In clay and glaze chemistry, the glass-formers, which combine with bases (fluxes) and neutrals (refractories or stabilizers).
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Agateware
Wares featuring swirling marbleized colors, resulting either from surface slip effects or marbleized colored clays.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Alkaline
Basic, opposite of acidic—chemical nature of many fluxes.
Source:
Clay: A Studio Handbook
Alkaline Earths
A category of high-fire fluxes, which includes calcium, magnesium, barium, and strontium. Encourage hard, durable, but often matt glaze surfaces.
Source:
Clay: A Studio Handbook
Alkaline Fluxes
In low-fire, boron fluxes. In high-fire, the feldspars.
Source:
Clay: A Studio Handbook
Alumina Hydrate
Al(OH)3—alumina source, rarely used in clay bodies 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.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Amorphous
Materials such as glass, which have no regular repeating crystalline structure.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Annealing
The process of cooling a heated object gradually to allow internal shrinkage stress to equalize without damage.
Source:
Clay: A Studio Handbook
Applique
Low-relief clay shapes added to scored, slurried leather-hard surface for decoration.
Source:
Clay: A Studio Handbook
Ash Slagging
Heavy deposition of fly-ash onto surface of wares, furniture, and kiln interior in a wood kiln.
Source:
Clay: A Studio Handbook
Avery Kaolin
Well-known flashing slips for wood fire, but no longer being mined.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Bag Wall
Refractory wall in some fuel-burning kilns to deflect heat and flames from direct contact with the wares.
Source:
Clay: A Studio Handbook
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 clay bodies, but in large quantities promotes high shrinkage.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Barnard Clay
Slip clay with very high iron content, often used as brown colorant for clay bodies.
Source:
Clay: A Studio Handbook
Bases
In glaze chemistry, the fluxes or melting agents that are combined with acids (glass-formers) and neutrals (refractories).
Source:
Clay: A Studio Handbook
Bentonite
Al2O3×5SiO2×7H2O —Montmorillonitic clay formed from decomposition of airborne volcanic ash.
Source:
Clay: A Studio Handbook
Bisque Firing
Initial kiln firing in which clay sinters without vitrifying, and though very porous, will no longer soften in water.
Source:
Clay: A Studio Handbook
Blistering
Glaze defect where fired glaze surface contains bubbles, which often break open to leave sharp-edged craters in surface.
Source:
Clay: A Studio Handbook
Bloating
Firing defect where blisters form within clay body, raising large lumps on the surface.
Source:
Clay: A Studio Handbook
Blunge
The process of mixing a glaze or slip with a motorized impeller-mixer.
Source:
Clay: A Studio Handbook
Blunger
A motorized impeller-mixer used in preparing casting slip.
Source:
Clay: A Studio Handbook
Body Reduction
Period of reduction atmosphere induced between cone 012 and 08 to bring out iron color and speckles in clay body, especially in high-fired stoneware.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Bone China
Translucent porcelain containing bone ash.
Source:
Clay: A Studio Handbook
Bone Dry
Completely dry (and very brittle) state clay must reach before firing.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Burnishing Clay
Method of achieving a shine by rubbing clay or slip with smooth hard object.
Source:
Clay: A Studio Handbook
Burnt Umber
Iron-manganese ore—good color source for basalt bodies.
Source:
Clay: A Studio Handbook
Calcine
To heat a material to a temperature high enough to drive off all chemically combined water.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Calipers
Adjustable tool for measuring inside/outside diameters, as in making lids.
Source:
Clay: A Studio Handbook
Carbon Coring
Firing defect where excessively fast bisque-firing and/or excessive early reduction retard outgassing, causing carbon and sulfur to be trapped within clay body.
Source:
Clay: A Studio Handbook
Carbondale Clay
Refractory red stoneware clay, used to obtain rich red and brown colors in high-fire clay bodies.
Source:
Clay: A Studio Handbook
Castable
Short for castable refractory—a refractory mix that can be cast into molds to form kiln parts.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Cedar Heights Redart
Brick-red earthenware clay, produced by Cedar Heights Clay Company.
Source:
Clay: A Studio Handbook
Celadon Glazes
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.
Source:
Clay: A Studio Handbook
Centering Clay
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.
Source:
Clay: A Studio Handbook
Ceramic Drawing
The removing of wares from a kiln.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Ceramic Flux
Low-melting component in clay or glaze that reacts with silica to form glass.
Source:
Clay: A Studio Handbook
Ceramic Glass
Super-cooled liquid, which softens and hardens over broad range of temperature and cools to form an amorphous, noncrystalline solid. Level and rate of hardness (viscosity) controlled by temperature and by addition of fluxes and refractories, making possible the wide temperature range of ceramic clays/glazes.
Source:
Clay: A Studio Handbook
Ceramic Greenware (Green)
Any dry, unfired clay form.
Source:
Clay: A Studio Handbook
Ceramic Kiln
A specialized oven or furnace used for firing clay for pottery.
Source:
Clay: A Studio Handbook
Ceramic Luster Glaze
Metallic overglaze finish created either by painting prepared luster (metallic salt in organic binder) over previously fired glaze and firing to cone 018 or by spraying metallic salt dissolved in water into kiln and/or on to wares at low red heat, either during cooling cycle of a glaze-firing, or in a separate firing heated to that temperature.
Source:
Clay: A Studio Handbook
Ceramic Stain
Ceramic colorants that have been fritted in order to eliminate solubility problems and give greater stability in firing and truer color before firing. Most are stable up to cone 5, many to cone 10. Can be mixed with 25—50% Ferro 3134 frit for Maiolica overglaze decoration. Most stains are ground glass and are highly toxic in inhalation.
Mixture of ceramic stains or pure coloring oxides (sometimes with a little flux) in water suspension, which can be used for overglaze brushwork, or as a patina on unglazed clay.
Source:
Clay: A Studio Handbook
Chamois
Very soft, pliable animal skin—when wet works well to smooth wet clay surfaces.
Source:
Clay: A Studio Handbook
Charge
A quantity of chemical material, usually salt or soda, which is inserted or injected into a hot kiln during vapor-glazing processes.
Source:
Clay: A Studio Handbook
Charging
Inserting or injecting a charge of chemical material into a kiln during vapor-glazing processes.
Source:
Clay: A Studio Handbook
Chemically Combined Water
Water in molecular combination within clay and glaze materials, which is driven off during the water-smoking period of the firing.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Chinoiserie
European pottery, eighteenth century and later, featuring decoration inspired by imported Chinese Ming Dynasty wares.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Chuck
On the wheel, a temporary wet-clay form or reusable bisque-fired form upon which wares may be inverted for trimming.
Source:
Clay: A Studio Handbook
Chun Glazes
A pale gray-blue feldspathic stoneware glaze featuring opalescence due to inclusions of phosphorous and/or other materials.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Clay Body
Clay mixture formulated of clays and other ceramic raw materials to give desired working characteristics.
Source:
Clay: A Studio Handbook
Clay Earthenware
Natural low-fire secondary clay—fluxed with iron, fires porous. Often called “common” clay, found almost everywhere, matures below 2000°F.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Closed Form
Wheel-thrown form that is closed off completely and then altered to form a vessel or sculptural component.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Coil Construction
Ceramic forming method utilizing ropelike coils of plastic clay, assembled in successive courses to build up wall of vessel or sculpture.
Source:
Clay: A Studio Handbook
Combing
Decoration where a toothed instrument is dragged over a soft clay surface, sometimes through a layer of slip.
Source:
Clay: A Studio Handbook
Combustion
Reaction initiated when fuel reaches kindling temperature, at which point oxidation of hydrocarbon gases releases heat, sustaining and accelerating reaction.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Convection Currents
Upwards rise of warm air currents due to the transference of heat.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Copper Sulfate
CuSO4—color source for saggar firing and pit-firing. Soluble, and highly toxic in absorption, ingestion, and inhalation.
Source:
Clay: A Studio Handbook
Cordierite
2MgO×2Al 2O3×5SiO2—magnesium/aluminum silicate clay mineral, used to make grog for refractory products—promotes formation of mullite.
Source:
Clay: A Studio Handbook
Cottles
Adjustable wooden forms used in casting plaster molds.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Crazing (Crackle)
Very fine surface cracks in fired glaze surface—technically a fault in glazed wares, but often sought after, especially in raku.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Crystalline Glaze
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.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Cullet
Crushed window or bottle glass, occasionally used as a glaze material.
Source:
Clay: A Studio Handbook
Custer Feldspar
K2O×Al2O3×6SiO2—a common potash feldspar—HT alkaline flux. See feldspar. Close match to G-200. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Damper
In fuel-burning kilns, adjustable refractory plate located in exhaust flue, allowing control of back-pressure and secondary air, regulating kiln atmosphere.
Source:
Clay: A Studio Handbook
Darvan
Common deflocculant for casting slips. Product of R.T. Vanderbilt Company.
Source:
Clay: A Studio Handbook
Devitrification
The phenomenon that occurs early in the glaze cooling cycle, when certain materials crystallize out of the vitrified (fused) mass.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Dissolution
Action of a solvent material on a solid, bringing it into liquid solution.
Source:
Clay: A Studio Handbook
Downdraft Kiln
Kiln where exhaust gases exit through flue at floor level.
Source:
Clay: A Studio Handbook
Draft
The flow of exhaust gases out of a fuel kiln, affecting intake of flames and secondary air.
Source:
Clay: A Studio Handbook
Drill Mixer
Electric-drill-mounted impeller-mixer excellent for mixing glazes, slips, and slurries and for blunging casting-slip.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Earthenware
Low-fired ware, usually still porous after firing—must be sealed with vitreous glaze to be functional.
Source:
Clay: A Studio Handbook
Egyptian Paste
A self-glazing clay body in which soluble alkaline fluxes effloresce to the surface as the piece dries, and subsequently form a thin glassy coating in the firing.
Source:
Clay: A Studio Handbook
Element
Any of a group of slightly over 100 substances on earth that may exist as individual atoms, and from which all materials on earth are composed.
Source:
Clay: A Studio Handbook
EPK: Edgar Plastic Kaolin
Al2O3×2SiO2×2H2O—pure white kaolin, less plastic than Tile-6 kaolin, frequently used in glazes.
Source:
Clay: A Studio Handbook
Epsom Salts (Magnesium Sulfate)
MgSO4—water soluble, rarely used as magnesium source in glazes. Most often used as flocculant for slips and glazes. Often added to porcelain and porcelaineous stoneware bodies (1/2 of 1% of dry materials weight) to counteract deflocculating alkalinity released by kaolins or fluxes.
Source:
Clay: A Studio Handbook
Ergonomics
The science of comfortable and effective utility, determining how well a functional object or device works with the human body.
Source:
Clay: A Studio Handbook
Eutectic
Chemical phenomenon where two materials in combination melt at lower temperature than either material by itself.
Source:
Clay: A Studio Handbook
Faceting
Decorating technique involving cutting or paddling flat facets in the clay surface.
Source:
Clay: A Studio Handbook
Faience
Widely used (and misused) term referring to any earthenware pottery glazed with an opaque glaze (usually white) and overglaze decoration.
Source:
Clay: A Studio Handbook
Fettling Knife
Long tapered knife useful for trimming cast or pressed pieces, and for separating mold components.
Source:
Clay: A Studio Handbook
Fire Clay
Highly refractory secondary clays with minimal fluxes and usually fairly coarse particle size—low shrinkage, buff-color, often non-plastic.
