give visual effects (e.g., specks from iron or rutile).
Iron Oxide - Common colorant in clays and glazes. Produces earth tones, reds, browns in oxidation; light blues, greens or grey in reduction (see also RIO).
Kaolin (China Clay) - A primary, white-firing clay with high alumina content. The basis of porcelain bodies.
Kaolinite - The crystalline clay mineral that makes up most kaolin deposits.
Kiln - A furnace for firing ceramics, built with refractory materials. May be electric, gas, wood, etc.
Kiln Furniture - Refractory shelves, posts, and supports inside a kiln.
Kiln Sitter - A mechanical device that uses a small pyrometric cone to trigger kiln shut-off at a certain temperature.
Kiln Wash - Protective coating (often a mix of kaolin and silica) applied to kiln shelves to prevent glaze drips from fusing ware to the shelf.
Leather-Hard - A semi-dry stage where clay is stiff yet slightly moist. Ideal for trimming, handle attachment, sgraffito, etc.
Limit Formulas - Guidelines defining acceptable oxide ranges in balanced glaze recipes.
Line Blend - A sequential glaze test method, varying one ingredient proportionally between two extremes.
Low-Fire - Firing typically below Cone 01–02 (~2000 °F) for earthenware or bisque.
Low-Midrange - Cone 01–3. Sometimes used for earthenware or specialized bodies.
Luster (Lustre) - Overglaze decoration leaving a thin metallic layer (e.g., gold, copper, pearl). Fired at low temperatures in a third firing.
Lye - Caustic alkali (sodium hydroxide) found in wood ash, can be corrosive/toxic.
Macrocrystalline - A glaze effect producing large, visible crystals. Often involves specific cooling cycles in zinc- or titanium-bearing glazes.
Maturity (Maturing Point) - Temperature or cone at which a clay body or glaze develops the desired properties (vitrification, gloss, etc.).
Midrange - Cone 4–7 (~2100–2200 °F). Common for many electric kiln stoneware glazes.
Modifiers - Glaze additives that alter color, opacity, surface texture, or flow (e.g., colorants, opacifiers).
Montmorillonite - A group of extremely fine-particle clays (like bentonite) known for high plasticity and shrinkage.
Mullite - Needle-like crystals formed in fully vitrified stoneware and porcelain, adding strength.
Multi-Fired - Ware that undergoes multiple firings (e.g., bisque, glaze, luster/overglaze).
Neutral Atmosphere - Kiln atmosphere neither oxidizing nor reducing.
Neutrals - In glaze chemistry, stabilizers like alumina that help balance the reaction between acids and bases.
Non-Plastics - Inert fillers or grogs in a clay body that reduce shrinkage and control structure.
Opacifier - Materials (e.g., tin oxide, zircon, titanium) that make glazes opaque.
Outgassing - Gas release from clay/glaze during firing. Controlled ramps help avoid pinholes.
Overfired - When ware is exposed to excessive heat/time, leading to warping, slumping, or overmelted glazes.
Oxide - A compound of oxygen with another element (e.g., Fe₂O₃, Al₂O₃). Essential in glaze chemistry.
Oxide Wash - A mix of metal oxide + water (and sometimes flux) brushed on clay or bisque for decorative effects.
Oxidation (Oxidation Firing) - An oxygen-rich atmosphere (typical in electric kilns) that yields bright, consistent colors in glazes and clay.
Paper Clay - Clay mixed with paper pulp or cellulose fibers for reduced shrinkage and stronger joins in the green state.
Partial Reduction (Climbing Reduction) - A slightly reducing atmosphere at certain stages of firing to achieve desired surfaces or color transitions.
Peeling - When glaze detaches in flakes (often near edges). Usually from poor adhesion or excessive shrinkage mismatch.
Phyllosilicates - Sheet-like structured minerals (e.g., kaolinite, montmorillonite).
Pinholes - Tiny holes in the fired glaze surface, often from outgassing or insufficient soak at peak temperature.
Pit-Firing - An ancient firing method placing ware in a pit with combustible materials (wood, sawdust).
Plasticity - Clay’s ability to be shaped without cracking. Related to clay particle size and water content.
Plasticizers - Additives (like bentonite) that improve clay’s plasticity and binding capacity.
Platelets - Thin, flat clay particles whose alignment affects plasticity.
Plucking - Glaze sticking to the kiln shelf, causing small chips on the foot or shelf. Often solved by adjusted kiln wash or changing foot ring designs.
Polishing - Smoothing or lightly burnishing greenware or bisque for a softer finish.
Posts - Vertical supports that hold kiln shelves.
