Glossary

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#6 Tile Kaolin; Tile-6 Kaolin: Al2O3×2SiO2×2H2O—air-floated secondary kaolin—broader particle distribution than primary kaolins, greater plasticity, green strength. Source: Clay: A Studio Handbook

A

Acid: 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.
Air-floated; Air-floating: Industrial method used in processing of raw clays, where powdered clay is floated in an airstream to settle out heavier particles. Source: Clay: A Studio Handbook
Air-set; Air-setting: Castable refractory or mortar, which becomes hard and strong while still damp as a result chemical interaction with water. Source: Clay: A Studio Handbook.
Air-shutter: Adjustable shutter on the rear opening of an atmospheric burner, or on the blower intake of a power burner, used to regulate entrainment of primary air. 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.
Albany Slip Substitute: Substitute for Albany slip. Highly fluxed with iron—true slip clay—will form glaze at HT. 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 claybodies 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
American Raku: See raku (western).
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
Antimony Oxide: Sb3O3—colorant—soluble, toxic, expensive, produces yellow with titanium. Highly toxic in absorption, ingestion, and inhalation. Source: Clay: A Studio Handbook
Applique: Low-relief clay shapes added to scored, slurried leather-hard surface for decoration. Source: Clay: A Studio Handbook
Arch Brick: Bricks with angled side faces, tapering across the width of the brick, which when laid together form a curved arch. Standard arch bricks give 4 1/2″ thick arch. 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
Atmospheric Burner: A gas burner that utilizes the effect of gas escaping from the orifice to entrain primary air for combustion, without the need for a mechanical blower. Source: Clay: A Studio Handbook
Atomic Vibration; Atomic Motion: Constant motion in atoms and molecules, present in all materials. Heat accelerates atomic vibration, eventually breaking atomic bonds, causing transition from solid to liquid to gas. Source: Clay: A Studio Handbook
Aventurine: A glaze featuring iridescent metallic flecks as a result of iron crystals just beneath the surface. Source: Clay: A Studio Handbook
Avery Kaolin: Well-known flashing slips for wood fire, but no longer being mined. Source: Clay: A Studio Handbook

B

Back-burning: In burners, when speed of air/fuel exiting tip of burner is slower than combustion, and flame jumps back down burner tube to fuel orifice, resulting in smoky orange flame and overheated burner tube. 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 claybodies, 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
Bank Kiln: Early form of East Asian kiln, where tubular kiln chamber was excavated in an earthen bank. Predecessor to anagama and tube kiln. Source: Clay: A Studio Handbook
Barnard Clay; Blackbird: Slip clay with very high iron content, often used as brown colorant for claybodies. Source: Clay: A Studio Handbook
Barrel Arch: Kiln arch that forms a perfect half circle, with the arch beginning and ending on a horizontal surface, using no skew bricks. Source: Clay: A Studio Handbook
Basalt Body; Basalt Ware: Claybody with enough content of dark clays and/or metallic oxides to fire dark brown or black. 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
Baso Valve: Safety valve used on most gas kilns, operates on minute electrical current generated by thermocouple. Natural-draft kilns equipped with Baso system often need no external electrical hookup at all. In operation, button on Baso valve is depressed and pilot flame is ignited, which heats up thermocouple, energizing small electromagnet in Baso valve, which holds valve open admitting gas to main burner. If pilot is extinguished, thermocouple cools, and Baso valve closes, shutting off gas. Source: Clay: A Studio Handbook
Beading Glaze; Beaded Glaze: A specialized controlled-crawl glaze designed to crack and crawl significantly during firing (due to extreme L.O.I. in glaze materials), and then melt into isolated beads on the surface. Source: Clay: A Studio Handbook
Bell Kiln: A kiln with a stationary floor but where the kiln body can be raised upwards on vertical tracks, making it easy to load and unload large forms. Source: Clay: A Studio Handbook
Bentonite: Al2O3×5SiO2×7H2O —Montmorillonitic clay formed from decomposition of airborne volcanic ash.
Bismuth Subnitrate: Soluble metallic salt—gives metallic luster under LT reducing conditions, especially in fuming and saggar applications. Toxic in inhalation and ingestion. 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
Black-figure Style: In ancient Greek ceramics, late Archaic and early Classical style in which figure first dominates, and where figures are in black against a red background. Source: Clay: A Studio Handbook
Blackbird; Barnard Clay: Slip clay with very high iron content, often used as brown colorant for claybodies. Source: Clay: A Studio Handbook
Blackware Firing: Bonfire firing that is smothered with manure and/or sawdust at maximum temperature and then buried in sand, dirt, or ash, to trap smoke, causing wares to turn black. 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.
Bloating: Firing defect where blisters form within claybody, raising large lumps on the surface.
Blow-off; Flame-off: In burners, when speed of air/fuel mixture exiting tip of burner is greater than combustion rate—flame jumps off tip of burner and often blows out. 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.
Body Reduction: Period of reduction atmosphere induced between cone 012 and 08 to bring out iron color and speckles in claybody, especially in high-fired stoneware. 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
Bonfire Firing: Earliest and most basic firing process, where wares are fired in an open bonfire.
Borax; Sodium Tetraborate: Na2O×2B2O3×10H2O—a major LT alkaline flux, available in granular or powdered form.
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.
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

C

Calcine

To heat a material to a temperature high enough to drive off all chemically combined water. Source: Clay: A Studio Handbook

Calcium Borate; Colemanite; Gerstley Borate

CaO×3B2O3—traditional important LT alkaline flux, but is no longer being mined. Replace with Ferro 3134 for LT glazes, commercial Gerstley borate substitutes for HT glazes. Test all substitutes. Source: Clay: A Studio Handbook

Calcium Carbonate; Whiting; Limestone; Marble; Chalk

CaCO3—alkaline earth, contributing calcium oxide to glaze—powerful AT flux—major HT flux for glazes—gives strong durable glass. Sometimes used in low-fire claybodies to extend firing range and give greater fired strength. 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

Calcium Silicate; Wollastonite

CaSiO3—used in partial replacement of silica and whiting in HT 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

Calipers

Adjustable tool for measuring inside/outside diameters, as in making lids. Source: Clay: A Studio Handbook

Car Kiln; Shuttle Kiln

Kiln where kiln floor and often the door are mounted on a car that may be rolled in and out of kiln on tracks. Sometimes has two cars, with doors in both ends of the kiln. 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 claybody.

