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Term

Glossary of Pottery Terms

Abrasives  Hard materials used for grinding,cutting or polishing e.g.alumina
Absorption  The amount of water that a body can draw into its surface at any stage of processing
Agate ware  Pottery made by incomplete mixing of 2 different coloured bodies. The fired result is a streaked effect resembling agate.
Airbrush  An atomiser that uses compressed air to apply colour or glaze
Alumina  Aluminium oxide. A hard non fusing powder used to stop ware sticking to the supports in kilns.
Amorphous  Non crystalline structure e.g.glass
Antimony oxide  Used to produce yellow colours in glaze or pigments. Now considered toxic.
Apparent porosity  Relation between the volume of item and the volume of water absorbed on immersion.
Ark  Large storage vessel or container e.g slip ark or glaze ark
Ash glaze  A glaze using wood ash as the flux . Sometimes mixed with other materials.
Autoclave  An air tight container used to heat articles under water pressure . e.g to test the craze resistance of a piece
Bagwall  The inner wall of kiln . Sometimes referred to as muffle it protects ware from direct flames on gas firing.
Ball clay  A sedimentary clay usually of fine particle size. This  clay is usually highly plastic
Ball mill  A ceramic lined steel cylinder filled with ceramic balls used to grind glaze or clay body.
Baluster  The name given to a vessel that is slender at the top and bulbous at the bottom.
Bamboo knife  A piece of bamboo that is sharpened to a chisel edge.Used in throwing.
Banding wheel  A turntable, usually made of metal, for applying decorative colour bands to vesssels or plates
Batt  A flat disc which attaches to the potters wheeel for throwing large articles. Also a refractory shelf.
Batt wash  A coating of refractory powder applied as a slurry to kiln furniture or saggars to stop sticking of ware.
Bentonite  A general term for montmorrllinite clay. Often added to glaze to stop sedimentation or to clay body to add plasticity
Biscuit  Pottery that has undergone its first firing. This gives the pottery sufficient strength to allow decoration and glazing.
Blistering Bubbles or crater faults in the surface of glaze after firing-caused by gas release from body or glaze
Bloating  Deformation of the body on firing . Produced by gas trapped in the partially fused mass.
Blunging  Mechanical mixing of clay body with water to produce a slip.
Body stain  Ceramic pigment used to colour the body on firing
Bone ash  Animal bones that have been processed and calcined to produce a ceramic powder,
Bone china  A thin, white translucent pottery made using bone ash as a body component.
Borax  Sodium borate-usually as the decahydrate form. Used in the manufacture of frits.
Bottle kiln  A huge bottle shaped brick kiln. Originally used in Stoke onTrent for firing of pottery-now museum exhibits exist only.
Bullers ring Special ceramic discs used to assess the firing in kilns. The measured contraction of the disc indicates the heat applied.
Burnish  To smooth the surface of a pot or precious metal decration to give a polished or shiny effect.
Calcium carbonate  Used a a glaze or frit component-also called limestone
Calcination  A high temperature heating process usually employed to change the crystalline form.e.g.flint
Caliper  A device for measuring the internal or external diameter of an object.
Casting  Forming pottery by pouring a slip into a porous mould
Casting slip  A very fluid slip of high solids content  made of clay body,deflocculant  and water. It is used in the casting process to form pottery shapes.
Celadon  A transparent green glaze pale in colour of Chinese origin
Ceramic  Derived from the greek word 'Keramos' meaning earthen vessel. Used nowadays to describe many high temperature formed produced using ceramic materials.
Cheesehard  Also called leatherhard-a clay body that is dry enough to retain its shape.
Chemically combined water  Water that is chemically attached to materials e.g. clay and is only removed by high temperature firing
China clay  Also known as kaolin. Made from pegmatite and mined in cornwall and other parts of the world.
Chinastone  Pegmatite. A feldspar and in its impure form the source of china clay in the UK.
Chrome oxide  Used to produce yellow or green pigments or glazes
Chuck  Tube like former to hold a pot in position on the wheel while the foot is being trimmed.
Chun A pale blue glaze used on stoneware
Clay body  The main pottery body composed of intimately mixed clays,feldspar,silica and other materials
Cobalt oxide  Used as a blue colorant in body glaze or decoration.
Coefficient of expansion  A measure of the reversible change of volume with temperature. can be quoted as linear or cubic expansion
Coiling  Rolled cylinders of clay build up to make a solid pottery form.
Colloid  A stable suspension of ultra fine particles.
Comminution  Size reduction by breaking, crushing or grinding
Compressive strength  The ability of an an article to withstand crushing loads
Cones  Sometimes called pyrometric cones, these ceramic pieces change shape and slump on exposure to heat. Used by potters as an indicator of a repeatable firing cycle.
Copper oxide  Used to produce green transparent colours in glaze. Under reducing fire can give bright red colours.
Cottle A retaining wall or side used to prepare plaster moulds
Crackle  Intentionally produced decorative crazing of the glaze.
Crawling  Shrinkage of the glaze leaving exposed body or pinholes after glaze firing-has many causes.
Crazing  Cracking of the glaze after firing. Caused by mismatch of body and glaze expansion.
Cryptocrystalline  Possessing crystals so small that they cannot be distinguished by optical microscope
Crystobalite A crystalline form of silica usually associated with a change from quartz at 225C
Cut glaze  Bare areas on glazed ware caused by mechanical damage of the glaze prior to firing.
Damper  A crude device or small hinged refractory door to control the flow of gases from the kiln.
De-airing  Removal of air from clay either by wedging by hand or using a special chamber in the pug mill.
Deflocculation  The dispersion of clay slip or glaze by the addition of an electrolyte e.g.sodium silicate or soda ash
Delft  Opaque lead glazed eathenware often decorated by metal oxides prior to firing
Devitrification  Crystallisation of a vitreous glaze after firing. Often at the surface of the glaze.
Dilatancy  The property of a suspension whereby it gets thicker as it is stirred and thinner as it stands.
Dipping  The application of glaze or decoration slip by immersion and ten allowing excess to drain off.
Dispersion  The separation of the clusters of fine particles in water into individual particles.
Draw  Absorption of glaze by an unglazed surface during firing.
Dropper  Globules of glaze that are found on glazed pieces after firing. These are caused by drops from the kiln roof.
Dunting  Cracking pottery on cooling. Often caused by silica inversions in the body or too fast firing.
Earthenware  Originally a moderately porous pottery formed by firing a mixture of ball clay, flint , china clay and feldspar.
Edge runner mill  At ype of mill for mixing or grinding materials using 2 large stone rollers and a stone pan.
Efforescence  A growth on the surface of bricks or other articles due to the the presence of soluble salts.
Electrical porcelain  Type of porcelain used for producing elelectrical insulators. Typical formula 28 ball clay,22 china clay.quartz25,feldspar 25
Elutriation  The separation of particles according to their size or density by a controlled velocity water stream.
Enamel  A fusible low temperature coating for metal or decoration for pottery.
Engobe

