Ceramic Glossary

See Articles - List of Tags, List of Categories, List of Articles, List of Glossaries, Nomenclature and Symbols, (See Ceramic)

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Abrasive - Hard substances (such as fused alumina) that are used for polishing, cutting or grinding.
Absorbency - The ability and capability of a material to soak up a liquid. In pottery and ceramics this would relate to a glaze prior to firing.
Alumina - A technical ceramic commonly used in engineering owing to its outstanding electrical insulation properties combined with rigidity and resistance to corrosion.
Advanced Ceramics - These types of ceramics are known as technical ceramics, high-tech ceramics, and high-performance ceramics. They are for industrial and commercial applications demanding high mechanical strength, abrasion and chemical resistance, electrical insulation or resistance to high temperatures.

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Ball Mill - Comprised of a hollow cylindrical shell that rotates about its axis. A ball mill is used to grind and blem materials for use in ceramics.
Bases - Fluxes in glaze chemistry that combine with acids and neutrals.
Batt - Depending upon its use a batt is either a type of tray to enable the movement of ceramic ware without it being handled directly. Or it can mean a kiln batt which acts as a kiln shelf support element upon which ware is positioned during firing.
Batt Wash - A type of kiln wash which is coated onto kiln shelves and furniture to stop the adhesion of ware during the firing process.
Bentonite - An extremely plastic clay rich in montmorillonite which can be added in small quantities to clays or clay bodies to increase plasticity.
Bisque / Biscuit - The first firing, converting clay to ceramic which cannot be reconstituted in water. Bisque is used to describe a first firing that is higher than the subsequent glaze firing. Biscuit describes the reverse where the glaze firing is the higher, also the ware produced by these firings.
Biscuit Firing - Is an initial firing process which dries and changes the composition of clay prior to glazing and/ or secondary firing. The term originates from the French bis-cuite, which means twice baked.
Blistering - Glaze defect causing bubbles and sharp craters, preventable with proper firing.
Bloating - Lumps on the surface of a fired piece caused by expansion of gasses within the piece generated by overfiring.
Blunger - Machine for mixing clay and water in the production of casting slips.
Body - A manufactured clay, where raw clays and minerals are blended to produce specific qualities.
Body Reduction - Early reduction atmosphere in firing, affecting the clay body.
Bone Ash - Calcinated animal bone used as an ingredient in bone china.
Bone China - A type of pale thin porcelain with a translucent quality made with a clay composite containing a minimum of 30% of phosphate derived from animal bone.
Bone Dry - the final stage of greenware dried to a near or fully dry state and ready to be fired. In this state, the article is very fragile, non-plastic and porous.
Burnishing - Polishing unfired clay using a smooth object.