Carbondale Clay

Refractory red stoneware clay, used to obtain rich red and brown colors in high-fire claybodies. Source: Clay: A Studio Handbook

Carpal Tunnel Syndrome (CTS)

Serious affliction of the wrist resulting from excessive and/or stressful repetitive-motion activity such as hand wedging clay. 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 Crackle Glaze

A glaze designed to craze for decorative effect. Appropriate primarily for nonfunctional objects and surfaces, as crazing is a flaw and weakens wares. 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 Frit

Combinations of ceramic materials that have been melted to a glass and crushed/ground back to a powder, in order to give greater chemical stability and to eliminate toxicity resulting from water solubility of raw material. All frits are ground glass and are toxic in inhalation. FERRO 3124—high-alumina calcium-borate frit, gives greater strength in LT claybodies. FERRO 3134—calcium-borate frit often used as substitute for Gerstley borate in low-fire glazes when greater reliability and/or long-term insolubility and/or greater transparency are desired. Makes good cone 04 transparent glaze by itself. FERRO 3110 and 3195—Both very similar to 3134—run tests to determine which works best for your needs. 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 Msds

MATERIAL SAFETY DATA SHEET, available from your supplier, giving toxicity information on each of the ceramic materials you use. Source: Clay: A Studio Handbook

Ceramic Pyrometer

Temperature gauge connected to a thermocouple, which indicates temperature within kiln. Pyrometers provide good general reference, but only respond to temperature, whereas clay and glazes (and cones) are affected by temperature, duration, and atmosphere of firing. See pyrometric cones. 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

Ceramics Handbuilding

Forming plastic clay by hand without the wheel, using pinching, coiling, and/or slab construction. Source: Clay: A Studio Handbook

Chalk; Whiting; Calcium Carbonate; Limestone; Marble

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 Clay; Kaolin

Al2O3×2SiO2×2H2O—Primary clay that fires pure white, very refractory, coarse particle size, low plasticity, high-temperature—major component of porcelain and some whiteware bodies. 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 Cone

See pyrometric cones. 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

Claybody

Clay mixture formulated of clays and other ceramic raw materials to give desired working characteristics. 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

Climbing Reduction; Partial Reduction

In a fuel kiln, atmosphere that is slightly reducing, but still allows increase in temperature. 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

Coefficient Of Expansion Formula

A measurement of a material’s tendency to expand when heated and contract when cooled. The higher the coefficient of expansion, the lower the thermal shock resistance. In firing dissimilar materials in contact with one another the coefficient of expansion must be matched. Source: Clay: A Studio Handbook

Coggle; Roulette

Small stamp wheel with raised pattern around the rim, which when rolled along a plastic clay surface leaves a band of relief pattern. Usually formed with damp or dry clay and bisque-fired. 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

Colemanite; Calcium Borate; Gerstley Borate

Collaring; Necking-in

Process of reducing the upper diameter of a thrown form by working the walls of the rotating form inwards with fingers or rib, as in a bottle shape. Source: Clay: A Studio Handbook

Colloid; Colloidal

Gaseous, liquid, or solid materials that remain suspended within glaze melt without dissolving into melt, and which often coagulate to form visible particles. An example is copper in a copper-red glaze—if firing is too fast, minute copper globules will not coagulate into visible masses, and glaze will be clear. Source: Clay: A Studio Handbook

Color-active Slip

Slip of a composition that affects color of glazes placed over it. Can result from coloring oxides, color modifiers, or textural qualities that influence color. 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

Continuous Kiln

Industrial tunnel car kilns or rolling-hearth kilns in which wares are slowly moved through a kiln that continuously remains at maturing temperature. Source: Clay: A Studio Handbook

Controlled-crawl Glaze

A glaze designed to crawl as glaze materials shrink during drying or during early red heat, producing alligator skin or beaded effect. Source: Clay: A Studio Handbook

Convection Currents

Upwards rise of warm air currents due to the transference of heat. Source: Clay: A Studio Handbook

Cooling Ramp

The profile or schedule of temperature change in the cooling of a kiln. 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 Oxide, Black; Cupric Oxide

CuO—alternate source of copper, coarser particle size, twice as powerful as copper carbonate. Toxic in inhalation and ingestion. Source: Clay: A Studio Handbook

Copper Red; Flambe; Oxblood; Sang De Boeuf

Popular mid-range and high-fire glazes featuring very small percentage of copper that, under correct light reducing conditions, gathers into colloidal particles of red copper oxide, producing bright rich red and red-purple colors. For good copper reds, start reduction at cone 012 or 010, and maintain partial up to maturation, with oxidation cleanup at end but no glaze reduction. 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

Cornish Stone; Cornwall Stone

K2O/Na2O/CaO×Al2O3×10SiO2—HT feldspathic alkaline flux containing calcium and potassium, but more refractory than potash feldspars. Substitution eight parts potash feldspar, two parts silica, one part kaolin. Toxic in inhalation. Source: Clay: A Studio Handbook

