Ball clay: Highly plastic clay which fires off-white; workable, fine-grained, sedimentary clay used in white earthenware, china, and porcelain bodies, engobes and glazes; found in the U.S.A. in the state of Kentucky and Tennessee.
Bat: Any slab used as a base for throwing or handbuilding clay; also applies to a though usually made of plaster, pressboard, plywood, or bisqued clay.
Batch: A mixture of materials or ingredients calculated by parts or weight.
Bisque/biscuit: unglazed but fired clay, usually accomplished in a low temperature firing prior to the glaze fire; also applies to unglazed ware fired high, as in porcelain bisque.
Blistering: Bubbles formed in the glaze during the firing due to liberation of gases or impurities, caused by firing that is too fast putting into the glaze a material such as trisodium phosphate, which will promote decorative bloats or blisters
Bloating: Occurs in clay bodies when they are overfired, contain air, or have added bloat ingredients.
Blunger: A mixer with revolving paddles for liquid mixes; can be used to make casting slip, to re-mix scrap clay, and to mix glaze.
Body: A combination of natural clays and non-plastics, especially formulated to have certain workability and firing characteristics.
Bone Ash: The mineral calcium phosphate, or ash from bones; found in Europe and the Orient and used in clay bodies; used as glaze flux in the U.S.A.
Bone China: Percelain of high translucency made with bone ash, produced mainly in /England and Japan; highly prized but not technically superior to feldspathic porcelain bodies made in the U.S.A.
Burnishing: Polishing with a smooth stone or tool on leather-hard clay or slip to make a surface sheen, bonfired or low-fired (the surface will not stay shiny at temperature above 2000º Fahrenheit [1093º C]); the traditional process used in North and South American Indian pottery.
Calcining: Firing in red heat to remove physical and chemical moisture; treatment for surface clay s and other high-shrink materials such as zinc oxide, colemanite, and trap rock, to prepare them for use in glazes
Calipers: Two-pronged device for measuring inside and outside diameters.
Candling: Leaving a kiln on very low heat for a long time without changing the input.
Casting: Process of forming shapes by pouring deflocculated liquid clay slip into plaster molds for duplication or mass production.
Celadon Glaze: French name for a sea-green glaze with a small percentage (1/2 to 2% ) of iron as the colorant, fired in reduced atmosphere; innumerable variations of green, gray-green, blue green and gray; a stoneware or porcelain glaze first used by Oriental potters.
Celsius: Measurement of temperature used many countries.
Centering: Pushing a mass of clay on center with the centrifugal motion of a potter’s wheel.
Ceramics: Art and science of forming objects from earth materials containing or combined with silica, produced with the aid of heat treatment at 1300º F (704º C) or more.
China: (1) A porcelain clay body, with up to 1 percent absorption , usually translucent; industry fires high to vitrification and glazes low; studio potters usually fire clay and glaze together to high temperatures by the traditional Oriental method, or make low-fire porcelain by the European method.
(2) Whiteware, vitreous and hard, sometimes translucent. (3) A general term used in the trade when discussing any kind of tableware.
China Clay: Primary or secondary kaolin, refractory, not very plastic, white-burning, rare in the world, found in the U.S.A. in a few south-eastern states; used in the blending of all whiteware and porcelain bodies.
Chuck: A cylindrical form used to put pots in or over, used upside-down for foot trimming on a potter’s wheel.
Clay: Theoretically Al2O3.2SiO2.6H2O; fine-grained earth materials formed by the decomposition of igneous rock; when combined with water, clay is plastic enough to be shaped; when dry, it is strong; and when subjected to re heat or above, it will become progressively more dense and rock-like.
Clay Slip: Liquid clay; clay substance that has been suspended in water which possesses the consistency of creamy buttermilk or thicker. Some slips have the addition of colorants ( generally metallic oxides) to be used as decoration and are applied on the surface of greenwares. Some slips have the addition of deflocculating agent to be used for casting in plaster molds.
Coiling, coil-building: Age old method of constructing hollow forms by rolling and attaching ropes of soft clay.
Cones: Pyrometric cones, Orton or /seger brand, are 2 inches (5 cm) pyramids made of clay and glaze constituents that soften and bend at specific temperatures. Cones are placed in the kiln during firing as a guide, and to indicate the final heat; they are classified by numbers coded to their softening point.