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Calcine - To heat a material such that certain temperature dependant changes. Ceramic raw materials which are calcined include clay, bone and talc.
Calcination/Calcining - Heating to purify and prepare materials for use.
Candling - Optional initial stage of firing in which the kiln is very gradually heated to eliminate all water and completely dry the ware.
Carbon-trapping - Effect that traps carbon in glaze, creating shading, often intentional.
Casting - A method where slip is poured into a mould to create more complex ceramic forms.
Ceramic - Clay that has been altered by firing to produce a hard substance that cannot be reconstituted in water. The general name given to all fired clayware.
Ceramics - This is a term which covers a broad range of clay based products from bricks to tableware.
Ceramic Change - When clay reaches 1100 fahrenheit / 593 centigrade the material is changed by its particles being fused together.
Cheesehard - A stage of drying where clay is hard enough to be moved without deforming but can still be worked on by adding elements like handles and decorating techniques.
China - The term in relation to porcelain is derived from 17th century Britain to describe ceramics imported from China. It could not be manufactured in Europe making it very expensive. Eventually a substitute was found, and animal bone ash was added to make the delicate fine porcelain we know today.
Chittering - The result of incorrect fettling, chittering is a collection of small irregularities that form on the outer rim of pottery ware.
Clay - Clay is dug up from underground and is a fine grained material usually found in areas where prehistoric rivers once flowed. Over time this sedimentary rock is formed from ancient flora and fauna as a result of water pressure and the microscopic grains settle into clay beds. The three most common types of clay are earthenware, stone ware and kaolin. It can be moulded and manipulated when wet, before being dried and kiln fired to make bricks, pottery, and ceramics.
Clay-Glaze Interface - Junction between clay and glaze, influenced by firing process.
Cmc Liquid - Method of mixing cmc gum into glaze or slip using hot water.
Coiling - Building up ceramic forms and walls of an object by layering and coiling thin rolls of clay.
Combustion - Oxidation process of fuel at kindling temperature, releasing sustaining heat.
Cones - These small pyramid items are designed to melt at an equal temperature to a specific glaze. They act as a visual indicator within a kiln and are positioned to be viewed from a peephole.
Crazing - A network of small cracks in the glaze surface caused by a poor glaze fit. The glaze contracts more than the clay and the resulting tension causes it to crack. If a glaze contracts after firing more than the clay, the glaze being unable to stretch, starts to crack under the tension. The glaze will continue to crack until the tension is eased as crazing covers the surface and becomes finer. Delayed crazing can occur days or even months after firing. The most common cause of crazing is underfired biscuit. If the clay does not reach its full maturing temperature either in the bisque or glaze firing, it will not contract sufficiently to fit the glaze.
Crawling - A parting and contraction of the glaze on the surface of ceramic ware during drying or firing, resulting in unglazed areas bordered by coalesced glaze. May be caused by uneven glazing, excessive glaze thickness or a greasy substrate.
Cristobalite - A high-expansion form of silicon dioxide, resulting in low thermal-shock-resistance, formed above 2200°f.
Crystalline - Solids with a geometric molecular structure and specific melting point, unlike amorphous materials.
Cullet - Crushed glass used in glazes.

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Deairing - The removal of air from clay, usually in a pug mill.
Decal - A design set on special paper or as a transfer to enable the decorative reproduction on to the surface of porcelain. This is an abbreviation of decalcomania and the decoration, once attached to the glazes ware is fired again at a lower temperature to complete the process.
Decorative - This is ceramic ware which is purely for decoration without any practical application.
Deflocculation - Separating and dispersing the clay particles in a slip making it more fluid. Sodium Carbonate and Sodium Silicate are deflocculants, when disolved in the slip they allow large proportions of clay to remain fluid giving a high density to the slip.
Devitrification - When a glaze recrystallise during the cooling stage of firing. Results in a fault unless the intention is the formation of a crystalline glaze.
Dipping - The method of applying glaze through immersion.
Dispersion - Movement of dissolved materials towards uniform distribution in glaze melt.
Dispersoids - Non-dissolving particles in glaze melt, like titanium or air bubbles.
Dryfooting - Leaving the base or footring of a pot without glaze so that it can stand on a kiln shelf during the firing process.
Dunting - Cracking of pots due to stress built up by Silica Inversion in the firing. Rapid and uneven heating and cooling of the pot in firing is the most common cause, pots made from uneven thicknesses of clay are also at risk and poor glaze fit can also be a cause.

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Earthenware - Porous ware, lower fired ware usually under 1200°C and the clay and glazes that it is made from. Earthenware is usually characterised by brighter colours, softer glazes and separate glaze layer over the body.
Extrusion - The clay is forced through an aperture, to reform it or extract air.
Electrical Porcelain - A type of technical ceramic used as an electrical insulating material made from clay, quartz or alumina and feldspar, and glazed to shed water.
Element - The component which carries electrical current for heating a kiln.
Enamel - The decorative and protective coating applied to ceramics which is made by heated and fused glass particles.
Engobe Usually used to mean decorating slip. A covering over the base clay not firing to a glassy finish.
Eutectic Mixture - The mixture of substances in proportions that produce the lowest possible melting point given the fluxing action in combinations of materials. The eutectic mixture of Alumina and Silica for instance is 10% Alumina and 90% Silica which gives a melting point of 1545°C. This is lower than the melting point of Alumina at 2050°C or Silica 1710°C.

