Useful Vocabulary
most definitions are sourced from: Clay: A Studio Handbook
Agateware – Wares featuring swirling marbleized colors resulting either from surface slip effects or marbleized colored clays.
Applique – Low-relief clay shapes added to scored, slurried leather-hard surface for decoration.
Bisque Firing – Initial kiln firing in which clay sinters without vitrifying, and though very porous, will no longer soften in water.
Bloating – Firing defect where blisters from within claybody, raising large lumps on the surface.
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 Dry – Completely dry (and very brittle) state clay must reach before firing.
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.
Burnishing Clay – A technique where leather hard clay is polished with a smooth hard instrument to force the smallest clay particles to the surface creating a soft sheen. This surface remains glossy after the pot is fired so long as the firing is cone 04 or below (1950° F).
Calipers – Adjustable tool for measuring inside/outside diameters for making lids.
Centering Clay – Critical step in throwing, occurring during and after wheel wedging, whereby the clay mass is formed into a symmetrical limp before penetrating and raising walls.
Chuck – On the wheel, a temporary wet-clay form or reusable bisque-fired form upon which wares may be inverted for trimming.
Clay – Widely occurring aluminum silicate material resulting from natural decomposition of feldspar and granite. Composed of microscopic disk-shaped platelets that give clay its slippery, plastic quality.
Coil Construction (Coiling) – Ceramic forming method utilizing ropelike coils of plastic clay, assembled in successive coursed to build-up wall of a vessel of sculpture.
Cones – Pyrometric cones are composed of glaze material and designed to melt and bend at specific temperatures. By observing them through a small peep hole in the kiln, it is possible to ascertain the exact conditions in the kiln. Cones are a better indicator than temperature alone as the degree of glaze melt is a combination of time over temperature (‘heat work done’), thus a fast firing needs to go to higher temperature to get the same results as a slow firing to a lower temperature.
Compression – In wheel throwing, the act of hand or finger pressure on the clay, resulting in lower moisture content and denser structure. Lack of compression in bottoms of pots can result in S-cracks.
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.
Crazing – Very fine surface cracks in fired glaze surface – technically a fault in glazed wares, but often sought after, especially in raku.
Drill Mixer – Electric drill 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 over-vitrified wares resulting from over-fluxing and/or over-firing.
Earthenware – Low-fired ware, usually still porous after firing – must be sealed with vitreous glaze to be functional
Engobe – An engobe has a similar make-up to slip but is produces with less clay than slip; the rest of the ingredients of an engobe are made up of flux or silica. You can usually tell the difference on a finished piece, as the engobe will have a sheen to it, where a slip will look dry.
Faceting – Decorating technique involving cutting or paddling flat facets in the clay surface.
Fettling Knife – Long tapered knife useful for trimming cast or pressed pieces, and for separating mold components. They are also useful for many trimming, carving, and sculpting tasks.
Fluting – Decorating technique involving carving or forming vertical flutes or grooves in surface of a piece.
Foot – The base of a ceramic piece
Glaze Firing – Kiln firing in which glazes are melted to form a smooth glassy surface. Typically, after bisque firing.
Greenware – Clay before it is fired.
Grog – Crushed high-fire clay graded in sizes from 15 mesh to 150 mesh added as a source of filler or tempering grit to claybodies to reduce shrinkage and give structure for throwing or handbuilding.
High Fire – High-temperature firing range usually including cone 8-12, for firing stoneware or porcelain.
Hydrometer – Laboratory device for measuring specific gravity in slips and glazes.
Incising – Decorating technique where design is formed by cutting or carving shallow lines in clay surface.
Kiln – A thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes.
Leather Hard – A stage in the drying process of clay when the clay is pliable but strong enough to handle. Soft leather-hard is ideal for forming, joining, thick slip decorating. Medium leather-hard is good for thin slip decorating, joining, incising, carving and piercing. Hard leather-hard is good for thin slip decorating, carving and scraping.
Low Fire – Low-temperature firing range, usually below cone 02 (2048° F), used for most bisque-firing and for glaze-firing terracotta and whiteware. If a low fire clay gets into a high fire kiln, it will melt and make a mess of the kiln.
Matt Glaze – Glaze featuring a dull, nonglossy surface.
Maturity – The point at which a glaze has reached complete fusion or clay has become completely vitrified.
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.
Mid Range – Glaze-firing ranged usually including cone 4 to cone 7. Very popular with electric kilns.
Mishima Ceramics – East Asian method of creting an inlaid effect by applying contrasting slip into a design incised in leather-had clay. When the slip stiffens, the excess is scraped off.
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.
Oxidation – A firing where there is either no combustion taking place (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).
Oxide Stain – Colorants that can be applied before or after the bisque firing, but most effectively the latter. They can also be applied over glazes.
