The ability of a fired clay to absorb water. Used as a gauge of vitrification.

In clay and glaze chemistry, the glass-formers, which combine with bases (fluxes) and neutrals (refractories or stabilizers).

A wood-extract product used as a clay additive to give greater plasticity, increase dry strength, and improve workability. Especially effective in bodies with a high percentage of kaolins. Does not change color of clay.

Wares featuring swirling marbleized colors, resulting either from surface slip effects or marbleized colored clays.

Industrial method used in processing of raw clays, where powdered clay is floated in an airstream to settle out heavier particles.

Castable refractory or mortar, which becomes hard and strong while still damp as a result chemical interaction with water.

Adjustable shutter on the rear opening of an atmospheric burner, or on the blower intake of a power burner, used to regulate entrainment of primary air.

Traditional dark brown slip clay used as liner-glaze in high-fired Early American wares. No longer being mined—use Alberta slip.

Substitute for Albany slip. Highly fluxed with iron—true slip clay—will form glaze at HT.

Basic, opposite of acidic—chemical nature of many fluxes.

A category of high-fire fluxes, which includes calcium, magnesium, barium, and strontium. Encourage hard, durable, but often matt glaze surfaces.

In low-fire, boron fluxes. In high-fire, the feldspars.

Al(OH)₃—alumina source, rarely used in claybodies or glazes, because all needed alumina comes from clay and feldspar. Used primarily for shelf wash and wadding—better adhesion and suspension than aluminum oxide (Al₂O₃). Small additions increase viscosity of glaze melt. Should not be used as matting agent in functional glazes—produces immature glaze.

Broad category of materials composed primarily of alumina and silica. Includes raw materials such as clay and feldspar, as well as fired clay and glaze.

Materials such as glass, which have no regular repeating crystalline structure.

Translates "cellar kiln"—Traditional Japanese kiln evolving from the bank-kiln, and featuring a long, swelling inclined tubular ware chamber, with lower extreme serving as firebox. Characteristically produces heavy flame-flashing and residual-ash effects.

The process of cooling a heated object gradually to allow internal shrinkage stress to equalize without damage.

Sb₃O₃—colorant—soluble, toxic, expensive, produces yellow with titanium. Highly toxic in absorption, ingestion, and inhalation.

Low-relief clay shapes added to scored, slurried leather-hard surface for decoration.

Bricks with angled side faces, tapering across the width of the brick, which when laid together form a curved arch. Standard arch bricks give 4 1/2" thick arch.

Heavy deposition of fly-ash onto surface of wares, furniture, and kiln interior in a wood kiln.

A gas burner that utilizes the effect of gas escaping from the orifice to entrain primary air, without the need for a mechanical blower.

Constant motion in atoms and molecules, present in all materials. Heat accelerates atomic vibration, eventually breaking atomic bonds, causing transition from solid to liquid to gas.

A glaze featuring iridescent metallic flecks as a result of iron crystals just beneath the surface.

Well-known flashing slips for wood fire, but no longer being mined.

In burners, when speed of air/fuel exiting tip of burner is slower than combustion, and flame jumps back down burner tube to fuel orifice, resulting in smoky orange flame and overheated burner tube.

In a fuel kiln, the internal pressure resulting from proper balance of combustion pressure and damper setting, resulting in more economical and efficient heatwork.

Refractory wall in some fuel-burning kilns to deflect heat and flames from direct contact with the wares.

 Al₂O₃×2SiO₂×2H₂O —Secondary clays deposited in marshy areas. Very fine particle size, high plasticity, high in organic contaminates, and fires white or off-white. Used as the primary plasticizing clay in most claybodies, but in large quantities promotes high shrinkage. 

A mechanically revolving vessel in which ceramic materials can be placed along with water and flint pebbles or high-fired porcelain slugs. Used to grind clay and glaze materials.

Hand-operated turntable for applying wax resist and banded decoration.

Early form of East Asian kiln, where tubular kiln chamber was excavated in an earthen bank. Predecessor to anagama and tube kiln.

BaCO₃—alkaline earth—active HT flux, but also promotes matt glaze surface. Unsafe for low-fire functional glazes. Controversial in HT functional glazes, but toxicity problems from balanced HT glaze not proven. Often used as additive in claybodies in very small percentages to render sulfates insoluble, reducing scumming. Toxic in inhalation and ingestion.

Slip clay with very high iron content, often used as brown colorant for claybodies.

Kiln arch that forms a perfect half circle, with the arch beginning and ending on a horizontal surface, using no skew bricks.

Claybody with enough content of dark clays and/or metallic oxides to fire dark brown or black.

In glaze chemistry, the fluxes or melting agents that are combined with acids (glass-formers) and neutrals (refractories).

Safety valve used on most gas kilns, operates on minute electrical current generated by thermocouple. Natural-draft kilns equipped with Baso system often need no external electrical hookup at all. In operation, button on Baso valve is depressed and pilot flame is ignited, which heats up thermocouple, energizing small electromagnet in Baso valve, which holds valve open admitting gas to main burner. If pilot is extinguished, thermocouple cools, and Baso valve closes, shutting off gas.