 A white or coloured coating of clay slip applied to the body prior to glazing. Often used for technical or decorative reasons.

Eutectic  A set mixture of 2 components having a lowest melting point of any ratio of the individual components.
Extrusion  The process of forcing a clay body thro an aperture or die to achieve a specific cross section or size.
Faience  Style of tin glazed earthenware produced in Europe.
Feathering  Effect obtained by trailing a feather thro still wet slip decoration
Feldspar  A crystalline mineral  caused by weathering of granite. Used as a high temperature flux in many bodies.
Fettling  The removal of the seams edges and blemishes from dried pots prior to glazing
Fillers  Minerals added to a body to give it rigidity or strength. Quartz and flint are termed as fillers.
Filter cloth  Used to filter press clay slip to produce solid filter cake
Filter pressing  Process used to change slip into solid clay. Clay slip is forced thro a special cloth to remove water.
Fireclay  Clay that is uesd for  refractory items. some fireclays contain naturalgrogs to improve shock resistance.
Firing  The increase in temperature used to produce a ceramic item.
Flocculation  The aggregation of ceramic particles in a suspension. A divalent electroltye such as CaCl2 is commonly used to create a thickening effect.
Flambe  A stronge red glaze produced by a reduced copper glaze.
Flatware  Plates,saucers,trays etc
Flint  Calcined powdered silica often used in earthenware bodies.
Fluting  Grooves cut into clay in parallel lines
Flux  A material added to a glaze body or colour to improve its melting properties
Foot  The base of the pot on which the piece stands.Sometimes called the footring

