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Glossary Of Ceramic Terms
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Airbrush: A device operated with an air compressor used for spraying on colors, either in overall coverage or decorative form.

Antiquing: A method of applying color and wiping it back to accentuate the detailed surface.

Banding Wheel: A turntable operated by hand, used for decorating purposes.

Basecoat: Generally an all over coat of color on bisque on top of which detailed decorating is done.

Bat: A plaster disk or slab for clay work.

Bisque: Clay that has been fired but not glazed. Sometimes referred to as "biscuit." Cadmium: Heavy metal used in producing red glazes and underglazes.

Burnishing: A Technique where the leather hard clay is polished with a hard instrument to force the smallest clay particals to the surface creating a soft sheen, this surface remains after the pot is fired.

Calcine: To purify a material through the action of heating to red heat 1292-1382 °F.

Casting: A clay form made from a mold. May also refer to plaster castings.

Casting Slip: A liquid clay used in the process of forming objects with molds. Also referred to as "slip."

Cavity (of a mold): The inside section of a mold where the casting is formed. Ceramics: Clay forms which are fired in a kiln.

Ceramic Change: The slow process of clay becoming ceramic. Clay which is exposed to heat 1112 °F, loses its chemically bound water molecules and can no lnoger be broken down by water. Once this change has occured it cannot be reversed.

China: A term which usually refers to the bone china of England, but also is associated with vitreous white wares and porcelain.

Clay: The Decomposition of Granite through the process of Kaolinization creates clay ( See Kaolinization). Clay is a minteral with a plate ( Platelet) like structure; it is these plates, (about 0.5 microns across) when lubricated with water, slide against each other to form the plastic mass we know as clay (see Water). 'Primary' clays are those found close to the area of Kaolinization and hence the purest (Kaolin or Chins Clays). Secondary clays are those moved by water away from the site of Kaolinization and get progressively more plastic and less pure (Ball Clays, Fire Clays, Earthenware's and Marls).

Clay Body: A clay designed for a special purpose. It is created by blending different clays or by adding other materials to clay, such as feldspar and flint in order to produce a desired workability, maturing temperature, or finished result. A clay body is the result of technology.

Cleaning Greenware: The process by which mold seam lines and surface imperfections are removed from unfired clay objects.

Coats: Applications of ceramic color by brush, sponge, and spray which cover an entire area or a specific area of a piece.

Cobalt: One of the strongest coloring oxides used by the potter. Cobalt creates a dark dense royal blue in most cases. Historically used in China as a painting pigment on blue and white wares.

Coiling: A meathod of handbuilding a form using long rolled out, or extruded, snake-like lengths of clay. Each coil of clay is integrated with the previous one to build the work up. The coils may be completely obliterated in the construction process or retained for their decorative qualities.

Conditioned Brush: A brush lightly coated or dampened with a fluid to prepare it for the application of a specific type of color or medium.

Conditioning Coat (glaze or underglaze): A thin coat of color that will soak into the greenware or bisque well.

Cone: (See Pyrometric Cones).

Cone Plaque: A small clay cone holder used when cones are placed on the shelf of the kiln.

Cone Temperature: The mixture of time and temperature at which the cone will bend.

Crackle: Decorative craze lines in a glaze.

Craters: Bubbles in the glaze finish which break.

Crazing: The fine network of small cracks that occurs on glazes. The Japanese encourage crazing and will stain cracks with concentrated tea.

Crawling: Glaze which pulls together and beads up, leaving bare spots of bisque. Also referred to as "separation."

Crystalline Glazes: Mosr glazes have no easily visable crystal structure. Crystalline Glazes have large and dramatic crystals up about three inches across. A high alkaline low alummina glaze is vital for crystals to develpoe. Additions of zinc and titanium also help seed the crystals. An extremely slow cool of the kiln is necessary to allow the crystals to grow. Because of the low alummina content in crystalline glazes they are very runny, often pots are supported in the iln on stilts to avoid them adhering to the kiln shelves, the stilts can be broken off after the firing.