Cordierite: A magnesium aluminum silicate with low expansion characteristics, used for kiln furniture.
Core: The interior of a piece, or a frame or stuffing on or over which work can be supported; combustible core materials can burn out in the kiln; rigid cores should be removed before the clay shrinks.
Crackle: Decorative and intentional fissures netting the surface of a glaze due to a variation in expansion and contraction of the glaze and the clay body.
Crawling: Glaze that has separated into mounds on the clay surface during firing, generally caused by fluffy or high-shring materials in the raw glaze; sometimes called alligator glaze.
Crazing: A faulty and unintentional cracking of the glaze due to a variation in body and glaze expansion and contraction. Crackle is the term used when these
Cupric, Cuprous: Pertaining to copper: cupric compounds contain CuO; Cuprous compounds contain Cu2O
Cross-draft kiln: Kiln with burners across from each other so that the atmosphere circulates in a criss-cross pattern and then up and down before exiting via a flue.
Crystalline glazes: Large crystals grown on the glaze surface during firing and cooling, primarily induced by high zinc oxide and low alumina content in the glaze.
Damper: Adjustable shutter to control draft at the kiln flue.
De-airing: Process of removing air from plastic clay mixtures either by hand wedging or by mechanical means in a vacuum chamber.
Deflocculant: Electrolyte or catalyst that causes clay and water mix to become liquid faster with a minimum addition of water; examples are sodium silicate, soda ash, Calgon, tannic acid, and Darvon.
Deflocculate: To function as an electrolyte in a clay casting slip mixture; to cause clay and water mix to become liquid faster with a minimum addition of water.
Delft Ware: Buff-colored earthenware covered with a white tin-enamel glaze and decorated with cobalt blue overglaze painting on the unfired glaze. First made in Holland and later in England in the 1600s, and still made today.
Devitrify: To overfire, or fire in such a way that a clay or other surface is spoilt.
Dipping: The application of an engobe or glaze by immersing areas of the piece quickly and allowing the excess liquid to drain off.
Downdraft kiln: Kiln with fire entering at the side or base, where heat is forced around, up, and down through the ware, and finally exits via a glue at the back of the chamber.
Dry foot: No glaze on the footrim; usual for stoneware and porcelain because the claybody is fired to density.
Dunting: The explosion-like cracking or breaking of pots from sudden changes of temperature on cooling, or from wrong clay body composition.
Earthenware: All ware with a permeable or porous body after firing; by definition earthenware has 10 to 15 percent absorption.
Elasticity: When referring to clay, its ability to be maneuvered without breaking.
Enamels: (1) As applied to pottery: low-temperature glazes, usually applied over the glazes. (2) As applied to metals; transparent or opaque glaze that melts lower than the copper, silver, or gold on which enamel is used as the decorative finish; usually fired about 1300º F (704º C)
Engobe: A liquid clay slip colored with metallic earth oxides or glaze stains or glaze stains applied to wet or leather-hard ware for decoration; also natural clays of different colors applied on raw ware for decoration. Engobe can be covered by glaze or used alone.
Extrusion: Forcing plastic clay through an auger or form, mechanically or by hand, to change its shape.
Fahrenheit ( ºF) : Measurement of temperature used in the U.S.A.
Faience: A general word covering low-fire colored clay bodies, such as Egyptian paste. Often a misused term, it is more particularly a French name for the tin-enameled earthenware made in the Italian town of Faenza during its period of Hispano-Moresque influence.
Feldspar: Mineral ground in granite which melts around 2300º F (1260º C), used as a flux in clay bodies and glazes. When feldspar rock loses its alkaline content through decomposition, it becomes kaolin and is thus the origin of clay.
Ferric, Ferrous: Pertaining to iron: ferric compounds contain tervalent iron; ferrous compounds contain bivalent iron.
Firebox: The chamber of certain kilns into which the fuel is fed and in which the initial combustion takes place.
Fire Clay: Secondary clay that withstands high temperature and has varying amounts of free silica in addition the clay molecule; prevalent throughout the world.
Firing: (1) Heating in a kiln to the required temperature for clay or glaze, at least to red heat, 1300º F (704 C); most clay and glaze matures between 2000º and 2300º F (1093º C and 1260º C).