Source:
Clay: A Studio Handbook
Firebox
The part of a fuel-burning kiln where fuel gases combust before contacting wares. Gas kilns need little if any firebox, whereas wood and oil kilns produce long hot flames whereas require a large firebox unless flame-flashing and ash-slagging (with wood) effects are sought.
Source:
Clay: A Studio Handbook
Firing Down
Maintaining some heat input after maturation, to slow down the cooling process, or to maintain reduction atmosphere during cooling. See reduction cooling.
Source:
Clay: A Studio Handbook
Firing Ramp
The profile or schedule for temperature change in a kiln-firing, often including both the heating and cooling ramps.
Source:
Clay: A Studio Handbook
Flame Flashing
Surface effects caused by direct flame contact on wares.
Source:
Clay: A Studio Handbook
Flameware
Wares made to stand stove-top heat. Explosions from trapped moisture, and resulting lawsuits have caused studio flameware to disappear from the domestic market.
Source:
Clay: A Studio Handbook
Flashing
Color change in fired clay or slip due to direct flame contact and residual ash deposition in wood firing, or due to variable currents of vapor deposition in salt and soda firing. Flashing can occur on almost any light-colored clay body, but is most dramatic on porcelain bodies and slips.
Source:
Clay: A Studio Handbook
Flashing Slip
Slip that is painted or dipped onto wares in order to promote flashing effects in the firing.
Source:
Clay: A Studio Handbook
Flocs
Commercial flocculant used in glazes—1/4 tsp. per gallon of glaze.
Source:
Clay: A Studio Handbook
Flue
Passages in kiln for flames or exhaust gases.
Source:
Clay: A Studio Handbook
Fluorspar
CaF2—limited use as flux. As with Cryolite, fluorine reacts w/silica at high temperatures, can cause pinholing, blisters. Useful in special-effect crater-glazes. Highly toxic in inhalation and ingestion.
Source:
Clay: A Studio Handbook
Fluting
Decorating technique involving carving or forming vertical flutes or grooves in surface of a piece.
Source:
Clay: A Studio Handbook
Fly Ash
Airborne ash in a wood-kiln.
Source:
Clay: A Studio Handbook
Foot
Base of a ceramic piece.
Source:
Clay: A Studio Handbook
Forced Air
Firing system in a fuel-burning kiln that uses power-driven blowers or other pressurized air source to entrain primary air.
Source:
Clay: A Studio Handbook
Fuming
Process of introducing metallic salts into kiln or onto wares at about cone 018, producing thin layer of metallic surface iridescence.
Source:
Clay: A Studio Handbook
G 200
K2O×Al2O3×6SiO2—common potash feldspar, HT alkaline flux—close match to Custer. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Glass Former
The primary material that, in combination with fluxes, forms the glass essential to all fired ceramics. Primary glass-former at all temperatures is silica.
Source:
Clay: A Studio Handbook
Glaze Compression
In high-fired wares, ideal state when clay body shrinks slightly more than glaze, putting glaze under slight compression, giving greater strength, resiliency. See crazing and shivering.
Source:
Clay: A Studio Handbook
Glaze Firing
Kiln firing in which glazes are melted to form a smooth glassy surface.
Source:
Clay: A Studio Handbook
Glaze Fit
The matching of glaze to clay body in terms of composition and coefficient of expansion so that it will adhere permanently.
Source:
Clay: A Studio Handbook
Glaze Melt
The chemically active state of the melted glaze.
Source:
Clay: A Studio Handbook
Glaze Resist
Decorating technique where resist materials are applied to prevent glaze from adhering to certain areas.
Source:
Clay: A Studio Handbook
Glomax
Calcined kaolin.
Source:
Clay: A Studio Handbook
Goldart
Buff stoneware clay, produced by Cedar Heights Clay Company.
Source:
Clay: A Studio Handbook
Grog
Crushed high-fired clay graded in sizes from 15-mesh (very coarse) to 150M (extra fine) added as a source of filler or tempering grit to clay bodies to reduce shrinkage and give structure for throwing or handbuilding. Does not shrink in firing, so in medium and coarse grades will show texture through thin to medium glaze. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Grolleg Kaolin
Al2O3×2SiO2×2H2O—English kaolin, more costly than other choices in the United States, but gives whiter porcelain. Less plastic than TILE-6. Best kaolin for translucent bone china.
Source:
Clay: A Studio Handbook
Groundhog Kiln
A long, low horizontal sprung-arch or barrel-arch wood-fired kiln, often banked with earth, traditionally used by potters on the eastern slope of the Appalachian Mountains.
Source:
Clay: A Studio Handbook
Gum Additives
CMC Gum, Veegum Cer—water soluble organic thickeners often added to glazes or slips as a suspension and/or brushing agent. Also used to allow glaze adhesion to vitrified or previously glaze-fired surfaces. Most commercial low-fire glazes and underglazes contain gum additives.
Source:
Clay: A Studio Handbook
Hardbrick
Hard, dense firebrick generally used only in high-stress areas of kiln (floor, burner ports, flues, bag wall) and for corrosive firing processes (salt, soda, wood).
Source:
Clay: A Studio Handbook
Heatwork
Work done by effective heat transfer to wares, resulting from ramp rate, temperature and duration of firing.
Source:
Clay: A Studio Handbook
Helmer Kaolin
Al2O3×2SiO2×2H2O—Kaolin that works especially well as flashing slip for salt, soda, and wood firing, especially since Avery kaolin is no longer being mined.
Source:
Clay: A Studio Handbook
HEPA (High Efficiency Particulate Arrestor)
Industrial designation that indicates that a filter or dust mask is capable of blocking microscopic particulates such as silica dust.
Source:
Clay: A Studio Handbook
High Fire
High-temperature firing range usually including cone 8 to cone 12, for firing stoneware or porcelain.
Source:
Clay: A Studio Handbook
Hispano Moresque
Decorative style originating in Moorish Spain, characterized by extremely elaborate patterning and curvilinear plant motifs; major influence on Italian maiolica.
Source:
Clay: A Studio Handbook
Hobs
Steps constructed in walls of wood-kiln firebox in place of grates. Wood cut to proper length catches on hobs and remains suspended above coal bed, insuring quick and complete combustion.
Source:
Clay: A Studio Handbook
Hot Face
The interior refractory surface of a kiln.
Source:
Clay: A Studio Handbook
Hydrocarbons
Combustible gases containing hydrogen and carbon, present in fuel gases and produced when carbon-based liquid or solid fuels are heated above kindling temperature.
Source:
Clay: A Studio Handbook
Hydrometer
Laboratory device for measuring specific gravity in slips and glazes. Does not give absolute measurement, and should be used only for comparing and reproducing results. Must be an appropriate glaze hydrometer for measuring suspensions heavier than water. Does not work well in thick glazes.
Source:
Clay: A Studio Handbook
Ilmenite
An iron ore with significant titanium—most often used in granular form to produce dark specks in clay or glaze. Higher iron concentration than in rutile.
Source:
Clay: A Studio Handbook
Impressing
Decorating technique where textured or patterned material or object is pressed into clay surface.
Source:
Clay: A Studio Handbook
Incising
Decorating technique where design is formed by cutting or carving shallow lines in clay surface.
Source:
Clay: A Studio Handbook
Inclusions
Any particles or bubbles of material that remain suspended within the glaze-melt, affecting the glaze appearance.
Source:
Clay: A Studio Handbook
Interface
Contact face between clay and glaze. On low-fired wares, primarily just a physical interlocking of glaze into pores in clay. In high-fired wares, an intimate interaction of clay and glaze, reinforced by mullite crystals, creating very strong bond.
Source:
Clay: A Studio Handbook
Iron
Most abundant coloring material on earth, responsible for brown and sometimes black color in natural rock and fired clays. In oxidation firings, the red ferric oxide remains very refractory, but in reduction firing and or at high-fire temperatures it transforms to the black ferrous oxide, a powerful flux.
Source:
Clay: A Studio Handbook
Iron Chromate
FeO×Cr2O3—glaze colorant producing pink or red with tin, brown with zinc, gray with alkaline fluxes. Highly toxic in absorption, inhalation, and ingestion.
Source:
Clay: A Studio Handbook
Iron Oxide Black Ferrous
FeO—reduced form of iron oxide—gives same results as red iron in the firing, dependent on oxidation/reduction.
Source:
Clay: A Studio Handbook
Kaki
High-fired Japanese saturated iron glaze usually containing at least 10% (of dry-batch weight) iron oxide, which produces iridescent microcrystalline rust red.
Source:
Clay: A Studio Handbook
Kanna
Japanese trimming tool, formed of a steel strap bent and sharpened on the ends.
Source:
Clay: A Studio Handbook
Kaolinite
Crystalline clay mineral forming the basis of most clays we use in ceramics.
Source:
Clay: A Studio Handbook
Keramos
Greek term meaning earthenware from which the word ceramics is derived.
Source:
Clay: A Studio Handbook
Kiln 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.
Source:
Clay: A Studio Handbook
Kiln Wash
Kiln wash is a refractory slip coating applied to the top surface of a kiln shelf to protect it from glaze runs, drips and other accidents that occur in red hot kilns, like pots that tip over, bloating or melting clay bodies, etc. It is also used to protect shelves from volatiles in atmospheric kilns like wood ash, or sodium oxide in salt and soda kilns.
There are many kiln wash recipes to choose from and it’s important to find the recipe that works best with your firing practices. What works well at one firing range or atmosphere, may not be well suited for another.
A good wash requires materials that have very high melting points and that, when combined, do not create eutectics that cause melting. Kiln wash is used in the full range of ceramics firing from cone 022 to cone 14. The type needed varies for each specific situation because some potters work in electric kilns at low-fire temperatures, while others work with fuel-fired kilns at very high temperatures.
A common recipe for gas and electric oxidation or reduction kilns is 50% kaolin and 50% silica (flint). However, that mixture must never be used in salt, soda, and wood firing. For those processes, use 40% kaolin, 10% ball clay, and 50% alumina hydrate.
Mix kiln wash to a thick creamy consistency and apply with a wide brush or paint roller. In some cases you may wish to build up successive coats, especially in any situation where significant glaze runs are fairly common.
In all cases, avoid getting your wash on the edges of the shelves where it can flake off in the firing and fall on glaze wares below.
Kiln wash is a refractory slip coating applied to the top surface of a kiln shelf to protect it from glaze runs, drips and other accidents that occur in red hot kilns, like pots that tip over, bloating or melting clay bodies, etc. It is also used to protect shelves from volatiles in atmospheric kilns like wood ash, or sodium oxide in salt and soda kilns.
Source:
Clay: A Studio Handbook
Kingman Feldspar
Potash spar no longer mined. Substitute Custer or G-200.
Source:
Clay: A Studio Handbook
Kona F 4 Feldspar
Na2O×Al2O3×6SiO2—a common soda feldspar—powerful HT alkaline flux. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Kyanite
3Al2O3×2SiO2—aluminum silicate used in place of alumina and silica to promote formation of mullite crystals, increase thermal shock resistance. Coarse-ground used as grog in refractory bodies for kiln furniture.
Source:
Clay: A Studio Handbook
Leather Hard
Condition of clay in which 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-decorating. Medium leather-hard good for thin slip- decorating, joining, incising, carving, piercing. Hard leather-hard good for thin slip- decorating, carving, scraping.