Defloculant: The alkaline substance which is added in extremely small amounts to slip to make it more fluid without adding excessive amounts of water.

Dipping: The process of dipping ware into glazes.

Dry-Brushing: A technique of applying color which produces a feather-like effect using a dry brush and liquid colors.

Dryfooting: The process by which glaze is removed from the bottom or foot of a clay object so it may be fired without stilting.

Dunting: Cracks, which occur on pottery during the heating or coolinf cycle of the firing. They are usually caused by the silica inversion at 1063°F (Alpha to Beta phase) or the Crystobalite (one of the 'phases' os silica) inversion at 428 °F in both cases there is an expansion and contraction, of around 2-3%, in the heating and cooling cycles.

Earthenware: A low-fire porous clay bodies which are fired to maturity at approx. 2000°F. They are not waterproof and must be glazed to be used.

Element: The heating coils of an electric kiln. (Tired or burned-out elements refer to elements which carry too little or no electrical current for producing heat).

Enamel: A form of low temperature glaze that is applied on top of an already fired higher temperature glaze. Enamels are often lead based, as it is a flux, which works at a low temperature.

Feldspar: One of the predominant naturally occurring fluxes used primarily in stoneware glazes. Feldspars can be the only flux present in a stoneware glaze although this is uncommon and additions of calcium usually supplement it.

Fettling Lines (seam lines): The ridges created on a casting where the mold comes together. They are usually removed during the cleaning process.

Finger-sand: Gentle rubbing of the glazed surface to remove ridges.

Firing: Clay is hardened by heating it to a high temperature, fusing the clay particals. Primitive pottery is usually fired on the ground or in pits with whatever flammable material is available. Kilns allow a more efficient use of materials and more control over the atmosphere during a firing. The two basic atmospheres, oxidation and reduction, affect the color of the final piece.

Fired Finish: A finish that must be fired to produce proper color and surface finish. Fired Products (fired color products): Products which must be fired.

Firing Chamber: The interior of a kiln in which the ceramic ware is fired (also referred to as a fire box).

Firing Cycle: A system of gradually raising and lowering the temperature of a kiln to properly fire ware.

Flocculate: To thicken.

Flowing Coats: An application of glaze applied with a fully-loaded brush so the color flows onto the surface of the ware.

Flux: A substance which causes or promotes melting.

Food-safe: A product that has been tested and determined to be safe for use on surfaces which come in contact with food or drink.

Foot: The base or the part of the piece of ceramic on which it rests. Glaze: A fired glassy coating on a piece of ceramic.

Frit: A combination of materials that have been melted into a glass, cooled, and reground into a powder prior to being added to a glaze recipe.

Fuse: To melt.

Gloss : A shiny, glass-like finish. Greenware: Unfired clay forms or shapes.

Greenware: Unfired pottery.

Grit Cloth: A rough scrubbing material used in the process of cleaning greenware.

Grog: Clay that has been fired and then ground into granules of more or less fineness. Grog is considered a filler, and is added to clay bodies for several reasons; it helps open a tight or dense body, promotes even drying, which reduces warping and cracking, and reduces overall shrinkage. Grog also adds tooth and texture to a clay body aiding in the ability of the body to maintain its form during construction.

Glaze: A coating of material applied to ceramics before firing, that forms a glass-like surface. Glazes can be colored, opaque, translucant or matte.

Hard Spot: An area on greenware or bisque surface that resists color application.

Hot Spot: A section of a kiln that fires to a hotter temperature than the rest of the kiln.

Incise: The process of carving a design into a greenware surface.

Inlay: A decorative technique where a pattern is carved into the clay at the leather hard stage and contrastingly colored soft clay is forced into the decoration. When the clay is a little drier the excess is scraped off to reveal the pattern.

Iron Oxide: One of a potters favorite colorants, when combined with the right glaze and firing, iron oxide can produce greens, browns, blacks, yellows, oranges, subtle blues and grays. Most of the best color responses for iron in a glaze need a reduction firing. Iron is also a useful colorant in clay bodies and is best introduced by adding high iron clays to the clay recipe.

