» » Glossary

absorption

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

acids

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

Additive A

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.

agateware

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

air-floated; air-floating

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

air-set; air-setting

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

air-shutter

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.

Albany slip clay

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

Alberta slip clay

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

alkaline

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

alkaline earths

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

alkaline fluxes

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

alumina hydrate

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.

aluminum silicate

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.

amorphous

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

anagama

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.

annealing

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

antimony oxide

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

applique

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

arch brick

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.

ash-slagging

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

atmospheric burner

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

atomic vibration; atomic motion

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.

aventurine

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

Avery kaolin

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

back-burning

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.

back-pressure

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

bag wall

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

ball clay

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.

ball mill

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.

banding wheel

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

bank kiln

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

barium carbonate

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

barnard; blackbird

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

barrel arch

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

basalt body; basalt ware

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

bases

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

Baso valve

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 [...]

bat

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

beading glaze; beaded glaze

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.

bell kiln

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.

bentonite

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.

bismuth subnitrate

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

bisque-firing

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

black-figure style

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.

blackbird; barnard

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

blackware firing

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.

blistering

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 [...]

bloating

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).

blow-off; flame-off

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.

blunge

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

blunger

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.

body reduction

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

bone ash; calcium phosphate

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.

bone china

Translucent porcelain containing bone ash.

bone dry

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

bonfire firing

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

borax; sodium tetraborate

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 [...]

Bourry-box

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 [...]

breaking

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.

burnishing

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

burnt umber

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

calcine

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

calcium borate; colemanite; Gerstley borate

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.

calcium carbonate; whiting; limestone; marble; chalk

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.

calcium phosphate; bone ash

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.

calcium silicate; wollastonite

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.

calipers

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

car kiln; shuttle kiln

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.

carbon coring

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.

carbon-trapping

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.

Carbondale clay

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

carpal tunnel syndrome (CTS)

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

castable

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

catenary arch

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

celadon

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.

centering

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 fiber

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.

chalk; whiting; calcium carbonate; limestone; marble

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.

chamois

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

charge

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

charging

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

chemically combined water

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

china clay; kaolin

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.

china paints; enamels

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.

chinoiserie

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

chrome oxide

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.

chuck

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

chun

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

clay

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.

claybody

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

climbing kiln

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.

climbing reduction; partial reduction

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

closed form

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

CMC gum

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.

cobalt oxide

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.

coefficient of expansion formula

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.

coggle; roulette

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.

coil construction

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

colemanite; calcium borate; Gerstley borate

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.

collaring; necking-in

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.

colloid; colloidal

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.

color-active slip

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.

combing

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

combustion

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

compression

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.

cone

See pyrometric cones.

continuous kiln

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

controlled-crawl glaze

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

convection currents

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

cooling ramp

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

copper carbonate

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.

copper oxide, black; cupric oxide

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

copper red; flambe; oxblood; sang de boeuf

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.

copper sulfate

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

cordierite

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

Cornish stone; Cornwall stone

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.

Cornwall stone; Cornish stone

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.

cottles

Adjustable wooden forms used in casting plaster molds.

crackle

See crazing.

crackle glaze

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

crawling

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.

crazing

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

cristobalite

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.

cross-draft

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.

cryolite; sodium aluminum fluoride

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.

crystalline

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

crystalline glazes

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.

cuerda seca

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.

cullet

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

Custer feldspar

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

cylinder kiln

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

damp cabinet

(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 [...]

damper

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

Darvan

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

deairing

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

deflocculate; deflocculation

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% [...]

deposited clays; sedimentary clays; secondary clays

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.

devitrification

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

dispersion

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.

dispersoids

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.

dissolution

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.

dolomite; calcium/magnesium carbonate

MgCO3×CaCO3—High temperature 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.

downdraft

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.

draft

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

draw rings; draw trials

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.

drawing

The removing of wares from a kiln.

drill-mixer

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

dunting

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.

earthenware

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

earthenware clay

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

effloresce, efflorescence

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

Egyptian paste

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

element

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.

enamels; china paints

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.

engobe; underglaze

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.

entrained air

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

envelope kiln

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.

EPK, calcined

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

EPK; Edgar Plastic Kaolin

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

epsom salts; magnesium sulfate

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.

ergonomics

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

eutectic

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°.

extruder, extrude, extruding

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

faceting

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

faience

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

feathering; feather-combing

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

feldspar

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.

feldspar

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.

ferric chloride; iron chloride

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

fettling knife

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

figure-ground

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

fillers; tempering materials; temper

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.

fire clay

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

fire-eye

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

firebox

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.

firing down

Maintaining some heat input after maturation, to slow down the cooling process, or to maintain reduction atmosphere during cooling. See reduction cooling.

firing ramp

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

flame-flashing

Surface effects caused by direct flame contact on wares.

flame-off; blow-off

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.

flame-rectification system

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.

flame-retention tip

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

flameware

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

flashing

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.

flashing slip

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

flint; quartz; silica; silicon dioxide

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

flocculation; flocculate

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 [...]

flocs

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

flue

Passages in kiln for flames or exhaust gases.

fluorspar

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.

fluting

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

flux

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

fly-ash

Airborne ash in a wood-kiln.

foot

Base of a ceramic piece.

forced-air

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

forced-draft

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 plane

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.

frit

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. [...]

fritting; fritted

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

fuming

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

fusion; fused

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.

G-200

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

geometric style

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

Gerstley borate; colemanite; calcium borate

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.

glass

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.

glass-former

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

glassy-phase

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

glaze compression

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.

glaze, glazes, reduction-fired glazes, celadon, temmoku, chun, crystalline glazes, macrocrystalline glazes

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.

glaze-firing

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

glaze-fit

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

glaze-melt

The chemically active state of the melted glaze.

glaze-resist

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

Glomax

Calcined kaolin. See EPK, calcined.

glycerin; glycerol

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.

Goldart

Buff stoneware clay, produced by Cedar Heights Clay Company.

graniteware

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

grate kiln

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.

green firing; single-firing

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

green glazing; raw glazing

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

green; greenware

Any dry, unfired clay form.

grog

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 [...]