Source:
Clay: A Studio Handbook
Lepidolite
Lithium feldspar—HT alkaline flux—used for thermal shock bodies and matching glazes. Contains fluorine, with associated problems. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Lithium Carbonate
Li2CO3—powerful AT alkaline flux, especially with soda or potash feldspars. Promotes hardness and recrystallization in LT glazes. Forms low-temperature eutectic with silica. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Low Fire
Low-temperature firing range, usually below cone 02 (2048° F), used for most bisque-firing and for glaze-firing terra cotta and whiteware.
Source:
Clay: A Studio Handbook
Low Mid Range
Firing range usually including cone 01 to cone 3, underused in studio ceramics, useful for functional earthenware, refractory sculpture bodies, and outdoor terra cotta work
Source:
Clay: A Studio Handbook
Lye
Potassium hydroxide or sodium hydroxide. Caustic alkaline soluble that leaches out of wood ashes when they are soaked in water. Occasionally used as a flux additive in terra sigillata, as in the black slip on Classical Greek wares. Toxic in liquid state, and may cause burns.
Source:
Clay: A Studio Handbook
Macaloid
Suspension agent/plasticizer similar to Bentonite and Veegum T. Up to 2% of dry-materials weight as plasticizer in high-kaolin clay bodies. Up to 1/2 of 1% of dry-materials weight as suspension agent, brushing medium in glazes, and slips. Mix with water before adding other ingredients.
Source:
Clay: A Studio Handbook
Macrocrystalline
Glaze effect featuring large patches of crystal development on glaze surface, as compared to microcrystalline effects.
Source:
Clay: A Studio Handbook
Maiolica
Earthenware, generally terra cotta, with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration. Originated from Islamic/Moorish techniques used on the Spanish island of Mallorca, which exported these wares throughout the Mediterranean in the 15th and 16th centuries. The technique was adopted by Italy during the Renaissance, which is when the term maiolica was introduced to describe these brightly decorated wares. Not to be confused with majolica, which is a trade name for an historic English style of brightly glazed earthenware vessels made in the forms of vegetables, animals, etc.
Source:
Clay: A Studio Handbook
Majolica
Trade name for an historic English style of brightly glazed earthenware vessels made in the form of vegetables, animals, etc. Not to be confused with maiolica, which is defined as earthenware with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration.
Source:
Clay: A Studio Handbook
Manganese Dioxide
MnO2—flexible colorant—with alkaline fluxes gives purple and red colors—by itself gives soft yellow-brown—with cobalt gives black. Used with iron to color basalt bodies. Concentrations of more than 5% may promote blistering. Toxic in inhalation and ingestion. Fumes from firing are especially toxic.
Source:
Clay: A Studio Handbook
Matte Glaze
Glaze featuring a dull, nonglossy surface.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Mica
K2O×3Al2O2×6SiO2—an aluminosilicate with a fine sheet-lattice structure, closely related to clay and feldspar, and often found as minute iridescent flakes in some clays.
Source:
Clay: A Studio Handbook
Microcrystalline
Crystal effects dependent on tiny crystals in the glaze surface, as compared to macrocrystalline effects.
Source:
Clay: A Studio Handbook
Mid Range
Glaze-firing range usually including cone 4 to cone 7, very popular with electric kilns
Source:
Clay: A Studio Handbook
Mishima Ceramics
East Asian method of creating an inlaid effect by applying contrasting slip into a design incised in leather-hard clay. When the slip stiffens, the excess is scraped off.
Source:
Clay: A Studio Handbook
Modifiers
Glaze materials that are added to a base glaze to modify surface qualities such as color, gloss, matteness, opacity, crystal development, etc.
Source:
Clay: A Studio Handbook
Molecule
A group of atoms chemically bonded together to form a compound.
Source:
Clay: A Studio Handbook
Montmorillonite
Clays such as bentonite, resulting from the breakdown of airborne volcanic ash. The finest particle size of all clays. Very high shrinkage, and generally used only as an additive to clay bodies or glazes.
Source:
Clay: A Studio Handbook
Mullite
3Al2O3×2SiO2—calcined Kyanite, but is also found naturally. Very desirable interlocking needlelike aluminum silicate crystals form above 1800° F and greatly add to the strength of high-fired vitrified clay bodies and glazes. In well-formulated high-fired wares, clay-glaze interface is extremely strong partially due to interlocking mullite crystals.
Source:
Clay: A Studio Handbook
Natural Draft
Firing system in a fuel-burning kiln that uses natural convection currents in kiln (updraft) or in chimney (downdraft), without any mechanical blowers, to draw in secondary air, circulate heat and atmosphere through kiln, and exhaust combustion gases.
Source:
Clay: A Studio Handbook
Nc 4 Feldspar
Na2O×Al2O3×6SiO2—soda feldspar, interchangeable with Kona F-4. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Neutral Atmosphere
In fuel-burning kiln, atmosphere that is neither oxidizing nor reducing—most efficient atmosphere for efficient climb and even temperature.
Source:
Clay: A Studio Handbook
Neutrals
In glaze chemistry, the refractories or stabilizers that are combined with bases (fluxes) and acids (glass-formers).
Source:
Clay: A Studio Handbook
Noborigama
Translates as “step-climbing kiln”—traditional Japanese multichamber downdraft/crossdraft climbing kiln, with initial firebox at bottom, and secondary fireboxes in each chamber. As kiln is fired, each chamber preheats the next. See climbing kiln.
Source:
Clay: A Studio Handbook
Nuka
An often-iridescent, milky-white Japanese glaze containing rice-hull ash.
Source:
Clay: A Studio Handbook
Ocmulgee
Rich golden-brown stoneware clay, no longer being mined.
Source:
Clay: A Studio Handbook
Opacifier
In glaze formulation, a material that produces inert inclusions or minute crystals in glaze, causing it to become opaque. Most common are tin oxide and zirconium silicate.
Source:
Clay: A Studio Handbook
Opax
Zircon opacifier. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Orifice
In a gas or oil burner, the restricted opening through which a jet of fuel emerges.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Oxidation
Any chemical reaction in which atoms or molecules combine with oxygen atoms. The combustion of fuel is an oxidation reaction.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Oxidize
To subject a material to a high-oxygen atmosphere, encouraging oxidation reactions.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Paper Resist
Decoration technique where strips of moist or adhesive paper are adhered to the surface to resist application of slip or glaze.
Source:
Clay: A Studio Handbook
Patina
An overall thin wash of glaze or oxide stain, allowing the color and texture of the clay body to show through.
Source:
Clay: A Studio Handbook
Periodic Table Of The Elements
A table listing all of the chemical elements (those substances that can theoretically be reduced to individual atoms), with the smallest and simplest across the top and the heaviest and most complex at the bottom and grouped by similar physical/chemical characteristics.
Source:
Clay: A Studio Handbook
Perlite
Porous expanded granular silica, useful in making insulating refractories.
Source:
Clay: A Studio Handbook
Petalite
Li2O×Al2O3×8SiO2—lithium feldspar—HT alkaline flux—good for reducing thermal expansion, increasing thermal-shock resistance. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Petuntse
Feldspathic rock, similar to Cornwall stone, found in China. Traditional Chinese flux for HT glazes and clay bodies.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Pit Firing
A type of bonfire-firing where wares are buried in sawdust in a pit in the ground and a bonfire is built on top, so that the fire and coals slowly burn away the sawdust and fire the wares. Not to be confused with sawdust smoking.
Source:
Clay: A Studio Handbook
Plastic Vitrox
K2O/Na2O/CaO×Al2O3×10SiO2—a plastic high-potash feldspathic clay, similar in structure to Cornwall stone—used in place of potash feldspar in some porcelain bodies to increase plasticity. Toxic in inhalation.
Source:
Clay: A Studio Handbook
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 clay body, adequate water content, and/or addition of accessory plasticizers, such as Veegum T or Macaloid.
Source:
Clay: A Studio Handbook
Plasticizers
Materials added to some clay bodies, especially those high in kaolins, to increase plasticity and dry strength—includes bentonite, Macaloid, Veegum-T.
Source:
Clay: A Studio Handbook
Platelets
Flat, thin crystals that make up clay. When wet they become sticky and slippery, creating the phenomenon we call plasticity.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Porcelain
High-fired vitreous clay body 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.
Source:
Clay: A Studio Handbook
Porcelaneous
White-firing stoneware clay bodies closely related to porcelain.
Source:
Clay: A Studio Handbook
Portland Cement
Calcium silicate aluminate—air-setting cement, often added in small quantities to homemade castable refractor mix in order to introduce air-setting qualities. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Potash Feldspar
Materials like G-200; Custer feldspar.
Source:
Clay: A Studio Handbook
Potassium Dichromate
K2Cr2O7—Occasionally used as acid-green colorant in raku glazes, but is highly toxic in absorption, ingestion, and inhalation.
Source:
Clay: A Studio Handbook
Pottery Bat
A pottery bat is a pottery throwing accessory that enables freshly thrown work to be removed from the throwing wheel without the damage or warpage that can occur from touching the pot directly. Pottery bats also make it possible to return a piece to exact center to work on later.
Throwing bats can be made from most any rigid material, but wood, wood composites, plastics, and plaster are the most common. Except for plastic, these materials are all porous so pots will release from them easily as they absorb water from the clay. If the material is not porous (e.g., plastic) the pot must be wired off before it sets up too much, or it will crack as it shrinks.
Some of the materials that pottery bats are made from include:
Duron® (aka Masonite® or tempered hardboard), a resin impregnated hardboard that’s water resistant and smooth on both sides.
Medex®, a fiber board material made with a formaldehyde-free adhesive that’s waterproof yet porous
Plastics of several different types (nonporous and waterproof, so they require wiring off pots)
Plywood (must be exterior- or marine-grade to avoid delaminating)
Plaster, one of the traditional materials for bats and one of the best for porosity
Hydro-Stone, a USG product containing cement and crystalline silica, is similar to plaster but 8 to 10 times stronger
Most wheel manufacturers pre-drill wheel heads with bat-pin holes that hold 3/8-inch (or 10-mm) bat pins on 10-inch centers. Most bat makers produce bats that fit this standard. While pottery bats can be stuck to the wheel head with clay or pottery slip, bat pins allow the user to quickly add and remove bats with ease.
Source:
Clay: A Studio Handbook
Pottery Wheel (Potter’s Wheel/Potters Wheel)
Machine with a horizontally spinning wheel head upon which potters form and trim clay vessels by hand.
Source:
Clay: A Studio Handbook
Press Mold
A mold, usually plaster, into which moist clay is pressed to crease multiples
Source:
Clay: A Studio Handbook
Primary Air
In a fuel-burning kiln, air that feeds initial combustion; in a wood-burning kiln, air that feeds base of flames; in a gas kiln air that enters back end of burner tube in an atmospheric burner, or via blower in a power burner. See secondary air.
Source:
Clay: A Studio Handbook
Pugmill
A machine similar to an oversized meat grinder, used to homogenize plastic clay bodies. Deairing pugmills have a vacuum pump attachment, which effectively removes all air from clay, eliminating need for hand wedging.
Source:
Clay: A Studio Handbook
Pyrophyllite
Al2O3×4SiO2×H2O—used in HT clay bodies (20% or less) to reduce thermal expansion, increase thermal shock resistance, reduce shrinkage, give stronger vitreous bodies. May reduce plasticity.
Source:
Clay: A Studio Handbook
Pyroplastic
Flexibility of clay or glaze when heated sufficiently, as a result of developing glassy-phase.
