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Glossary

Vince Pitelka, Glossary of Ceramics Terminologyopen in new window

Lakeside Pottery Ceramic and Pottery Glossaryopen in new window

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Acronyms

Terms

  • Absorbency - the ability of a material to soak up water, for example when applying glaze to bisque ware.
  • Absorption - the quantity of water that fired clay can absorb, indicating the adequacy of firing and suitability for functional use.
  • Acids - substances that form glass by combining with bases (fluxes) and stabilizers.
  • Alkaline - referring to substances that are basic or non-acidic, with many fluxes having an alkaline nature.
  • Alkaline earths - high-fire fluxes including calcium, magnesium, barium, and strontium that promote hard, durable surfaces.
  • Alkaline fluxes - fluxes containing elements like boron, sodium, potassium, and lithium.
  • Alumina - aluminum oxide, a stabilizing agent in clay and glazes.
  • Aluminum silicate - a material group mainly of alumina and silica, found in substances like clay and feldspar.
  • Amorphous - materials without a crystalline structure, such as glass.
  • Annealing - controlled cooling to reduce internal stresses without causing thermal shock.
  • Atmosphere - the gaseous environment within the kiln during the firing process. Can be oxidising (an excess of oxygen) or reducing (a deficiency of oxygen). Influences the fired colour of bodies and glazes. Common atmospheres are reduction, oxidation, neutral, wood, salt and soda.
  • Atmospheric firing - firing where the kiln atmosphere deposits a glaze, as in salt or soda-firing.
  • Back pressure - internal pressure in a fuel kiln, balancing combustion and damper setting.
  • Ball clay - a secondary clay. Ball clays invariably also contain various non-clay minerals, and sometimes organic matter. They commonly exhibit high plasticity and high dry strength.
  • Ball mill - a cylindrical grinder used to grind, or mill, raw materials for use in ceramic bodies or glazes. Size reduction of the feed materials is achieved by a combination of impact and attrition resulting from the tumbling of hard media, such as pebbles, inside the mill during the rotation of the mill. Ceramic raw materials which are commonly milled include quartz, feldspar and calcined alumina. Ball mills can also be used to mix ceramic bodies.
  • Basalt body; basalt ware - clay body resulting in dark brown or black appearance when fired.
  • Bases - fluxes in glaze chemistry that combine with acids and neutrals.
  • Bentonite - an extremely plastic clay rich in montmorillonite which can be added in small quantities to clays or clay bodies to increase plasticity.
  • Bisque, biscuit - pottery that has been fired but not yet glazed.
  • Bisque-firing - initial firing that hardens clay without vitrifying, preparing it for glazing.
  • Blistering - glaze defect causing bubbles and sharp craters, preventable with proper firing.
  • Bloating - a clay defect creating surface lumps, often due to trapped carbon or over-firing.
  • Body reduction - early reduction atmosphere in firing, affecting the clay body.
  • Bone ash - calcined animal bone used in the production of bone china. Synthetic alternatives are available.
  • Bone china - translucent porcelain containing bone ash.
  • Bone dry - the final stage of greenware dried to a near or fully dry state and ready to be fired. In this state, the article is very fragile, non-plastic and porous.
  • Burnishing - polishing unfired clay using a smooth object.
  • Calcine - to heat a material such that certain temperature dependant changes occur, examples being oxididation, reduction, phase changes or the loss of chemically-bound water. Ceramic raw materials which are calcined include clay, bone and talc.
  • Candling - optional initial stage of firing in which the kiln is very gradually heated to eliminate all water and completely dry the ware.
  • Carbon-coring; black-coring - defect from rapid firing or early reduction, leading to trapped carbon and bloating.
  • Carbon-trapping - effect that traps carbon in glaze, creating shading, often intentional.
  • Chemically-combined water - water within clay and glaze, escaping during firing.
  • Chamois, chammy, shammy - very soft leather used for smoothing surfaces like the lips of pots while forming.
  • China clay - another term for kaolin.
  • China stone - a pottery stone that was formerly mined in cornwall in the uk. Traditionally was used at around 25% in bone china bodies. Also known as cornish stone.
  • Clay - a group of hydrous aluminium phyllosilicate minerals. Often also used to refer to the clay body, which sometimes may only contain small amounts of clay minerals.
  • Clay-glaze interface - junction between clay and glaze, influenced by firing process.