Grolleg kaolin

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.

groundhog kiln

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.

gum additives

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.

handbuilding

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

hard-paste

Traditional European term for high-fired porcelain.

hardbrick

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).

Hawthorn Bond

Refractory stoneware clay or fireclay, used in stoneware claybodies.

heatwork

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

Helmer kaolin

HEPA

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-fire

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

Hispano-Moresque

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

hobs

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.

hot face

The interior refractory surface of a kiln.

hydrocarbons

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

hydrometer

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.

IFB; insulating firebrick; softbrick

Porous firebrick with insulating values much higher than hardbrick.

IFB; insulating firebrick; softbrick

Porous firebrick with insulating values much higher than hardbrick.

ilmenite

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.

impressing

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

incising

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

inclusions

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

interface

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.

iron

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.

iron chloride; ferric chloride

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

iron chromate

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

iron oxide, black ferrous

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

iron oxide, red, ferric

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.

iron oxide, yellow

See yellow ochre.

Jasper ware

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

jiggering; jolleying

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.

jolleying; jiggering

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.

jug finger; potter’s finger

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.

kaki

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.

kanna

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

kaolin; china clay

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.

kaolinite

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

keramos

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

keys; registration marks

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.

kiln

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

kiln furniture

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

kiln sitter

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

kiln wash

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.

kindling temperature

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.

Kingman feldspar

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

Kona F-4 feldspar

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

kyanite

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.

L.O.I.; loss on ignition

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

lead carbonate; white lead

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.

lead oxide; red lead

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

lead silicate

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.

leather-hard

Condition of clay in which 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.

lepidolite

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

limestone; marble; chalk; whiting; calcium carbonate

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.

limit formulas

A set of formulas giving ideal ranges of glaze oxides that can be present in glaze unity formulas at different firing temperatures. Used in conjunction with glaze calculation to predict or evaluate glaze performance. See unity formulas.

line blend graph

Method for testing glaze materials, where proportional amounts vary through a series of samples between two limits. May involve change in a glaze material, or addition of a colorant or modifier. See triaxial blend.

lithium carbonate

Li2CO3—powerful AT alkaline flux, especially with soda or potash feldspars. Promotes hardness and recrystallization in LT glazes. Forms low-temperature eutectic with silica. Toxic in inhalation.

Lizella clay

High-iron (4%) orange-red stoneware clay—substitute for Ocmulgee, which is no longer being mined.

loss on ignition; L.O.I.

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

low mid-range

Firing range usually including cone 01 to cone 3, underused in studio ceramics, useful for functional earthenware, refractory sculpture bodies, and outdoor terracotta work.

low-fire

Low-temperature firing range, usually below cone 02 (2048° F), used for most bisque-firing and for glaze-firing terracotta and whiteware.

luster

Metallic overglaze finish created either by painting prepared luster (metallic salt in organic binder) over previously fired glaze and firing to cone 018 or by spraying metallic salt dissolved in water into kiln and/or on to wares at low red heat, either during cooling cycle of a glaze-firing, or in a separate firing heated to [...]

luting

English word for attaching clay forms with slurry.

lye

Potassium hydroxide or sodium hydroxide. Caustic alkaline soluble that leaches out of wood ashes when they are soaked in water. Occasionally used as a flux additive in terra sigillata, as in the black slip on Classical Greek wares. Toxic in liquid state, and may cause burns.

Macaloid

Suspension agent/plasticizer similar to Bentonite and Veegum T. Up to 2% of dry-materials weight as plasticizer in high-kaolin claybodies. Up to 1/2 of 1% of dry-materials weight as suspension agent, brushing medium in glazes, and slips. Mix with water before adding other ingredients.

macrocrystalline

Glaze effect featuring large patches of crystal development on glaze surface, as compared to microcrystalline effects. See crystaline glazes.

magnesium carbonate

MgCO3—alkaline earth—HT flux, promotes mattness and opacity in LT glazes, smooth, hard, buttery surface in HT glazes—promotes purples or pinks with cobalt. High L.O.I., used to promote controlled crawl glaze effects.

magnesium silicate; steatite; soapstone; talc

3MgO×4SiO2×H2O—HT alkaline earth flux in glaze, promotes smooth buttery surfaces, partial opacity—similar composition to clay, but in LT claybodies gives low shrinkage and high thermal shock resistance, as in standard 50/50 talc/ball clay whiteware body. Highly toxic in inhalation and ingestion.

magnesium sulfate; epsom salts

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.

maiolica

Earthenware, generally terracotta, with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration. Originated from Islamic/Moorish techniques used on the Spanish island of Mallorca, which exported these wares throughout the Mediterranean in the 15th and 16th centuries. The technique was adopted by Italy during the Renaissance, which is when the term [...]

majolica

Trade name for an historic English style of brightly glazed earthenware vessels made in the form of vegetables, animals, etc. Not to be confused with maiolica, which is defined as earthenware with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration.

manganese dioxide

MnO2—flexible colorant—with alkaline fluxes gives purple and red colors—by itself gives soft yellow-brown—with cobalt gives black. Used with iron to color basalt bodies. Concentrations of more than 5% may promote blistering. Toxic in inhalation and ingestion. Fumes from firing are especially toxic.

manometer

Primitive gauge for measuring water column inches, consisting of a U-shaped glass tube partially filled with water, with the pressure source connected to one end. See water column inches.

marble; chalk; whiting; calcium carbonate; limestone

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.

matt glaze

Glaze featuring a dull, nonglossy surface.

maturity; maturing point

Firing point at which a claybody or glaze reaches its desired condition of color, hardness, density, etc.

memory

During drying and firing of clay, the phenomenon whereby a clay piece will “remember” the way it was formed and will often shrink specifically according to the forming method. If not accommodated, memory can aggravate a variety of drying and firing faults.

mica

K2O×3Al2O2×6SiO2—an aluminosilicate with a fine sheet-lattice structure, closely related to clay and feldspar, and often found as minute iridescent flakes in some clays.