Source:
Clay: A Studio Handbook
Ramp
Profile of the firing of a kiln, including speed, duration, soaking periods, etc. of both the heating and cooling cycle, as in firing ramp and cooling ramp.
Source:
Clay: A Studio Handbook
Red Iron Oxide
Fe2O3—powdered rust—refractory red in oxidation, converts to black iron (flux) in reduction and/or high-fire. Low quantities in clear glaze produces celadon-green—high quantities produce temmoku black or saturated iron red—powerful flux. More than 5% in a glaze significantly increases fluxing in reduction.
Source:
Clay: A Studio Handbook
Reduce
To induce reduction.
Source:
Clay: A Studio Handbook
Reduction
Chemical reaction in which oxygen atoms are removed from a compound.
Source:
Clay: A Studio Handbook
Reduction Cooling
In a fuel kiln, maintaining slight reduction atmosphere during cooling cycle, from maturing temperature down to approximately 1400° F, in order to minimize reoxidation of clay and/or glazes.
Source:
Clay: A Studio Handbook
Refractories
Any materials highly resistant to the effects of heat. In kiln construction, high-temperature insulating materials, such as firebrick, ceramic fiber, etc. In clay and glaze chemistry, the neutrals or stabilizers that are resistant to melting and that combine with the fluxes (bases) and glass-formers (acids). Primary refractory material in ceramics is alumina, most often introduced in the form of clay (aluminum silicate).
Source:
Clay: A Studio Handbook
Residual Ash
In a wood kiln, the surface effects created by fly-ash settling upon the wares. Effect is maximized by turbulence in firebox, considerable draft through kiln, and/or active stirring of the coal bed. In short wood firings ash doesn’t have time to melt, and leaves a rough, crusty surface.
Source:
Clay: A Studio Handbook
Resist
Material used in glazing and decorating that can be applied to surface to prevent adhesion of slip or glaze.
Source:
Clay: A Studio Handbook
Rib
Wide, flat handheld 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.
Source:
Clay: A Studio Handbook
Rolled Edge
Beveled edge obtained by rolling the outer edge of the foot of a soft leather-hard pot at an angle against a hard, flat surface.
Source:
Clay: A Studio Handbook
Rutile
Titanium ore, used as source of titanium dioxide, contains iron, other trace minerals—gives tan color, promotes crystallization giving mottled multicolor effects in some HT glazes, or in overglaze stain (very refractory, use sparingly). Gives rich mottled medium blue in some HT glazes. Dark rutile contains higher percentage of iron.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Saggar
A refractory clay enclosure in which wares are fired.
Source:
Clay: A Studio Handbook
Saggar Firing
Contemporary firing process in which wares are placed within refractory saggars, along with chemicals and combustibles, in order to achieve certain surface effects.
Source:
Clay: A Studio Handbook
Salt (Sodium Chloride)
NaCl—table salt, rock salt—used in salt-firing. Skin irritant.
Source:
Clay: A Studio Handbook
Salt Firing
Vapor-glazing process where salt (sodium chloride) is introduced into kiln firebox at high temperature.
Source:
Clay: A Studio Handbook
Sand
Granular silica. Major grit besides grog used to give clay bodies structure for throwing and hand building, high shrinkage in high-temperature clay bodies, but gives smoother fired surface than grog. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Scoring
Process of incising surface of wet or leather-hard clay in crosshatch pattern before applying slurry and joining pieces.
Source:
Clay: A Studio Handbook
Secondary Air
In fuel-burning kilns, air that enters after initial combustion and feeds flames as they leave primary combustion source.
Source:
Clay: A Studio Handbook
Set
To place wares in a kiln. Or, in a loaded kiln, the entire structure of shelves, furniture, and wares.
Source:
Clay: A Studio Handbook
Setting
Process of loading a kiln, or technically, of placing the set.
Source:
Clay: A Studio Handbook
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 clay body beneath.
Source:
Clay: A Studio Handbook
Shard
A broken fragment of pottery.
Source:
Clay: A Studio Handbook
Shino Glaze
Classic Japanese glaze ranging from gray to white to orange, composed primarily of feldspar.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Shrinkage
Permanent contraction of the clay in both drying and firing stages. Overall may be as much as 18%.
Source:
Clay: A Studio Handbook
Sieve
A container with fine-mesh wire screen in the bottom, available in different mesh sizes, used for straining slips and glazes in order to achieve complete mixing of raw materials and removal of large particles or contaminants.
Source:
Clay: A Studio Handbook
Sieving
The process of working a glaze or slip through a wire-mesh sieve to strain out impurities and to break up clumped raw materials.
Source:
Clay: A Studio Handbook
Silicon Carbide
SiC — Extremely refractory material used to form kiln shelves.
Source:
Clay: A Studio Handbook
Sintering
In heating clays and glazes, a solid-state reaction where particles stick together permanently, and mass can be considered fired.
Source:
Clay: A Studio Handbook
Slab Roller
A mechanized or manually operated device for rolling out large uniform slabs of clay.
Source:
Clay: A Studio Handbook
Slip
Clay suspended in water, usually the consistency of thick cream.
Source:
Clay: A Studio Handbook
Slip Casting
The creation of ceramic forms by casting slip in plaster molds.
Source:
Clay: A Studio Handbook
Slip Clay
A naturally occurring clay that contains a high enough component of flux to form a glaze at high-fire temperatures.
Source:
Clay: A Studio Handbook
Slip Glaze
A glaze with slip-clay or earthenware clay contributing the primary flux.
Source:
Clay: A Studio Handbook
Slip Resist
Decorating technique where resist materials are applied to prevent slip from adhering to some areas.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Slump Mold
A typically shallow frame or mold into which a slab of clay is allowed to fall or settle in order to form a vessel. These can be frames with no bottom.
Source:
Clay: A Studio Handbook
Slumping
Process of creating ceramic wares with a slump-mold. Firing defect where glassy-phase begins to dissolve sintered structure in clay, causing it to sag and deform.
Source:
Clay: A Studio Handbook
Soaking
During firing or cooling ramp, the act of holding kiln at steady temperature for a period of time to allow proper formation or maturation of certain clay and glaze effects.
Source:
Clay: A Studio Handbook
Soap
A specialized hard brick shape, half the width of a standard brick. Often used for posts in salt and wood firings.
Source:
Clay: A Studio Handbook
Soda Feldspar
Na2O×Al2O3×6SiO2—feldspars contributing sodium (and potassium), primarily as a high-temperature flux.
Source:
Clay: A Studio Handbook
Softbrick
Insulating firebrick.
Source:
Clay: A Studio Handbook
Solution
A liquid mix where the components are fully dissolved (as in a water/soda ash solution for vapor-glazing), where materials will not settle out via gravity.
Source:
Clay: A Studio Handbook
Solvent
An agent that acts to accelerate the dissolution of a more resistant material. In glaze maturation, the flux-glass-former combination acts as a solvent on alumina.
Source:
Clay: A Studio Handbook
Specific Gravity
The weight or density of a liquid measured in proportion to that of water. A glaze with a specific gravity of 1.2 is 1.2 times as dense as water per unit of measure.
Source:
Clay: A Studio Handbook
Split
A specialized hardbrick, half the thickness of a standard brick.
Source:
Clay: A Studio Handbook
Spodumene
Li2O×Al2O3×4SiO2—lithium feldspar—powerful high-temperature alkaline flux. Promotes copper blues—good for thermal-shock bodies and matching glaze. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Spray Booth
Open-front enclosure with an exhaust fan at the rear, designed to draw off all overspray and other toxic dust or fumes.
Source:
Clay: A Studio Handbook
Spraying
Application of liquid slip, engobe, glaze, or stain, using mechanized spray equipment. Gives smooth glaze coating, but with less glaze pooling in recesses.
Source:
Clay: A Studio Handbook
Sprigging Ceramics
Surface decorating technique in which small coils or balls of clay are affixed to the damp or leather-hard surface, usually with a layer of slip.
Source:
Clay: A Studio Handbook
Sprung Arch
Kiln arch representing less than 180 degrees of curvature, and requiring buttressing to support outward thrust of the arch.
Source:
Clay: A Studio Handbook
Stoneware
High-fired vitreous ware, literally as hard and durable as stone. Matures from 2200-2400° F. (cone 5–11).
Source:
Clay: A Studio Handbook
Strontium Carbonate
SrCO3 Alkaline earth, high-temperature flux, similar to barium, slightly more powerful.
Source:
Clay: A Studio Handbook
Superpax
Zircon opacifier. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Suspension
A liquid mix where insoluble particles are distributed throughout without dissolving and may settle out from gravity, as in a glaze or slip.
Source:
Clay: A Studio Handbook
Tenmoku
Classic East Asian high-iron gloss glaze giving black where thick, breaking to brown or red-brown where thin.
Source:
Clay: A Studio Handbook
Terra Cotta Clay
Low temperature, porous earthenware clay body, fires red-brown due to high iron content, which also fluxes clay, making it the most durable low-fired clay after firing.
Source:
Clay: A Studio Handbook
Terra Sigillata
Ultra-refined 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.
Source:
Clay: A Studio Handbook
Thermal Expansion
The physical expansion and contraction that accompanies the heating and cooling of most materials. See coefficient of expansion.
Source:
Clay: A Studio Handbook
Thermal Shock
Effect of sudden temperature changes during firing or during subsequent heating and cooling in daily use.
Source:
Clay: A Studio Handbook
Thermocouple
Temperature probe that produces minute variable electrical current dependent on degree of heat exposure—used in pyrometers and Baso valves.
Source:
Clay: A Studio Handbook
Tin Oxide
SnO2—most powerful opacifier, but expensive—inert dispersoid in glaze melt—5–7% will produce opaque white in a clear glaze. Toxic in inhalation and ingestion.
Source:
Clay: A Studio Handbook
Titanium Dioxide
TiO2—matting/opacifying agent. Promotes crystal growth, visual texture in glazes.
Source:
Clay: A Studio Handbook
Tombo
A T-shaped Japanese throwing gauge, used to measure the depth and rim diameter of a vessel, usually when throwing off the hump.
Source:
Clay: A Studio Handbook
Triaxial Blend
Method for testing three-way combinations of glaze materials, where proportional amounts vary through a series of samples between three limits. May involve change in glaze materials, or addition of colorants or modifiers.
Source:
Clay: A Studio Handbook
Trimming
At the leather-hard stage, removal of excess clay from a piece, using any of a variety of sharp cutting tools.
Source:
Clay: A Studio Handbook
Ultrox
Zircon opacifier. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Undercut
Common flaw in plaster or bisque molds, where the clay or casting catches and will not pull free without breaking or distorting.
Source:
Clay: A Studio Handbook
Underglaze Decoration
Process of applying any decoration to the bare, (usually bisque-fired) clay surface directly before glazing.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Vanadium Pentoxide
V2O5—weak yellow colorant—toxic, expensive—usually fritted with tin to produce stronger yellow. Highly toxic in inhalation and ingestion.
Source:
Clay: A Studio Handbook
Vapor Glazing
Any glaze process where the glaze results from vapor deposited within the kiln—includes salt glazing, soda glazing, fuming and wood firing (where fly ash is deposited on the ware and melted into a glaze).
Source:
Clay: A Studio Handbook
Veegum Cer
Combination of Veegum T and carboxymethylcellulose gum, used as a suspension and adhesion agent in glazes. Product of H.T. Vanderbilt Company.