Porcelain - A high-fire, white, often translucent, vitreous clay body, rich in kaolin. Noted for strength, purity, and a characteristic “ring” when tapped.
Porosity - Measure of a fired ceramic’s ability to absorb water. Earthenware is more porous; porcelain is nearly vitreous.
Primary Clays (Residual Clays) - Clays found at the site of parent rock (kaolin). Usually coarser but purer.
Pyrometer - Instrument measuring kiln temperature, though it doesn’t account for duration and other variables (unlike cones).
Pyrometric Cones - Triangular test pieces formulated to melt at certain heatwork levels, indicating kiln maturity.
Pyroplastic (Pyroplasticity) - Clay’s ability to deform when hot enough (beyond red heat). Overfired thin forms may slump or warp.
Quartz Inversion - The abrupt expansion/contraction of silica (quartz) at ~1063 °F (573 °C). Rapid crossing of this range can cause dunting.
Raku - A rapid, low-fire technique derived from Japanese traditions, involving post-firing reduction in combustible materials.
Ram-Pressing - Industrial method shaping clay with hydraulic pressure against a mold.
Raw Glazing (Green Glazing) - Applying glaze to unfired (green) clay for a single firing cycle.
Re-Crystallization - Formation of crystalline structures in the glaze during cooling—often deliberate in crystal glazes.
Reduce (Reduction) - To starve a kiln atmosphere of oxygen, causing chemical changes in clays and glazes.
Reducing Agent - A substance (e.g., unburnt fuel) that removes oxygen from compounds in the kiln atmosphere.
Reduction Cooling - Maintaining a slightly reduced atmosphere during cool-down for specific color or surface effects.
Reduction Firing - Firing with limited oxygen (typical of gas or wood kilns). Yields special effects like copper reds, celedons, shinos.
Refractory - Able to withstand high temperatures without melting or degrading (e.g., kiln shelves, bricks).
Residual Ash - Residue from wood firing that settles on wares, forming natural ash glaze or deposits.
Resist - Any substance (wax, latex, paper, etc.) that masks areas from slip or glaze.
Rhyolite (Rhyolitic) - Silica-rich volcanic rock sometimes associated with halloysite deposits.
“S”-Cracks - Throwing cracks on the pot’s bottom shaped like an S, often from inadequate compression on the wheel.
Saggar - A refractory container shielding ware from direct flame or atmospheric effects in the kiln.
Saggar-Firing - Firing wares sealed inside a saggar with various combustible or color-inducing materials.
Score and Slip - Technique to join separate pieces of clay: scratch (score) both surfaces, apply slip, and press together for a solid bond.
Scumming - White crystalline deposits on bisqueware (commonly sulfates). Treat with barium carbonate or adjust water chemistry.
Secondary Clays (Deposited/Sedimentary) - Clays transported from their original site, typically finer and more plastic (e.g., ball clay).
Secondary Kaolins - Kaolin deposits moved and redeposited—still relatively pure but typically more plastic.
Shivering - Glaze defect where small flakes shear away from edges, caused by a too-low expansion glaze or excessive compression.
Short - Describes clay lacking plasticity. Tends to crack easily during forming.
Shrinkage - Reduction in size of clay from wet to fired states (can be up to ~18%). Controlled by clay composition and process.
Sieve / Sieving - Straining glazes or slips through a mesh to remove lumps or contaminants.
Silica (SiO₂) - Essential glass-former for glazes; also in clay bodies for structure. Known as quartz or flint in ceramic contexts.
Silicon Carbide - Very refractory compound used for kiln shelves or other specialized applications (often avoided in electric kilns due to conductivity).
Single-Firing (Green-Firing) - Combining the bisque and glaze firing into one process, applying glaze onto unfired ware.
Sintering - Initial stage of particle bonding in ceramics, occurring even before full vitrification.
Slaking (Slake Down) - Rehydrating dry or scrap clay by soaking in water until it disintegrates into slurry.
Slip - A fluid suspension of clay in water, used for casting, decorating, or joining.
Slip-Casting - Forming shapes by pouring deflocculated slip into a plaster mold. Plaster absorbs water, leaving a clay layer.
Slumping - Sagging or losing shape when clay becomes pyroplastic (overfired or too fluxed).
Slurry - A thick slip used to join or patch clay at the same stage of dryness.
Soak (Soaking) - Holding a kiln at a set temperature for a period, allowing even heat or glaze healing.
Soft-Paste Porcelain - Lower-temperature “porcelain,” historically made with a frit-based body that matures below the highest porcelain range.
Solution - A fully dissolved mixture. In ceramics, certain soluble salts or colorants can affect surfaces if they re-crystallize upon drying.
Solvent - A liquid that dissolves certain substances. In ceramics, typically just water, though organic binders may act similarly.