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Faience - The term for earthenware which is decoratively tin glazed over an opaque white glaze.
Feldspar - A group of alumino-silicate minerals. After crushing and grinding to give very fine particles are commonly used as fluxes in bodies and glazes.
Feldspathic - Pertaining to feldspars or similar minerals.
Flatware - This refers to plates and dishes instead of jugs, mugs and pots which is known as holloware.
Fettling - Removing any excess clay using a fettling knife from the casting ware especially where it has gathered in the seams and joins of moulds.
Fire Clay - Coarse, highly refractory clays with low shrinkage and buff color.
Firing - The process of heating pottery and ceramics in a kiln to harden and change its composition to become a completed product.
Firing Down - The process of controlling and slowing down the cooling process by maintaining heat in the kiln once the ware has reached maturity. This can be used to create effects in the glaze or reduce the risk of problems like dunting or pin holing.
Firing Ramp - Kiln heating schedule, including speed, duration, soaking, and cooling periods.
Fixed Props - These are supports which enable the construction of multi layered structure for kiln firing.
Floculation - The opposite of deflocculation, the thickening of slips or slops to give a bulkier and more viscous liquid. In flocculation the particles collect together rather than repel each other as in deflocculation. Calcium Chloride and vinegar are commonly used flocculants.
Fluidity - The liquidity of slips and molten glazes. Used to describe free flowing slips and glazes which melt to a runny consistency in the firing. The opposite of viscosity.
Flux - Substances that encourage fusion in the firing to produce ceramic. An essential ingredient in both clays and glazes, fluxes interact with Silica glass formers producing the solid, fused ceramic of the pot and fused glass of the glaze. Some of the commonest fluxes are: Boric Oxide, Calcia, Lead Oxide, Potash, Soda.
Flameware - Stove-top heat-resistant ceramics, generally avoided due to risks.
Frit - A fusion of soluble or harmful glaze materials in a glass that renders them insoluble and safe so that they can easily be incorporated in the glaze.
Fritting - The creation of a frit by melting and grinding ceramic materials.
Frizzling - A decorating fault caused by high temperatures at the start of firing, which causes organic media to erupt or boil off.
Fusion - Combining constituent parts so that they bond together. These could be glaze ingredients or the surfaces of clay and glaze.

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Gauge - A potters gauge is a device which ensures the uniformity of ware.
Glass - A super-cooled liquid becoming a non-crystalline solid, manipulated through temperature and materials.
Glass Former - Primary ingredient like silica that combines with fluxes to create glass.
Glassy Phase - Stage in heating ceramics when glass-formers and fluxes combine to form glass.
Glaze - A coating that has been matured to the glassy state on a formed ceramic article, or the material or mixture from which the coating is made. Often consists of a flux, silica and optional modifiers like colorants.
Glaze Compression - State where the clay body shrinks more than the glaze during firing, affecting strength and causing potential defects.
Glaze Firing - Process of melting glazes in a kiln to create a glassy surface.
Glaze Fit - The matching of the thermal expansion and contraction of the glaze to the clay body. If glaze and body contract at different rates the faults of crazing, shivering or dunting can occur.
Glaze Melt - The melted phase of a glaze during firing.
Glaze Resist - Technique using resist materials to prevent selective glaze adhesion.
Glaze Sgraffito - Decorative carving in glaze to expose the underlying clay body.
Glost - Glaze firing, especially where the glaze firing is lower than the bisque.
Glost Firing - Otherwise known as glaze firing, this is the process where the glaze is fired to form a smooth glass like surface.
Greenware - This refers to unfired clay items.
Grog - Otherwise known as chamotte. This is where clay material is fired to remove moisture and then crushed into a grain to be used as an additive.

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Halloysite - A clay mineral used in porcelain and bone china outside north america.
Hardening-on - The firing of underglaze colours before applying glaze to prevent crawling, glaze smudging of the colour and contamination of the glaze and to aid glaze application.
Holloware - Shaped and formed ware such as jugs, bowls and containers. The opposite of flatware.
Hardbrick - Also known as kiln bricks, fire bricks, or refractory bricks. These are used to build supports and structures within a kiln and to line furnaces because of their dense structure and capacity to withstand extremely high temperatures.
Heat Work - Temperature x Time. To produce ceramic change in firing it is necessary not only to reach certain temperatures but to expose the ware to temperature for a period of time. Heatwork is measured by Pyrometric cones, Bullers rings and Minibars.
Hepa - Industrial filtration systems trapping microscopic particles like silica dust.
Hire Fire - The high-temperature firing range which includes cone 8 to cone 12 in the creation of stoneware or porcelain.
Hot Face - The internal refractory surface area of the kiln.
Hygroscopic - Ability to absorb atmospheric water, observed in plaster or limestone.
Hygroscopic - Ability to absorb atmospheric water, observed in plaster or limestone.

