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Glossary
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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.
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.
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.
Carbondale clay
Refractory red stoneware clay, used to obtain rich red and brown colors in high-fire claybodies.
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.
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.
drawing
The removing of wares from a kiln.
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.
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.