Specific Gravity (SG) - Ratio of a glaze/slip’s density to water. Helps standardize glaze thickness and application.
Split - A kiln brick half the usual thickness.
Spooze - A greenware repair slip of vinegar, corn syrup, and powdered clay. Helps fuse small cracks or reattach broken bits before firing.
Spraying - Applying slip or glaze with compressed air. Allows even coverage or layering of multiple glazes.
Spyhole (Peephole) - An opening in a kiln wall or door for visual inspection or reading cones.
Stabilizers - Oxides like alumina that help strengthen the silica network in glazes, raising viscosity and adding durability.
Stain - Commercially manufactured ceramic colorants, more stable and consistent than raw oxides.
Stoneware - A mid-fire or high-fire clay body (~Cone 5–10) that becomes vitreous or nearly so, often buff, gray, or brown. Durable for functional ware.
Suspension - A liquid mixture (like a glaze) with insoluble particles evenly dispersed. Flocculation/deflocculation affect stability.
Terracotta - Low-fired, iron-bearing clay that fires to a characteristic red-brown color. Porous unless glazed.
Terra Sigillata - A very refined slip that can be polished to a silky or glossy sheen at low temperatures.
Thermal Expansion - Changes in volume with temperature fluctuations. Key factor in preventing glaze fit issues.
Thermal Shock - Rapid temperature change causing stress (and possibly cracking). Slow heating/cooling reduces the risk of dunting.
Thermal-Shock Resistance - How well a ceramic tolerates sudden changes in temperature. Some specialized flameware or grogged clay bodies have better resistance.
Thermocouple - A kiln sensor measuring temperature via voltage differences in two alloy wires.
Throwing - Forming clay on a potter’s wheel. Involves centering, opening, and pulling walls.
Titanium Dioxide (TiO₂) – Opacifier, can promote crystalline or variegated glaze effects.
Triaxial Blend - Test method exploring three-component mixes (like clay-flux-silica) across multiple samples.
Trimming - Removing excess clay from a wheel-thrown piece at the leather-hard stage. Creates foot rings, refines shape.
Tube Burner - A cylindrical atmospheric burner commonly used in gas kilns.
Tube Kiln - A long, sloping, tube-like wood or soda kiln design found historically in some Korean traditions.
Tube-Lining - Piping thick slip (like icing) for decoration, reminiscent of stained glass outlines.
Undercut - In mold-making, an area preventing easy release of the casting or clay form.
Underfired - Ware not fully matured or glazes not melted sufficiently, leading to weak, porous clay or matte, unstable glazes.
Underglaze - Colored slip or oxide decoration applied to leather-hard or bisque ware, typically covered by a clear glaze.
Underglaze Pencils - Pencils made of ceramic pigments for drawing on bisque. Fused permanently once glazed and fired.
Unity Formula (Seger Formula) - A way to express glaze recipes in molar oxide ratios. Guides the balance of fluxes, stabilizers, and glass-formers.
Updraft - A kiln where hot gases exit at the top, drawing flame/heat upward.
Vapor Glazing - Glaze derived from atmospheric interactions with salt, soda, or wood ash rather than an applied glaze layer.
Venturi Burner - Gas burner using constricted airflow (venturi effect) to mix fuel and air without a blower.
Viscosity - Resistance to flow. In molten glazes, lower viscosity means more fluid melting; higher viscosity means stiffer melt.
Vitreous / Vitrified / Vitrification - The formation of a glassy, non-porous structure in clay or glaze. A fully vitrified body has minimal water absorption.
Volcanic Ash - Fine pyroclastic material used in certain glazes or as a source of clay (e.g., bentonite formation).
Volatilization - Conversion of materials to gas during firing, potentially creating outgassing issues (pinholes, bubbles).
Wads (Wadding) - Refractory lumps placed between ware and shelves in wood or salt firing to prevent sticking.
Warning Cone - A slightly lower cone indicating approach to the target temperature.
Warping - Distortion of clay shape during drying or firing, often from uneven stresses or overfiring.
Water of Plasticity - The water content in clay that allows plastic deformation without cracking.
Water Smoking Stage - Early bisque-firing phase where mechanical and pore water is driven off.
Wax Resist - Wax applied to repel glaze or slip in decorative patterns. Also used on foot rings to keep them free of glaze.
Wedging - Kneading or cutting-and-recompressing clay to remove air pockets and homogenize moisture/content.
Whiteware - Generic term for lighter-fired ceramic bodies (often earthenware) that are not high-fire porcelain.
Zinc Oxide (ZnO) – Flux in mid-fire oxidation glazes; can assist in crystal glazes.