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Inclusions - Undissolved particles in glaze melt affecting appearance.
In-Glaze - Decoration of colour or oxide applied to the unfired glaze surface. The decoration is therefore neither underglaze nor on-glaze but in-glaze. Majolica and Delft ar examples of in-glaze ware.
Iron Oxide - A common oxide in glazes and some clays. The fired colour depends on a number of factors, such as concentration and firing atmosphere.

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Jigger/Jolley - Clay forming machine. The Jigger rotates clay over a mould which forms the inside shape of the pot, the outside shape is produced by a former being pressed against the clay as it revolves. The Jolley is th reverse, it forms clay by rotating it while being held in a mould which shapes the outside of the pot, a former in the profile of the required design is forced into the clay creating the inside shape.

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Kaolinite - Crystalline clay mineral forming most clays.
Kiln - A pottery oven in which ceramic ware is fired.
Kiln Furniture - A broad term for shelves and posts used within a kiln to support the ware inside.
Kiln Sitter - A mechanical device for shutting off kilns when the firing is complete. Kiln Sitters are activated by the bending of Minibars when they reach their melting point.
Kiln Wash - Protective coating for kiln shelves, varying by firing techniques.
Kneading - The manual working of clay to produce a homogenous smooth and plastic mass ready for throwing, hand building or modelling. Kneading follows wedging in clay preparation and is often referred to as wedging.

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Lawn - The fine wire mesh used for sieving materials. Usually made from phosphor-bronze and measured by the number of apertures per linear inch. A 60's mesh lawn would have 60 apertures per inch.
Loss of Ignition - The percentage of weight lost in clay when it is heated under certain conditions.
Low Sol (Low Solubility) - The name given to glazes incorporating metals, especially lead, which are present in a form that is not readily dissolved by the action of acid on the fired ware. Low Sol glazes conform to Health and Safety requirements and are therefore safe for dinnerware when fired correctly.
Lustre - Precious metals in liquid suspensions that are applied to glaze fired ware and fired to low on-glaze temperatures to produce a metalic surface.
Lustres - Made by coating ceramics with metallic substances which are then fired on to the glaze. These are usually iridescent in appearance.

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Majolica - Painted decoration of oxides or decorating colour applied to a white tin glaze before the glaze is fired.
Maturity - The temperature, or heatwork, range at which Clays reach their optimum strength and glazes achieve the desired surface quality and colour.
Minibars - A form of Pyrometric cone but in a bar shape specifically designed for use with Kiln Sitters.
Model - The first cast, or prototype, of a piece. Models are usually made in clay, or sometimes plaster.
Moisture Expansion - How much a porous ceramic material will expand when it absorbs water.
Muffle Kiln - A box within a furnace that is used to move pieces out of reach of flames or other combustible materials.

N

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Opacifier - Material for making transparent glazes opaque or white. Tin and Zircon are two common opacifiers.
On-Glaze - Colour decoration applied to fired glaze and fired again to between 680°C and 880°C
Once-Fired - Ware that has been fired once.
Oxidising - Firing in a kiln atmosphere where oxygen is present. Electric kiln firings are always considered to be in oxidising atmospheres.

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Peeling - A defect that occurs in glazed ware, peeling occurs when the glaze flakes away from the body.
Pin Holes - Small holes in the glaze surface resulting from bubbles of gas coming through the glaze in its molten state and leaving craters that have had sufficent time to heal over. Causes are; poorly deflocculated casting slips, underfired biscuit and not long enough firings.
Pint Weight - A measure of density used to enable slips or glazes of matching density to be reproduced. It is the weight of a pint of the liquid expressed in ounces e.g. 30 oz pt. The metric equivalent is gramms per 100ml. 30oz pt. equals 150 g/100ml.
Porcelain - Traditional ware created from china clay, ball clay, quartz and feldspar.
Porosity - The quality of fired ware to absorb water. Lower fired ware is generally more porous and absorbs water readily while ware fired to vitrification will have practically no porosity. The correct level of porosity is important in glazing where the rate of water absorbtion afects the thickness of the glaze layer.
Pugmill - A machine for processing and reconstituting clay. Scraps of used clay are compressed by an auger through a tapering barrel and extruded. A De-airing pugmill has the same function with the addition of a vacuum pump to remove greater amounts of air.
Pyrometer - A temperature measuring instrument. Pyrometers measure kiln temperatures by interpreting the micro-volt output of a thermocouple.
Pyrometric Cones - Usually made from ceramic materials, they are used to show accurate firing temperatures and heat work.