Rigid flat disc of wood, plastic, or plaster placed on wheelhead. When throwing is finished, bat is lifted off wheelhead, avoiding damage or warpage.

A specialized controlled-crawl glaze designed to crack and crawl significantly during firing (due to extreme L.O.I. in glaze materials), and then melt into isolated beads on the surface.

A kiln with a stationary floor but where the kiln body can be raised upwards on vertical tracks, making it easy to load and unload large forms.

Al₂O₃×5SiO₂×7H₂O —Montmorillonitic clay formed from decomposition of airborne volcanic ash. Finest particles of all clay, extremely plastic (3-times as powerful as ball clay), but shrinkage is too high to be used alone, add 2% to 3% to clay to increase plasticity. When added to glaze, will help keep in suspension and improve raw glaze adhesion.

Soluble metallic salt—gives metallic luster under LT reducing conditions, especially in fuming and saggar applications. Toxic in inhalation and ingestion.

Initial kiln firing in which clay sinters without vitrifying, and though very porous, will no longer soften in water.

Slip clay with very high iron content, often used as brown colorant for claybodies.

In ancient Greek ceramics, late Archaic and early Classical style in which figure first dominates, and where figures are in black against a red background.

Bonfire firing that is smothered with manure and/or sawdust at maximum temperature and then buried in sand, dirt, or ash, to trap smoke, causing wares to turn black.

Glaze defect where fired glaze surface contains bubbles, which often break open to leave sharp-edged craters in surface. Most often caused if volatization of materials in clay and glaze is still occurring when kiln is shut off, freezing bubbles in place. Best solution in high-firing is brief oxidation soak at end of firing to allow surface defects to heal. Same may be accomplished in low-firing by slowing down firing or by soaking kiln near maturation.

Firing defect where blisters form within claybody, raising large lumps on the surface. Caused by expansion of gases within clay, as a result of excessive early reduction (trapped carbon—carbon coring), excessively fast bisque-firing (trapped carbon and sulfur), or overfiring (volatization of fluxes).

In burners, when speed of air/fuel mixture exiting tip of burner is greater than combustion rate—flame jumps off tip of burner and often blows out.

The process of mixing a glaze or slip with a motorized impeller-mixer.

A motorized impeller-mixer used in preparing casting slip. Unlike the handheld drill-mixer, a blunger is mounted on a heavy bracket or clamp, so that slip can be left blunging for extended time needed for proper casting slip.

Period of reduction atmosphere induced between cone 012 and 08 to bring out iron color and speckles in claybody, especially in high-fired stoneware.

Ca₃(PO₄)₂—HT flux—opacifier in LT glazes—translucence in HT glazes (from colloidal phosphorus globules) and especially in bone china (from supercharged glassy-phase). Toxic in inhalation. 

Translucent porcelain containing bone ash.

Completely dry (and very brittle) state clay must reach before firing.

Earliest and most basic firing process, where wares are fired in an open bonfire. See blackware firing.

Na₂O×2B₂O₃×10H₂O—a major LT alkaline flux, available in granular or powdered form. Gives smooth finish, bright colors. Water soluble, so is often used in fritted form. In excessive amounts creates brittle glass and can cause blistering and pinholing. Available in granular and powdered form. Sometimes used with salt or soda in vapor glazing to lower firing temperature and/or achieve a smoother, shinier surface. Toxic in inhalation and ingestion. 

Type of wood-kiln firebox where primary air enters at top of firebox, passes down through wood, and combustion occurs at level of grates or hobs, and is supercharged by the heat of the coal bed. Heat and flames enter firing chamber below grate-level. In true Bourry-box, wood can be piled above grates and is gravity-fed down to combustion zone.

In glazes, phenomenon where a glaze gives different colors in thick and thin areas—the color breaks from thick to thin. Effect accentuated in reduction firing when glazes reoxidize to different color in thinner areas, as in Temmoku breaking from black to brown, or copper red from red to clear.

Method of achieving a shine by rubbing clay or slip with smooth hard object.

Iron-manganese ore—good color source for basalt bodies.

To heat a material to a temperature high enough to drive off all chemically combined water.

CaO×3B₂O₃—traditional important LT alkaline flux, but is no longer being mined. Replace with Ferro 3134 for LT glazes, commercial Gerstley borate substitutes for HT glazes. Test all substitutes.  

CaCO₃—alkaline earth, contributing calcium oxide to glaze—powerful AT flux—major HT flux for glazes—gives strong durable glass. Sometimes used in low-fire claybodies to extend firing range and give greater fired strength.

Ca₃(PO₄)₂—HT flux—opacifier in LT glazes—translucence in HT glazes (from colloidal phosphorus globules) and especially in bone china (from supercharged glassy-phase). Toxic in inhalation. 

CaSiO₃—used in partial replacement of silica and whiting in HT bodies, improves thermal shock resistance. In some cases, it is used in place of whiting to eliminate L.O.I. Toxic in inhalation.

Adjustable tool for measuring inside/outside diameters, as in making lids.

Kiln where kiln floor and often the door are mounted on a car that may be rolled in and out of kiln on tracks. Sometimes has two cars, with doors in both ends of the kiln.

