Glossary of Ceramic Materials

Note: AT = all temperature, LT = low temperature, HT = high temperature.

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.

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. ALBERTA SLIP CLAY - substitute for Albany slip. Highly fluxed with iron - true slip clay – will form glaze at HT.

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.

ANTIMONY OXIDE - Sb2O3 - colorant - soluble, toxic, expensive, produces yellow with titanium. Highly toxic in ingestion and inhalation.

AVERY KAOLIN - Well-known flashing slips for woodfire, but no longer being mined.

BALL CLAY - Al2O3.2SiO2.2H2O - fine particle-size secondary clay, extremely plastic – primary plasticizing clay in most claybodies - in large quantities promotes high shrinkage.

BARIUM CARBONATE - BaCO3 - alkaline earth - active HT flux, but also promotes matt glaze surface. Unsafe for low-fire functional glazes. Controversial in HT functional glazes, but toxicity problems from balanced HT glaze not proven. Often used as additive in claybodies in very small percentages to render sulfates insoluble, reducing scumming. Toxic in inhalation and ingestion.

BENTONITE - Al2O3.5SIO2.7H2O - montmorillonitic clay formed from decomposition of airborne volcanic ash - finest particle-size of all clays - plasticizer (3-times as powerful as ball clay), suspension agent, should be used in quantities no more than 3% of dry materials weight.

BISMUTH SUBNITRATE - soluble metallic salt - gives metallic luster under LT reducing conditions, especially in fuming and saggar applications. Toxic in inhalation and ingestion.

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).

BORAX; SODIUM TETRABORATE - Na2O.2B2O3.10H2O - a major LT alkaline flux, available in granular or powdered form. Gives smooth finish, bright colors - water soluble, so is often used in fritted form. In excessive amounts creates brittle glass, and can cause blistering and pinholing. Available in granular and powdered form. Sometimes used with salt or soda in vapor glazing to lower firing temperature and/or achieve a smoother, shinier surface.

BLACKBIRD/BARNARD - slip-clay with very high iron content, often used as brown colorant for claybodies.

BURNT UMBER - iron-manganese ore - good color source for basalt bodies.

CARBONDALE CLAY - refractory red stoneware clay, used to obtain rich red and brown colors in high-fire claybodies.

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.

CMC GUM - carboxymethylcellulose - 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.

COBALT CARBONATE - CoCO3 - standard blue colorant for slips and glazes - very powerful - 5% will give dark blue in glaze or slip. Will cause crawling is used raw for underglaze brushwork. Toxic in inhalation and ingestion.

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.


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.

COPPER OXIDE, BLACK; CUPRIC OXIDE - CuO - alternate source of copper, coarser particle size, twice as powerful as copper carbonate. Toxic in inhalation and ingestion.

COPPER OXIDE, RED; CUPROUS - Cu2O - alternate source of copper, may help promote copper reds in LT and HT glazes. Rarely used, because has no affinity for water, and floats back to surface, but a few drops of detergent will break surface tension. Toxic in inhalation and ingestion.

COPPER SULFATE - CuSO4 - color source for sagger firing and pit firing. Soluble, and highly toxic in absorption, ingestion, and inhalation.

CORDIERITE - 2MgO.2Al2O3.5SiO2 - magnesium/aluminum silicate clay mineral, used to make grog for refractory products - promotes formation of MULLITE.

CORNWALL STONE; CORNISH 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.

CRYOLITE; SODIUM ALUMINUM FLUORIDE - Na3.AlF6 - small amounts promote crackle effects, larger amounts become very volatile with silica, and may cause blistering. Used for special effect crater glazes.

CUSTER FELDSPAR - K2O.Al2O3.6SiO2 - a common potash feldspar - HT alkaline flux. See FELDSPAR. Close match to G-200. Toxic in inhalation.

DARVAN - common deflocculant for casting slips. Product of R.T. Vanderbilt Company.

DOLOMITE; CALCIUM/MAGNESIUM CARBONATE - MgCO3.CaCO3 - HT alkaline earth flux, promotes hard, durable surfaces and re-crystallization/matting in glazes. Often added to claybodies to give longer firing range, and can promote more durable low-fire bodies.