  • Claybody, clay body - the material used to form an article of pottery. Thus a potter might prepare, or order from a supplier, such an amount of earthenware body, stoneware body or porcelain body.
  • Cmc liquid - method of mixing cmc gum into glaze or slip using hot water.
  • Coefficient of expansion - material's expansion or contraction rate with temperature changes, affecting thermal resistance.
  • Combustion - oxidation process of fuel at kindling temperature, releasing sustaining heat.
  • Crawling - a parting and contraction of the glaze on the surface of ceramic ware during drying or firing, resulting in unglazed areas bordered by coalesced glaze. May be caused by uneven glazing, excessive glaze thickness or a greasy substrate.
  • Crazing, crackle - a glaze fault characterised by the cracking of fired glazes and due to high tensile stresses. Can also be caused by the moisture expansion of porous bodies. Sometimes used for decorative effect.
  • Cristobalite - a high-expansion form of silicon dioxide, resulting in low thermal-shock-resistance, formed above 2200°f.
  • Crystalline - solids with a geometric molecular structure and specific melting point, unlike amorphous materials.
  • Cullet - crushed glass used in glazes.
  • Deairing - the removal of entrapped air from a clay, clay body or slurry, often by the application of a vacuum.
  • Deflocculate; deflocculation - to separate agglomerates in a slurry by the addition of small amounts of particular chemicals, and so decrease viscosity. Examples include sodium carbonate and sodium silicate.
  • Devitrification - when a glaze recrystallise during the cooling stage of firing. Results in a fault unless the intention is the formation of a crystalline glaze.
  • Dispersion - movement of dissolved materials towards uniform distribution in glaze melt.
  • Dispersoids - non-dissolving particles in glaze melt, like titanium or air bubbles.
  • Draw-rings; draw trials - clay rings for gauging glaze deposition in vapor-glaze processes.
  • Dunting - significant cracking in kiln-cooling wares from quick cooling or glaze compression.
  • Element - one of over 100 atomic substances comprising all materials on earth.
  • Elements - heating coils in electric kilns.
  • Enamels - synonymous with china paints.
  • Engobe - a slip coating applied to a ceramic body for imparting colour, opacity or other characteristics. Sometimes distinguished from slip by the addition of non-clay materials, and sometimes not. It may subsequently be covered with a glaze.
  • Eutectic - an invariant point on an equilibrium diagram. A mixture of two substances which has the lowest melting point in the whole series of possible compositions.
  • Feldspar - a group of alumino-silicate minerals. After crushing and grinding to give very fine particles are commonly used as fluxes in bodies and glazes.
  • Feldspathic - pertaining to feldspars or similar minerals.
  • Fire clay - coarse, highly refractory clays with low shrinkage and buff color.
  • Firing cone; target cone - cone in a pack signaling the target kiln temperature.
  • Firing down - controlled kiln cooling to foster crystal growth or maintain atmosphere.
  • Firing ramp - kiln heating schedule, including speed, duration, soaking, and cooling periods.
  • Flameware - stove-top heat-resistant ceramics, generally avoided due to risks.
  • Flint - often used synonymously with silica. Calcined flint, crushed then ground to fine particle size. A raw material in various ceramic bodies, used as a filler to attenuate drying shrinkage but it also modifies the fired thermal expansion. Traditionally used in the uk, but has largely been replaced by quartz.
  • Flocculate; flocculation - the opposite of deflocculate. Inducing particle attraction in suspension to alter properties in clays, slips, or glazes.
  • Flux - a material with a comparatively low melting point, and used to promote fusion in a given mixture of raw materials. Examples used in ceramics bodies include feldspar and nepheline syenite.
  • Frit - a product made by quenching and breaking up a glass of a specific composition. Common uses include as components of a glaze or enamel, or for the body of fritware, when it usually mixed with larger quantities of quartz sand.
  • Fritting; fritted - the creation of a frit by melting and grinding ceramic materials.
  • Fusion; fused - the point in glazes where structures dissolve into liquid.
  • Glass - a super-cooled liquid becoming a non-crystalline solid; manipulated through temperature and materials.
  • Glass-former - primary ingredient like silica that combines with fluxes to create glass.
  • Glassy-phase - stage in heating ceramics when glass-formers and fluxes combine to form glass.
  • Glaze - a coating that has been matured to the glassy state on a formed ceramic article, or the material or mixture from which the coating is made. Often consists of a flux, silica and optional modifiers like colorants.
