Ceramic Terms:
BALL CLAY: Secondary clays deposited in marshy areas. Very fine particle size, high plasticity,
high drying shrinkage, high in organic contaminates. Fire white or off-white.
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 for fine-grinding of clay and glaze
materials.
BANDING WHEEL: Hand-operated turntable for applying wax resist, banded decoration, etc.
BAT: Rigid flat disc of wood, plastic, or plaster placed on wheel-head. When throwing is finished,
bat is lifted off wheelhead, avoiding damage or warpage.
BISQUE-FIRING: Initial kiln firing in which clay sinters without vitrifying, and though very
porous, will no longer soften in water.
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 defects to heal. Same may be accomplished in lowfiring by slowing down firing or by soaking kiln near maturation.
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 over-firing
(volatization of fluxes).
BONE DRY: Completely dry (and very brittle) state clay must reach before firing.
BURNISHING: Method of achieving a shine by rubbing clay or slip with smooth hard object.
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.
CHUCK: On the wheel, a temporary wet-clay form or re-useable bisque-fired form upon which
wares may be inverted for trimming.
CLAY: Widely occurring aluminum silicate mineral resulting from natural decomposition of feldspar
and granite. Composed of microscopic disk-shaped platelates, which give clay its slippery, plastic
quality.
CLAYBODY: Clay mixture formulated of clays and other ceramic raw materials to give desired
working characteristics.
COIL CONSTRUCTION: Ceramic forming method utilizing rope-like coils of plastic clay,
assembled in successive courses to build up wall of vessel or sculpture.
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.
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.
COTTLES: Adjustable wooden forms used in casting plaster molds.
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.
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 slipcasting. Extremely low percentage of deflocculant additive is needed - 1/4 of 1% (of dry batch
weight) soda ash and/or sodium silicate.
EARTHENWARE: Low-fired ware, usually still porous after firing - must be sealed with vitreous
glaze to be functional.
ENGOBE: Slip formulated with less raw clay content in order to reduce drying shrinkage, to allow
application to bone-dry or bisque-fired clay. See UNDERGLAZE.
EXTRUDER: Machine which forces plastic clay through a die to produce extruded clay shapes.
FETTLING KNIFE: Long tapered knife useful for trimming cast or pressed pieces, for separating
mold components, and for many other clay studio applications.
FIRE CLAY: Highly refractory secondary clays with minimal fluxes and usually fairly coarse particle
size - Low shrinkage, buff-color, often non-plastic.
FLUX: Low-melting component in clay or glaze which reacts with silica to form glass.
FOOT: Base of a ceramic piece.
GLASS-FORMER: The primary material which, in combination with fluxes, forms the glass
essential to all fired ceramics. Primary glass-former at all temperatures is silica.
GLAZE: Coating of powdered ceramic materials, usually prepared and applied in water suspension,
which melts smooth and bonds to clay surface in glaze firing.
GLAZE-FIRING: Kiln firing in which glazes are melted to form a smooth glassy surface.
GREEN; GREENWARE: Any dry, unfired clay form.
GROG: Filler or tempering grit formed by grinding high-fired clay; added to claybodies to reduce
shrinkage and give structure for throwing or handbuilding.
HANDBUILDING: Forming plastic clay by hand without the wheel, using pinching, coiling,
and/or slab construction.
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.
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 FURNITURE: Refractory shelves, posts, and stilts used in a kiln to support the wares.
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.
LEATHER-HARD: Condition of clay where it has stiffened but is still damp. Point at which
pieces are joined and most surface modification and trimming are done. Soft-leather-hard ideal for
forming, joining, thick slip-decoratin. Medium-leather-hard good for thin slip-decorating, joining,
incising, carving, piercing. Hard-leather-hard good for thin slip-decorating, carving, scraping.
MATURITY; MATURING POINT: Firing point at which a claybody or glaze reaches it's desired
condition of color, hardness, density, etc.
MOLD-RELEASE COMPOUNDS - In making plaster molds, compounds which 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.
OUT-GASSING: The escape of gases from clay and glazes during firing. Carbonates, sulfates,
nitrates, organic contaminants, and chemically-combined water volatize between 600 and 1200
degrees F. Other compounds, especially fluxes, volatize later during glaze-melt.