Fit: Good or bad adjustment of a fired glaze to a clay body.
Flange: The tiny shelf on which a lid sits; or the extension on the lid to fit into the vessel.
Flatware: Dishes, plates, saucers, and low bowls are called flatrware in the pottery industry to distinguish them from the hollow ware.
Flint: Quartz, silica, sand, and flint are terms applying to minerals used in clay bodies and glazes containing nearly 100 percent silica oxide; the minerals are similar but not identical.
Flocculate: To thicken a clay suspension by the addition of an acid, as acetic acid.
Flue: (1) The passageway for flames in kilns-essentially the combustion space; the flue is the area around the stacking space. (2) The place of escape for the products of combustion from the chamber.
Flux: A material or mixture having a low melting point or lowering the melting point of other materials. One of the three main components of glaze; also used to increase density in clay bodies; examples include lead, borax, lime, feldspsar, and frit.
Foot: Base or bottom of a piece
Frit: Mixture that is melted, cooled quickly by quenching the molten mass in cold water, and ground to a fine powder. Fritting renders soluble glaze ingredients, such as soda ash, insoluble, and poisonous materials, such as lead, non-poisonous. Made commercially or in the studio.
Glaze: Glassy melted coating developed by chemicals and heat on a clay or metal surface; technically, an impervious silicate coating formed by the fusion of inorganic materials. Glaze has a similar oxide composition to glass, but also includes a binder. Glaze provides decoration and color, prevents penetration of liquids or acids, and yields a matt or glassy, easily cleaned, functional surface.
Glaze stains: Fabricated ceramic colorants from metallic oxides mixed in combination with other elements to widen the glaze decorating palette, manufactured to be stable at various temperatures; sold by code number, color, and company.
Greenware: Finished leather-hard or bone-dry clay pieces not yet fired; raw ware.
Grog: Crushed or ground-up fired clay, purchased commercially or made by the potter; used to reduce shrinkage, yield texture, give fired clay more resistance to temperature change, help in even drying and firing, and help large pieces to stand up during construction. More than 30 percent grog addition may cause too much porosity and reduce fired strength.
Gum Arabic: A natural tree gum used as a drying and adhering agent when applying overglaze enamenls or when reglazing a piece has previously been glaze-fired. A synthetic gum such as CMC or metho-cellulose can be used instead; it has the advantage of being water soluble and will not ferment.
Hard-paste porcelain: Sometimes called true porcelain; a body composed of kaolin, filler, and flux that when fired to density is white, vitreous, and translucent where thin. Hard paste porcelain fires the clay body and glaze together to the top temperature soft-paste porcelain is bisqued high and glazed low. As in the dinnerware industry today.
Hollow Casting: Pouring liquid clay slip into a gollow plaster mold to create a shell of specific shape.
Hollow Ware: A trade term for hollow dinnerware forms.
Impermeability: In ceramics this term refers to the property which results when clay forms have been rendered non-porous by vitrification, i.e.,which have achieved maximum density without meltin in a kiln.
Jigger and jolly: A jigger is an adjustable arm that holds a profile-tool or template for one side of a shape pressed against clay on a plaster mold; the mold revolves on a jolly or power-driven spindle. Many commerdial wares are made by this process, either on hand –jigger machines or with total automation.
Jomon: Blackened, coiled, textured, sometimes cord-marked Japanese ware thought to date from 10,000 to 200 B.C.
Kaolin: Anglicized form of the Chinese word for china clay. Pure kaolin is rare; it is a perfectly white-burning, high-firing natural clay that can be either primary or secondary in terms of geological formation; it is the essential component of porcelain bodies and an ingredient in many glazes.
Kickwheel: A potter’s machine for working clay with a centrifugal motion propelled by kicking.
Kiln: Furnace for firing clay, slumping glass, or melting enamels; studio kilns can achieve temperatures up to 2500 F(1371 C), depending on their construction materials; they can be fueled carbonaceously, organically, or electrically.
Kiln Furniture: Refractory slabs, posts, and setters for supporting ware in the kiln; handmade or purchased.
Kiln wash: Half clay, half silica, mixed with water to coat kiln shelves.