  • Glaze compression - state where the clay body shrinks more than the glaze during firing, affecting strength and causing potential defects.
  • Glaze-firing - process of melting glazes in a kiln to create a glassy surface.
  • Glaze-fit - the compatibility between glaze and clay body for permanent adhesion.
  • Glaze-melt - the melted phase of a glaze during firing.
  • Glaze-resist - technique using resist materials to prevent selective glaze adhesion.
  • Glaze sgraffito - decorative carving in glaze to expose the underlying clay body.
  • Green; greenware - unfired clay articles.
  • Green glazing; raw glazing - applying glaze to unfired pottery for single firing.
  • Green-packing density; gpd - particle size distribution in clay affecting plasticity and shrinkage.
  • Grog - ground fired clay used to reduce shrinkage and improve structure in clay bodies.
  • Guard cone - cone in a pack indicating target temperature exceeded.
  • Gum additives; cmc gum - water-soluble thickeners used in glazes or slips.
  • Gum arabic - a natural gum used as a binder to enable the glaze to increase the adherence to the body. Also, small percentages can be added to bodies with low green strength, such as bone china, to increase their strength prior to firing.
  • Halloysite - a clay mineral used in porcelain and bone china outside north america.
  • Hard-paste - european term for high-fired porcelain which had been fired up to 1400°c in a reducing atmosphere.
  • Heatwork - effectiveness of heat transfer during firing, dependent on various factors. Combined temperature and time.
  • Hepa - industrial filtration systems trapping microscopic particles like silica dust.
  • Highfire, high-fire - firing at high temperatures, used for stoneware and porcelain, typically above cone 8.
  • Hydrometer - instrument to measure specific gravity of liquids, less accurate in glazes.
  • Hydrothermal - related to processes influenced by hot water or steam.
  • Hygroscopic - ability to absorb atmospheric water, observed in plaster or limestone.
  • Inclusions - undissolved particles in glaze melt affecting appearance.
  • Iron oxide - a common oxide in glazes and some clays. The fired colour depends on a number of factors, such as concentration and firing atmosphere.
  • Kaolin; china clay - white or off-white firing kaolinitic clay, although rocks consisting of other kaolinite group minerals may also be called kaolin. Can be either of primary or secondary deposits. A key raw material for many pottery bodies. Also known as china clay.
  • Kaolinite - crystalline clay mineral forming most clays.
  • Kiln furniture - refractory materials inside a kiln for supporting wares.
  • Kiln sitter - automatic shut-off for electric kilns, triggered by a pyrometric cone.
  • Kiln wash - protective coating for kiln shelves, varying by firing techniques.
  • Leather-hard - the condition of a clay or clay body when it has been partially dried, ideal for forming and decorating.
  • Limit formulas - standardized formulas defining the ideal range of oxides in glaze.
  • Line blend - glaze testing through proportional variations between two limits.
  • Loss on ignition; l.O.I. - fraction of material that volatilizes during firing.
  • Lowfire - low-temperature firing, usually below cone-02, for bisque-firing or earthenware glazing.
  • Low-midrange - firing range from cone 01 to 3, often for outdoor earthenware.
  • Luster, lustre - a type of decoration originally developed in persia which leaves a thin layer of metal on the decorated portions of pottery. Has an iridescent appearance. Similar to using gold leaf, it comes as a liquid and is a third firing completed after glaze. Common varieties are pearl, copper, gold, silver, and platinum.
  • Lye - caustic alkaline substance, possibly toxic, found in wood ashes.
  • Macrocrystalline - glaze effect with large, visible crystals on the surface.
  • Maturity; maturing point - firing temperature where clay or glaze reaches intended characteristics.
  • Midrange - glaze-firing range from cone-4 to cone-7, common in electric kilns.
  • Modifiers - substances that alter glaze qualities, not part of the base glaze.
  • Molecule - structure of two or more bonded atoms.
  • Montmorillonite - clays like bentonite, with fine particle size and high shrinkage.
  • Mullite - needle-like crystals in vitrified clay bodies, enhancing strength.
  • Multi-fired - wares fired multiple times for specific surface effects.
  • Neutral atmosphere - neither oxidizing nor reducing environment in a kiln.
  • Neutrals - substances stabilizing the combination of bases and acids in glaze.
  • Non-plastics - non-clay components within a clay body.