microcrystalline

Crystal effects dependent on tiny crystals in the glaze surface, as compared to macrocrystalline effects.

mid-range

Glaze-firing range usually including cone 4 to cone 7, very popular with electric kilns.

mishima

East Asian method of creating an inlaid effect by applying contrasting slip into a design incised in leather-hard clay. When the slip stiffens, the excess is scraped off.

modifiers

Glaze materials that are added to a base glaze to modify surface qualities such as color, gloss, mattness, opacity, crystal development, etc.

mold-release compounds

In making plaster molds, compounds that are applied to all surfaces except damp clay, in order to prevent plaster from sticking. Liquid hand soap works great. Paint it on and let it dry before pouring plaster. Never use oil-base release compounds on plaster molds for press-molding or slip-casting.

molecule

A group of atoms chemically bonded together to form a compound.

montmorillonite

Clays such as bentonite, resulting from the breakdown of airborne volcanic ash. The finest particle size of all clays. Very high shrinkage, and generally used only as an additive to claybodies or glazes. See bentonite.

mouse-hole

In some wood kilns, a small air-port and passage that admits air directly beneath the coal bed.

MSDS

MATERIAL SAFETY DATA SHEET, available from your supplier, giving toxicity information on each of the ceramic materials you use.

muffle kiln

A fuel-burning kiln in which flames and combustion gases are ducted through sealed refractory flues within the ware chamber, heating the wares while maintaining a clean oxidizing atmosphere.

mullite

3Al2O3×2SiO2—calcined Kyanite, but is also found naturally. Very desirable interlocking needlelike aluminum silicate crystals form above 1800° F and greatly add to the strength of high-fired vitrified claybodies and glazes. In well-formulated high-fired wares, clay-glaze interface is extremely strong partially due to interlocking mullite crystals.

multifired

Wares where more than one glaze firing is required to achieve particular surface effects, as in lusters and china paints.

natural draft

Firing system in a fuel-burning kiln that uses natural convection currents in kiln (updraft) or in chimney (downdraft), without any mechanical blowers, to draw in secondary air, circulate heat and atmosphere through kiln, and exhaust combustion gases.

NC-4 feldspar

Na2O×Al2O3×6SiO2—soda feldspar, interchangeable with Kona F-4. Toxic in inhalation.

necking-in; collaring

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.

nepheline syenite

K2O×3Na2O×4AlO3×9SiO2—a common feldspathic flux, high in both soda and potash, used in claybodies and glazes. Less silica than soda feldspars, and therefore more powerful. Increases firing range of low-fire and mid-range glazes. Toxic in inhalation.

Neuman red clay

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

neutral atmosphere

n fuel-burning kiln, atmosphere that is neither oxidizing nor reducing—most efficient atmosphere for efficient climb and even temperature.

neutrals

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

nickel carbonate, green

Ni2CO3—weaker nickel colorant, reduces to green nickel oxide in the firing. Toxic in ingestion and inhalation.

nickel oxide, black

Ni2O3—reduces to green nickel oxide early in firing, and produces similar effects. Toxic in ingestion and inhalation.

nickel oxide, green

NiO—colorant or modifier—can give blues, tan, browns, greens, grays, dependent on fluxes present. Often used to mute the effects of cobalt, copper, and other colorants. Toxic in ingestion and inhalation.

noborigama

Translates as “step-climbing kiln”—traditional Japanese multichamber downdraft/crossdraft climbing kiln, with initial firebox at bottom, and secondary fireboxes in each chamber. As kiln is fired, each chamber preheats the next. See climbing kiln.

nuka

An often iridescent milky white Japanese glaze containing rice-hull ash.

Ocmulgee

Rich golden-brown stoneware clay, no longer being mined. See Lizella clay.

OM-4 (old mine #4)

A well known Kentucky ball clay.

opacifier

In glaze formulation, a material that produces inert inclusions or minute crystals in glaze, causing it to become opaque. Most common are tin oxide and zirconium silicate.

Opax

Zircon opacifier. See ziconium silicate. Toxic in inhalation.

orifice

In a gas or oil burner, the restricted opening through which a jet of fuel emerges.

outgassing

The escape of gases from clay and glazes during firing. Carbonates, sulfates, nitrates, organic contaminants, and chemically combined water volatize between 600 and 1200° F. Other compounds, especially fluxes, volatize later during glaze-melt. See carbon coring, oxidation cleanup, water-smoking stage.

overglaze

Any surface decoration applied over the glaze surface, either as an oxide wash applied over raw glaze surface before glaze-firing, or as a lower-temperature medium fired onto a previously higher-fired glaze surface, as in china paints and lusters.

overspray

In spraying glazes or other mediums, the small droplets and/or dust that do not settle on the object being sprayed. An adequate spray booth is essential in order to exhaust all overspray.

Oxford feldspar

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

oxidation

Any chemical reaction in which atoms or molecules combine with oxygen atoms. The combustion of fuel is an oxidation reaction.

oxidation cleanup; oxidation soak

After glaze reduction, a short period of oxidation to allow outgassing to stop and glazes to heal.

oxidation firing

Any kiln atmosphere with an abundance of oxygen to combust the fuel and oxidize the ceramic materials. Includes all electric firings and any gas firing with adequate air to insure complete combustion of the fuel close to the burner.

oxide

A molecule combining any element with oxygen.

oxide stain

A mixture of coloring oxide and water, sometimes including a little flux, used as an overall patina (often on unglazed work) or for overglaze brushwork. See patina.

oxidize

To subject a material to a high-oxygen atmosphere, encouraging oxidation reactions.

paddle and anvil

Forming technique where a soft leather-hard form may be shaped and thinned with a wood or bisqued clay paddle on the outside and a rounded “anvil” of wood, stone, or bisqued clay applying corresponding pressure on the inside.

paddling

Technique of shaping a soft or medium leather-hard piece by gently hitting with a wooden paddle (sometimes textured) to create flat facets or to resolve irregularities in the surface.

paper clay

Technique popularized by Rosette Gault, utilizing a claybody or slip containing paper pulp, which reduces shrinkage in drying stage, and encourages extremely strong joinery, allowing unconventional joinery such as wet to dry.