Source:
Clay: A Studio Handbook
Veegum T
Suspension agent/plasticizer similar to bentonite and macaloid. Up to 2% of dry materials weight as plasticizer in high-kaolin clay bodies, and up to 1/2 of 1% of dry-materials weight as suspension agent, brushing medium in glazes and slips. Mix with water before adding other ingredients. Product of H.T. Vanderbilt Company.
Source:
Clay: A Studio Handbook
Venturi Burner
Natural-draft gas burner featuring streamlined restriction in burner tube, which increases entrainment of primary air and the efficiency of gas-air mixing.
Source:
Clay: A Studio Handbook
Vermiculite
A porous expanded mica product used as filler in clay bodies and in insulating refractory layer on kiln exterior.
Source:
Clay: A Studio Handbook
Vitreous Engobe
An engobe containing sufficient flux to form to a vitreous clay coating.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Waster
European term referring to a fired piece discarded due to damage in firing.
Source:
Clay: A Studio Handbook
Water Of Plasticity
Amount of water required to bring a dry clay to its state of ideal plasticity. Common clay test used to determine this amount. The more water needed, the finer the particle size, the more plastic the clay, and the greater the drying shrinkage.
Source:
Clay: A Studio Handbook
Wax Resist
Melted wax or wax emulsion used to prevent slip or glaze from adhering to a clay surface, either in decorating, or in preparing work for glazing.
Source:
Clay: A Studio Handbook
Wedging
Process of kneading the clay with the hands to remove air bubbles and ensure homogenous mass.
Source:
Clay: A Studio Handbook
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.
Source:
Clay: A Studio Handbook
Whiteware
Low-temperature white clay body.
Source:
Clay: A Studio Handbook
Wollastonite (Calcium Silicate)
CaSiO3—used in partial replacement of silica and whiting in high-temperature bodies, improves thermal shock resistance. In some cases, it is used in place of whiting to eliminate L.O.I. Toxic in inhalation.
Source:
Clay: A Studio Handbook
XX Sagger
A plastic refractory stoneware clay, often used in sculpture and raku bodies.
Source:
Clay: A Studio Handbook
Yellow Iron Oxide (Yellow Ochre)
High-iron yellow clay mineral, used as colorant in glazes and slips, converts to red iron oxide in oxidation or black iron oxide in reduction and/or high-fire.
Source:
Clay: A Studio Handbook
Zinc Oxide
ZnO—High-temperature flux that promotes brilliant glossy surfaces. In some glazes can encourage opacity. With titanium in low-alumina glaze can encourage macrocrystalline growth (crystalline glazes). Volatizes in high-fire reduction. Toxic in inhalation.
Source:
Clay: A Studio Handbook
Zirconium Oxide
Common opacifier, often used in place of the more expensive tin oxide. Sold as Opax, Superpax, Ultrox, etc., use 7% in a slip and 10 to 12% in a clear glaze to get pure opaque white. Zirconium particles are extremely refractory and remain as inert inclusions in the glaze-melt. They will not affect matteness or gloss, but will increase viscosity.
Source:
Clay: A Studio Handbook
Zirconium Silicate
ZrSiO4—zircon opacifier—inert dispersoid in glaze melt—low-cost substitute for tin oxide—use double the recipe weight of tin. Includes Zircopax, Opax, Superpax, Ultrox. All are toxic in inhalation.
Source:
Clay: A Studio Handbook
Zircopax
Zircon opacifier, no longer being manufactured. Toxic in inhalation.
In clay and glaze chemistry, the glass-formers, which combine with bases (fluxes) and neutrals (refractories or stabilizers).
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Wares featuring swirling marbleized colors, resulting either from surface slip effects or marbleized colored clays.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Materials such as glass, which have no regular repeating crystalline structure.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
The process of cooling a heated object gradually to allow internal shrinkage stress to equalize without damage.
Source: Clay: A Studio Handbook
Low-relief clay shapes added to scored, slurried leather-hard surface for decoration.
Source: Clay: A Studio Handbook
Heavy deposition of fly-ash onto surface of wares, furniture, and kiln interior in a wood kiln.
Source: Clay: A Studio Handbook
Well-known flashing slips for wood fire, but no longer being mined.
Source: Clay: A Studio Handbook
In a fuel kiln, the internal pressure resulting from proper balance of combustion pressure and damper setting, resulting in more economical and efficient heatwork.
Source: Clay: A Studio Handbook
Refractory wall in some fuel-burning kilns to deflect heat and flames from direct contact with the wares.
Source: Clay: A Studio Handbook
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 clay bodies, but in large quantities promotes high shrinkage.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Slip clay with very high iron content, often used as brown colorant for clay bodies.
Source: Clay: A Studio Handbook
In glaze chemistry, the fluxes or melting agents that are combined with acids (glass-formers) and neutrals (refractories).
Source: Clay: A Studio Handbook
Al2O3×5SiO2×7H2O —Montmorillonitic clay formed from decomposition of airborne volcanic ash.
Source: Clay: A Studio Handbook
Initial kiln firing in which clay sinters without vitrifying, and though very porous, will no longer soften in water.
Source: Clay: A Studio Handbook
Glaze defect where fired glaze surface contains bubbles, which often break open to leave sharp-edged craters in surface.
Source: Clay: A Studio Handbook
Firing defect where blisters form within clay body, raising large lumps on the surface.
Source: Clay: A Studio Handbook
The process of mixing a glaze or slip with a motorized impeller-mixer.
Source: Clay: A Studio Handbook
A motorized impeller-mixer used in preparing casting slip.
Source: Clay: A Studio Handbook
Period of reduction atmosphere induced between cone 012 and 08 to bring out iron color and speckles in clay body, especially in high-fired stoneware.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Translucent porcelain containing bone ash.
Source: Clay: A Studio Handbook
Completely dry (and very brittle) state clay must reach before firing.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Method of achieving a shine by rubbing clay or slip with smooth hard object.
Source: Clay: A Studio Handbook
Iron-manganese ore—good color source for basalt bodies.
Source: Clay: A Studio Handbook
To heat a material to a temperature high enough to drive off all chemically combined water.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Adjustable tool for measuring inside/outside diameters, as in making lids.
Source: Clay: A Studio Handbook
Firing defect where excessively fast bisque-firing and/or excessive early reduction retard outgassing, causing carbon and sulfur to be trapped within clay body.
Source: Clay: A Studio Handbook
Refractory red stoneware clay, used to obtain rich red and brown colors in high-fire clay bodies.
Source: Clay: A Studio Handbook
Short for castable refractory—a refractory mix that can be cast into molds to form kiln parts.
Source: Clay: A Studio Handbook
A parabolic kiln arch requiring no buttressing or steel frame; laid out by hanging a chain from two points and marking the resulting curve.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
The removing of wares from a kiln.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Low-melting component in clay or glaze that reacts with silica to form glass.
Source: Clay: A Studio Handbook
Super-cooled liquid, which softens and hardens over broad range of temperature and cools to form an amorphous, noncrystalline solid. Level and rate of hardness (viscosity) controlled by temperature and by addition of fluxes and refractories, making possible the wide temperature range of ceramic clays/glazes.
Source: Clay: A Studio Handbook
Any dry, unfired clay form.
Source: Clay: A Studio Handbook
A specialized oven or furnace used for firing clay for pottery.
Source: Clay: A Studio Handbook
Metallic overglaze finish created either by painting prepared luster (metallic salt in organic binder) over previously fired glaze and firing to cone 018 or by spraying metallic salt dissolved in water into kiln and/or on to wares at low red heat, either during cooling cycle of a glaze-firing, or in a separate firing heated to that temperature.
Source: Clay: A Studio Handbook
Ceramic colorants that have been fritted in order to eliminate solubility problems and give greater stability in firing and truer color before firing. Most are stable up to cone 5, many to cone 10. Can be mixed with 25—50% Ferro 3134 frit for Maiolica overglaze decoration. Most stains are ground glass and are highly toxic in inhalation.
Mixture of ceramic stains or pure coloring oxides (sometimes with a little flux) in water suspension, which can be used for overglaze brushwork, or as a patina on unglazed clay.
Source: Clay: A Studio Handbook
Very soft, pliable animal skin—when wet works well to smooth wet clay surfaces.
Source: Clay: A Studio Handbook
A quantity of chemical material, usually salt or soda, which is inserted or injected into a hot kiln during vapor-glazing processes.
Source: Clay: A Studio Handbook
Inserting or injecting a charge of chemical material into a kiln during vapor-glazing processes.
Source: Clay: A Studio Handbook
Water in molecular combination within clay and glaze materials, which is driven off during the water-smoking period of the firing.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
European pottery, eighteenth century and later, featuring decoration inspired by imported Chinese Ming Dynasty wares.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
On the wheel, a temporary wet-clay form or reusable bisque-fired form upon which wares may be inverted for trimming.
Source: Clay: A Studio Handbook
A pale gray-blue feldspathic stoneware glaze featuring opalescence due to inclusions of phosphorous and/or other materials.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Clay mixture formulated of clays and other ceramic raw materials to give desired working characteristics.
Source: Clay: A Studio Handbook
Natural low-fire secondary clay—fluxed with iron, fires porous. Often called “common” clay, found almost everywhere, matures below 2000°F.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Wheel-thrown form that is closed off completely and then altered to form a vessel or sculptural component.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Ceramic forming method utilizing ropelike coils of plastic clay, assembled in successive courses to build up wall of vessel or sculpture.
Source: Clay: A Studio Handbook
Decoration where a toothed instrument is dragged over a soft clay surface, sometimes through a layer of slip.
Source: Clay: A Studio Handbook
Reaction initiated when fuel reaches kindling temperature, at which point oxidation of hydrocarbon gases releases heat, sustaining and accelerating reaction.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Upwards rise of warm air currents due to the transference of heat.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
CuSO4—color source for saggar firing and pit-firing. Soluble, and highly toxic in absorption, ingestion, and inhalation.
Source: Clay: A Studio Handbook
2MgO×2Al 2O3×5SiO2—magnesium/aluminum silicate clay mineral, used to make grog for refractory products—promotes formation of mullite.
Source: Clay: A Studio Handbook
Adjustable wooden forms used in casting plaster molds.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Very fine surface cracks in fired glaze surface—technically a fault in glazed wares, but often sought after, especially in raku.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Crushed window or bottle glass, occasionally used as a glaze material.
Source: Clay: A Studio Handbook
K2O×Al2O3×6SiO2—a common potash feldspar—HT alkaline flux. See feldspar. Close match to G-200. Toxic in inhalation.
Source: Clay: A Studio Handbook
In fuel-burning kilns, adjustable refractory plate located in exhaust flue, allowing control of back-pressure and secondary air, regulating kiln atmosphere.
Source: Clay: A Studio Handbook
Common deflocculant for casting slips. Product of R.T. Vanderbilt Company.
Source: Clay: A Studio Handbook
The phenomenon that occurs early in the glaze cooling cycle, when certain materials crystallize out of the vitrified (fused) mass.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Action of a solvent material on a solid, bringing it into liquid solution.
Source: Clay: A Studio Handbook
Kiln where exhaust gases exit through flue at floor level.
Source: Clay: A Studio Handbook
The flow of exhaust gases out of a fuel kiln, affecting intake of flames and secondary air.
Source: Clay: A Studio Handbook
Electric-drill-mounted impeller-mixer excellent for mixing glazes, slips, and slurries and for blunging casting-slip.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Low-fired ware, usually still porous after firing—must be sealed with vitreous glaze to be functional.