Kaolin: A china clay in its purest form, primary clay.

Kaolinization: The natural formation of kaolin from the decomposition of feldspar.

Keys (of a mold): The series of notches and bumps carved in the excess plaster around the cavity of the mold to !nsure a proper fit.

Kiln: Basically and insulated box, which is heated to fire clay. They can be either, cross draft, down draft, or up draft. The draft refers to the direction the combustion gasses have to travel from input to exit flues, since no combustion takes place in an electric kiln there are no input or exit flues and they are genuinely heated boxes. The fuels used to heat a kiln are gas, oil, wood, coal (now almost obsolete) and electricity. Each fuel sorce used to fire a kiln offers different possible outcomes for the clay fired in them. The maximum operating temperature for most pottery kilns is about 2372°F, although many woodfired kilns may be fired up to 2462°F.

Kiln Furniture: The series of posts, stilts, and shelves on which the ceramic ware rests in order to take full advantage of the interior space of the kiln.

Kiln Sitter (automatic shut-off): A device used with a pyrometric cone to shut off the kiln when conditions inside the kiln cause the cone to bend.

Kiln Wash: The refractory coating applied to the top of the kiln shelves to protect them from glaze drips.

Lead Release: The amount of lead that is dissolved from the surface of a glaze which has been in contact with acid solutions.

Leather-hard: A stage in the drying process of clay, when the clay is pliable but strong enough to handle. It is ideal for trimming and the addition of appendages such as handles and spouts. Relatively wet clay can be attached to the piece durring this stage and the resulting bond will not form cracks.

Matt : A soft finish with little or no shine.

Matured Bisque: A bisque that has been fired at the proper rate of heating and cooling to produce an even state of hardness throughout.

Mini Bars: Pyrometric cones used to measure the firing temperature of a kiln. They are shaped like bars rather than cone-shaped.

Mold Strap (mold bands): Dev~ces made of cloth, rubber, or metal used to tightly secure parts of a mold together during the pouring process.

Nesting: The procedure of stacking greenware in a kiln during the bisque firing.

Non-fired Finish: A color that is applied to bisque. These colors are never fired in a kiln.

OK Dinnerware: A product that when applied and fired according to label directions is safe for use on surfaces that come in contact with food.

Opaque : Color which does not allow other colors to show through.

Once Fired: A clay piece that has undergone a single glaze firing. The glaze is applied directly on to the dry or leather hard piece thus avaoiding the bisque firing. This approach, although offering certain economic and aesthetic advantages, can create technical problems for the potter.

Open Pour Mold: A mold that is made up of only one section or piece of plaster. Also referred to as an one piece mold because of the lack of a pouring gate.

Oxide: Any element combined with oxygen.

Oxidation: A firing where there is either no combustion occurring (electric kiln) or where there is sufficient oxygen in the kiln to allow the fuel to burn cleanly. The atmosphere of the kiln (oxidation, or reduction) dramatically affects the resulting clay and glaze colors, for example; copper in oxidation is green (as is copper oxide) in reduction it becomes red (more like copper metal).

Peep Holes (vent holes): Small holes in the side of a kiln used for viewing shelf cones and ventilating the kiln during the firing process.

Pin Holes: Tiny holes in the final surface finish of a glaze or underglaze. Plasticity: Refers to the ability of clay to be formed into a shape and retain it.

Pinching: A method of forming clay into a shape by pinching it with your fingers or between objects.

Plaster: An invaluable mold-making tool. It can be poured or carved into virtually any shape. When it is dry it can be used to press clay into or to slipcast with, forming a specific shape.

Plasticity: The properties of a material that allow it to be shaped and to retain its shape. The plastic properties of clay are principally determined by the size of the platelets. The smaller the platelets the more plastic the clay is. Aging or souring is also relevent to a clays plasticity; with time bacterial action creates a colloidal gel, which aids the lubrication of the platelets.