paper resist

Decoration technique where strips of moist or adhesive paper are adhered to the surface to resist application of slip or glaze.

partial reduction; climbing reduction

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

pate-sur-pate

Paste-on-paste—decoration in which sculptural relief decoration is built up with contrasting slip on surface of leather-hard ware.

patina

An overall thin wash of glaze or oxide stain, allowing the color and texture of the claybody to show through.

pearl ash; potassium carbonate

K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides.

peephole; spyhole

Small holes in kiln door or wall, allowing viewing of cones, flames, wares, and/or atmosphere during firing.

periodic kiln; intermittent kiln

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

periodic table of the elements

A table listing all of the chemical elements (those substances that can theoretically be reduced to individual atoms), with the smallest and simplest across the top and the heaviest and most complex at the bottom and grouped by similar physical/chemical characteristics.

perlite

Porous expanded granular silica, useful in making insulating refractories.

petalite

Li2O×Al2O3×8SiO2—lithium feldspar—HT alkaline flux—good for reducing thermal expansion, increasing thermal-shock resistance. Toxic in inhalation.

petuntse

Feldspathic rock, similar to Cornwall stone, found in China. Traditional Chinese flux for HT glazes and claybodies.

pinching

Handbuilding method where clay objects are formed by pinching repeatedly between thumb and fingers or between fingers of one hand and palm of opposing hand.

pinholing

Glaze defect characterized by fine pinholes in the surface—often caused by pinholes already present in dry unfired glaze coating. Can also be caused by burst bubbles in glaze surface that are not given opportunity to “heal” at end of firing.

Pioneer kaolin

Al2O3×2SiO2×2H2O—plastic secondary kaolin—wider particle distribution gives greater plasticity, green strength.

pit-firing

A type of bonfire-firing where wares are buried in sawdust in a pit in the ground and a bonfire is built on top, so that the fire and coals slowly burn away the sawdust and fire the wares. Not to be confused with sawdust smoking.

plastic vitrox

K2O/Na2O/CaO×Al2O3×10SiO2—a plastic high-potash feldspathic clay, similar in structure to Cornwall stone—used in place of potash feldspar in some porcelain bodies to increase plasticity. Toxic in inhalation.

plasticity

Quality of moldable flexibility in damp clay—superior plasticity depends on smaller clay particle size, slight acidity, less nonplastic additives, aging of damp claybody, adequate water content, and/or addition of accessory plasticizers, such as Veegum T or Macaloid.

plasticizers

Materials added to some claybodies, especially those high in kaolins, to increase plasticity and dry strength—includes bentonite, Macaloid, Veegum-T.

platelets

Flat, thin crystals that make up clay. When wet they become sticky and slippery, creating the phenomenon we call plasticity.

polishing

As compared to burnishing, the act of creating a shiny surface on terra sigillata (or any clay or slip) by rubbing with soft cloth, a soft brush, or a piece of plastic film.

porcelain

High-fired vitreous claybody containing kaolin, silica, fluxes, and often ball clay to increase plasticity, with total clay component not more than 50%. Usually pure white or “eggshell” in color; some porcelains may fire translucent where thin.

porcelaneous

White-firing stoneware claybodies closely related to porcelain.

Portland cement

Calcium silicate aluminate—air-setting cement, often added in small quantities to homemade castable refractor mix in order to introduce air-setting qualities. Toxic in inhalation.

post-firing smoking

See raku.

potash feldspar

See G-200; Custer feldspar.

potassium carbonate; pearl ash

K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides.

potassium dichromate

K2Cr2O7—Occasionally used as acid-green colorant in raku glazes, but is highly toxic in absorption, ingestion, and inhalation.

pottery wheel; potter’s wheel

Machine with a horizontally spinning wheel head upon which potters form and trim clay vessels by hand.

potter’s finger; jug finger

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.

power burner

Any burner in which a mechanical blower or other pressurized air source entrains the primary air for combustion.

press-mold

A mold, usually plaster, into which moist clay is pressed to crease multiples.

pressing; press molding

Forming of multiples by pressing plastic clay into a plaster press-mold.

primary air

In a fuel-burning kiln, air that feeds initial combustion; in a wood-burning kiln, air that feeds base of flames; in a gas kiln air that enters back end of burner tube in an atmospheric burner, or via blower in a power burner. See secondary air.

primary clays; residual clays

Clays that remains at the site of the parent rock, without being transported by wind or water—tend to be free of contaminants but have coarse particle size and therefore low plasticity. Includes the purest kaolins.

pugmill

A machine similar to an oversized meat grinder, used to homogenize plastic claybodies. Deairing pugmills have a vacuum pump attachment, which effectively removes all air from clay, eliminating need for hand wedging. See carpal tunnel syndrome.

pumice; volcanic ash

Fine volcanic particulate expelled from surface vents in a volcanic eruption. HT alkaline flux, similar in composition to potash feldspar, but higher in silica, with at least 1% iron. May be substituted for 7 parts potash spar, 3 parts flint. Toxic in inhalation.

Pyrax HS

A manufactured pyrophyllite product, promotes plasticity more than other pyrophyllite products. Product of the H.T. Vanderbilt Company.

pyrometer

Temperature gauge connected to a thermocouple, which indicates temperature within kiln. Pyrometers provide good general reference, but only respond to temperature, whereas clay and glazes (and cones) are affected by temperature, duration, and atmosphere of firing. See pyrometric cones.

pyrometric cones

Small slender pyramidal-shaped indicators made of ceramic material formulated to bend at a specific temperature—standard method for determining maturing temperature of firing. Like clay and glazes, cones respond to temperature, duration, and atmosphere of firing far more accurately than mechanical measurement.

pyrophyllite

Al2O3×4SiO2×H2O—used in HT claybodies (20% or less) to reduce thermal expansion, increase thermal shock resistance, reduce shrinkage, give stronger vitreous bodies. May reduce plasticity.

pyroplastic

Flexibility of clay or glaze when heated sufficiently, as a result of developing glassy-phase.