Firing defect where excessively fast bisque-firing and/or excessive early reduction retard outgassing, causing carbon and sulfur to be trapped within claybody. May cause discoloration of glaze, and as clay vitrifies and becomes thermoplastic, trapped carbon and sulfur may expand, causing bloating. 

Refractory red stoneware clay, used to obtain rich red and brown colors in high-fire claybodies.

Usually purposeful effect where carbon is trapped within surface of the glaze, giving smoky shaded areas, especially in shino glazes. Encouraged by slightly early body reduction; can be promoted in high-fire by brushing saturated soda ash solution over glaze.

Serious affliction of the wrist resulting from excessive and/or stressful repetitive-motion activity such as hand wedging clay. See pugmill.

Short for castable refractory—a refractory mix that can be cast into molds to form kiln parts.

A parabolic kiln arch requiring no buttressing or steel frame; laid out by hanging a chain from two points and marking the resulting curve.

Classic East Asian transparent or translucent glaze with small percentages of iron and/or copper and/or chrome, giving range of soft greens, blue-greens, and gray-greens. Most desirable Chinese celadons often contain minute air-bubble inclusions, giving slight opalescence.

Critical step in throwing, occurring during and after wheel wedging, whereby the clay mass is formed into a symmetrical lump before penetrating and raising walls.

Ceramic insulating material composed of spun kaolin fibers—available in blanket form, braided tape, rigid board, and tubular flue liners. Highest insulating rating of standard refractories, but can release carcinogenic fibers.

CaCO₃—alkaline earth, contributing calcium oxide to glaze—powerful AT flux—major HT flux for glazes—gives strong durable glass. Sometimes used in low-fire claybodies to extend firing range and give greater fired strength.

Very soft, pliable animal skin—when wet works well to smooth wet clay surfaces.

A quantity of chemical material, usually salt or soda, which is inserted or injected into a hot kiln during vapor-glazing processes.

Inserting or injecting a charge of chemical material into a kiln during vapor-glazing processes.

Water in molecular combination within clay and glaze materials, which is driven off during the water-smoking period of the firing.

Al₂O₃×2SiO₂×2H₂O—Primary clay that fires pure white, very refractory, coarse particle size, low plasticity, high-temperature—major component of porcelain and some whiteware bodies.  See EPK, Grolleg, Helmer, Tile-6.

Very low temperature (cone 018) glaze colors applied over a previously fired higher-temperature glaze. Allow greater detail, brighter colors than other ceramic glaze effects, but are vulnerable to surface abrasion.

European pottery, eighteenth century and later, featuring decoration inspired by imported Chinese Ming Dynasty wares.

Cr₂O₃—standard vivid green colorant—often softened with a little iron or manganese. Very refractory. With tin produces pink. May go gray-brown in reduction. Highly toxic in inhalation and ingestion.

On the wheel, a temporary wet-clay form or reusable bisque-fired form upon which wares may be inverted for trimming.

A pale gray-blue feldspathic stoneware glaze featuring opalescence due to inclusions of phosphorous and/or other materials.

Widely occurring aluminum silicate mineral resulting from natural decomposition of feldspar and granite. Composed of microscopic disk-shaped platelates that give clay its slippery, plastic quality.

Clay mixture formulated of clays and other ceramic raw materials to give desired working characteristics.

Any one of a variety of East Asian kiln designs featuring single or multiple chambers that climb up a slope, creating adequate draft often with little or no chimney. See anagama, noborigama, tube kiln.

In a fuel kiln, atmosphere that is slightly reducing, but still allows increase in temperature. See reduction firing.

Wheel-thrown form that is closed off completely and then altered to form a vessel or sculptural component.

Carboxymethylcellulose (CMC) — an organic gum used as a suspension/adhesion agent in glazes. Normally, a small amount of gum is added to a quart or so of warm water and left overnight. Once dissolved, this solution may be added in small doses to glazes, slips, and engobes to improve application performance.  See gum additives.

Co₃O₄—calcined cobalt carbonate—twice as powerful—coarser than carbonate, and may give mottling in glaze. Works well for underglaze brushwork, with few crawling problems. Toxic in inhalation and ingestion.

A measurement of a material’s tendency to expand when heated and contract when cooled. The higher the coefficient of expansion, the lower the thermal shock resistance. In firing dissimilar materials in contact with one another the coefficient of expansion must be matched. See thermal expansion.

Small stamp wheel with raised pattern around the rim, which when rolled along a plastic clay surface leaves a band of relief pattern. Usually formed with damp or dry clay and bisque-fired.

Ceramic forming method utilizing ropelike coils of plastic clay, assembled in successive courses to build up wall of vessel or sculpture.

CaO×3B₂O₃—traditional important LT alkaline flux, but is no longer being mined. Replace with Ferro 3134 for LT glazes, commercial Gerstley borate substitutes for HT glazes. Test all substitutes.

Process of reducing the upper diameter of a thrown form by working the walls of the rotating form inwards with fingers or rib, as in a bottle shape.

Gaseous, liquid, or solid materials that remain suspended within glaze melt without dissolving into melt, and which often coagulate to form visible particles. An example is copper in a copper-red glaze—if firing is too fast, minute copper globules will not coagulate into visible masses, and glaze will be clear.