Porcelain: A white highly virtified clay body that is translucent where thin (often fired up to 2462°F). The translucency is a result of silica glass fused into the fired clay. To achieve this, a high amount of flux is added to a kaolin based clay body. The flux to clay ratio is often flux > clay, indeed some of the original Chinese porcelains had as lillte as 20% clay like minerals. The low clay content makes porcelain very difficult to throw and trimming wares is almost unavoidable. The plasticity of porcelain can be improved by small additions (2%) of white bentonite.

Posts (kiln): Articles made of refractory material which support kiln shelves during firing.

Pouring Spare: The excess clay formed at the pouring hole of a mold during the casting process.

Pour Hole (pour gate): A section at the opening of the mold used for pouring the slip into the mold cavity.

Prop: (1 ) A device of clay or refractory material used for supporting greenware (usually porcelain) during the firing process. (2) Term applied to the slight opening of the kiln cover during the first stages of the firing process, kiln prop. (3) Another word for kiln posts.

Pyrometric Cone: A small piece of clay compound that reacts to time and temperature used to indicate maturity of ceramic clays and glazes.

Raku:Origianally a Japanese seal given to a prominent family of potters (1598) who developed the technique. The term describes a low fire form of pottery where the pieces are removed from the kiln as soon as the glaze has melted and then left to cool or doused with water. In the mid 20th century Paul Soldner introduced the now popular process of post-firing rediction. In this case the red hot piece is placed in a lidded bin filled with staw or sawdust. The glazes are dramatically altered by the reduction particularly noteworthy are the colors achieved with Copper.

Reduction:A firing atmosphere with inadequate oxygen and large amounts of carbon (smoke or unburned fuel). What would have been Copper Oxide in an oxidation atmosphere will be pure copper in reduction. Reduction allowed the Chinese to develope the sangue de beouf red glazes and gives Raku its metalic finishes. In Indian pottery, Maria's black pieces are the result of heavy reduction; the same piece in oxidation would be terra cotta color.

Refractory Material: Substances that have a resistance to high temperatures.

Seam Lines: Small lines on greenware produced where two sections of a mold are locked together during the pouring process. Also referred to as fettlings.

Semi-Matt : A satin-like surface which has a slight sheen to it.

Semi-Opaque : Colors which generally allow only dark colors to show through.

Semi-Transparent: Slightly colored and/or speckled colors which allow most colors to show through with only slight distortions.

Separation: See crawling.

Shelf Cone Temperature: The cone temperature that is fired on the shelf of a kiln. The amount of heating the ware actually receives.

Slip: See casting slip.

Soaking Cycle: A short cycle at the end of the regular firing cycle which maintains the level of heating in the kiln, and enhances many glaze finishes.

Spray Gun: See airbrush.

Sponge: Usually refers to a natural sponge used for cleaning and decorating greenware. There are also synthetic sponges available for ceramics.

Stain: Unfired colors used for decorating.

Stilts: Small shapes of bisque with metal or wire spurs used for supporting glazed greenware during firing.

Stoneware: A combination of clays which form a stone-like vitreous body during firing.

Test Cone Plaque: See cone plaque.

Three-piece Mold: A mold that has three pieces.

Thermal Expansion: The expansion that occurs in glazes and clays when heated in a kiln.

Thermal Shock: Sudden changes which occur in a clay or glaze which causes damage, usually through sudden heating or cooling.

Transparent : Clear base colors which are free from cloudiness or distortion.

Two-piece Mold: A mold that has two parts.

Underglaze: A color which is usually applied to greenware and in most cases is covered with a glaze.

Underfiring: Not firing hot enough or long enough, or both.

Vitrified: Usually refers to porcelain and stoneware that are fired at a high temperature. The clay begins to become glass-like in nature, although not necessarily waterproof.

Vent Holes: Small holes made by puncturing the wet greenware with a needle tool when two pieces of ware have been attached. These small holes allow the air and gases to escape during firing. Also refers to the peep holes in the side of the kiln.

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