Cottles

Adjustable wooden forms used in casting plaster molds. Source: Clay: A Studio Handbook

Crackle

See crazing. 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

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

Cryolite; Sodium Aluminum Fluoride

Na3AlF6—small amounts promote crackle effects, larger amounts become very volatile with silica and may cause blistering. Used for special effect crater glazes. Toxic in inhalation. 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. See glass, super-cooled liquid 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

Cylinder Kiln

First stage in kiln evolution beyond bonfire, consisting of cylindrical clay wall containing fire and wares, covered with layer of shards. Source: Clay: A Studio Handbook

D

Damp Cabinet: (also damp box, damp closet, damp room) A reasonably airtight cabinet or room in which damp ceramic forms can be stored temporarily to slowly dry, protected from harmful air currents.
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
Deairing: The process of removing the air from a plastic clay mass, usually accomplished through wedging, or far more effectively with a vacuum deairing pugmill. Source: Clay: A Studio Handbook
Deflocculate; Deflocculation: Process of adding an alkaline (usually) material (deflocculant) to a suspension, which introduces like electrical charges to all particles, causing them repel one another and remain in suspension.
Deposited Clays; Sedimentary Clays; Secondary Clays: Clays that have been transported away from their point of geologic origins by wind or water.
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
Dispersoids: In glaze-melt, inclusions that disperse throughout the melt without actually dissolving into the glassy-phase. Source: Clay: A Studio Handbook
Dissolution: Action of a solvent material on a solid, bringing it into liquid solution. Source: Clay: A Studio Handbook
Dolomite; Calcium/magnesium Carbonate: MgCO3×CaCO3—High temperature alkaline earth flux, promotes hard, durable surfaces and recrystallization/matting in glazes. Often added to claybodies to give longer firing range and can promote more durable low-fire bodies. 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
Draw Rings; Draw Trials: In vapor-glaze firing, small rings of clay, formed to stand vertically, which are placed inside a peephole and may be removed with an iron rod during the firing, quenched in water, and examined to determine the degree of glaze deposition. 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

E

Earthenware: Low-fired ware, usually still porous after firing—must be sealed with vitreous glaze to be functional. Source: Clay: A Studio Handbook
Effloresce, Efflorescence: Formation of crystalline deposits on surface of clay or concrete as soluble compounds migrate to surface during drying. See Egyptian paste. 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.
Enamels; China Paints: 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
Engobe; Underglaze: Colored slips formulated to have low drying shrinkage, allowing application to bone-dry or bisque-fired surface before glazing.
Entrained Air: Primary air drawn into an atmospheric burner, or mechanically injected into a power burner. Source: Clay: A Studio Handbook
Envelope Kiln: A kiln in which the firing platform is stationary, but the body of the kiln rolls out of the way horizontally on tracks. Often equipped with two firing platforms to be fired alternately. Source: Clay: A Studio Handbook
EPK, Calcined: Al2O3×2SiO2—used in place of regular kaolin to adjust raw fit (reduce glaze drying-shrinkage) in glazes and engobes. 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.
Extruder, Extrude, Extruding: Machine that forces plastic clay through a die to produce extruded clay shapes. Source: Clay: A Studio Handbook

F

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
Feathering; Feather-combing: Decorating technique where a soft, fine pointed tool is drawn through adjacent contrasting-colored bands of liquid slip applied to a damp clay surface. Source: Clay: A Studio Handbook
Ferric Chloride; Iron Chloride: FeCl2×6H2O—soluble metallic salt—fuming agent used to produce lusters on glazed surface. Highly toxic in inhalation and ingestion. 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
Figure-ground: The fundamental design relationship between foreground imagery and background or surrounding area. Foreground shapes subdivide background, creating additional important shapes. Source: Clay: A Studio Handbook
Fillers; Tempering Materials; Temper: Gritty materials like sand, grog, volcanic ash, crushed seashells, etc., added to clay to open up body, give physical structure in wet-working and increase thermal shock resistance. 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 nonplastic. Source: Clay: A Studio Handbook
Fire-eye: Ultraviolet sensor used on industrial burner systems to monitor burner flame. See flame-rectification system. 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
Flame-off; Blow-off: In burners, when speed of air/fuel mixture exiting tip of burner is greater than combustion rate—flame jumps off tip of burner and often blows out. Source: Clay: A Studio Handbook
Flame-rectification System: A burner system with automated reignition feature that immediate restores flame should it become extinguished. Usually incorporates a fire-eye ultraviolet sensor to monitor burner flame. Source: Clay: A Studio Handbook
Flame-retention Tip: Gas burner tip that causes turbulence in moving stream of gas and air, intermixing them, speeding combustion, holding flame at burner tip, and preventing both flame-off and back-burning. 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 claybody, 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
Flint; Quartz; Silica; Silicon Dioxide: SiO2 —The primary glass-former in clay and glazes—vitrification, fluidity, transparency/opacity controlled by adding fluxes and/or refractories. Highly toxic in inhalation. Source: Clay: A Studio Handbook
Flocculation; Flocculate: The process of adding an acidic (usually) substance (flocculant) that gives particles in suspension opposite electrical charges, causing them to attract one another (to flock together)—a disadvantage in a casting slip but a great advantage in a claybody or a decorating slip. Usually only claybodies high in kaolin need to be flocculated by adding 1/2 of 1% (of dry-batch weight) epsom salts. Flocculation also often used to thicken up a glaze to help keep it in suspension and to improve application properties. See deflocculation. 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
Forced-draft: Direct-connected exhaust system equipped with suction fan, used on commercial furnaces, but never on studio ceramic kilns. Term often mistakenly used to refer to forced-air system. Source: Clay: A Studio Handbook
Fracture Plane: Fracture zone that results when clay components are pressed straight together without disrupting the surface (by scoring and adding slurry or by smearing together) to intermix the platelates. Parts hold together while wet and tacky, but will separate easily when dry or fired. Source: Clay: A Studio Handbook
Fritting; Fritted: The process of melting particular ceramic materials to a glass and then crushing and grinding to form a frit. 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
Fusion; Fused: In the glaze-melt, the point where dissolution of sintered structure is complete, and all refractory particles are dissolved into the glaze melt, forming a fused material—one that has melted to liquid. Source: Clay: A Studio Handbook