Source: Clay: A Studio Handbook
A self-glazing clay body in which soluble alkaline fluxes effloresce to the surface as the piece dries, and subsequently form a thin glassy coating in the firing.
Source: Clay: A Studio Handbook
Any of a group of slightly over 100 substances on earth that may exist as individual atoms, and from which all materials on earth are composed.
Source: Clay: A Studio Handbook
Al2O3×2SiO2×2H2O—pure white kaolin, less plastic than Tile-6 kaolin, frequently used in glazes.
Source: Clay: A Studio Handbook
MgSO4—water soluble, rarely used as magnesium source in glazes. Most often used as flocculant for slips and glazes. Often added to porcelain and porcelaineous stoneware bodies (1/2 of 1% of dry materials weight) to counteract deflocculating alkalinity released by kaolins or fluxes.
Source: Clay: A Studio Handbook
The science of comfortable and effective utility, determining how well a functional object or device works with the human body.
Source: Clay: A Studio Handbook
Chemical phenomenon where two materials in combination melt at lower temperature than either material by itself.
Source: Clay: A Studio Handbook
Decorating technique involving cutting or paddling flat facets in the clay surface.
Source: Clay: A Studio Handbook
Widely used (and misused) term referring to any earthenware pottery glazed with an opaque glaze (usually white) and overglaze decoration.
Source: Clay: A Studio Handbook
Long tapered knife useful for trimming cast or pressed pieces, and for separating mold components.
Source: Clay: A Studio Handbook
Highly refractory secondary clays with minimal fluxes and usually fairly coarse particle size—low shrinkage, buff-color, often non-plastic.
Source: Clay: A Studio Handbook
The part of a fuel-burning kiln where fuel gases combust before contacting wares. Gas kilns need little if any firebox, whereas wood and oil kilns produce long hot flames whereas require a large firebox unless flame-flashing and ash-slagging (with wood) effects are sought.
Source: Clay: A Studio Handbook
Maintaining some heat input after maturation, to slow down the cooling process, or to maintain reduction atmosphere during cooling. See reduction cooling.
Source: Clay: A Studio Handbook
The profile or schedule for temperature change in a kiln-firing, often including both the heating and cooling ramps.
Source: Clay: A Studio Handbook
Surface effects caused by direct flame contact on wares.
Source: Clay: A Studio Handbook
Wares made to stand stove-top heat. Explosions from trapped moisture, and resulting lawsuits have caused studio flameware to disappear from the domestic market.
Source: Clay: A Studio Handbook
Color change in fired clay or slip due to direct flame contact and residual ash deposition in wood firing, or due to variable currents of vapor deposition in salt and soda firing. Flashing can occur on almost any light-colored clay body, but is most dramatic on porcelain bodies and slips.
Source: Clay: A Studio Handbook
Slip that is painted or dipped onto wares in order to promote flashing effects in the firing.
Source: Clay: A Studio Handbook
Commercial flocculant used in glazes—1/4 tsp. per gallon of glaze.
Source: Clay: A Studio Handbook
Passages in kiln for flames or exhaust gases.
Source: Clay: A Studio Handbook
CaF2—limited use as flux. As with Cryolite, fluorine reacts w/silica at high temperatures, can cause pinholing, blisters. Useful in special-effect crater-glazes. Highly toxic in inhalation and ingestion.
Source: Clay: A Studio Handbook
Decorating technique involving carving or forming vertical flutes or grooves in surface of a piece.
Source: Clay: A Studio Handbook
Airborne ash in a wood-kiln.
Source: Clay: A Studio Handbook
Base of a ceramic piece.
Source: Clay: A Studio Handbook
Firing system in a fuel-burning kiln that uses power-driven blowers or other pressurized air source to entrain primary air.
Source: Clay: A Studio Handbook
Process of introducing metallic salts into kiln or onto wares at about cone 018, producing thin layer of metallic surface iridescence.
Source: Clay: A Studio Handbook
K2O×Al2O3×6SiO2—common potash feldspar, HT alkaline flux—close match to Custer. Toxic in inhalation.
Source: Clay: A Studio Handbook
The primary material that, in combination with fluxes, forms the glass essential to all fired ceramics. Primary glass-former at all temperatures is silica.
Source: Clay: A Studio Handbook
In high-fired wares, ideal state when clay body shrinks slightly more than glaze, putting glaze under slight compression, giving greater strength, resiliency. See crazing and shivering.
Source: Clay: A Studio Handbook
Kiln firing in which glazes are melted to form a smooth glassy surface.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
The chemically active state of the melted glaze.
Source: Clay: A Studio Handbook
Decorating technique where resist materials are applied to prevent glaze from adhering to certain areas.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Buff stoneware clay, produced by Cedar Heights Clay Company.
Source: Clay: A Studio Handbook
Crushed high-fired clay graded in sizes from 15-mesh (very coarse) to 150M (extra fine) added as a source of filler or tempering grit to clay bodies to reduce shrinkage and give structure for throwing or handbuilding. Does not shrink in firing, so in medium and coarse grades will show texture through thin to medium glaze. Toxic in inhalation.
Source: Clay: A Studio Handbook
Al2O3×2SiO2×2H2O—English kaolin, more costly than other choices in the United States, but gives whiter porcelain. Less plastic than TILE-6. Best kaolin for translucent bone china.
Source: Clay: A Studio Handbook
A long, low horizontal sprung-arch or barrel-arch wood-fired kiln, often banked with earth, traditionally used by potters on the eastern slope of the Appalachian Mountains.
Source: Clay: A Studio Handbook
CMC Gum, Veegum Cer—water soluble organic thickeners often added to glazes or slips as a suspension and/or brushing agent. Also used to allow glaze adhesion to vitrified or previously glaze-fired surfaces. Most commercial low-fire glazes and underglazes contain gum additives.
Source: Clay: A Studio Handbook
Hard, dense firebrick generally used only in high-stress areas of kiln (floor, burner ports, flues, bag wall) and for corrosive firing processes (salt, soda, wood).
Source: Clay: A Studio Handbook
Work done by effective heat transfer to wares, resulting from ramp rate, temperature and duration of firing.
Source: Clay: A Studio Handbook
Al2O3×2SiO2×2H2O—Kaolin that works especially well as flashing slip for salt, soda, and wood firing, especially since Avery kaolin is no longer being mined.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
High-temperature firing range usually including cone 8 to cone 12, for firing stoneware or porcelain.
Source: Clay: A Studio Handbook
Decorative style originating in Moorish Spain, characterized by extremely elaborate patterning and curvilinear plant motifs; major influence on Italian maiolica.
Source: Clay: A Studio Handbook
Steps constructed in walls of wood-kiln firebox in place of grates. Wood cut to proper length catches on hobs and remains suspended above coal bed, insuring quick and complete combustion.
Source: Clay: A Studio Handbook
The interior refractory surface of a kiln.
Source: Clay: A Studio Handbook
Combustible gases containing hydrogen and carbon, present in fuel gases and produced when carbon-based liquid or solid fuels are heated above kindling temperature.
Source: Clay: A Studio Handbook
Laboratory device for measuring specific gravity in slips and glazes. Does not give absolute measurement, and should be used only for comparing and reproducing results. Must be an appropriate glaze hydrometer for measuring suspensions heavier than water. Does not work well in thick glazes.
Source: Clay: A Studio Handbook
An iron ore with significant titanium—most often used in granular form to produce dark specks in clay or glaze. Higher iron concentration than in rutile.
Source: Clay: A Studio Handbook
Decorating technique where textured or patterned material or object is pressed into clay surface.
Source: Clay: A Studio Handbook
Decorating technique where design is formed by cutting or carving shallow lines in clay surface.
Source: Clay: A Studio Handbook
Any particles or bubbles of material that remain suspended within the glaze-melt, affecting the glaze appearance.
Source: Clay: A Studio Handbook
Contact face between clay and glaze. On low-fired wares, primarily just a physical interlocking of glaze into pores in clay. In high-fired wares, an intimate interaction of clay and glaze, reinforced by mullite crystals, creating very strong bond.
Source: Clay: A Studio Handbook
Most abundant coloring material on earth, responsible for brown and sometimes black color in natural rock and fired clays. In oxidation firings, the red ferric oxide remains very refractory, but in reduction firing and or at high-fire temperatures it transforms to the black ferrous oxide, a powerful flux.
Source: Clay: A Studio Handbook
FeO×Cr2O3—glaze colorant producing pink or red with tin, brown with zinc, gray with alkaline fluxes. Highly toxic in absorption, inhalation, and ingestion.
Source: Clay: A Studio Handbook
FeO—reduced form of iron oxide—gives same results as red iron in the firing, dependent on oxidation/reduction.
Source: Clay: A Studio Handbook
High-fired Japanese saturated iron glaze usually containing at least 10% (of dry-batch weight) iron oxide, which produces iridescent microcrystalline rust red.
Source: Clay: A Studio Handbook
Japanese trimming tool, formed of a steel strap bent and sharpened on the ends.
Source: Clay: A Studio Handbook
Crystalline clay mineral forming the basis of most clays we use in ceramics.
Source: Clay: A Studio Handbook
Greek term meaning earthenware from which the word ceramics is derived.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Kiln wash is a refractory slip coating applied to the top surface of a kiln shelf to protect it from glaze runs, drips and other accidents that occur in red hot kilns, like pots that tip over, bloating or melting clay bodies, etc. It is also used to protect shelves from volatiles in atmospheric kilns like wood ash, or sodium oxide in salt and soda kilns.
There are many kiln wash recipes to choose from and it’s important to find the recipe that works best with your firing practices. What works well at one firing range or atmosphere, may not be well suited for another.
A good wash requires materials that have very high melting points and that, when combined, do not create eutectics that cause melting. Kiln wash is used in the full range of ceramics firing from cone 022 to cone 14. The type needed varies for each specific situation because some potters work in electric kilns at low-fire temperatures, while others work with fuel-fired kilns at very high temperatures.
A common recipe for gas and electric oxidation or reduction kilns is 50% kaolin and 50% silica (flint). However, that mixture must never be used in salt, soda, and wood firing. For those processes, use 40% kaolin, 10% ball clay, and 50% alumina hydrate.
Mix kiln wash to a thick creamy consistency and apply with a wide brush or paint roller. In some cases you may wish to build up successive coats, especially in any situation where significant glaze runs are fairly common.
In all cases, avoid getting your wash on the edges of the shelves where it can flake off in the firing and fall on glaze wares below.
Kiln wash is a refractory slip coating applied to the top surface of a kiln shelf to protect it from glaze runs, drips and other accidents that occur in red hot kilns, like pots that tip over, bloating or melting clay bodies, etc. It is also used to protect shelves from volatiles in atmospheric kilns like wood ash, or sodium oxide in salt and soda kilns.
Source: Clay: A Studio Handbook
Potash spar no longer mined. Substitute Custer or G-200.
Source: Clay: A Studio Handbook
Na2O×Al2O3×6SiO2—a common soda feldspar—powerful HT alkaline flux. Toxic in inhalation.
Source: Clay: A Studio Handbook
3Al2O3×2SiO2—aluminum silicate used in place of alumina and silica to promote formation of mullite crystals, increase thermal shock resistance. Coarse-ground used as grog in refractory bodies for kiln furniture.
Source: Clay: A Studio Handbook
Condition of clay in which 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-decorating. Medium leather-hard good for thin slip- decorating, joining, incising, carving, piercing. Hard leather-hard good for thin slip- decorating, carving, scraping.