Pyrotrol

A manufactured pyrophyllite product, made by the Resco Company. See Pyrophylite.

quartz inversion

Abrupt expansion in heating and corresponding contraction in cooling that occurs in silica crystals in all clay and glazes at around 1063° F. Greatest risk is in cooling high-fired wares and in refiring previously high-fired wares.

quartz; silica; silicon dioxide; flint

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

raku (japanese)

Traditional raku or Japanese raku is a low-fired glazed pottery by a direct process that involved putting the pots into and removing them from a red-hot kiln. The potter Chojiro is credited with being the first to produce raku ware in 1580. The term raku is translated as “enjoyment” or “ease.” From The Potters Dictionary of Materials and Techniques, by Frank and Janet Hamer. See raku (Western).

raku (western)

Inspired by traditional Japanese raku firing, Western (typically referred to as American) raku is a relatively low-temperature firing process using clay that is either under fired or otherwise formulated to withstand the thermal shock of being removed from a kiln at top temperature. Work is removed from the kiln at bright red heat and subjected to post-firing reduction (or [...]

ramp

Profile of the firing of a kiln, including speed, duration, soaking periods, etc. of both the heating and cooling cycle, as in firing ramp and cooling ramp.

raw glazing; green glazing

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

recrystallization

During initial cooling, the formation of microcrystalline or macrocrystalline structure in the glaze surface.

red lead; lead oxide

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

red-figure style

In ancient Greek ceramics, Classical stage of vase painting, featuring figures in red against black background, with highlights painted in black and white.

Redart

Brick-red earthenware clay, produced by Cedar Heights Clay Company.

reduce

To induce reduction.

reducing agent

Material such as silicon carbide used in clay or glaze to create localized reducing conditions in an oxidation firing.

reduction

Chemical reaction in which oxygen atoms are removed from a compound.

reduction cooling

In a fuel kiln, maintaining slight reduction atmosphere during cooling cycle, from maturing temperature down to approximately 1400° F, in order to minimize reoxidation of clay and/or glazes. See reoxidation.

reduction firing

In fuel-burning kilns, firing atmosphere with insufficient oxygen to completely combust fuel, introducing abundance of unoxidized carbon and hydrogen, which extract oxygen molecules from surface of wares, altering appearance of clay and glaze. See climbing reduction.

refractories

Any materials highly resistant to the effects of heat. In kiln construction, high-temperature insulating materials, such as firebrick, ceramic fiber, etc. In clay and glaze chemistry, the neutrals or stabilizers that are resistant to melting and that combine with the fluxes (bases) and glass-formers (acids). Primary refractory material in ceramics is alumina, most often introduced in the form of clay (aluminum [...]

refractory

Capable of withstanding very high temperatures.

registration marks; keys

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.

reoxidation

In a reduction firing, oxidation that occurs after fuel is shut off at end of firing.

residual ash

In a wood kiln, the surface effects created by fly-ash settling upon the wares. Effect is maximized by turbulence in firebox, considerable draft through kiln, and/or active stirring of the coal bed. In short wood firings ash doesn’t have time to melt, and leaves a rough, crusty surface.

residual salt/soda

In a well-used salt/soda kiln, the light salt/soda effect that may be obtained by firing wares in the kiln without introducing any additional salt or soda, due to the revolatization of salt/soda from the walls of the kiln.

resist

Material used in glazing and decorating that can be applied to surface to prevent adhesion of slip or glaze.

rheostat

Variable electric switch often found on forced-air burners to control blower speed. Small inexpensive rheostats used on most power burners are imprecise, and adjustments should always be made on basis of flame at burner tip, preferably with an air- shutter on the blower-intake for fine adjustments.

rib

Wide, flat handheld tool used to shape, smooth, and/or scrape clay surfaces; usually wood, rubber, plastic, or metal, either rigid or flexible, with straight, curved, or profiled edge.

rolled edge

Beveled edge obtained by rolling the outer edge of the foot of a soft leather-hard pot at an angle against a hard flat surface.

Roman arch

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

roulette; coggle

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.

rutile

Titanium ore, used as source of titanium dioxide, contains iron, other trace minerals—gives tan color, promotes crystallization giving mottled multicolor effects in some HT glazes, or in overglaze stain (very refractory, use sparingly). Gives rich mottled medium blue in some HT glazes. Dark rutile contains higher percentage of iron.

s-cracks

S-shaped cracks that occasionally appear in the bottoms of wheel-thrown pots, resulting from inadequate compression of the bottom and/or excessive water left in bottom. Occur most often in fine-grain gritless claybodies, especially thrown off the hump.

saggar

A refractory clay enclosure in which wares are fired. Originally developed to protect wares from ash-slagging and flame-flashing in wood firings. In contemporary use, with clean-burning gas firings, process used in exactly opposite way.

saggar firing

Contemporary firing process in which wares are placed within refractory saggars, along with chemicals and combustibles, in order to achieve certain surface effects.

salt firing; salt glazing

Vapor-glazing process where salt (sodium chloride) is introduced into kiln firebox at high temperature. Salt vaporizes, and sodium vapor combines with silica in clay surface, forming extremely hard sodium-silicate glaze. See soda firing.

salt; sodium chloride

NaCl—table salt, rock salt—used in salt-firing. Skin irritant.

sand

Granular silica. Major grit besides grog used to give clay bodies structure for throwing and hand building, high shrinkage in high-temperature clay bodies, but gives smoother fired surface than grog. Toxic in inhalation.

sawdust smoking

scoring

Process of incising surface of wet or leather-hard clay in crosshatch pattern before applying slurry and joining pieces.

secondary air

In fuel-burning kilns, air that enters after initial combustion and feeds flames as they leave primary combustion source. In gas kiln, secondary air enters burner port around burner tip. In wood kiln, secondary air enters above grates in conventional firebox, and below grates or hobs in Bourry-box. See primary air.

secondary clays; deposited clays; sedimentary clays

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.

secondary kaolins

Kaolins that have been transported some distance from the parent rock and are still very pure, but much more plastic than primary kaolins.

sedimentary clays; secondary clays; deposited clays

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.