Pinching – A method of forming clay, which is well described by its name.
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.
Plaster – An invaluable mold making tool for the potter. It can be poured or carved to virtually any shape. When it is dry, it can be used to press clay into or slipcast with.
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. Aging or souring is also relevant to a clay’s plasticity, with time bacterial action creates a colloidal gel with aids the lubrication of the platelets.
Platelets – Flat, thin crystals that make up clay. When wet they become sticky and slippery, creating the phenomenon we call plasticity.
Porcelain – A white, highly vitrified clay body that is translucent where thin. 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 greater than clay. The low clay content makes porcelain very difficult to thrown and trimming wares is almost unavoidable.
Pottery – A term used to describe ceramic ware.
Pottery Bat – A throwing accessory that enables freshly thrown work to be removed from the potter’s wheel without damage or warpage that can occur when touching the pot directly.
Potter’s Wheel – A machine used to create thrown vessels.
Press Mold – A mold, usually plaster or bisque clay, into which moist clay is pressed to crease multiples.
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 the need for hand wedging.
Raku – Originally a Japanese seal given to a prominent family of potters (1598) who developed the technique. The term describes a lowfire form of pottery where the pots 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 Reduction. In this case, the red hot pot is placed in a lidded bin filled with straw or sawdust. The glazes are dramatically altered by the Reduction; particularly noteworthy are the colors achieved with Copper.
Reduction – Also see Oxidation. A situation where too much fuel is introduced into the kiln to be able to burn with the available oxygen, consequently oxygen is ‘stolen’ from the pots in the kiln, it affects the clay and the glaze color. A good example is iron—even the tiny amount of iron present in porcelain changes it hue from a creamy color in Oxidation to a slight grey blue in Reduction.
Resist – A decorative technique where a wax-based medium is used to create a pattern which is then covered in another coat of glaze or slip. The wax resists the subsequent coating creating the pattern. Paper stencils or tape can create a similar effect.
Rib – A wide, flat handled 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 that occasionally appear in the bottoms of wheel-thrown pots, resulting from inadequate compression of the bottom and/or excessive water left in bottom.
Salt Firing – Vapor-glazing process where salt (sodium chloride) is introduced into kiln firebox at high temperature.
Scoring – Process of incising 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 before firing to exist contrasting claybody beneath.
Shard – A broken fragment of pottery.
Shivering – Serious and dangerous glaze defect where excessive glaze compression causes small razor-sharp chips of glaze to pop off along outer edges, corners, or rims.
Short – Clay with insufficient plasticity – tends to fragment during forming.
Shrinkage – Permanent contraction of the clay in both drying and firing stages.
Silica – A natural compound, found all around us in nature—Quartz, sand, flint, sandstone and many other types of rocks contain high levels of silica. It is one of the 3 main ingredients of clay and glaze materials. It is a glass-former. Inhaling finely divided crystalline silica is toxic and can lead to severe inflammation of the lung tissue.
Slab Construction – A technique of creating pottery or sculpture by rolling out flat pieces of clay and joining them to create height and width.
Slab Roller – A mechanized or manually operated device for rolling out uniformed slabs of clay.
Slip – Clay suspended in water, usually the consistency of thick cream. Used for joining clay together. Can also be colored with oxides and stains and painted or poured onto pots for decoration.
Slip Trailing – Application of decoration to wet or leather-hard clay by flowing on lines of slip with fine pointed dispenser, such as a rubber syringe.
Slump Mold / Slumping – A typically shallow frame or mold—most often made with plaster or bisqueware, into which a slab of clay is allowed to fall or settle in order to form a vessel.
Spray Booth – Open-front enclosure with an exhaust fan and/or waterfall, at the rear, designed to draw off overspray and other toxic dust or fumes.
Stains – Ceramic colorants used to color overglazes, engobes, slips, low-fire glazes and body colorants.
Stoneware – High vitrified ceramic fired above 2192°F. Most of the silica in a fired stoneware body is melted into a glassy matrix and the resulting body is of high density and usually has a water absorption rate of less than 1%.
Terra Sigillate – Ultarefined 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.
Throwing – (or turning) The process of using a potter’s wheel to coax clay upward to form a centered, symmetrical vessel.
Trimming – At the leather-hard stage, removal of excess clay from a piece, using any of a variety of sharp cutting tools.
Underglaze – Colored slips formulated to have low drying shrinkage, allowing application to bone-dry or bisque-fried surface before glazing. Commercial underglazes are available in a wide palette of colors primarily for low-fire, but many will survive high-fire.
Vitrification – The degree of melt in a clay body as a silica forms a glass with fluxes present. Responsible for its impermeability 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.
Wax Resist – Wax emulsion used to prevent slip or glaze from adhering to a clay surface, either in decorating, or in preparing work for glazing.