Slip of a composition that affects color of glazes placed over it. Can result from coloring oxides, color modifiers, or textural qualities that influence color.

Decoration where a toothed instrument is dragged over a soft clay surface, sometimes through a layer of slip.

Reaction initiated when fuel reaches kindling temperature, at which point oxidation of hydrocarbon gases releases heat, sustaining and accelerating reaction.

In wheel throwing, the act of hand or finger pressure on the clay, resulting in lower moisture content and a denser structure. Lack of compression in bottoms of pots can result in S-cracks.

See pyrometric cones.

Industrial tunnel car kilns or rolling-hearth kilns in which wares are slowly moved through a kiln that continuously remains at maturing temperature.

A glaze designed to crawl as glaze materials shrink during drying or during early red heat, producing alligator skin or beaded effect. See crawling.

Upwards rise of warm air currents due to the transference of heat.

The profile or schedule of temperature change in the cooling of a kiln. See firing ramp.

CuCO₃—a major glaze colorant to produce greens in LT and HT, copper reds in HT reduction, and greens and metallic effects in raku. Toxic in inhalation and ingestion.

CuO—alternate source of copper, coarser particle size, twice as powerful as copper carbonate. Toxic in inhalation and ingestion.

Popular mid-range and high-fire glazes featuring very small percentage of copper that, under correct light reducing conditions, gathers into colloidal particles of red copper oxide, producing bright rich red and red-purple colors. For good copper reds, start reduction at cone 012 or 010, and maintain partial up to maturation, with oxidation cleanup at end but no glaze reduction.

CuSO₄—color source for saggar firing and pit-firing. Soluble, and highly toxic in absorption, ingestion, and inhalation.

2MgO×2Al ₂O₃×5SiO₂—magnesium/aluminum silicate clay mineral, used to make grog for refractory products—promotes formation of mullite.

K₂O/Na₂O/CaO×Al₂O₃×10SiO₂—HT feldspathic alkaline flux containing calcium and potassium, but more refractory than potash feldspars. Substitution eight parts potash feldspar, two parts silica, one part kaolin. Toxic in inhalation.

K₂O/Na₂O/CaO×Al₂O₃×10SiO₂—HT feldspathic alkaline flux containing calcium and potassium, but more refractory than potash feldspars. Substitution eight parts potash feldspar, two parts silica, one part kaolin. Toxic in inhalation.

Adjustable wooden forms used in casting plaster molds.

See crazing.

A glaze designed to craze for decorative effect. Appropriate primarily for nonfunctional objects and surfaces, as crazing is a flaw and weakens wares.

Glaze fault where glaze recedes away from an area in the firing, leaving bare clay. Usually caused by dusty, dirty, or oily surface beneath glaze or by excessively powdery glaze. In some cases results from very high L.O.I. in glaze materials, causing high glaze-shrinkage and resulting cracking during firing. Used intentionally in controlled crawl and beading glazes.

Very fine surface cracks in fired glaze surface—technically a fault in glazed wares, but often sought after, especially in raku.

Crystalline form of silica, which can form in clay and glaze above 2200°F; has very high coefficient of expansion, producing low thermal shock resistance. Promoted by excessive free silica in clay and/or glaze, by repeated firing, and/or by excessive soaking or slow firing/cooling at high temperatures.

Fuel-burning kiln, usually downdraft, where heat enters at floor level at one side of ware chamber and exits at floor level at opposite side of chamber.

Na₃AlF₆—small amounts promote crackle effects, larger amounts become very volatile with silica and may cause blistering. Used for special effect crater glazes. Toxic in inhalation.

Solid material characterized by regular repeating geometric molecular structure or lattice, with specific melting point, as compared to glass, an amorphous, non crystalline material that softens over broad temperature range. See glass, super-cooled liquid.

Glazes in which significant macrocrystalline structure forms in surface of low-alumina glaze seeded with zinc or titanium. Crystalline glazes feature large, visible crystal development, vs. microcrystalline effects, as in matt glazes and saturated-iron glazes.

Technique where a design is outlined in oxide-tinted wax resist, and the intervening spaces coated with glazes. Finished results show areas of glaze divided by dark unglazed lines.

Crushed window or bottle glass, occasionally used as a glaze material.

K₂O×Al₂O₃×6SiO₂—a common potash feldspar—HT alkaline flux. See feldspar. Close match to G-200. Toxic in inhalation.

First stage in kiln evolution beyond bonfire, consisting of cylindrical clay wall containing fire and wares, covered with layer of shards.

(also damp box, damp closet, damp room) A reasonably airtight cabinet or room in which damp ceramic forms can be stored temporarily to slowly dry, protected from harmful air currents. High humidity is easy to maintain when the cabinet is full of damp wares, but sometimes damp sponges or bowls of water are placed in the damp cabinet for extra humidity.

In fuel-burning kilns, adjustable refractory plate located in exhaust flue, allowing control of back-pressure and secondary air, regulating kiln atmosphere.

Common deflocculant for casting slips. Product of R.T. Vanderbilt Company.