Lead: The most active glaze flux at low temperatures; found in red lead, litharge, and lead carbonate; poisonous in the raw, unfritted state; the flux in crystal glass mixtures.
Leather-hard: Cheese-hard stage which clay reaches before being bone-dry; stiff enough to support itself, but can still be altered.
Limestone: Impure calcium carbonate, whiting, CaCO3, chalk; a much-used high-temperature glaze ingredient and an auxiliary body flux in porcelains.
Luster: A brilliant iridescent metallic film on glaze, formed from certain metallic salts in reduction; the technique was developed by the potters of Persia and Valencia during the Middle Ages.
Luting: A cross-hatch and moistening method of putting together coils, slabs, or other clay forms in the wet or leather-hard stage; the same as scoring.
Majolica: The decorative application of coloring oxides and stains over an unfired glaze that fuses into the base glaze during firing, leaving fuzzy edges. The term come from the island of Majorca. The della Robbia workshop was famous for Majolica during the 15th. Century. Also the term for a kind of white color, and historically for certain white undecorated earthenwares.
Majolica Glaze: An opaque glaze with a glossy surface, usually white, generally opacified by tinoxide; a base for colored stain overglaze decoration; traditionally thought of as Italian and Spanish, also used at Delft, Holland, and Persia.
Matt: Dull, non-reflective surface; in the case of glaze, due to deliberate composition or to immature firing.
Maturing: Reaching the temperature in a kiln which develops desired properties in the ware; or the stage that materials, bodies or glazes need to reach in order to be durable. Potters talk about mature glazes, mature bodies, referring to the look and feel of density. Most clays and glazes have a long maturing range, at any point of which they could be deemed mature.
Mica: A sheet-like mineral found in small flakes in some natural clays; the material that shimmers in many low-fire pots of primeval cultures.
Millefiore: Similar to Neriage; a traditional technique in glass and clay where several, or many slabs of color are combined in patterns, drawings, or shapes and cut through in cross-section to make many similar onesl
Mimbres: A group of Indians in Southwestern U.S.A. who made a unique contribution to clay art about A.D. 900 to 1200.
Mishima: Carved decoration in leather-hard clay, covered with engobe and ribbed off when drier, leaving engobe inlaid in the carving.
Mold: Usually a plaster form, single or multi-pieced, which will be used to reproduce any number of copies of the original model in clay or plaster.
Mosaic: A pictorial composition made of many small shapes, usually ceramic, glass, or stone.
Mullite: The aluminum silicate crystal created during firing to vitrification that gives strength to stoneware and porcelain as opposed to earthenware; can be formed as low as 1832 F (1148 C).
Neutral atmosphere: An atmosphere in a kiln that is neither completely oxidizing nor completely reducing.
Noborigama: Climbing kiln, the basic tube kiln, with chambers which can be fired individually.
Off the Hump: Method of throwing many small forms consecutively from one large mound of clay.
Once firing: Glazing leather-hard or dry ware and firing to maturing temperature (this skips the first bisquing); frequently used in commercial production; often the method in salt and wood-firing.
Opalescent Glaze: Glaze with a milky moonstone or translucent quality.
Opacifier: A material that causes a glaze to become opaque bu producing minute crystals. Tin, zirconium, and titanium oxides are used as opacifiers in comgination with various oxides.
Open firing: Firing thai is not done in an enclosed kiln
Orifice (gas burner) The opening through which the gas fuel comes from the gas line and is mixed with air as it enters the burner.
Overfire: to fire a clay body or glaze above its maturing point.
Overglaze: A low-temperature ceramic enamel painted on a previously glazed and fired surface, then fired for a second time at a lower temperature, usually as the final firing process. Bright colors like red and orange that would burn out at high temperatures will be maintained in the lower firing (around 1300° F/705° C). Often called enamel or china paint.
Oxidation (Oxidation firing): The firing of a kiln or open fire with complete combustion so that the firing atmosphere contains enough oxygen to allow the metals in clays and glazes to produce their oxide colors. Electric kilns always produce oxidizing firings unless reducing materials are added. Bright and clear low-fire colors are often associated with glazes and clays fired in an oxidation atmosphere.
Oxide: A combination of an element with oxygen. In ceramics, oxides are used in formulating glazes and for coloring glazes and clays. They are also used for decorating ware.