EPK; EDGAR PLASTIC KAOLIN - Al2O3.2SiO2.2H2O - pure white kaolin, less plastic than Tile-6 kaolin, frequently used in glazes.

EPK, CALCINED - Al2O3.2SiO2 - used in place of regular kaolin to adjust raw fit (reduce glaze drying-shrinkage) in glazes and engobes.

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.

FELDSPAR - HT alkaline fluxes - insoluble aluminum silicates of potassium, sodium, calcium, and/or lithium - inexpensive flux for clay and glaze. Substitution of soda spar for potash spar can lower vitrification by 100 degrees. Toxic in inhalation. See CUSTER, G-200, KONA F-4, SPODUMENE, NEPHYLENE SYENITE.

FIRE CLAY - very refractory clay, for sculpture and raku bodies. Tremendous variation between different brands.

FLINT - see SILICA. Highly toxic in inhalation.

FLOCS - Commercial flocculant used in glazes - 1/4 tsp. per gallon of glaze.

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.

FRIT - fluxes which have been melted to a glass, cooled and ground, in order to stabilize soluble and/or toxic components during handling of unfired 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.

G-200 - K2O.Al2O3.6SiO2 - common potash feldspar, HT alkaline flux - close match to Custer. Toxic in inhalation.

GERSTLEY BORATE; COLEMANITE; CALCIUM BORATE - 2CaO.3B2O3 - major LT alkaline flux - often gives slight opalescence in glaze - for greater transparency and long-term stability substitute Ferro 3134 or other Gerstley borate substitute.

GLOMAX - calcined kaolin. See EPK, CALCINED.

GLYCERIN; GLYCEROL - an organic gum derived from fats and oils, sometimes used as brushing medium for engobes or for re-glazing 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.

GOLDART- buff stoneware clay, produced by Cedar Heights Clay Company.

GROG (crushed hard-fired clay) - source of grit for claybodies - graded in sizes from 15-mesh (very coarse) to 150M (extra fine). Does not shrink in firing, so in medium and coarse grades will show texture through thin to medium glaze.

GROLLEG KAOLIN - Al2O3.2SiO2.2H2O - English kaolin, more costly than other choices, but gives whiter porcelain. Less plastic than TILE-6. Best kaolin for translucent bone china.

HAWTHORN BOND - refractory stoneware clay or fireclay, used in stoneware claybodies.

HELMER KAOLIN - Al2O3.2SiO2.2H2O - Kaolin which works especially well as flashing slip for salt, soda, and wood firing, especially since Avery Kaolin is no longer being mined.

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.

IRON CHLORIDE - FeCl2.6H2O - soluble metallic salt - fuming agent used to produce lusters on glazed surface. Highly toxic in inhalation and ingestion.

IRON CHROMATE - FeO.Cr2O3 - glaze colorant producing pink or red with tin, brown with zinc, gray with alkaline fluxes. Highly toxic in inhalation and ingestion.

IRON OXIDE, RED, FERRIC - 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.

IRON OXIDE, BLACK, FERROUS - FeO - reduced form of iron oxide - gives same results as red iron in the firing, dependent on oxidation/reduction.


KAOLIN; CHINA CLAY - Al2O3.2SiO2.2H2O - very refractory white primary clay – essential ingredient of porcelain and whiteware - less plastic than most other clays. See EPK, GROLLEG, HELMER, TILE-6.

KINGMAN FELDSPAR - Potash spar no longer mined. Substitute CUSTER or G-200.

KONA F-4 FELDSPAR - Na2O.Al2O3.6SiO2 - a common soda feldspar - powerful HT alkaline flux. Toxic in inhalation.

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.

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.

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.

LEAD SILICATE - 3PbO.2SiO2 - fritted lead compounds - LT flux, but resulting glazes may be toxic, and should cannot be used on functional wares. Most powerful of all fluxes, promotes smooth flowing, self-healing glazes. Highly toxic in inhalation and ingestion.

LEPIDOLITE - lithium feldspar - HT alkaline flux - used for thermal shock bodies and matching glazes. Contains fluorine, with associated problems. Toxic in inhalation.