The process of removing the air from a plastic clay mass, usually accomplished through wedging, or far more effectively with a vacuum deairing pugmill.

Process of adding an alkaline (usually) material (deflocculant) to a suspension, which introduces like electrical charges to all particles, causing them repel one another and remain in suspension. A deflocculated suspension gives flowing consistency with less water content, meaning lower drying shrinkage—especially important in slip-casting. Extremely low percentage of deflocculant additive is needed—1/4 of 1% (of dry-batch weight) soda ash and/or sodium silicate. See flocculation.

Clays that have been transported away from their point of geologic origins by wind or water. Finer particle-size gives greater plasticity—ball clays, stoneware clays, fireclays, etc.

The phenomenon that occurs early in the glaze cooling cycle, when certain materials crystallize out of the vitrified (fused) mass.

Natural tendency of materials in liquid solution to go from area of high concentration to area of lower concentration, resulting in even distribution of materials throughout the glaze melt.

In glaze-melt, inclusions that disperse throughout the melt without actually dissolving into the glassy-phase. Examples are titanium or zirconium particles, colloidal copper, or minute air bubbles.

Action of a solvent material on a solid, bringing it into liquid solution. During glaze firing, the point at which the glassy-phase dissolves the sintered structure of the glaze, producing a true glass.

MgCO₃×CaCO₃—HT alkaline earth flux, promotes hard, durable surfaces and recrystallization/matting in glazes. Often added to claybodies to give longer firing range and can promote more durable low-fire bodies.

Kiln where exhaust gases exit through flue at floor level. Requires chimney to develop convection currents needed to draw off exhaust gases. Best system for reduction firing.

The flow of exhaust gases out of a fuel kiln, affecting intake of flames and secondary air.

In vapor-glaze firing, small rings of clay, formed to stand vertically, which are placed inside a peephole and may be removed with an iron rod during the firing, quenched in water, and examined to determine the degree of glaze deposition.

The removing of wares from a kiln.

Electric-drill-mounted impeller-mixer excellent for mixing glazes, slips, and slurries and for blunging casting-slip.

Traditional term referring to serious cracking occurring in cooling, resulting from drawing too soon, from extreme excessive glaze-compression, or from low thermal shock-resistance in overvitrified wares resulting from overfluxing and/or over-firing. 

Low-fired ware, usually still porous after firing—must be sealed with vitreous glaze to be functional.

Natural low-fire secondary clay—fluxed with iron, fires porous. Often called "common" clay, found almost everywhere, matures below 2000°F. 

Formation of crystalline deposits on surface of clay or concrete as soluble compounds migrate to surface during drying. See Egyptian paste.

A self-glazing claybody in which soluble alkaline fluxes effloresce to the surface as the piece dries, and subsequently form a thin glassy coating in the firing. 

Any of a group of slightly over 100 substances on earth that may exist as individual atoms, and from which all materials on earth are composed. See periodic table of the elements. In electric kilns, the heating coils. 

Very low temperature (cone 018) glaze colors applied over a previously fired higher-temperature glaze. Allow greater detail, brighter colors than other ceramic glaze effects, but are vulnerable to surface abrasion.

Colored slips formulated to have low drying shrinkage, allowing application to bone-dry or bisque-fired surface before glazing. Commercial underglazes are available in a wide palette of colors primarily for low-fire, but many will survive high-fire.

Primary air drawn into an atmospheric burner, or mechanically injected into a power burner. 

A kiln in which the firing platform is stationary, but the body of the kiln rolls out of the way horizontally on tracks. Often equipped with two firing platforms to be fired alternately.

Al₂O₃×2SiO₂—used in place of regular kaolin to adjust raw fit (reduce glaze drying-shrinkage) in glazes and engobes.

Al₂O₃×2SiO₂×2H₂O—pure white kaolin, less plastic than Tile-6 kaolin, frequently used in glazes. 

MgSO₄—water soluble, rarely used as magnesium source in glazes. Most often used as flocculant for slips and glazes. Often added to porcelain and porcelaineous stoneware bodies (1/2 of 1% of dry materials weight) to counteract deflocculating alkalinity released by kaolins or fluxes.

The science of comfortable and effective utility, determining how well a functional object or device works with the human body.

Chemical phenomenon where two materials in combination melt at lower temperature than either material by itself. For example, lithium carbonate and silica each melt at cone 32 (3100°F) but mixture of 55% silica and 45% lithium develops a eutectic and melts at cone 06—1830°. 

Machine that forces plastic clay through a die to produce extruded clay shapes.

Decorating technique involving cutting or paddling flat facets in the clay surface.

Widely used (and misused) term referring to any earthenware pottery glazed with an opaque glaze (usually white) and overglaze decoration.

Decorating technique where a soft, fine pointed tool is drawn through adjacent contrasting-colored bands of liquid slip applied to a damp clay surface.

HT alkaline fluxes—insoluble aluminum silicates of potassium, sodium, calcium, and/or lithium—inexpensive flux for clay and glaze. Substitution of soda spar for potash spar can lower vitrification by 100 degrees. Toxic in inhalation. See Custer feldspar, G-200, Kona F-4 feldspar, nepheline syenite, spodumene. 