G

G-200: K2O×Al2O3×6SiO2—common potash feldspar, HT alkaline flux—close match to Custer. Toxic in inhalation. Source: Clay: A Studio Handbook
Geometric Style: In ancient Greek ceramics, early pre-Classical (Archaic) style featuring intricate geometric slip decoration. Source: Clay: A Studio Handbook
Gerstley Borate; Colemanite; Calcium Borate: CaO×3B2O3—traditional important LT alkaline flux, but is no longer being mined. Replace with Ferro 3134 for LT glazes, commercial Gerstley borate substitutes for HT glazes. Test all substitutes. 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
Glassy-phase: In heating ceramic materials, point where glass-formers and fluxes combine and soften to begin forming a glass. See sintering, dissolution. Source: Clay: A Studio Handbook
Glaze Compression: In high-fired wares, ideal state when claybody 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 claybody in terms of composition and coefficient of expansion so that it will adhere permanently . See glaze compression, interface. 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
Glaze, Glazes, Reduction-fired Glazes, Celadon, Temmoku, Chun, Crystalline Glazes, Macrocrystalline Glazes: Coating of powdered ceramic materials, usually prepared and applied in water suspension, which melts smooth and bonds to clay surface in glaze firing. See interface. Source: Clay: A Studio Handbook
Glomax: Calcined kaolin. See EPK, calcined. Source: Clay: A Studio Handbook
Glycerin; Glycerol: An organic gum derived from fats and oils, sometimes used as brushing medium for engobes or for reglazing fired wares. Also used as a lubricant in burnishing. For applying glazes to glaze-fired or vitrified surfaces, add one teaspoon glycerin to 100 grams dry glaze. Source: Clay: A Studio Handbook
Goldart: Buff stoneware clay, produced by Cedar Heights Clay Company. Source: Clay: A Studio Handbook
Graniteware: Ware with mottled slip or glaze treatment to create appearance of granite. Source: Clay: A Studio Handbook
Grate Kiln: An early form of kiln originating in the Middle East, with a below-ground firebox, a grate supporting the wares within a cylindrical enclosure, and a piled shard roof. Source: Clay: A Studio Handbook
Green Firing; Single-firing: Process of glaze-firing glazed greenware without a bisque-firing. Source: Clay: A Studio Handbook
Green Glazing; Raw Glazing: Glazing leather-hard or bone-dry wares for single-firing. 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 claybodies 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

H

Hard-paste: Traditional European term for high-fired porcelain. 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
Hawthorn Bond: Refractory stoneware clay or fireclay, used in stoneware claybodies. 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

I

IFB; Insulating Firebrick; Softbrick: Porous firebrick with insulating values much higher than hardbrick. 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 Chloride; Ferric Chloride: FeCl2×6H2O—soluble metallic salt—fuming agent used to produce lusters on glazed surface. Highly toxic in inhalation and ingestion. 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

J

Jasper Ware: Ware developed by Wedgwood in which colored stoneware is decorated with contrasting raised relief slip or clay imagery. Source: Clay: A Studio Handbook
Jiggering; Jolleying: A partially mechanized vessel-forming process on the potter’s wheel, where a pivoting arm (a jigger or jolley) with a mounted profile rib is brought down against a centered clay ball within or upon a wheel-mounted mold. Source: Clay: A Studio Handbook
Jug Finger; Potter’s Finger: A long wooden tool with a knob on the end, often covered with chamois, which is inserted in a bottle or jug on the wheel after the top has been necked in and is used to broaden and refine the shape. Source: Clay: A Studio Handbook

K

Kaki

High-fired Japanese saturated iron glaze usually containing at least 10% (of dry-batch weight) iron oxide, which produces iridescent microcrystalline rust red. See tessha. 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

Kaolin; China Clay

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

Key; Registration Marks

In plaster slip-casting molds, small rounded depressions carved in each parting face of a mold segment, so that when adjacent segments are cast a corresponding bump will form. In use, the keys allow the mold segments to be aligned and assembled securely after the original prototype is removed. 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.

Source: Clay: A Studio Handbook Since this was expanded using text from another source, should we delete this? Or could we put it in the first paragraph like this:

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 (Source: Clay: A Studio Handbook). It is also used to protect shelves from volatiles in atmospheric kilns like wood ash, or sodium oxide in salt and soda kilns.