seger formula; unity formula

A formula generated by glaze calculation, which lists all glaze oxides present in separate columns for fluxes, refractories, and glass-formers.

sequential firing

Firing in a sloped tube kiln or noborigama, where as each chamber or zone reaches maturity, the fire is moved up to the next, and each one preheats subsequent chambers or zones.

set

To place wares in a kiln. Or, in a loaded kiln, the entire structure of shelves, furniture, and wares. Also, an aqueous solution’s ability to cling to a vertical surface without running, as in a wet, raw glaze on a pot.

setting

Process of loading a kiln, or technically, of placing the set. See set.

sgraffito

Decorating technique achieved by scratching or carving through a layer of slip or glaze (helps to apply wax-resist over glaze before carving) before firing to expose contrasting claybody beneath.

shard

A broken fragment of pottery.

shino

Classic Japanese glaze ranging from gray to white to orange, composed primarily of feldspar. The orange color is achieved with thinner glaze coating when fluxes in the glaze activate iron content in the claybody. In the West, shino glazes often contain spodumene or other source of lithium, and/or nepheline syenite. Many potters seek shinos that [...]

shivering

Serious and dangerous glaze defect where excessive glaze compression causes small razor-sharp chips of glaze to pop off along outer edges, corners, and rims. All wares showing shivering must be destroyed. The remedy is to slightly increase flux and/or decrease the silica in the glaze.

short

Clay with insufficient plasticity—tends to fragment during forming.

shrinkage

Permanent contraction of the clay in both drying and firing stages. Overall may be as much as 18%.

shuttle kiln

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. See car kiln.

sieve

A container with fine-mesh wire screen in the bottom, available in different mesh sizes, used for straining slips and glazes in order to achieve complete mixing of raw materials and removal of large particles or contaminants.

sieving

The process of working a glaze or slip through a wire-mesh sieve to strain out impurities and to break up clumped raw materials.

silica; silicon dioxide; flint; quartz

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

silicon carbide

SiC — Extremely refractory material used to form kiln shelves. Highly resistant to corrosive atmospheres, and therefore suitable for salt, soda, and wood firing. Silicon carbide kiln shelves conduct electricity and should never be used in electric kilns. Occasionally used in powdered form as local reducing agent in oxidation firings, used in very small quantities [...]

single-firing; green firing

Process of firing glazed greenware without a bisque firing.

sintering

In heating clays and glazes, a solid-state reaction where particles stick together permanently, and mass can be considered fired. With increasing temperature, fluxes and glass-formers begin to interact, increasing the strength of bond between refractory particles, still leaving open, porous structure, as in bisque-fired and low-fired wares, which are sintered but not vitrified.

skew bricks

Special angled bricks used to support the first course of arch bricks on either side of a sprung arch.

slabroller

A mechanized or manually operated device for rolling out large uniform slabs of clay.

slaking; slake down

The process of returning dry unfired clay to a slip by soaking in water.

slip

Clay suspended in water, usually the consistency of thick cream. May be colored and used to decorate surfaces, or may be cast into plaster molds to create ceramic forms.

slip casting

The creation of ceramic forms by casting slip in plaster molds.

slip trailing

Application of decoration to wet or leather-hard clay by flowing on lines of slip with a fine pointed dispenser, such as a rubber syringe.

slip-clay

A naturally occurring clay that contains a high enough component of flux to form a glaze at high-fire temperatures.

slip-glaze

A glaze with slip-clay or earthenware clay contributing the primary flux.

slip-resist

Decorating technique where resist materials are applied to prevent slip from adhering to some areas.

slump mold

A typically shallow frame or mold into which a slab of clay is allowed to fall or settle in order to form a vessel. These can be frames with no bottom.

slumping

Process of creating ceramic wares with a slump-mold. Firing defect where glassy-phase begins to dissolve sintered structure in clay, causing it to sag and deform.

soaking

During firing or cooling ramp, the act of holding kiln at steady temperature for a period of time to allow proper formation or maturation of certain clay and glaze effects.

soap

A specialized hardbrick shape, half the width of a standard brick. Often used for posts in salt and wood firings.

soapstone; talc; magnesium silicate; steatite

3MgO×4SiO2×H2O High-temperature alkaline earth flux in glaze that contibute magnesia and silica. Promotes smooth buttery surfaces, partial opacity. Similar composition to clay, but in low-temperature clay bodies gives low shrinkage and high thermal-shock resistance, as in standard 50/50 talc/ball clay whiteware body. Highly toxic in inhalation and ingestion.

soda ash; sodium carbonate

Na2CO3—soluble source of soda, used as deflocculant, or to supply soda in vapor-glazing process. Source of flux in Egyptian paste recipes. Brush solution over hi-fire glaze to increase carbon trapping. Highly toxic in inhalation and ingestion. Skin irritant.

soda feldspar

Na2O×Al2O3×6SiO2—feldspars contributing sodium (and potassium), primarily as a high-temperature flux. Kona F-4 feldspar, NC-4 feldspar and nepheline syenite. Toxic in inhalation.

soda firing; soda glaze

Vapor-glazing process touted as modern-day nontoxic replacement for salt firing. Gives slightly less gloss and orange-peel. Instead of salt, soda ash (sodium carbonate) in water solution is sprayed into kiln at maturing temperature, and sodium vapor combines with silica in clay to form sodium-silicate glaze. Recent research indicates that effluent gases are highly corrosive and [...]

sodium aluminum fluoride; cryolite

Na3×AlF6—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.

sodium silicate; waterglass

Na2SiO3—comes as a liquid—used as a deflocculant in slips, as an air-setting binder for low-temperature refractories. Many clay artists use it in “magic water” in place of slurry for joining wet or soft leather-hard ceramic forms. Add 1.5% sodium silicate and 1.5% soda ash by weight to a measure of water. Toxic in ingestion. Skin [...]

sodium tetraborate; borax

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 [...]

soft paste

Traditional European term for a dense, white claybody that fires at lower temperature and was the precursor to true porcelain.

softbrick

See insulating firebrick.