  • Opacifier - a group of raw materials used in glazes which, after firing, reduce the transmission of light; to make opaque. Useful to mask the colour of the underlying body. Tin oxide and zircopax re often used as opacifiers.
  • Outgassing - release of gases from clay or glazes during firing.
  • Overfired - when an article has been subject to excessive firing, either time or temperature, such that the physical properties have been adversely affected.
  • Oxide - molecule containing an element combined with oxygen.
  • Oxide wash - coloring blend of oxide and water, sometimes with flux.
  • Oxidize - exposure to oxygen, facilitating oxidation reactions.
  • Oxidation - chemical reaction involving the gain of oxygen atoms.
  • Oxidation cleanup; oxidation soak - short oxidation period in firing for vibrant colors and flaw healing.
  • Oxidation firing - oxygen-rich kiln atmosphere used to oxidize materials.
  • Paper clay - clay with paper pulp, reducing shrinkage and improving joinery.
  • Partial reduction; climbing reduction - slightly reducing kiln atmosphere maintaining reduced effects while raising temperature.
  • Peeling - glaze flaw marked by the contraction and peeling of slip or glaze in firing.
  • Phylosillicates - sheet-like structured minerals including kaolin and mica.
  • Pinholes; pinholing - glaze defect of small openings in fired glaze, often from outgassing.
  • Pit-firing - firing technique involving burying wares in a pit and building a bonfire above.
  • Plasticity - the property of clay that allows it to be manipulated and retain its shape without cracking after the shaping force has been removed.
  • Plasticizers - additives in clay to increase plasticity and dry strength.
  • Platelets - flat, thin particles in clay, associated with its plasticity.
  • Plucking - glaze flaw of remnants adhering to the kiln shelf, often solvable with specific mixtures.
  • Polishing - making unfired clay glossy through gentle rubbing or burnishing.
  • Posts - pillar-like supports for bearing kiln shelves.
  • Primary clays; residual clays - clays at the site of parent rock, pure but coarse.
  • Pyrometer - kiln device that measures temperature, not considering duration or atmosphere.
  • Pyrometric cones - cones that bend after a certain amount of heatwork, standardizing firing assessment.
  • Pyroplastic; pyroplasticity - clay's flexibility when heated beyond red heat.
  • Quartz inversion - thermal expansion and contraction in silica around 1063°f, relevant in cooling.
  • Raku - japanese ware or firing method, inspiring a western process for post-firing smoking.
  • Ram-pressing - shaping clay using an industrial hydraulic ram-press.
  • Raw glazing - also known as green-glazing.
  • Re-crystallization - micro or macrocrystalline structure formation in glaze during cooling.
  • Reduce - act of prompting reduction in fuel kilns.
  • Reducing agent - substance removing oxygen atoms in reduction firing, like hydrogen.
  • Reduction - chemical reaction involving oxygen atom removal from a compound. Firing in an oxygen deficient atmosphere.
  • Reduction cooling - maintaining a light reduction atmosphere in cooling to preserve effects.
  • Reduction firing - firing with inadequate oxygen, altering clay and glaze appearances.
  • Refractory - able to withstand extremely high temperatures.
  • Refractories - elements stabilizing clay and glazes, or materials like firebrick used in kiln construction.
  • Residual ash - surface effects on woodfired wares from settling fly-ash.
  • Resist - substances applied to clay to block slip or glaze adhesion.
  • Rhyolite; rhyolitic - silica-rich extrusive rocks; a source for halloysite clays.
  • “s”-cracks - “s”-shaped cracks in wheel-thrown pots, often from insufficient compression or water.
  • Sagger; saggar - a refractory ceramic box used to protect wares from direct flame, fumes, fuel-ash or cinders during firing.
  • Sagger-firing - firing wares in saggers with chemicals for desired surface effects.
  • Scumming - discoloration on bisque-fired wares from sulfates, treatable with barium carbonate.
  • Secondary clays; deposited clays; sedimentary clays - finer, more plastic clays, transported from origin, containing contaminants.
  • Secondary kaolins - transported kaolins, pure but more plastic than primary kaolins.
  • Shivering - glaze defect of chips detaching from edges; correctable by adjusting content.
  • Short - clay lacking plasticity, prone to breaking during shaping.
  • Shrinkage - permanent clay contraction during drying and firing, up to 18%.
  • Sieve - fine-mesh pan for straining slips and glazes.
  • Sieving - straining glaze or slip through a sieve to remove impurities.