Q

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Raku - A method of firing where pots are removed from the kiln at maturing temperature. After removing from the kiln Raku pots are often enclosed in sawdust or other combustible material to create a reducing atmosphere where bright metallic colours can often be produced.
Raw Glaze - A glaze made of un-fritted materials.
Raw Glazing - Once-firing.
Reducing Agent - Substance removing oxygen atoms in reduction firing, like hydrogen.
Reduction - Firing in an atmosphere where the oxygen content is reduced and oxygen is removed from metal oxides creating different colours in clay and glaze. The immersion of Raku pots in saw dust is described as post-firing reduction.
Reduction Firing - Firing with inadequate oxygen, altering clay and glaze appearances.
Refractory - Used to describe how capable a material is of withstanding high temperatures. This is usually related to kiln shelves, cones, or stilts.
Residual Ash - Surface effects on woodfired wares from settling fly-ash.
Resist - Substances applied to clay to block slip or glaze adhesion.

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Saggars - Clay boxes that are used to protect ware from contamination during firing.
Salt Glazing - Glazing with common salt which is thrown into the kiln at temperatures above 1100°C. The salt turns to vapour and the Soda content combines with Alumina and Silica in the clay to produce a glaze.
Sgraffito - Decorating technique involving scratching through a slip or glaze layer.
Shivering - A glaze fault occurring when the body contracts at a greater rate than the glaze, putting the glaze under compression and causing it to separate from the body and flake and peel off, particularly on edges of potts. Shivering is the opposite fault to crazing.
Short - Clay lacking plasticity, prone to breaking during shaping.
Shrinkage - Permanent clay contraction during drying and firing, up to 18%.
Sieve - Fine-mesh pan for straining slips and glazes.
Sieving - Straining glaze or slip through a sieve to remove impurities.
Silicon Carbide - Refractory material for kiln shelves, not for electric kilns.
Sintering - The sticking together of materials prior to melting.
Sintered Mass - Porous bonded particles at contact points.
Slip - Clay in a liquid form for casting or decorating.
Slip Casting - Method of reproducing shapes in clay by moulding liquid slip in plaster moulds. The plaster absorbs water from the slip leaving a coating of clay next to the mould surface taking the shape of the mould.
Slop - The name given to a glaze after it has been mixed with water ready for glazing.
Slumping - Firing defect where clay deforms from overfiring or too much flux.
Slurry - Thick clay slip used to join clay pieces.
Soak - Holding the temperature for a given time during the firing to enable the kiln temperature to even out and to allow glazes to become smooth while molten without overfiring.
Soft Paste Porcelain - White claybody with glassy frit, maturing at lower temperatures.
Solution - Liquid mixture where dissolved components don't settle.
Solvent - Substance that speeds up the dissolution of a material.
Spalling - Used to describe the disintegration of ware when it is subjected to sudden or unexpected temperature changes.
Spit Out - Calcium impurities in a clay body can absorb moisture over time which causes them to swell and burst through the glaze layer leaving a crater in the pot. The bottom of the crater will typically have a small white centre confirming the presence of Calcium.
Spraying - Applying liquid slip or glaze with compressed-air equipment, similar to that for applying paint to cars.
Sprigging - Embossed decoration on clay ware, usually press moulded shapes applied to the pot to give a raised decoration.
Stabilizers - Materials that structure clays and influence glaze viscosity.
Stain - Ceramic colorants known for color accuracy, range, and stability.
Stoneware - Ceramic ware and the clays and glazes from which it is made. Stoneware is fired high enough to produce a low porosity body. Usually taken to be ware fired above 1200°C. Stoneware is also characterised by the integration of the glaze and the body.
Stoneware Clay - Clay fired within the stoneware temperature range.
Suspension - Liquid mixture with evenly distributed, insoluble particles.