soluble salts

Range of soluble metallic salts like bismuth subnitrate, silver nitrate, copper sulfate, cobalt sulfate, ferric chloride, and stannous chloride, used to create fumed luster effects on glazed wares and to create surface effects on unglazed saggar-fired wares. Most are water soluble and highly toxic.

solution

A liquid mix where the components are fully dissolved (as in a water/soda ash solution for vapor-glazing), where materials will not settle out via gravity.

solvent

An agent that acts to accelerate the dissolution of a more resistant material. In glaze maturation, the flux-glass-former combination acts as a solvent on alumina.

spalling

Defect in repeatedly fired clay materials where outer layers shrink and peel away—most often occurs in kiln hot-face when refractories are repeatedly fired beyond their rated temperature.

specific gravity

The weight or density of a liquid measured in proportion to that of water. A glaze with a specific gravity of 1.2 is 1.2 times as dense as water per unit of measure. See hydrometer.

split

A specialized hardbrick, half the thickness of a standard brick.

spodumene

Li2O×Al2O3×4SiO2—lithium feldspar—powerful high-temperature alkaline flux. Promotes copper blues—good for thermal-shock bodies and matching glaze. Toxic in inhalation.

spooze

A repair medium for bone-dry greenware, made of equal parts vinegar and corn syrup mixed with dry powdered claybody to the desired consistency.

spray booth

Open-front enclosure with an exhaust fan at the rear, designed to draw off all overspray and other toxic dust or fumes.

spraying

Application of liquid slip, engobe, glaze, or stain, using mechanized spray equipment. Gives smooth glaze coating, but with less glaze pooling in recesses.

sprigging

Surface decorating technique in which small coils or balls of clay are affixed to the damp or leather-hard surface, usually with a layer of slip.

sprung arch

Kiln arch representing less than 180 degrees of curvature, and requiring buttressing to support outward thrust of the arch.

spyhole

See peephole.

stain

Ceramic colorants that have been fritted in order to eliminate solubility problems and give greater stability in firing and truer color before firing. Most are stable up to cone 5, many to cone 10. Can be mixed with 25—50% Ferro 3134 frit for Maiolica overglaze decoration. Most stains are ground glass and are highly toxic [...]

stannous chloride; tin chloride

Soluble metallic salt. Fuming agent to create mother-of-pearl luster on glazed surfaces. Introduced in cooling ramp at dull red heat, which will affect everything in kiln. Highly toxic in inhalation and ingestion. Fumes are especially toxic.

stilts

Term often applied to all kiln posts, but more correctly referring to specialized refractory furniture pieces equipped with ceramic or metallic points designed to support fully glazed wares during firing.

stoneware

High-fired vitreous ware, literally as hard and durable as stone. Matures from 2200-2400° F. (cone 5–11). Also naturally occurring refractory clays with adequate fluxes to fire in stoneware temperature range.

strontium carbonate

SrCO3 Alkaline earth, high-temperature flux, similar to barium, slightly more powerful. Gives semimatt surfaces. Nontoxic in balanced glaze. Substitute 75 parts strontium to one part barium.

super-cooled liquid

A material that, in its solid form, maintains the amorphous physical structure of a liquid rather than the ordered structure of a crystalline material. A crystalline material softens or solidifies at a specific freezing point, whereas a super-cooled liquid like glass remains amorphous, softening or solidifying gradually over a broad temperature range.

Superpax

Zircon opacifier. See zirconium silicate. Toxic in inhalation.

suspension

A liquid mix where insoluble particles are distributed throughout without dissolving and may settle out from gravity, as in a glaze or slip.

talc; magnesium silicate; steatite; soapstone

3MgO×4SiO2×H2O High-temperature alkaline earth flux in glaze, promotes smooth buttery surfaces, partial opacity. Similar composition to clay, but in LT claybodies gives low shrinkage and high thermal shock resistance, as in standard 50/50 talc/ball clay whiteware body. Highly toxic in inhalation and ingestion.

temper; tempering materials; fillers

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

Tenmoku

Classic East Asian high-iron gloss glaze giving black where thick, breaking to brown or red-brown where thin.

terra sigillata

Ultrarefined clay slip that can give a soft sheen when applied to bone-dry wares and if polished or burnished while still damp may give a high gloss. All ancient Greek red-black pottery and Roman red wares were finished with this technique, without the use of glaze.

terracotta

Low temperature, porous earthenware claybody, fires red-brown due to high iron content, which also fluxes clay, making it the most durable low-fired clay after firing.

tessha

A Japanese glaze similar to kaki, in which re-oxidation and crystallization of iron in the glaze results in significant areas of purplish-red, often with some black or brown surface showing.

thermal expansion

The physical expansion and contraction that accompanies the heating and cooling of most materials. See coefficient of expansion.

thermal shock

Effect of sudden temperature changes during firing or during subsequent heating and cooling in daily use.

thermocouple

Temperature probe that produces minute variable electrical current dependent on degree of heat exposure—used in pyrometers and Baso valves.

thermoplastic

See Pyroplastic.

throwing stick

Wooden tool traditionally used by Japanese potters as an extension of the hand for raising the inside of pots, especially tall narrow forms.

Tile-6 kaolin

Al2O3×2SiO2×2H2O—air-floated secondary kaolin—broader particle distribution than primary kaolins, greater plasticity, green strength.

tin chloride; stannous chloride

Soluble metallic salt—fuming agent to create mother-of-pearl luster on glazed surfaces. Introduced in cooling ramp at dull red heat, it will affect everything in kiln. Highly toxic in inhalation and ingestion. Fumes are especially toxic.

tin oxide

SnO2—most powerful opacifier, but expensive—inert dispersoid in glaze melt—5–7% will produce opaque white in a clear glaze. Toxic in inhalation and ingestion.

titanium dioxide

TiO2—matting/opacifying agent. Promotes crystal growth, visual texture in glazes.

tombo

A t-shaped Japanese throwing gauge, used to measure the depth and rim diameter of a vessel, usually when throwing off the hump.

triaxial blend

Method for testing three-way combinations of glaze materials, where proportional amounts vary through a series of samples between three limits. May involve change in glaze materials, or addition of colorants or modifiers. See line blend.

trimming

At the leather-hard stage, removal of excess clay from a piece, using any of a variety of sharp cutting tools.

tube burner

A simple atmospheric burner with no venturi effect present.

tube kiln

Sequential-firing Korean kiln design evolving from Chinese bank kiln, with long sloping tubular firing chamber. See anagama.