Source: Clay: A Studio Handbook
Lithium feldspar—HT alkaline flux—used for thermal shock bodies and matching glazes. Contains fluorine, with associated problems. Toxic in inhalation.
Source: Clay: A Studio Handbook
Li2CO3—powerful AT alkaline flux, especially with soda or potash feldspars. Promotes hardness and recrystallization in LT glazes. Forms low-temperature eutectic with silica. Toxic in inhalation.
Source: Clay: A Studio Handbook
Low-temperature firing range, usually below cone 02 (2048° F), used for most bisque-firing and for glaze-firing terra cotta and whiteware.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Potassium hydroxide or sodium hydroxide. Caustic alkaline soluble that leaches out of wood ashes when they are soaked in water. Occasionally used as a flux additive in terra sigillata, as in the black slip on Classical Greek wares. Toxic in liquid state, and may cause burns.
Source: Clay: A Studio Handbook
Suspension agent/plasticizer similar to Bentonite and Veegum T. Up to 2% of dry-materials weight as plasticizer in high-kaolin clay bodies. Up to 1/2 of 1% of dry-materials weight as suspension agent, brushing medium in glazes, and slips. Mix with water before adding other ingredients.
Source: Clay: A Studio Handbook
Glaze effect featuring large patches of crystal development on glaze surface, as compared to microcrystalline effects.
Source: Clay: A Studio Handbook
Earthenware, generally terra cotta, with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration. Originated from Islamic/Moorish techniques used on the Spanish island of Mallorca, which exported these wares throughout the Mediterranean in the 15th and 16th centuries. The technique was adopted by Italy during the Renaissance, which is when the term maiolica was introduced to describe these brightly decorated wares. Not to be confused with majolica, which is a trade name for an historic English style of brightly glazed earthenware vessels made in the forms of vegetables, animals, etc.
Source: Clay: A Studio Handbook
Trade name for an historic English style of brightly glazed earthenware vessels made in the form of vegetables, animals, etc. Not to be confused with maiolica, which is defined as earthenware with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration.
Source: Clay: A Studio Handbook
MnO2—flexible colorant—with alkaline fluxes gives purple and red colors—by itself gives soft yellow-brown—with cobalt gives black. Used with iron to color basalt bodies. Concentrations of more than 5% may promote blistering. Toxic in inhalation and ingestion. Fumes from firing are especially toxic.
Source: Clay: A Studio Handbook
Glaze featuring a dull, nonglossy surface.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
K2O×3Al2O2×6SiO2—an aluminosilicate with a fine sheet-lattice structure, closely related to clay and feldspar, and often found as minute iridescent flakes in some clays.
Source: Clay: A Studio Handbook
Crystal effects dependent on tiny crystals in the glaze surface, as compared to macrocrystalline effects.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
East Asian method of creating an inlaid effect by applying contrasting slip into a design incised in leather-hard clay. When the slip stiffens, the excess is scraped off.
Source: Clay: A Studio Handbook
Glaze materials that are added to a base glaze to modify surface qualities such as color, gloss, matteness, opacity, crystal development, etc.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Clays such as bentonite, resulting from the breakdown of airborne volcanic ash. The finest particle size of all clays. Very high shrinkage, and generally used only as an additive to clay bodies or glazes.
Source: Clay: A Studio Handbook
3Al2O3×2SiO2—calcined Kyanite, but is also found naturally. Very desirable interlocking needlelike aluminum silicate crystals form above 1800° F and greatly add to the strength of high-fired vitrified clay bodies and glazes. In well-formulated high-fired wares, clay-glaze interface is extremely strong partially due to interlocking mullite crystals.
Source: Clay: A Studio Handbook
Firing system in a fuel-burning kiln that uses natural convection currents in kiln (updraft) or in chimney (downdraft), without any mechanical blowers, to draw in secondary air, circulate heat and atmosphere through kiln, and exhaust combustion gases.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
In fuel-burning kiln, atmosphere that is neither oxidizing nor reducing—most efficient atmosphere for efficient climb and even temperature.
Source: Clay: A Studio Handbook
In glaze chemistry, the refractories or stabilizers that are combined with bases (fluxes) and acids (glass-formers).
Source: Clay: A Studio Handbook
Translates as “step-climbing kiln”—traditional Japanese multichamber downdraft/crossdraft climbing kiln, with initial firebox at bottom, and secondary fireboxes in each chamber. As kiln is fired, each chamber preheats the next. See climbing kiln.
Source: Clay: A Studio Handbook
An often-iridescent, milky-white Japanese glaze containing rice-hull ash.
Source: Clay: A Studio Handbook
Rich golden-brown stoneware clay, no longer being mined.
Source: Clay: A Studio Handbook
In glaze formulation, a material that produces inert inclusions or minute crystals in glaze, causing it to become opaque. Most common are tin oxide and zirconium silicate.
Source: Clay: A Studio Handbook
Zircon opacifier. Toxic in inhalation.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Any chemical reaction in which atoms or molecules combine with oxygen atoms. The combustion of fuel is an oxidation reaction.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
To subject a material to a high-oxygen atmosphere, encouraging oxidation reactions.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Decoration technique where strips of moist or adhesive paper are adhered to the surface to resist application of slip or glaze.
Source: Clay: A Studio Handbook
An overall thin wash of glaze or oxide stain, allowing the color and texture of the clay body to show through.
Source: Clay: A Studio Handbook
A table listing all of the chemical elements (those substances that can theoretically be reduced to individual atoms), with the smallest and simplest across the top and the heaviest and most complex at the bottom and grouped by similar physical/chemical characteristics.
Source: Clay: A Studio Handbook
Porous expanded granular silica, useful in making insulating refractories.
Source: Clay: A Studio Handbook
Li2O×Al2O3×8SiO2—lithium feldspar—HT alkaline flux—good for reducing thermal expansion, increasing thermal-shock resistance. Toxic in inhalation.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
A type of bonfire-firing where wares are buried in sawdust in a pit in the ground and a bonfire is built on top, so that the fire and coals slowly burn away the sawdust and fire the wares. Not to be confused with sawdust smoking.
Source: Clay: A Studio Handbook
K2O/Na2O/CaO×Al2O3×10SiO2—a plastic high-potash feldspathic clay, similar in structure to Cornwall stone—used in place of potash feldspar in some porcelain bodies to increase plasticity. Toxic in inhalation.
Source: Clay: A Studio Handbook
Quality of moldable flexibility in damp clay—superior plasticity depends on smaller clay particle size, slight acidity, less non-plastic additives, aging of damp clay body, adequate water content, and/or addition of accessory plasticizers, such as Veegum T or Macaloid.
Source: Clay: A Studio Handbook
Materials added to some clay bodies, especially those high in kaolins, to increase plasticity and dry strength—includes bentonite, Macaloid, Veegum-T.
Source: Clay: A Studio Handbook
Flat, thin crystals that make up clay. When wet they become sticky and slippery, creating the phenomenon we call plasticity.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
High-fired vitreous clay body 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.
Source: Clay: A Studio Handbook
White-firing stoneware clay bodies closely related to porcelain.
Source: Clay: A Studio Handbook
Calcium silicate aluminate—air-setting cement, often added in small quantities to homemade castable refractor mix in order to introduce air-setting qualities. Toxic in inhalation.
Source: Clay: A Studio Handbook
Materials like G-200; Custer feldspar.
Source: Clay: A Studio Handbook
K2Cr2O7—Occasionally used as acid-green colorant in raku glazes, but is highly toxic in absorption, ingestion, and inhalation.
Source: Clay: A Studio Handbook
A pottery bat is a pottery throwing accessory that enables freshly thrown work to be removed from the throwing wheel without the damage or warpage that can occur from touching the pot directly. Pottery bats also make it possible to return a piece to exact center to work on later.
Throwing bats can be made from most any rigid material, but wood, wood composites, plastics, and plaster are the most common. Except for plastic, these materials are all porous so pots will release from them easily as they absorb water from the clay. If the material is not porous (e.g., plastic) the pot must be wired off before it sets up too much, or it will crack as it shrinks.
Some of the materials that pottery bats are made from include:
- Duron® (aka Masonite® or tempered hardboard), a resin impregnated hardboard that’s water resistant and smooth on both sides.
- Medex®, a fiber board material made with a formaldehyde-free adhesive that’s waterproof yet porous
- Plastics of several different types (nonporous and waterproof, so they require wiring off pots)
- Plywood (must be exterior- or marine-grade to avoid delaminating)
- Plaster, one of the traditional materials for bats and one of the best for porosity
- Hydro-Stone, a USG product containing cement and crystalline silica, is similar to plaster but 8 to 10 times stronger
Most wheel manufacturers pre-drill wheel heads with bat-pin holes that hold 3/8-inch (or 10-mm) bat pins on 10-inch centers. Most bat makers produce bats that fit this standard. While pottery bats can be stuck to the wheel head with clay or pottery slip, bat pins allow the user to quickly add and remove bats with ease.Source: Clay: A Studio Handbook
Machine with a horizontally spinning wheel head upon which potters form and trim clay vessels by hand.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
In a fuel-burning kiln, air that feeds initial combustion; in a wood-burning kiln, air that feeds base of flames; in a gas kiln air that enters back end of burner tube in an atmospheric burner, or via blower in a power burner. See secondary air.
Source: Clay: A Studio Handbook
A machine similar to an oversized meat grinder, used to homogenize plastic clay bodies. Deairing pugmills have a vacuum pump attachment, which effectively removes all air from clay, eliminating need for hand wedging.
Source: Clay: A Studio Handbook
Al2O3×4SiO2×H2O—used in HT clay bodies (20% or less) to reduce thermal expansion, increase thermal shock resistance, reduce shrinkage, give stronger vitreous bodies. May reduce plasticity.
Source: Clay: A Studio Handbook
Flexibility of clay or glaze when heated sufficiently, as a result of developing glassy-phase.
Source: Clay: A Studio Handbook
Profile of the firing of a kiln, including speed, duration, soaking periods, etc. of both the heating and cooling cycle, as in firing ramp and cooling ramp.
Source: Clay: A Studio Handbook
Fe2O3—powdered rust—refractory red in oxidation, converts to black iron (flux) in reduction and/or high-fire. Low quantities in clear glaze produces celadon-green—high quantities produce temmoku black or saturated iron red—powerful flux. More than 5% in a glaze significantly increases fluxing in reduction.
Source: Clay: A Studio Handbook
To induce reduction.
Source: Clay: A Studio Handbook
Chemical reaction in which oxygen atoms are removed from a compound.
Source: Clay: A Studio Handbook
In a fuel kiln, maintaining slight reduction atmosphere during cooling cycle, from maturing temperature down to approximately 1400° F, in order to minimize reoxidation of clay and/or glazes.
Source: Clay: A Studio Handbook
Any materials highly resistant to the effects of heat. In kiln construction, high-temperature insulating materials, such as firebrick, ceramic fiber, etc. In clay and glaze chemistry, the neutrals or stabilizers that are resistant to melting and that combine with the fluxes (bases) and glass-formers (acids). Primary refractory material in ceramics is alumina, most often introduced in the form of clay (aluminum silicate).
Source: Clay: A Studio Handbook
In a wood kiln, the surface effects created by fly-ash settling upon the wares. Effect is maximized by turbulence in firebox, considerable draft through kiln, and/or active stirring of the coal bed. In short wood firings ash doesn’t have time to melt, and leaves a rough, crusty surface.