Kindling Temperature

In combustion of fuels, temperature that must be reached before oxidation reaction will produce enough heat to sustain and accelerate. For wood and paper, kindling temperature is 451° F. 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

L

L.O.I.; Loss On Ignition: The fraction of any material that volatizes and outgases during firing—includes carbonates, sulfates, nitrates, organic contamination, and chemically combined water. Source: Clay: A Studio Handbook
Lead Carbonate; White Lead: 2(PbCO3)×Pb(OH)2—former source of lead for glazes. Highly toxic in absorption, inhalation, and ingestion. Remove from studio and dispose of in a responsible fashion. Source: Clay: A Studio Handbook
Lead Oxide; Red Lead: Pb3O4—Former source of lead for glazes. Highly toxic in absorption, inhalation, and ingestion. Remove from studio and dispose of in a responsible fashion. Source: Clay: A Studio Handbook
Lead Silicate: 3PbO×2SiO2—fritted lead compounds—LT flux, but resulting glazes may be toxic and should not be used on functional wares. Most powerful of all fluxes, promotes smooth-flowing, self-healing glazes. Highly toxic in inhalation and ingestion. 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
Limestone; Marble; Chalk; Whiting; Calcium Carbonate: CaCO3—alkaline earth, contributing calcium oxide to glaze—powerful AT flux—major HT flux for glazes—gives strong durable glass. Sometimes used in low-fire claybodies to extend firing range and give greater fired strength. Source: Clay: A Studio Handbook
Limit Formulas: A set of formulas giving ideal ranges of glaze oxides that can be present in glaze unity formulas at different firing temperatures. Used in conjunction with glaze calculation to predict or evaluate glaze performance. See unity formulas. Source: Clay: A Studio Handbook
Line Blend Graph: Method for testing glaze materials, where proportional amounts vary through a series of samples between two limits. May involve change in a glaze material, or addition of a colorant or modifier. See triaxial blend. 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
Lizella Clay: High-iron (4%) orange-red stoneware clay—substitute for Ocmulgee, which is no longer being mined. 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 terracotta work. 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 terracotta and whiteware. Source: Clay: A Studio Handbook
Luting: English word for attaching clay forms with slurry. 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

M

Macaloid: Suspension agent/plasticizer similar to Bentonite and Veegum T. Up to 2% of dry-materials weight as plasticizer in high-kaolin claybodies. 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. See crystaline glazes. Source: Clay: A Studio Handbook
Magnesium Silicate; Steatite; Soapstone; Talc: 3MgO×4SiO2×H2O—HT alkaline earth flux in glaze, promotes smooth buttery surfaces, partial opacity—similar composition to clay, but in LT claybodies gives low shrinkage and high thermal shock resistance, as in standard 50/50 talc/ball clay whiteware body. Highly toxic in inhalation and ingestion. Source: Clay: A Studio Handbook
Magnesium Sulfate; Epsom Salts: 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
Maiolica: Earthenware, generally terracotta, 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
Manometer: Primitive gauge for measuring water column inches, consisting of a U-shaped glass tube partially filled with water, with the pressure source connected to one end. See water column inches. Source: Clay: A Studio Handbook
Matt Glaze: Glaze featuring a dull, nonglossy surface. Source: Clay: A Studio Handbook
Maturity; Maturing Point: Firing point at which a claybody or glaze reaches its desired condition of color, hardness, density, etc. 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, mattness, opacity, crystal development, etc. Source: Clay: A Studio Handbook
Mold-release Compounds: In making plaster molds, compounds that are applied to all surfaces except damp clay, in order to prevent plaster from sticking. Liquid hand soap works great. Paint it on and let it dry before pouring plaster. Never use oil-base release compounds on plaster molds for press-molding or slip-casting. 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 claybodies or glazes. See bentonite. Source: Clay: A Studio Handbook
Mouse-hole: In some wood kilns, a small air-port and passage that admits air directly beneath the coal bed. Source: Clay: A Studio Handbook
Muffle Kiln: A fuel-burning kiln in which flames and combustion gases are ducted through sealed refractory flues within the ware chamber, heating the wares while maintaining a clean oxidizing atmosphere. 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 claybodies 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
Multifired: Wares where more than one glaze firing is required to achieve particular surface effects, as in lusters and china paints. Source: Clay: A Studio Handbook

N

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
Necking-in; Collaring: Process of reducing the upper diameter of a thrown form by working the walls of the rotating form inwards with fingers or rib, as in a bottle shape. Source: Clay: A Studio Handbook
Neuman Red Clay: Refractory red stoneware clay, used to obtain rich red and red-brown colors in high-fire claybodies. 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
Nickel Carbonate, Green: Ni2CO3—weaker nickel colorant, reduces to green nickel oxide in the firing. Toxic in ingestion and inhalation. Source: Clay: A Studio Handbook
Nickel Oxide, Black: Ni2O3—reduces to green nickel oxide early in firing, and produces similar effects. Toxic in ingestion and inhalation. Source: Clay: A Studio Handbook
Nickel Oxide, Green: NiO—colorant or modifier—can give blues, tan, browns, greens, grays, dependent on fluxes present. Often used to mute the effects of cobalt, copper, and other colorants. Toxic in ingestion and inhalation. 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

O

Ocmulgee: Rich golden-brown stoneware clay, no longer being mined. See Lizella clay. Source: Clay: A Studio Handbook
OM-4 (old Mine #4): A well known Kentucky ball clay. 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. See ziconium silicate. 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
Outgassing: The escape of gases from clay and glazes during firing. Carbonates, sulfates, nitrates, organic contaminants, and chemically combined water volatize between 600 and 1200° F. Other compounds, especially fluxes, volatize later during glaze-melt. See carbon coring, oxidation cleanup, water-smoking stage. 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
Oxford Feldspar: Potash feldspar no longer mined. Substitute Custer or G-200. 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
Oxidation Cleanup; Oxidation Soak: After glaze reduction, a short period of oxidation to allow outgassing to stop and glazes to heal. Source: Clay: A Studio Handbook
Oxide Ceramics: A molecule combining any element with oxygen. 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. See patina. Source: Clay: A Studio Handbook
Oxidize: To subject a material to a high-oxygen atmosphere, encouraging oxidation reactions. Source: Clay: A Studio Handbook