FeCl₂×6H₂O—soluble metallic salt—fuming agent used to produce lusters on glazed surface. Highly toxic in inhalation and ingestion.

Long tapered knife useful for trimming cast or pressed pieces, and for separating mold components.

The fundamental design relationship between foreground imagery and background or surrounding area. Foreground shapes subdivide background, creating additional important shapes.

Gritty materials like sand, grog, volcanic ash, crushed seashells, etc., added to clay to open up body, give physical structure in wet-working and increase thermal shock resistance.

Highly refractory secondary clays with minimal fluxes and usually fairly coarse particle size—low shrinkage, buff-color, often nonplastic.

The part of a fuel-burning kiln where fuel gases combust before contacting wares. Gas kilns need little if any firebox, whereas wood and oil kilns produce long hot flames whereas require a large firebox unless flame-flashing and ash-slagging (with wood) effects are sought.

Ultraviolet sensor used on industrial burner systems to monitor burner flame. See flame-rectification system.

Maintaining some heat input after maturation, to retard cooling, or to maintain reduction atmosphere during cooling. See reduction cooling.

The profile or schedule for temperature change in a kiln-firing, often including both the heating and cooling ramps.

Surface effects caused by direct flame contact on wares.

In burners, when speed of air/fuel mixture exiting tip of burner is greater than combustion rate—flame jumps off tip of burner and often blows out.

A burner system with automated reignition feature that immediate restores flame should it become extinguished. Usually incorporates a fire-eye ultraviolet sensor to monitor burner flame.

Gas burner tip that causes turbulence in moving stream of gas and air, intermixing them, speeding combustion, holding flame at burner tip, and preventing both flame-off and back-burning.

Wares made to stand stove-top heat. Explosions from trapped moisture, and resulting lawsuits have caused studio flameware to disappear from the domestic market.

Color change in fired clay or slip due to direct flame contact and residual ash deposition in wood firing, or due to variable currents of vapor deposition in salt and soda firing. Flashing can occur on almost any light-colored claybody, but is most dramatic on porcelain bodies and slips.

Slip that is painted or dipped onto wares in order to promote flashing effects in the firing.

SiO₂ —The primary glass-former in clay and glazes—vitrification, fluidity, transparency/opacity controlled by adding fluxes and/or refractories. Highly toxic in inhalation.

The process of adding an acidic (usually ) substance (flocculant) that gives particles in suspension opposite electrical charges, causing them to attract one another (to flock together)—a disadvantage in a casting slip but a great advantage in a claybody or a decorating slip. Usually only claybodies high in kaolin need to be flocculated by adding 1/2 of 1% (of dry-batch weight) epsom salts. Flocculation also often used to thicken up a glaze to help keep it in suspension and to improve application properties. See deflocculation.

Commercial flocculant used in glazes—1/4 tsp. per gallon of glaze.

Passages in kiln for flames or exhaust gases.

CaF₂—limited use as flux. As with Cryolite, fluorine reacts w/silica at high temperatures, can cause pinholing, blisters. Useful in special-effect crater-glazes. Highly toxic in inhalation and ingestion.

Decorating technique involving carving or forming vertical flutes or grooves in surface of a piece.

Low-melting component in clay or glaze that reacts with silica to form glass.

Airborne ash in a wood-kiln.

Base of a ceramic piece.

Firing system in a fuel-burning kiln that uses power-driven blowers or other pressurized air source to entrain primary air.

Direct-connected exhaust system equipped with suction fan, used on commercial furnaces, but never on studio ceramic kilns. Term often mistakenly used to refer to forced-air system.

Fracture zone that results when clay components are pressed straight together without disrupting the surface (by scoring and adding slurry or by smearing together) to intermix the platelates. Parts hold together while wet and tacky, but will separate easily when dry or fired.

Combinations of ceramic materials that have been melted to a glass and crushed/ground back to a powder, in order to give greater chemical stability and to eliminate toxicity resulting from water solubility of raw material. All frits are ground glass and are toxic in inhalation. FERRO 3124—high-alumina calcium-borate frit, gives greater strength in LT claybodies. FERRO 3134—calcium-borate frit often used as substitute for Gerstley borate in low-fire glazes when greater reliability and/or long-term insolubility and/or greater transparency are desired. Makes good cone 04 transparent glaze by itself. FERRO 3110 and 3195—Both very similar to 3134—run tests to determine which works best for your needs.

The process of melting particular ceramic materials to a glass and then crushing and grinding to form a frit.

Process of introducing metallic salts into kiln or onto wares at about cone 018, producing thin layer of metallic surface iridescence.

In the glaze-melt, the point where dissolution of sintered structure is complete, and all refractory particles are dissolved into the glaze melt, forming a fused material—one that has melted to liquid.

K₂O×Al₂O3×6SiO₂—common potash feldspar, HT alkaline flux—close match to Custer. Toxic in inhalation.

In ancient Greek ceramics, early pre-Classical (Archaic) style featuring intricate geometric slip decoration.

CaO×3B₂O₃—traditional important LT alkaline flux, but is no longer being mined. Replace with Ferro 3134 for LT glazes, commercial Gerstley borate substitutes for HT glazes. Test all substitutes.