LITHIUM CARBONATE - Li2CO3 - powerful AT alkaline flux, especially with soda or potash feldspars. Promotes hardness and re-crystallization in LT glazes. Forms low-temperature eutectic with silica.

LIZELLA CLAY - high-iron (4%) stoneware clay - substitute for OCMULGEE , which is no longer being mined.

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 ½ of 1% of dry materials weight as suspension agent, brushing medium in glazes and slips. Mix with water before adding other ingredients.

MAGNESIUM CARBONATE - MgCO3 - alkaline earth - HT flux, promotes matness and opacity in LT glazes, smooth, hard, buttery surface in HT glazes - promotes purples or pinks with cobalt. High L.O.I., used to promote controlled crawl glaze effects.


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.

MICA - K2O.3Al2O3.6SiO2 - an aluminosilicate with a fine sheet-lattice structure, closely related to clay and feldspar, and often found as minute iridescent flakes in some clays.

MOLOCHITE - porcelain grog - source of grit for pure white claybodies.

MONTMORILLONITE - clay resulting from decomposition of volcanic ash, includes BENTONITE.

MULLITE - 3Al2O3.2SiO2 - calcined KYANITE, but is also found naturally. Interlocking needlelike aluminum silicate crystals which form in clays and glazes above 1800 degrees, increasing strength of body, glaze, and clay-glaze interface.

NC-4 FELDSPAR - Na2O.Al2O3.6SiO2 - Soda feldspar, interchangeable with KONA F-4. Toxic in inhalation.

NEPHELINE SYENITE - K2O.3Na2O.4Al2O3.9SiO2 - a common feldspathic flux, high in both soda and potash, used in claybodies and glazes. Less silica than soda feldspars, and therefore more powerful. Increases firing range of low-fire and midrange glazes. Toxic in inhalation.

NEUMAN RED CLAY - refractory red stoneware clay, used to obtain rich red and red-brown colors in high-fire claybodies.

NICKEL CARBONATE, GREEN - Ni2CO3 - weaker nickel colorant, reduces to GREEN NICKEL OXIDE in the firing.

NICKEL OXIDE, BLACK - Ni2O3 - reduces to GREEN NICKEL OXIDE early in firing, and produces similar effects.

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.

OCMULGEE - rich golden-brown stoneware clay, no longer being mined. See LIZELLA.

OM-4 - (Old Mine #4) - a well known Kentucky BALL CLAY.

OPAX - zircon opacifier. See ZIRCONIUM SILICATE. Toxic in inhalation.

OXFORD FELDSPAR - Potash feldspar no longer mined. Substitute CUSTER or G-200.

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

PERLITE - porous expanded granular silica, useful in making insulating refractories.

PETALITE - Li2O.Al2O3.8SiO2 - lithium feldspar - HT alkaline flux - good for reducing thermal expansion, increasing thermal-shock resistance.

PETUNTSE - feldspathic rock, similar to CORNWALL STONE, found in China . Traditional Chinese flux for HT glazes and claybodies.

PIONEER KAOLIN - Al2O3.2SiO2.2H2O - plastic secondary kaolin - wider particle distribution gives greater plasticity, green strength.

PLASTER - CaSO4.2H2O - hydrated calcium sulfate, manufactured from gypsum, which will set to a hard solid after being mixed properly with water. Used for making molds for pressing or casting ceramic forms.

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.

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.



POTASSIUM DICHROMATE -K2Cr2O7 - Occasionally used as acid-green colorant in raku glazes, but is highly toxic in absorption, ingestion, and inhalation.


PYRAX HS - a manufactured PYROPHYLLITE product, promotes plasticity more than other pyrophyllite products. Product of the H.T. Vanderbilt Company.

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.

PYROTROL - a manufactured PYROPHYLLITE product, made by the Resco Company.

REDART - brick-red earthenware clay, produced by Cedar Heights Clay Company.

RUTILE - Titanium ore, used as source of TITANIUM DIOXIDE, contains iron, other trace minerals - gives tan color, promotes crystallization giving mottled multi-color 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.