P

Paddle And Anvil

Forming technique where a soft leather-hard form may be shaped and thinned with a wood or bisqued clay paddle on the outside and a rounded “anvil” of wood, stone, or bisqued clay applying corresponding pressure on the inside. Source: Clay: A Studio Handbook

Paper Clay

Technique popularized by Rosette Gault, utilizing a claybody 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

Partial Reduction; Climbing Reduction

In a fuel kiln, atmosphere that is slightly reducing, but still allows increase in temperature. See reduction firing. Source: Clay: A Studio Handbook

Pate-sur-pate

Paste-on-paste—decoration in which sculptural relief decoration is built up with contrasting slip on surface of leather-hard ware. Source: Clay: A Studio Handbook

Patina

An overall thin wash of glaze or oxide stain, allowing the color and texture of the claybody to show through. Source: Clay: A Studio Handbook

Pearl Ash; Potassium Carbonate

K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides. Source: Clay: A Studio Handbook

Peephole; Spyhole

Small holes in kiln door or wall, allowing viewing of cones, flames, wares, and/or atmosphere during firing. Source: Clay: A Studio Handbook

Periodic Kiln; Intermittent Kiln

A kiln that is loaded cold, brought to temperature, cooled and unloaded. See continuous kiln. 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 claybodies. 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

Pioneer Kaolin

Al2O3×2SiO2×2H2O—plastic secondary kaolin—wider particle distribution gives greater plasticity, green strength. 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 nonplastic additives, aging of damp claybody, 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 claybodies, 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 claybody containing kaolin, silica, fluxes, and often ball clay to increase plasticity, with total clay component not more than 50%. Usually pure white or “eggshell” in color; some porcelains may fire translucent where thin. Source: Clay: A Studio Handbook

Porcelaneous

White-firing stoneware claybodies 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

Post-firing Smoking

See raku. Source: Clay: A Studio Handbook

Potash Feldspar

See G-200; Custer feldspar. Source: Clay: A Studio Handbook

Potassium Carbonate; Pearl Ash

K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides. 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

Potter’s Finger; Jug Finger

A long wooden tool with a knob on the end, often covered with chamois, which is inserted in a bottle or jug on the wheel after the top has been necked in and is used to broaden and refine the shape. Source: Clay: A Studio Handbook

Pottery Banding Wheel

Hand-operated turntable for applying wax resist and banded decoration. 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 wheelheads 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.

Pottery Wheel; Potter's Wheel

Machine with a horizontally spinning wheel head upon which potters form and trim clay vessels by hand. Source: Clay: A Studio Handbook

Power Burner

Any burner in which a mechanical blower or other pressurized air source entrains the primary air for combustion. 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

Pressing; Press Molding

Forming of multiples by pressing plastic clay into a plaster press-mold. 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

Primary Clays; Residual Clays

Clays that remains at the site of the parent rock, without being transported by wind or water—tend to be free of contaminants but have coarse particle size and therefore low plasticity. Includes the purest kaolins. Source: Clay: A Studio Handbook

Pugmill

A machine similar to an oversized meat grinder, used to homogenize plastic claybodies. Deairing pugmills have a vacuum pump attachment, which effectively removes all air from clay, eliminating need for hand wedging. See carpal tunnel syndrome. Source: Clay: A Studio Handbook

Pumice; Volcanic Ash

Fine volcanic particulate expelled from surface vents in a volcanic eruption. HT alkaline flux, similar in composition to potash feldspar, but higher in silica, with at least 1% iron. May be substituted for 7 parts potash spar, 3 parts flint. Toxic in inhalation. Source: Clay: A Studio Handbook

Pyrax HS

A manufactured pyrophyllite product, promotes plasticity more than other pyrophyllite products. Product of the H.T. Vanderbilt Company. Source: Clay: A Studio Handbook

Pyrophyllite

Al2O3×4SiO2×H2O—used in HT claybodies (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

Pyrotrol

A manufactured pyrophyllite product, made by the Resco Company. See Pyrophylite. Source: Clay: A Studio Handbook

Q

Quartz; Silica; Silicon Dioxide; Flint: SiO2 —The primary glass-former in clay and glazes—vitrification, fluidity, transparency/opacity controlled by adding fluxes and/or refractories. Highly toxic in inhalation. Source: Clay: A Studio Handbook