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Tensile Strength - How strong, or resistant, a material is against being torn apart by tension.
Terracotta - Durable, low-temperature clay, firing to red-brown.
Terra Sigillata - Refined clay slip that can be polished to a sheen or gloss.
Thermal Conductivity - How effective a material is at letting heat pass through it.
Thermal Expansion - A physical occurrence in which the dimensions (size, volume, length, etc.) of a substance change in response to changes in temperature. When an object is heated, its particles gain kinetic energy and move more vigorously, leading to an increase in the average separation between the particles.
Thermal Shock - Sudden change of temperature in a fired pot that creates stress due to expansion and contraction. Oven ware must be made of a clay that withstands the thermal shock of repeated cooking.
Thermocouple - An instrument for measuring temperature in the kiln. Thermocouples are the ceramic probes that protrude into the kiln. Wires inside the thermocouple generate a small current which is measured by a Pyrometer which displays the temperature.
Thixotropy - The thickening of liquids when they are left undisturbed. A slip left for a few days will become less viscous as soon as it is stirred. Thixotropy enables the clay in a mould to keep its shape when newly cast and still wet.
Throwing - The forming of round pots by rotating clay on a potters' wheel. The clay is driven through the potters' hands by the force of the wheel and the position of the hands determines the shape of the pot.
Triaxial Blend - Testing method for three-way glaze combinations, varying proportions in samples.
Trimming - Removal of surplus clay at the leather-hard stage with cutting tools.
Tube Burner - Tube-shaped atmospheric burner without venturi effect.
Turning - The trimming off of surplus clay on thrown pots. This is done at the leather hard stage and is usually confined to the outside and base of pots.

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Undercut - Defect in molds where clay becomes trapped, risking breakage.
Underfired - When an article has been subject to insufficient firing, either time or temperature, such that the physical properties have been adversely affected.
Underglaze - Decoration applied to pots which are subsequently glazed with a transparent glaze. Underglaze colours are protected from wear by the glaze layer on top of them.
Underglaze Decoration - Decorating bare clay before glazing.
Underglaze Pencils - Engobe pigments in pencil form for marking or drawing.
Updraft - Kiln configuration where exhaust exits through a roof flue.

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Vapor Glazing - Glaze derived from atmospheric firing processes like salt-glazing.
Venting - Allowing the escape of gasses both from the kiln during firing and the kiln room. It is important that all kilns and kiln rooms are vented adequately to prevent the build up of harmful gasses.
Venturi Burner - Natural-draft gas burner with restricted tube design to enhance mixing.
Viscosity - The opposite of fluidity, the stiffness of a slip or a molten glaze. Viscous slips pour slowly and viscous glazes move very little in the firing.
Vitrification - The point at which clay becomes virtually non-porous and solid, almost glassy. Vitrification is the stage before melting when the pot would slump under its own weight.
Volatilisation (Volatilize) - Some glaze materials volatilise in the kiln atmosphere becoming gasses which can then be lost through the kiln flue or they can combine with materials on the surface of other pots giving flashes of colour. Chrome and Copper are materials prone to volatilisation and salt glazing relies on this phenomenom.
Volcanic Ash - Fine volcanic particles used in glazes or bentonite clays.

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Wads (Wadding) - Refractory clay mixtures preventing adhesion in atmospheric firing.
Warning Cone - Cone signaling proximity to target temperature in firing.
Warping - Clay distortion due to uneven stresses, drying, or heating.
Water of Plasticity - Water needed to bring dry clay to ideal plasticity.
Water Smoking Stage - Bisque-firing stage when compounds volatilize and outgas.
Wax Resist - A decorating technique where liquid wax is used to create areas of pattern that repel a covering colour or glaze. Wax resist can be applied in several layers, each layer blocking out more of the origional colour. The wax is burned away during firing.
Wedging - The process that compresses clay removes air bubbles and prepares it for use. A lump of clay is thrown down on to a work surface, cut in half with a wire and the half piece thrown down on the remainder. This process is the manual equivalent pugging and is followed by kneading.
Whiteware - White clay bodies fired below porcelain temperatures.