  • Silica; silicon dioxide; flint; quartz - essential in forming glass in clay and glazes.
  • Silicon carbide - refractory material for kiln shelves, not for electric kilns.
  • Single-firing; green-firing - glaze-firing un-bisque-fired greenware.
  • Sintering - particle bonding into a weak mass at around 900°f.
  • Sintered mass - porous bonded particles at contact points.
  • Slaking; slake down - reconstituting dry clay into a slurry by soaking in water.
  • Slip - water-suspended clay, used for decoration or casting.
  • Slip-casting - forming ceramics by pouring deflocculated casting slip in plaster molds.
  • Slumping - firing defect where clay deforms from overfiring or too much flux.
  • Slurry - thick clay slip used to join clay pieces.
  • Soak; soaking - a period during a firing cycle when a set temperature is maintained. The period of time at the maintained temperature is called the soak, hold or dwell.
  • Soft paste porcelain - white claybody with glassy frit, maturing at lower temperatures.
  • Solution - liquid mixture where dissolved components don't settle.
  • Solvent - substance that speeds up the dissolution of a material.
  • Specific gravity - comparative weight of a liquid to water, for measuring.
  • Split - hardbrick half the thickness of a standard brick.
  • Spooze - greenware repair mix of vinegar, corn-syrup, and powdered claybody.
  • Spraying - applying liquid slip or glaze with compressed-air equipment, similar to that for applying paint to cars.
  • Spyhole; peephole - kiln openings for observing firing processes.
  • Stabilizers - materials that structure clays and influence glaze viscosity.
  • Stain - ceramic colorants known for color accuracy, range, and stability.
  • Stoneware - ware fired to 2200-2400°f, with stone-like hardness.
  • Stoneware clay - clay fired within the stoneware temperature range.
  • Suspension - liquid mixture with evenly distributed, insoluble particles.
  • Terracotta - durable, low-temperature clay, firing to red-brown.
  • Terra sigillata - refined clay slip that can be polished to a sheen or gloss.
  • Thermal expansion - expansion and contraction with temperature changes.
  • Thermal shock - high stress in an article due to the sudden creation of a large thermal gradient. If the stress is sufficiently high it can result in the formation of a crack, such as a dunt.
  • Thermal-shock resistance - ability to withstand thermal shock effects.
  • Thermocouple - a probe generating electrical currents based on heat level, used with specific devices.
  • Triaxial blend - testing method for three-way glaze combinations, varying proportions in samples.
  • Trimming - removal of surplus clay at the leather-hard stage with cutting tools.
  • Tube burner - tube-shaped atmospheric burner without venturi effect.
  • Tube kiln - korean kiln design with a long sloping tubular firing chamber.
  • Tube-lining - decoration using thick slip trailed from a squeeze bulb, akin to stained glass.
  • Undercut - defect in molds where clay becomes trapped, risking breakage.
  • Underfired - when an article has been subject to insufficient firing, either time or temperature, such that the physical properties have been adversely affected.
  • Underglaze - commercially prepared engobe.
  • Underglaze decoration - decorating bare clay before glazing.
  • Underglaze pencils - engobe pigments in pencil form for marking or drawing.
  • Unity formula; seger formula - glaze calculation technique categorizing oxides by function.
  • Updraft - kiln configuration where exhaust exits through a roof flue.
  • Vapor glazing - glaze derived from atmospheric firing processes like salt-glazing.
  • Venturi burner - natural-draft gas burner with restricted tube design to enhance mixing.
  • Viscosity; viscous - description of material's resistance to flow; more viscous means less flow.
  • Vitreous; vitrified; vitrification - process by which ceramic raw materials bond to become non-permeable after firing.
  • Volcanic ash - fine volcanic particles used in glazes or bentonite clays.
  • Volatilization; volatilize - conversion to gas during firing, causing outgassing.
  • Wads; wadding - refractory clay mixtures preventing adhesion in atmospheric firing.
  • Warning cone - cone signaling proximity to target temperature in firing.
  • Warping - clay distortion due to uneven stresses, drying, or heating.
  • Water of plasticity - water needed to bring dry clay to ideal plasticity.
  • Water smoking stage - bisque-firing stage when compounds volatilize and outgas.
  • Wax resist - wax used in resist processes for decorative effects.
  • Wedging - kneading clay to remove air and create uniform mass.
  • Whiteware - white clay bodies fired below porcelain temperatures.
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