R

Raku (japanese): Traditional raku or Japanese raku is a low-fired glazed pottery by a direct process that involved putting the pots into and removing them from a red-hot kiln. The potter Chojiro is credited with being the first to produce raku ware in 1580. The term raku is translated as “enjoyment” or “ease.” From The Potters Dictionary of Materials and Techniques, by Frank and Janet Hamer. See raku (Western).
Raku (western): Inspired by traditional Japanese raku firing, Western (typically referred to as American) raku is a relatively low-temperature firing process using clay that is either under fired or otherwise formulated to withstand the thermal shock of being removed from a kiln at top temperature. Work is removed from the kiln at bright red heat and subjected to post-firing reduction (or smoking) by placing in containers of combustible materials, which blackens raw clay and craze cracks in glaze. American raku is not usually considered food-safe, partly because of the under-fired nature of the clay body. Some of the most popular glazes used in raku tend to be unstable glasses that can leach glaze material when in contact with food acids. This is particularly true of glazes with high copper content, which are some of the most popular in American raku. See raku (Japanese). 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
Raw Glazing; Green Glazing: Glazing leather-hard or bone-dry wares for single-firing. Source: Clay: A Studio Handbook
Recrystallization: During initial cooling, the formation of microcrystalline or macrocrystalline structure in the glaze surface. 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
Red Lead; Lead Oxide: Pb3O4—Former source of lead for glazes. Highly toxic in absorption, inhalation, and ingestion. Remove from studio and dispose of in a responsible fashion. Source: Clay: A Studio Handbook
Red-figure Style: In ancient Greek ceramics, Classical stage of vase painting, featuring figures in red against black background, with highlights painted in black and white. Source: Clay: A Studio Handbook
Reduce: To induce reduction. Source: Clay: A Studio Handbook
Reducing Agent: Material such as silicon carbide used in clay or glaze to create localized reducing conditions in an oxidation firing. 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. See reoxidation. 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
Registration Marks; Keys: In plaster slip-casting molds, small rounded depressions carved in each parting face of a mold segment, so that when adjacent segments are cast a corresponding bump will form. In use, the keys allow the mold segments to be aligned and assembled securely after the original prototype is removed. Source: Clay: A Studio Handbook
Reoxidation: In a reduction firing, oxidation that occurs after fuel is shut off at end of firing. 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
Residual Salt/soda: In a well-used salt/soda kiln, the light salt/soda effect that may be obtained by firing wares in the kiln without introducing any additional salt or soda, due to the revolatization of salt/soda from the walls of the kiln. 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
Rheostat: Variable electric switch often found on forced-air burners to control blower speed. Small inexpensive rheostats used on most power burners are imprecise, and adjustments should always be made on basis of flame at burner tip, preferably with an air- shutter on the blower-intake for fine adjustments. 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
Roman Arch: Kiln arch that forms a perfect half circle, with the arch beginning and ending on a horizontal surface, using no skew bricks. Source: Clay: A Studio Handbook
Roulette; Coggle: Small stamp wheel with raised pattern around the rim, which when rolled along a plastic clay surface leaves a band of relief pattern. Usually formed with damp or dry clay and bisque-fired. 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

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.
Saggar: A refractory clay enclosure in which wares are fired.
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 Firing; Salt Glazing: Vapor-glazing process where salt (sodium chloride) is introduced into kiln firebox at high temperature.
Salt; Sodium Chloride: NaCl—table salt, rock salt—used in salt-firing. Skin irritant. 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
Sawdust Smoking: Cosmetic smoking process where unglazed wares are buried in sawdust in a brick enclosure or a perforated steel drum.
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.
Secondary Clays; Deposited Clays; Sedimentary Clays: Clays that have been transported away from their point of geologic origins by wind or water.
Secondary Kaolins: Kaolins that have been transported some distance from the parent rock and are still very pure, but much more plastic than primary kaolins. Source: Clay: A Studio Handbook
Seger Formula; Unity Formula: A formula generated by glaze calculation, which lists all glaze oxides present in separate columns for fluxes, refractories, and glass-formers. Source: Clay: A Studio Handbook
Sequential Firing: Firing in a sloped tube kiln or noborigama, where as each chamber or zone reaches maturity, the fire is moved up to the next, and each one preheats subsequent chambers or zones. 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.
Setting: Process of loading a kiln, or technically, of placing the set. See 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 claybody 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.
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.
Short: Clay with insufficient plasticity—tends to fragment during forming. 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.
Single-firing; Green Firing: Process of firing glazed greenware without a bisque firing. 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.
Skew Bricks: Special angled bricks used to support the first course of arch bricks on either side of a sprung arch. Source: Clay: A Studio Handbook
Slabroller: A mechanized or manually operated device for rolling out large uniform slabs of clay. Source: Clay: A Studio Handbook
Slaking; Slake Down: The process of returning dry unfired clay to a slip by soaking in water. Source: Clay: A Studio Handbook
Slip: Clay suspended in water, usually the consistency of thick cream.
Slip Casting: The creation of ceramic forms by casting slip in plaster molds. 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 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
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-resist: Decorating technique where resist materials are applied to prevent slip from adhering to some areas. 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 hardbrick shape, half the width of a standard brick. Often used for posts in salt and wood firings. Source: Clay: A Studio Handbook
Soapstone; Talc; Magnesium Silicate; Steatite: 3MgO×4SiO2×H2O High-temperature alkaline earth flux in glaze that contibute magnesia and silica.
Soda Ash; Sodium Carbonate: Na2CO3—soluble source of soda, used as deflocculant, or to supply soda in vapor-glazing process.
Soda Feldspar: Na2O×Al2O3×6SiO2—feldspars contributing sodium (and potassium), primarily as a high-temperature flux.
Soda Firing; Soda Glaze: Vapor-glazing process touted as modern-day nontoxic replacement for salt firing.
Sodium Aluminum Fluoride; Cryolite: Na3×AlF6—small amounts promote crackle effects, larger amounts become very volatile with silica and may cause blistering. Used for special effect crater glazes. Toxic in inhalation. Source: Clay: A Studio Handbook
Sodium Silicate; Waterglass: Na2SiO3—comes as a liquid—used as a deflocculant in slips, as an air-setting binder for low-temperature refractories.
Sodium Tetraborate; Borax: Na2O×2B2O3×10H2O—a major LT alkaline flux, available in granular or powdered form. Gives smooth finish, bright colors.
Soft Paste: Traditional European term for a dense, white claybody that fires at lower temperature and was the precursor to true porcelain. Source: Clay: A Studio Handbook
Softbrick: See insulating firebrick. Source: Clay: A Studio Handbook
Soluble Salts Ceramics: Range of soluble metallic salts like bismuth subnitrate, silver nitrate, copper sulfate, cobalt sulfate, ferric chloride, and stannous chloride, used to create fumed luster effects on glazed wares and to create surface effects on unglazed saggar-fired wares. Most are water soluble and highly toxic. 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
Spalling: Defect in repeatedly fired clay materials where outer layers shrink and peel away—most often occurs in kiln hot-face when refractories are repeatedly fired beyond their rated temperature. 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. See hydrometer. 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
Spooze: A repair medium for bone-dry greenware, made of equal parts vinegar and corn syrup mixed with dry powdered claybody to the desired consistency. 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
Spyhole: See peephole. Source: Clay: A Studio Handbook
Stannous Chloride; Tin Chloride: Soluble metallic salt. Fuming agent to create mother-of-pearl luster on glazed surfaces.
Stoneware: High-fired vitreous ware, literally as hard and durable as stone. Matures from 2200-2400° F. (cone 5–11).
Strontium Carbonate: SrCO3 Alkaline earth, high-temperature flux, similar to barium, slightly more powerful.
Super-cooled Liquid: A material that, in its solid form, maintains the amorphous physical structure of a liquid rather than the ordered structure of a crystalline material.
Superpax: Zircon opacifier. See zirconium silicate. 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