SAND - granular silica (usually) - source of grit for claybodies - high shrinkage in HT claybodies, but gives smoother fired surface than grog. Toxic in inhalation.

SILICA; SILICON DIOXIDE; FLINT ; QUARTZ - SiO2 - main glass-former - vitrification, fluidity, transparency/opacity controlled by adding fluxes and/or refractories. Highly toxic in inhalation.

SILICON CARBIDE - SiC - used in very small quantities for localized reduction of copper reds - larger amounts for frothing glazes.

SODA ASH; SODIUM CARBONATE - Na2CO3 - soluble source of soda, used as deflocculant, or to supply soda in vapor-glazing process. Source of flux in Egyptian paste recipes. Brush solution over hi-fire glaze to increase carbon trapping. Highly toxic in inhalation and ingestion. See “magic water” under SODIUM SILICATE.

SODA FELDSPAR - Na2O.Al2O3.6SiO2 - feldspars contributing sodium (and potassium), primarily as a HT flux - includes KONA F-4, NC-4 and NEPHYLINE SYENITE. Toxic in inhalation.


SODIUM CHLORIDE; SALT - NaCl - table salt, rock salt - used in salt-firing.

SODIUM SILICATE; WATERGLASS - Na2SiO3 - comes as a liquid - used as a deflocculant in slips, as an air-setting binder for LT refractories. Lana Wilson (and many clay artists/artisans) promotes its use in “magic water” for use in place of slurry for joining wet or soft-leather-hard ceramic forms. Add 1.5% sodium silicate and 1.5% soda ash by weight to a measure of water.

SPODUMENE - Li2O.Al2O3.4SiO2 - lithium feldspar - powerful HT alkaline flux – promotes copper blues - good for thermal-shock bodies and matching glazes. Toxic in inhalation.

STAINS - MASON, HARSHAW, PEMBCO, FERRO, ETC. - stable fritted ceramic colorants available in wide range of colors, suitable for coloring clays, slips, engobes, and glazes. 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.

STANNOUS CHLORIDE; TIN CHLOIDE - 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.

STRONTIUM CARBONATE - SrCO3 - alkaline earth, HT flux, similar to barium, slightly more powerful - gives semi-matt surfaces. Non-toxic in balanced glaze. Substitute .75 parts strontium to one part barium.

SUPERPAX - zircon opacifier. See ZIRCONIUM SILICATE. Toxic in inhalation.

TALC; MAGNESIUM SILICATE; STEATITE; SOAPSTONE - 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.

TILE-6 KAOLIN - Al2O3.2SiO2.2H2O - air-floated secondary kaolin - broader particle distribution than primary kaolins, greater plasticity, green strength.


TIN OXIDE - SnO2 - most powerful opacifier, but expensive - inert dispersoid in glaze melt - 5-7% will produce opaque white in a clear glaze.

TITANIUM DIOXIDE - TiO2 - matting/opacifying agent. Promotes crystal growth, visual texture in glazes.

ULTROX - zircon opacifier. See ZIRCONIUM SILICATE. Toxic in inhalation.

VANADIUM PENTOXIDE - V2O5 - weak yellow colorant - toxic, expensive - usually fritted with tin to produce stronger yellow. Highly toxic in inhalation and ingestion.

VEEGUM-CER - Combination of VEEGUM-T and carboxymethylcellulose gum, used as a suspension and adhesion agent in glazes. Product of H. T. Vanderbilt Company.

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 ½ 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.

VERMICULITE - A porous expanded mica product used to introduce mica flecks in claybodies, and as filler in insulating refractory layer on kiln exterior.

VOLCANIC ASH; PUMICE - 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.

WHITING; CALCIUM CARBONATE; 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.

WOLLASTONITE; CALCIUM SILICATE - 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.

XX-SAGGER - a plastic refractory stoneware clay, often used in sculpture and raku bodies.

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.

ZINC OXIDE - ZnO - HT flux which 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.

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.

ZIRCOPAX - zircon opacifier, no longer being manufactured. See ZIRCONIUM SILICATE. Toxic in inhalation.

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