Super-cooled liquid, which softens and hardens over broad range of temperature and cools to form an amorphous, noncrystalline solid. Level and rate of hardness (viscosity) controlled by temperature and by addition of fluxes and refractories, making possible the wide temperature range of ceramic clays/glazes.

The primary material that, in combination with fluxes, forms the glass essential to all fired ceramics. Primary glass-former at all temperatures is silica.

In heating ceramic materials, point where glass-formers and fluxes combine and soften to begin forming a glass. See sintering, dissolution.

Coating of powdered ceramic materials, usually prepared and applied in water suspension, which melts smooth and bonds to clay surface in glaze firing. See interface.

In high-fired wares, ideal state when claybody shrinks slightly more than glaze, putting glaze under slight compression, giving greater strength, resiliency. See crazing and shivering.

Kiln firing in which glazes are melted to form a smooth glassy surface.

The matching of glaze to claybody in terms of composition and coefficient of expansion so that it will adhere permanently. See glaze compression, interface.

The chemically active state of the melted glaze.

Decorating technique where resist materials are applied to prevent glaze from adhering to certain areas.

Calcined kaolin. See EPK, calcined.

An organic gum derived from fats and oils, sometimes used as brushing medium for engobes or for reglazing fired wares. Also used as a lubricant in burnishing. For applying glazes to glaze-fired or vitrified surfaces, add one teaspoon glycerin to 100 grams dry glaze.

An organic gum derived from fats and oils, sometimes used as brushing medium for engobes or for reglazing fired wares. Also used as a lubricant in burnishing. For applying glazes to glaze-fired or vitrified surfaces, add one teaspoon glycerin to 100 grams dry glaze.

Buff stoneware clay, produced by Cedar Heights Clay Company.

Ware with mottled slip or glaze treatment to create appearance of granite.

An early form of kiln originating in the Middle East, with a below-ground firebox, a grate supporting the wares within a cylindrical enclosure, and a piled shard roof.

Process of glaze-firing glazed greenware without a bisque-firing.

Glazing leather-hard or bone-dry wares for single-firing.

Any dry, unfired clay form.

Any dry, unfired clay form.

Crushed high-fired clay graded in sizes from 15-mesh (very coarse) to 150M (extra fine) added as a source of filler or tempering grit to claybodies to reduce shrinkage and give structure for throwing or handbuilding. Does not shrink in firing, so in medium and coarse grades will show texture through thin to medium glaze. Toxic in inhalation.

Al₂O₃×2SiO₂×2H₂O—English kaolin, more costly than other choices in the United States, but gives whiter porcelain. Less plastic than TILE-6. Best kaolin for translucent bone china.

A long, low horizontal sprung-arch or barrel-arch wood-fired kiln, often banked with earth, traditionally used by potters on the eastern slope of the Appalachian Mountains.

CMC Gum, Veegum Cer—water soluble organic thickeners often added to glazes or slips as a suspension and/or brushing agent. Also used to allow glaze adhesion to vitrified or previously glaze-fired surfaces. Most commercial low-fire glazes and underglazes contain gum additives.

Forming plastic clay by hand without the wheel, using pinching, coiling, and/or slab construction.

Hard, dense firebrick generally used only in high-stress areas of kiln (floor, burner ports, flues, bag wall) and for corrosive firing processes (salt, soda, wood).

Traditional European term for high-fired porcelain.

Refractory stoneware clay or fireclay, used in stoneware claybodies.

Work done by effective heat transfer to wares, resulting from ramp rate, temperature and duration of firing.

Al₂O₃×2SiO₂×2H₂O—Kaolin that works especially well as flashing slip for salt, soda, and wood firing, especially since Avery kaolin is no longer being mined.

High Efficiency Particulate Arrestor—industrial designation that indicates that a filter or dust mask is capable of blocking microscopic particulates such as silica dust.

High-temperature firing range usually including cone 8 to cone 12, for firing stoneware or porcelain.

Decorative style originating in Moorish Spain, characterized by extremely elaborate patterning and curvilinear plant motifs; major influence on Italian maiolica.

Steps constructed in walls of wood-kiln firebox in place of grates. Wood cut to proper length catches on hobs and remains suspended above coal bed, insuring quick and complete combustion.

The interior refractory surface of a kiln.

Combustible gases containing hydrogen and carbon, present in fuel gases and produced when carbon-based liquid or solid fuels are heated above kindling temperature.

Laboratory device for measuring specific gravity in slips and glazes. Does not give absolute measurement, and should be used only for comparing and reproducing results. Must be an appropriate glaze hydrometer for measuring suspensions heavier than water. Does not work well in thick glazes.

Porous firebrick with insulating values much higher than hardbrick.

An iron ore with significant titanium—most often used in granular form to produce dark specks in clay or glaze. Higher iron concentration than in rutile.

Decorating technique where textured or patterned material or object is pressed into clay surface.

Decorating technique where design is formed by cutting or carving shallow lines in clay surface.

Any particles or bubbles of material that remain suspended within the glaze-melt, affecting the glaze appearance.