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.
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.
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 clay body or slip containing
paper pulp, which reduces shrinkage in drying stage, and encourages extremely strong joinery,
allowing unconventional joinery such as wet to dry.
PATINA: An overall thin wash of glaze or oxide stain, allowing the color and texture of the
claybody to show through.
PINCHING: Hand-building 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 which are not given opportunity to “heal” at end of firing.
PLASTICITY: Quality of moldable flexibility in damp clay - superior plasticity depends on smaller
clay particle size, slight acidity, less non-plastic additives, aging of damp claybody, adequate water
content, and/or addition of accessory plasticizers, such as veegum-T or macaloid.
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.
POSTS: Refractory columns used as kiln furniture to support kiln shelves. See STILTS.
PUG-MILL: A machine similar to an over-sized meat-grinder, used to homogenize plastic
claybodies. De-airing pugmill has vacuum pump attachment, effectively removes all air from clay,
eliminating need for hand-wedging.
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.
QUARTZ INVERSION: Abrupt expansion in heating and corresponding contraction in cooling
which occurs in silica crystals in all clay and glazes at around 1063 degrees F. Greatest risk is in
cooling high-fired wares, and in re-firing previously high-fired wares.
RAKU: In the West, firing process inspired by traditional Japanese raku firing. In American raku,
work is removed from kiln at bright red heat and subjected to post-firing smoking by placing in
containers of combustible materials, which blackens raw clay and craze-cracks in glaze.
REDUCTION FIRING: In fuel-burning kilns, firing atmosphere with insufficient oxygen to
completely combust fuel, introducing abundance of un-oxidized carbon and hydrogen, which extract
oxygen molecules from surface of wares, altering appearance of clay and glaze.
REFRACTORY: Capable of withstanding very high temperatures.
RESIST: Material used in glazing and decorating which can be applied to surface to prevent
adhesion of slip or glaze. Resists may be in the form of tape or adhesive-backed paper, or liquid.
Common liquid resists include water-emulsion WAX RESIST, hot wax, and LATEX.
RIB: Wide, flat hand-held 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.
“S”-CRACKS: “S”-shaped cracks which 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.
SCORING: Process of incising shallow grooves into surface of wet or leather hard clay in crosshatch pattern before applying slurry and joining pieces.
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.
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. Cure is to slightly increase flux and/or decrease silica in 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%.
SLAB-ROLLER: A mechanized but usually manually operated device for rolling out large uniform
slabs of clay.
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.
SLUMP-MOLD: A mold over which a moist clay slab is slumped in order to create a vessel form.
SPRAYING: Application of liquid slip, engobe, glaze, or stain, using mechanized spray
equipment. Gives smooth glaze coating, but with less glaze-pooling in recesses.
SPRAY-BOOTH: Open-front enclosure with an exhaust fan at the rear, designed to draw off all
overspray and other toxic dust or fumes.
STAIN: Commercial ceramic colorants which have been fritted in order to eliminate solubility
problems and give greater stability in firing and truer color before firing. Mixture of ceramic stains or
pure coloring oxides (sometimes with a little flux) in water suspension, which can for overglaze
brushwork, or as a patina on unglazed clay.
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 degrees F. (cone 5-11).
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.
TERRA SIGILLATA: Ultra-refined clay slip which 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 redblack pottery and Roman red wares were finished with this technique, without the use of glaze.
THERMAL EXPANSION: The physical expansion and contraction which accompanies the
heating and cooling of most materials.
TRIMMING: At the leather-hard stage, removal of excess clay from a piece, using any of a variety
of sharp cutting tools.
UNDERCUT: Common flaw in plaster or bisque molds, where the clay or casting catches and will
not pull free without breaking or distorting. See DRAFT.
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.
UNDERGLAZE PENCILS: Underglaze pigments in pencil form, excellent for marking wares and
test-tiles, and for "pencil-drawn" decorative effects.
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.
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.
WEDGING: Process of kneading the clay with the hands to remove air-bubbles and insure
homogenous mass.
WHEEL-WEDGING: Working the clay up and down in a cone shape on the wheel to align the
platelets in a spiral formation and thus increase control in centering and throwing.