Ultrox

Zircon opacifier. See zirconium silicate. Toxic in inhalation.

undercut

Common flaw in plaster or bisque molds, where the clay or casting catches and will not pull free without breaking or distorting.

underglaze decoration

Process of applying any decoration to the bare, (usually bisque-fired) clay surface directly before glazing.

underglaze; engobe

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.

unity formula; seger formula

A formula generated by glaze calculation, which lists all glaze oxides present in separate columns for fluxes, refractories, and glass-formers.

vanadium pentoxide

V2O5—weak yellow colorant—toxic, expensive—usually fritted with tin to produce stronger yellow. Highly toxic in inhalation and ingestion.

vapor glazing

Any glaze process where the glaze results from vapor deposited within the kiln—includes salt glazing, soda glazing, fuming and wood firing (where fly ash is deposited on the ware and melted into a glaze).

Veegum Cer

Combination of Veegum T and carboxymethylcellulose gum, used as a suspension and adhesion agent in glazes. Product of H.T. Vanderbilt Company.

Veegum T

Suspension agent/plasticizer similar to bentonite and macaloid. Up to 2% of dry materials weight as plasticizer in high-kaolin claybodies, and up to 1/2 of 1% of dry-materials weight as suspension agent, brushing medium in glazes and slips. Mix with water before adding other ingredients. Product of H.T. Vanderbilt Company.

venturi burner

Natural-draft gas burner featuring streamlined restriction in burner tube, which increases entrainment of primary air and the efficiency of gas-air mixing.

vermiculite

A porous expanded mica product used as filler in clay bodies and in insulating refractory layer on kiln exterior.

viscosity; viscous

Reference to a material’s resistance to flowing. A viscous glaze flows less. As glass is heated it slowly becomes less viscous over a broad temperature range.

vitreous engobe

An engobe containing sufficient flux to form to a vitreous clay coating.

vitreous; vitrified; vitrification

Fired clay that has fused together completely, so that the pores between refractory particles are filled with glass and the body is impervious to water. Vitrification is sintering in the presence of a fully developed glassy-phase.

volatization; volatize

Change from solid or liquid to gas during firing, resulting in outgassing from the clay or glaze. See outgassing, blistering.

volcanic ash; pumice

Fine volcanic particulate expelled from surface vents in a volcanic eruption. HT alkaline flux, similar in composition to potash feldspar, but higher in silica, with at least 1% iron. May be substituted for 7 parts potash spar, 3 parts flint. Toxic in inhalation.

W.C.I.; water column inches

Standard low-pressure measure of gas pressure, based on pressure required to lift a column of water. Household natural gas pressure is usually 7″ or 7 W.C.I. Industrial natural gas pressure may be 12″ to 15″ or more. Propane household systems usually carry 11″ of pressure, but propane kiln systems may be equipped with adjustable regulators [...]

wadding; wads

Small balls or rolls of refractory clay mixture (40 alumina, 10 ball clay, 50 kaolin) placed under wares and posts, and between pots, lids, etc., in vapor-glazing and wood firing processes, to keep surfaces from sticking together.

warping

Distortion of clay forms caused by uneven stresses within clay due to forming method, uneven drying, uneven support in firing, or uneven or excessive heat in firing.

waster

European term referring to a fired piece discarded due to damage in firing.

water of plasticity

Amount of water required to bring a dry clay to its state of ideal plasticity. Common clay test used to determine this amount. The more water needed, the finer the particle size, the more plastic the clay, and the greater the drying shrinkage.

water-smoking stage

The stage during bisque-firing or single-firing, below 1200°F, when carbonates, nitrates, sulfates, organic contaminants, and chemically combined water volatize and outgas. See carbon coring, outgassing.

wax resist

Melted wax or wax emulsion used to prevent slip or glaze from adhering to a clay surface, either in decorating, or in preparing work for glazing.

wedge brick

Bricks with angled side faces, tapering along the length of the brick, which when laid together form a curved arch. Standard wedge bricks give 9″ thick arch.

wedging

Process of kneading the clay with the hands to remove air bubbles and ensure homogenous mass.

wheel-wedging

Working the clay up and down in a cone shape on the wheel to align the platelates in a spiral formation and thus increase control in centering and throwing.

white lead; lead carbonate

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.

whiteware

Low-temperature white claybody.

whiting; calcium carbonate; limestone; marble; chalk

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

wollastonite; calcium silicate

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

XX-Sagger

A plastic refractory stoneware clay, often used in sculpture and raku bodies.

yellow ochre

High-iron yellow clay mineral, used as colorant in glazes and slips, converts to red iron oxide in oxidation or black iron oxide in reduction and/or high-fire.

zinc oxide

ZnO—High-temperature flux that promotes brilliant glossy surfaces. In some glazes can encourage opacity. With titanium in low-alumina glaze can encourage macrocrystalline growth (crystalline glazes). Volatizes in high-fire reduction. Toxic in inhalation.

zirconium oxide

Common opacifier, often used in place of the more expensive tin oxide. Sold as Opax, Superpax, Ultrox, etc., use 7% in a slip and 10 to 12% in a clear glaze to get pure opaque white. Zirconium particles are extremely refractory and remain as inert inclusions in the glaze-melt. They will not affect mattness or [...]

zirconium silicate

ZrSiO4—zircon opacifier—inert dispersoid in glaze melt—low-cost substitute for tin oxide—use double the recipe weight of tin. Includes Zircopax, Opax, Superpax, Ultrox. All are toxic in inhalation.

Zircopax

Zircon opacifier, no longer being manufactured. See zirconium silicate. Toxic in inhalation.

zone of crystallization

Period during cooling ramp when a particular material tends to crystallize out of glaze-melt.

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