Peephole: A hole in the door or wall of a kiln through which the ceramist can watch the pyrometric cones, the color of heat in the kiln, and the process of the firing.
Pinholes: Small holes in a glaze caused by the bursting of blisters formed by gases as they escape through the glaze during firing.
Plastic, Plasticity: The ability of a damp clay body to yield under pressure without cracking and to retain the formed shape after the pressure is released.
Porcelain: A transluscent, nonabsorbent body fired at high temperature. White and hard, it was first developed in China. Traditionally fired in the 2370°-2640° F range, some porcelain bodies have been developed that mature in the 2230-2340° F range.
Pottery: Originally a term for earthenware, now loosely used to refer to any type of ceramic ware, as well as to the workshop where it is made.
Press mold: Any mold made from plaster, fired clay, wood, or a found object into which damp clay can be pressed to reproduce the shape of the mold.
Primary clay: Clay found in nature that was formed in place rather than transported by the action of water. Also called residual clay. Kaolin is a primary clay.
Pug mill: A machine used to blend clay into a moist, workable consistency. Also used to recycle clay scraps and, when equipped with a vacuum pump system, to de-air clay.
Pyrometer: A device for measuring and recording out the firing and cooling process.
Pyrometric Cones: Small pyramids of ceramic materials formulated to bend over and melt at designated temperatures. Orton cones in the United States and Seger cones in England and Europe have different ranges.
Quartz Inversion point: The point at which the silica crystals in clay change in structure and volume during the rise and fall of the temperature in the kiln. This development influences the fit of glaze to clay body.
Raku: Originally a name used by a Japanese family that has made tea ceremony ware since the seventeenth century. Now refers to both the process of raku firing and to ware glazed in such a firing.
Soft and porous, traditional raku ware was lead-glazed, placed in a red-hot kiln, and quickly withdrawn when the glaze melted.
In the West, lead is now rarely used in raku glazes. Leadless frits and Gerstley borate are now commonly used fluxes in place of lead. Raku ware is often reduced after firing by burying it in straw, sawdust, paper, or other combustible material, then covering it with an airtight lid to create a reducing atmosphere that aids in producing luster or opalescent colors.
Reduction (reduction firing, reduction atmosphere): A firing in which insufficient air is supplied to the kiln for complete combustion. Under these conditions, thecarbon monoxide in the kiln combines with the oxygen in the oxides of the clay body and glaze, causing the oxides to change color. Commonly associated with high-fire stoneware, porcelain, raku, and lusters.
Refractory: Resistance to heat and melting. Refractory materials are used in porcelain and stoneware, also used for building kilns and kiln furniture, and in combination with other materials, as kiln insulation.
Resist: A method of applying a covering material such as wax, latex, or special luster resist to bisque or glazed ware, then coating the piece with a glaze or a second glaze.
The resist materials will not accept the glaze so on firing, the color of the covered area will remain intact.
Rib: Acurved tool made of wood, metal, or plastic, used for shaping , scraping, or smoothing clay objects.
Saggar: A refractory container in which glazed ware is placed during firing to protect it from the kiln fire. Saggers are also used to introduce local reducing material by placing leaves, seaweed, cow dung, or other organic material in the sagger with the ware.
Salt glaze: A glaze formed by introducing salt into a hot kiln. The vaporized salt combines with the silica in the clay body, forming a sodium silicate glaze on the surface. It also combines with the silica in the kiln bricks, coating them with a glaze that will be transferred to any ware fired in the kiln later. Salt glazing releases noxious and toxic fumes, so many potters now use alternatives for vapor firing.
Secondary clay: Natural clay that has been moved by water or wind from its source and settled elsewhere in deposits.
Setting: Placing the ware in the kiln in preparation for firing.
Sgraffito: Decoration of pottery made by scratching through a layer of colored slip to the differently colored clay body underneath.
Short: clay that is not plastic. Cracks will form on handling after brief manipulation.
Silica: Oxide of silicon, SIO². Found in nature as quartz or flint sand, it is the most common of all ceramic materials.
Silicate of Soda: A solution of sodium silicate that is used as a deflocculant to help in the suspension of clay materials in slip.
Silicon Carbide: Used in a glaze to produce local reduction in an electric kiln. Also used in making kiln furniture for high-fire ware.