Contact face between clay and glaze. On low-fired wares, primarily just a physical interlocking of glaze into pores in clay. In high-fired wares, an intimate interaction of clay and glaze, reinforced by mullite crystals, creating very strong bond.

A kiln that is loaded cold, brought to temperature, cooled and unloaded. See continuous kiln.

Most abundant coloring material on earth, responsible for brown and sometimes black color in natural rock and fired clays. In oxidation firings, the red ferric oxide remains very refractory, but in reduction firing and or at high-fire temperatures it transforms to the black ferrous oxide, a powerful flux.

FeCl₂×6H₂O—soluble metallic salt—fuming agent used to produce lusters on glazed surface. Highly toxic in inhalation and ingestion. 

FeO×Cr₂O₃—glaze colorant producing pink or red with tin, brown with zinc, gray with alkaline fluxes. Highly toxic in absorption, inhalation, and ingestion.

FeO—reduced form of iron oxide—gives same results as red iron in the firing, dependent on oxidation/reduction.

Fe₂O₃—powdered rust—refractory red in oxidation, converts to black iron (flux) in reduction and/or high-fire. Low quantities in clear glaze produces celadon-green—high quantities produce temmoku black or saturated iron red—powerful flux. More than 5% in a glaze significantly increases fluxing in reduction.

  See yellow ochre.

Ware developed by Wedgwood in which colored stoneware is decorated with contrasting raised relief slip or clay imagery.

A partially mechanized vessel-forming process on the potter’s wheel, where a pivoting arm (a jigger or jolley) with a mounted profile rib is brought down against a centered clay ball within or upon a wheel-mounted mold.

A partially mechanized vessel-forming process on the potter’s wheel, where a pivoting arm (a jigger or jolley) with a mounted profile rib is brought down against a centered clay ball within or upon a wheel-mounted mold.

A long wooden tool with a knob on the end, often covered with chamois, which is inserted in a bottle or jug on the wheel after the top has been necked in and is used to broaden and refine the shape.

High-fired Japanese saturated iron glaze usually containing at least 10% (of dry-batch weight) iron oxide, which produces iridescent microcrystalline rust red. See tessha.

Japanese trimming tool, formed of a steel strap bent and sharpened on the ends.

  Al₂O₃×2SiO₂×2H₂O—Primary clay that fires pure white, very refractory, coarse particle size, low plasticity, high-temperature—major component of porcelain and some whiteware bodies.  See EPK, Grolleg, Helmer, Tile-6.

Crystalline clay mineral forming the basis of most clays we use in ceramics.

Greek term meaning earthenware from which the word ceramics is derived.

In plaster slip-casting molds, small rounded depressions carved in each parting face of a mold segment, so that when adjacent segments are cast a corresponding bump will form. In use, the keys allow the mold segments to be aligned and assembled securely after the original prototype is removed. 

A specialized oven or furnace used for firing clay for pottery.

Refractory shelves, posts, and stilts used in a kiln to support the wares.

Automatic shutoff device mounted on many electric kilns; accepts a small-size pyrometric cone, and shuts off kiln when cone deforms.

Refractory slip coating applied to top surface of kiln shelf to protect from glaze runs. For all but salt and wood firings, 50-50 kaolin and silica. For salt and wood, 60% alumina, 30% kaolin, 10% ball clay.

In combustion of fuels, temperature that must be reached before oxidation reaction will produce enough heat to sustain and accelerate. For wood and paper, kindling temperature is 451° F.

Potash spar no longer mined. Substitute Custer or G-200.

Na₂O×Al₂O₃×6SiO₂—a common soda feldspar—powerful HT alkaline flux. Toxic in inhalation.

3Al₂O₃×2SiO₂—aluminum silicate used in place of alumina and silica to promote formation of mullite crystals, increase thermal shock resistance. Coarse-ground used as grog in refractory bodies for kiln furniture.

The fraction of any material that volatizes and outgases during firing—includes carbonates, sulfates, nitrates, organic contamination, and chemically combined water.

2(PbCO₃)×Pb(OH)₂—former source of lead for glazes. Highly toxic in absorption, inhalation, and ingestion. Remove from studio and dispose of in a responsible fashion.

Pb₃O₄—Former source of lead for glazes. Highly toxic in absorption, inhalation, and ingestion. Remove from studio and dispose of in a responsible fashion.

3PbO×2SiO₂—fritted lead compounds—LT flux, but resulting glazes may be toxic and should not be used on functional wares. Most powerful of all fluxes, promotes smooth-flowing, self-healing glazes. Highly toxic in inhalation and ingestion.

Condition of clay where it has stiffened but is still damp. Point at which pieces are joined and most surface modification and trimming are done. Soft leather-hard ideal for forming, joining, thick slip-decorating. Medium leather-hard good for thin slip- decorating, joining, incising, carving, piercing. Hard leather-hard good for thin slip- decorating, carving, scraping.

Lithium feldspar—HT alkaline flux—used for thermal shock bodies and matching glazes. Contains fluorine, with associated problems. Toxic in inhalation.

CaCO₃—alkaline earth, contributing calcium oxide to glaze—powerful AT flux—major HT flux for glazes—gives strong durable glass. Sometimes used in l