Source: Clay: A Studio Handbook
Material used in glazing and decorating that can be applied to surface to prevent adhesion of slip or glaze.
Source: Clay: A Studio Handbook
Wide, flat handheld 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.
Source: Clay: A Studio Handbook
Beveled edge obtained by rolling the outer edge of the foot of a soft leather-hard pot at an angle against a hard, flat surface.
Source: Clay: A Studio Handbook
Titanium ore, used as source of titanium dioxide, contains iron, other trace minerals—gives tan color, promotes crystallization giving mottled multicolor effects in some HT glazes, or in overglaze stain (very refractory, use sparingly). Gives rich mottled medium blue in some HT glazes. Dark rutile contains higher percentage of iron.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
Contemporary firing process in which wares are placed within refractory saggars, along with chemicals and combustibles, in order to achieve certain surface effects.
Source: Clay: A Studio Handbook
NaCl—table salt, rock salt—used in salt-firing. Skin irritant.
Source: Clay: A Studio Handbook
Vapor-glazing process where salt (sodium chloride) is introduced into kiln firebox at high temperature.
Source: Clay: A Studio Handbook
Granular silica. Major grit besides grog used to give clay bodies structure for throwing and hand building, high shrinkage in high-temperature clay bodies, but gives smoother fired surface than grog. Toxic in inhalation.
Source: Clay: A Studio Handbook
Process of incising surface of wet or leather-hard clay in crosshatch pattern before applying slurry and joining pieces.
Source: Clay: A Studio Handbook
In fuel-burning kilns, air that enters after initial combustion and feeds flames as they leave primary combustion source.
Source: Clay: A Studio Handbook
To place wares in a kiln. Or, in a loaded kiln, the entire structure of shelves, furniture, and wares.
Source: Clay: A Studio Handbook
Process of loading a kiln, or technically, of placing the set.
Source: Clay: A Studio Handbook
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 clay body beneath.
Source: Clay: A Studio Handbook
A broken fragment of pottery.
Source: Clay: A Studio Handbook
Classic Japanese glaze ranging from gray to white to orange, composed primarily of feldspar.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Permanent contraction of the clay in both drying and firing stages. Overall may be as much as 18%.
Source: Clay: A Studio Handbook
A container with fine-mesh wire screen in the bottom, available in different mesh sizes, used for straining slips and glazes in order to achieve complete mixing of raw materials and removal of large particles or contaminants.
Source: Clay: A Studio Handbook
The process of working a glaze or slip through a wire-mesh sieve to strain out impurities and to break up clumped raw materials.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
In heating clays and glazes, a solid-state reaction where particles stick together permanently, and mass can be considered fired.
Source: Clay: A Studio Handbook
A mechanized or manually operated device for rolling out large uniform slabs of clay.
Source: Clay: A Studio Handbook
Clay suspended in water, usually the consistency of thick cream.
Source: Clay: A Studio Handbook
The creation of ceramic forms by casting slip in plaster molds.
Source: Clay: A Studio Handbook
A naturally occurring clay that contains a high enough component of flux to form a glaze at high-fire temperatures.
Source: Clay: A Studio Handbook
A glaze with slip-clay or earthenware clay contributing the primary flux.
Source: Clay: A Studio Handbook
Decorating technique where resist materials are applied to prevent slip from adhering to some areas.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
A typically shallow frame or mold into which a slab of clay is allowed to fall or settle in order to form a vessel. These can be frames with no bottom.
Source: Clay: A Studio Handbook
Process of creating ceramic wares with a slump-mold. Firing defect where glassy-phase begins to dissolve sintered structure in clay, causing it to sag and deform.
Source: Clay: A Studio Handbook
During firing or cooling ramp, the act of holding kiln at steady temperature for a period of time to allow proper formation or maturation of certain clay and glaze effects.
Source: Clay: A Studio Handbook
A specialized hard brick shape, half the width of a standard brick. Often used for posts in salt and wood firings.
Source: Clay: A Studio Handbook
Na2O×Al2O3×6SiO2—feldspars contributing sodium (and potassium), primarily as a high-temperature flux.
Source: Clay: A Studio Handbook
Insulating firebrick.
Source: Clay: A Studio Handbook
A liquid mix where the components are fully dissolved (as in a water/soda ash solution for vapor-glazing), where materials will not settle out via gravity.
Source: Clay: A Studio Handbook
An agent that acts to accelerate the dissolution of a more resistant material. In glaze maturation, the flux-glass-former combination acts as a solvent on alumina.
Source: Clay: A Studio Handbook
The weight or density of a liquid measured in proportion to that of water. A glaze with a specific gravity of 1.2 is 1.2 times as dense as water per unit of measure.
Source: Clay: A Studio Handbook
A specialized hardbrick, half the thickness of a standard brick.
Source: Clay: A Studio Handbook
Li2O×Al2O3×4SiO2—lithium feldspar—powerful high-temperature alkaline flux. Promotes copper blues—good for thermal-shock bodies and matching glaze. Toxic in inhalation.
Source: Clay: A Studio Handbook
Open-front enclosure with an exhaust fan at the rear, designed to draw off all overspray and other toxic dust or fumes.
Source: Clay: A Studio Handbook
Application of liquid slip, engobe, glaze, or stain, using mechanized spray equipment. Gives smooth glaze coating, but with less glaze pooling in recesses.
Source: Clay: A Studio Handbook
Surface decorating technique in which small coils or balls of clay are affixed to the damp or leather-hard surface, usually with a layer of slip.
Source: Clay: A Studio Handbook
Kiln arch representing less than 180 degrees of curvature, and requiring buttressing to support outward thrust of the arch.
Source: Clay: A Studio Handbook
High-fired vitreous ware, literally as hard and durable as stone. Matures from 2200-2400° F. (cone 5–11).
Source: Clay: A Studio Handbook
SrCO3 Alkaline earth, high-temperature flux, similar to barium, slightly more powerful.
Source: Clay: A Studio Handbook
Zircon opacifier. Toxic in inhalation.
Source: Clay: A Studio Handbook
A liquid mix where insoluble particles are distributed throughout without dissolving and may settle out from gravity, as in a glaze or slip.
Source: Clay: A Studio Handbook
Classic East Asian high-iron gloss glaze giving black where thick, breaking to brown or red-brown where thin.
Source: Clay: A Studio Handbook
Low temperature, porous earthenware clay body, fires red-brown due to high iron content, which also fluxes clay, making it the most durable low-fired clay after firing.
Source: Clay: A Studio Handbook
Ultra-refined 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.
Source: Clay: A Studio Handbook
The physical expansion and contraction that accompanies the heating and cooling of most materials. See coefficient of expansion.
Source: Clay: A Studio Handbook
Effect of sudden temperature changes during firing or during subsequent heating and cooling in daily use.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
SnO2—most powerful opacifier, but expensive—inert dispersoid in glaze melt—5–7% will produce opaque white in a clear glaze. Toxic in inhalation and ingestion.
Source: Clay: A Studio Handbook
TiO2—matting/opacifying agent. Promotes crystal growth, visual texture in glazes.
Source: Clay: A Studio Handbook
A T-shaped Japanese throwing gauge, used to measure the depth and rim diameter of a vessel, usually when throwing off the hump.
Source: Clay: A Studio Handbook
Method for testing three-way combinations of glaze materials, where proportional amounts vary through a series of samples between three limits. May involve change in glaze materials, or addition of colorants or modifiers.
Source: Clay: A Studio Handbook
At the leather-hard stage, removal of excess clay from a piece, using any of a variety of sharp cutting tools.
Source: Clay: A Studio Handbook
Zircon opacifier. Toxic in inhalation.
Source: Clay: A Studio Handbook
Common flaw in plaster or bisque molds, where the clay or casting catches and will not pull free without breaking or distorting.
Source: Clay: A Studio Handbook
Process of applying any decoration to the bare, (usually bisque-fired) clay surface directly before glazing.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
V2O5—weak yellow colorant—toxic, expensive—usually fritted with tin to produce stronger yellow. Highly toxic in inhalation and ingestion.
Source: Clay: A Studio Handbook
Any glaze process where the glaze results from vapor deposited within the kiln—includes salt glazing, soda glazing, fuming and wood firing (where fly ash is deposited on the ware and melted into a glaze).
Source: Clay: A Studio Handbook
Combination of Veegum T and carboxymethylcellulose gum, used as a suspension and adhesion agent in glazes. Product of H.T. Vanderbilt Company.
Source: Clay: A Studio Handbook
Suspension agent/plasticizer similar to bentonite and macaloid. Up to 2% of dry materials weight as plasticizer in high-kaolin clay bodies, and up to 1/2 of 1% of dry-materials weight as suspension agent, brushing medium in glazes and slips. Mix with water before adding other ingredients. Product of H.T. Vanderbilt Company.
Source: Clay: A Studio Handbook
Natural-draft gas burner featuring streamlined restriction in burner tube, which increases entrainment of primary air and the efficiency of gas-air mixing.
Source: Clay: A Studio Handbook
A porous expanded mica product used as filler in clay bodies and in insulating refractory layer on kiln exterior.
Source: Clay: A Studio Handbook
An engobe containing sufficient flux to form to a vitreous clay coating.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
European term referring to a fired piece discarded due to damage in firing.
Source: Clay: A Studio Handbook
Amount of water required to bring a dry clay to its state of ideal plasticity. Common clay test used to determine this amount. The more water needed, the finer the particle size, the more plastic the clay, and the greater the drying shrinkage.
Source: Clay: A Studio Handbook
Melted wax or wax emulsion used to prevent slip or glaze from adhering to a clay surface, either in decorating, or in preparing work for glazing.
Source: Clay: A Studio Handbook
Process of kneading the clay with the hands to remove air bubbles and ensure homogenous mass.
Source: Clay: A Studio Handbook
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.
Source: Clay: A Studio Handbook
Low-temperature white clay body.
Source: Clay: A Studio Handbook
CaSiO3—used in partial replacement of silica and whiting in high-temperature bodies, improves thermal shock resistance. In some cases, it is used in place of whiting to eliminate L.O.I. Toxic in inhalation.
Source: Clay: A Studio Handbook
Source: Clay: A Studio Handbook
High-iron yellow clay mineral, used as colorant in glazes and slips, converts to red iron oxide in oxidation or black iron oxide in reduction and/or high-fire.
Source: Clay: A Studio Handbook
ZnO—High-temperature flux that promotes brilliant glossy surfaces. In some glazes can encourage opacity. With titanium in low-alumina glaze can encourage macrocrystalline growth (crystalline glazes). Volatizes in high-fire reduction. Toxic in inhalation.
Source: Clay: A Studio Handbook
Common opacifier, often used in place of the more expensive tin oxide. Sold as Opax, Superpax, Ultrox, etc., use 7% in a slip and 10 to 12% in a clear glaze to get pure opaque white. Zirconium particles are extremely refractory and remain as inert inclusions in the glaze-melt. They will not affect matteness or gloss, but will increase viscosity.
Source: Clay: A Studio Handbook
ZrSiO4—zircon opacifier—inert dispersoid in glaze melt—low-cost substitute for tin oxide—use double the recipe weight of tin. Includes Zircopax, Opax, Superpax, Ultrox. All are toxic in inhalation.
Source: Clay: A Studio Handbook
Zircon opacifier, no longer being manufactured. Toxic in inhalation.
Source: Clay: A Studio Handbook