Sintering: The stage in glaze firing during which the heat converts a powder into a cohesive mass before melting it into a glassy material.
Slab Roller: A mechanical device for rolling out slabs to a set, consistent thickness.
Slip: A suspension of clay in water used for casting pottery or sculpture in molds. Slip (sometimes called engobe) can also be used for painted decoration or for the sgraffito technique.
Slip casting: Forming objects by pouring slip into a plaster mold. The mold absorbs the water in the slip so that solid clay walls are formed to create a positive of the original.
Slip glaze: A glaze that contains a large proportion of clay. Generally one that contains enough flux to form a glaze with few or no additives. Albany slip was widely used as slip glaze in traditional American potteries.
Slip trailer: A rubber syringe used to apply decorations of slip on ware.
Soaking: Maintaining a certain temperature in the kiln for a period of time to achieve heat saturation.
Sprigging: The process of attaching low-relief decorations of damp clay on to already formed greenware.
Stains: Commercially processed and refined raw chemicals yielding ceramic stains offer a wide range shades for coloring clays and glazes. They are generally color stable than oxides.
Stilts: Triangular support with either clay (for low fire) or heat-resistant metal points (for low or high-fire), used to support pieces of glazed pottery during glaze firing. They support the ware above the shelves to keep the glaze from sticking the ware to the shelf.
Stoneware: A type of clay-body fired to a temperature at which the body becomes vitrified, dense, and non-absorptive, but not translucent. Natural stoneware is usually brownish in color because of the presence of iron, but there are formulated white stoneware bodies. Usually mature at temperatures above 2192° F.
Temper: Any material, such as sand, mica, or crushed fired pottery fragments (grog), added to a clay body to make it more porous and less likely to shrink and warp.
Tenmoku (temmoku, tienmu): High fired, saturated iron glaze; black, brown, and yellowish. Used by the Chinese and Japanese, especially on tea ware. Still a popular glaze.
Terra-cotta: A low-fire, porous, reddish clay body, frequently containing grog or other temper. Used throughout history for common, utilitarian ware; also used for sculpture.
Terra sigillata: A fine slip glaze used byu the Greeks, Etruscans and Romans to coat their pottery. It fired black or red according to the kiln atmosphere. Now used in a wide variety of colors by many potters and sculptors to surface their ware or sculpture.
Thermal shock: The stress to which ceramic material issubjected when sudden changes occur in the heat during firing or cooling.
Throwing: Forming objects on the potter’s wheel using clay body with plastic qualities.
Tin glaze (tin lead glaze): A low fire, opaque glaze containing tin oxide.
Trailing: a method of decorating in which a slip or glaze is squeezed out of a syringe. Historically decoration trail from a quill inserted in a narrow-neck clay cup.
Undercut: A negative space in a solid form, creating an over hang. Casting a form with an undercutting requires a multipart mold in order to release the mold from the cast.
Underfire: to fire a clay or glaze accidentally or deliberately- to a point below its maturing point. Under-firing can turn a normally glossy glaze into a matt surface.
Underglaze: Any coloring material used under a glaze. The color can be provided by oxides or by commercially prepared glaze and clay-body stains.
Updraft kilns: A kiln in which the heat goes up through the chamber and is vented through the top of the kiln.
Viscosity: The ability to resist running or flow. A glaze must have enough viscosity to avoid flowing off the ware when it is melted under heat. China clay in a glaze assists in stabilizing it.
Vitreous: Pertaining to or having the nature of glass. In ceramics, a vitreous glaze or clay body has been fired to a dense, hard, and nonabsorbent condition. Earthenware clay-bodies do not reach this stage.
Vitrification: The state of being vitrified, or glassy.
Ware: A gejneral term applied to any ceramic- earthenware, stoneware, or porcelain- in the green, bisque or fired state.
Wax resist: A method of decoration in which melted wax or oil emulsion is painted onto the clay body or onto a glazed piece.
Wedging: Any one of various methods of kneading a mass of clay to expel the air, get rid of lumps, and prepare a homogeneous material.
Wedging table: A table of plaster, wood, or concrete often covered with canvas, on which clay can be wedged. A stretched wire attached to the table allows one to cut the clay to check for air bubbles, lumps, or lack or homogeneity.