T

Temper; Tempering Materials; Fillers: Gritty materials like sand, grog, volcanic ash, crushed seashells, etc., added to clay to open upthe body, give physical structure in wet-working and increase thermal shock resistance. 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 Sigillata: Ultrarefined 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.
Terracotta Clay: Low temperature, porous earthenware claybody, fires red-brown due to high iron content, which also fluxes clay, making it the most durable low-fired clay after firing. Source: Clay: A Studio Handbook
Tessha: A Japanese glaze similar to kaki, in which re-oxidation and crystallization of iron in the glaze results in significant areas of purplish-red, often with some black or brown surface showing. 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
Thermoplastic: See Pyroplastic.
Tin Chloride; Stannous Chloride: Soluble metallic salt—fuming agent to create mother-of-pearl luster on glazed surfaces. Introduced in cooling ramp at dull red heat, it will affect everything in kiln. Highly toxic in inhalation and ingestion. Fumes are especially toxic. 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. See line blend. 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
Tube Burner: A simple atmospheric burner with no venturi effect present. Source: Clay: A Studio Handbook
Tube Kiln: Sequential-firing Korean kiln design evolving from Chinese bank kiln, with long sloping tubular firing chamber. See anagama. Source: Clay: A Studio Handbook

U

Ultrox: Zircon opacifier. See zirconium silicate. 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
Unity Formula; Seger Formula: A formula generated by glaze calculation, which lists all glaze oxides present in separate columns for fluxes, refractories, and glass-formers. Source: Clay: A Studio Handbook

V

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 claybodies, 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
Viscosity; Viscous: Reference to a material’s resistance to flowing. A viscous glaze flows less. As glass is heated it slowly becomes less viscous over a broad temperature range. Source: Clay: A Studio Handbook
Vitreous Engobe: An engobe containing sufficient flux to form to a vitreous clay coating. Source: Clay: A Studio Handbook
Vitreous; Vitrified; Vitrification: Fired clay that has fused together completely, so that the pores between refractory particles are filled with glass and the body is impervious to water. Vitrification is sintering in the presence of a fully developed glassy-phase. Source: Clay: A Studio Handbook
Volatization; Volatize: Change from solid or liquid to gas during firing, resulting in outgassing from the clay or glaze. See outgassing, blistering. Source: Clay: A Studio Handbook
Volcanic Ash; Pumice: Fine volcanic particulate expelled from surface vents in a volcanic eruption. HT alkaline flux, similar in composition to potash feldspar, but higher in silica, with at least 1% iron. May be substituted for 7 parts potash spar, 3 parts flint. Toxic in inhalation. Source: Clay: A Studio Handbook

W

W.C.I.; Water Column Inches: Standard low-pressure measure of gas pressure, based on pressure required to lift a column of water. Household natural gas pressure is usually 7″ or 7 W.C.I. Industrial natural gas pressure may be 12″ to 15″ or more. Propane household systems usually carry 11″ of pressure, but propane kiln systems may be equipped with adjustable regulators delivering up to 20 P.S.I. (pounds per square inch). See manometer. Source: Clay: A Studio Handbook
Wadding; Wads: Small balls or rolls of refractory clay mixture (40 alumina, 10 ball clay, 50 kaolin) placed under wares and posts, and between pots, lids, etc., in vapor-glazing and wood firing processes, to keep surfaces from sticking together. 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
Water-smoking Stage: The stage during bisque-firing or single-firing, below 1200°F, when carbonates, nitrates, sulfates, organic contaminants, and chemically combined water volatize and outgas. See carbon coring, outgassing. 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
Wedge Brick: Bricks with angled side faces, tapering along the length of the brick, which when laid together form a curved arch. Standard wedge bricks give 9″ thick arch. 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 platelates in a spiral formation and thus increase control in centering and throwing. Source: Clay: A Studio Handbook
White Lead; Lead Carbonate: 2(PbCO3)×Pb(OH)2—former source of lead for glazes. Highly toxic in absorption, inhalation, and ingestion. Remove from studio and dispose of in a responsible fashion. Source: Clay: A Studio Handbook
Whiteware: Low-temperature white claybody. 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

X

XX-Sagger: A plastic refractory stoneware clay, often used in sculpture and raku bodies. Source: Clay: A Studio Handbook

Y

Yellow Iron Oxide: See yellow ochre. Source: Clay: A Studio Handbook
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

Z

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 mattness 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. See zirconium silicate. Toxic in inhalation. Source: Clay: A Studio Handbook
Zone Of Crystallization: Period during cooling ramp when a particular material tends to crystallize out of glaze-melt. Source: Clay: A Studio Handbook

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