Frit

 A special glass ground to a powder and used to reduce the melting point of a glaze.
Frizzling  A decorating fault where the decoration curls or moves on firing. There are many causes.
Fusibility  The melting behaviour of a ceramic glaze,frit,flux or decoration with increase in temperature.
Glaze   a thin glaze coating formed on the surface of pottery to render it impermeable and provide decoration.
Glaze-body fit  The relationship between the thermal expansion of rhe body and glaze. Ideally the glaze should have a lower expansion than the body.
Glaze stain  A colouring pigmant added to a glaze to produce a coloured glaze.
Glost  A glazed and fired piece is referred to as 'glost'.
Greenware  Clay pieces that have been formed but not fired.
Grog  Ceramic material that is relatively coarse compared to the rest of body components. Generally used to give texture or improved thermal shock properties.
Gypsum  Plaster of paris used to make moulds
Hakeme  Oriental technique of applying white slip with a brush made of straw-allows slip to be applied thickly.
Handbuilding  Constructing pottery from premade components. These components might be made by moulding,coiling or by hand.
Hardening on  A firing process normally 650-700C to remove organic components of underglaze decoration.
Hot pressing  Densification of a ceramic by simultaneous application of heat and pressure.
Holloware  cups,jugs,bowls etc
Impressed A design stamped into the leatherhard clay using a die or tool
Incised  Pattern cut into the clay with a sharp tool
Jiggering  Machine process using a mould to form the inside of the plate and the tool the outside
Jolleying  Machine process using a mould to form the outside of the pot and the tool the outside
Kaolin  Also called china clay. White firing clay used in many bodies.
Kiln  High temperature oven or furnace used for firing of pottery.
Kiln furniture  General term used to describe refractory pieces used to separate or support items during firing
Kiln wash  Refractory slurry applied to kiln furniture to stop sticking during firing
Lamination  A fault structure in clay before or after firing where the clay is aligned in layered clusters.
Lawn  A sieve
Lead solubility  The solubility of lead containng glaze or colour powders when exposed to aqueous hydrochloric acid
Leatherhard  Or cheesehard. Clay that has dried to a point where it retains its shape.
Limestone  Calcium carbonate-often used as a glaze component
Lithograph  A method of decoration involving transfers.Allows fine detailed printing.
Loss on ignition  The loss in weight of a clay body when fired to a high temperature (usually 1000C) expressed as a %
Lustres   A metallic thin film applied to pottery either as a glaze or decorative effect
Lute To join 2 pottery surfaces together with slip
Majolica  The technique of applying low fired tin glazes with colours to produce a multi coloured effect.
Manganese oxide  Colouring oxide used in glaze or decoration to give brown to purple colours.
Medium The liquid carrier used to apply colour,glaze or slip. This may be simply water,oil or complex polymer solution.
Model The original or prototype article
Modulus of elasticity  The term used to define the extent a material can be distorted under stress withot fracture.
Modulus of rupture  The term is used to define the strength of unfired clay body or the fired ceramic.
Moisture expansion  The extent to which a porous ceramic expands when it absorbs moisture or water vapour.
Molochite A calcined kaolin
Mould  The shape usually made of plaster used to form the article either by casting or pressing.
Muffle kiln  A type of kiln where the ware is protected from the flame by an inner refractory box.
 Nickel oxide  Used to produce brown,green or violet colours in glaze. Now considered toxic.
Onglaze  Decoration applied on top of the fired glaze. Overglaze or enamel terms are sometimes used.

Opacifier

 An additive to the glaze to give opacity e.g. zircon or tin oxide
Oxidation The firing of a kiln with an excess of oxygen
Particle size  The distribution of particle sizes either as a powder or a slurry. Important measure in manufacture of glaze or colour.
Peeling  A defect in glazed and fired ware.The glaze flakes off in pieces particularly on edges.
Peephole  Small hole in a kiln door for the potter to observe the progress of firing.
Pinching  Indenting a pot with fingers or thumb before it is hard.
Pinholes  A common fault in the body or glaze. Small holes appear in the glaze or body as gases erupt thro the surface.
Pitchers  Fired or broken scrap pottery (of course also a large jug)
Plaster of paris  Gypsum. Used to make moulds.
Plasticity  The property of clay which allows it  to be moulded and retain its shape after the force is removed.
Plucked ware  A fault caused by the ware sticking to the refractory support during firing
Porcelain  A vitrified and white translucent ware made predominantly in Europe and Asia.Usually strong in use.
Porosity  The amount of pore space in a ceramic body-consists of closed and open pores.
Pouncing  The ancient technique known as pouncing is when 'pounce', powder graphite or charcoal, is rubbed through a series of small holes punched in a paper pattern to transfer a design to an item to be decorated.
Pugging  The intimate mixing and extrusion of plastic clay body. Also the machine used to carry out this process.
Pulling  Shaping a handle by hand to the correct size and shape.
Pyrometer  High temperature measuring device
Pyrometric cones  Special refractory cones placed next to the ware to confirm repeatable firing.
Quenching  Method of making a frit where the molten glass is poured into cold water to produce small granules of frit
Raku  Low fired  glazed earthenware produced normally by rapid firing. Raku is japanese for'enjoyment' and this type of pottery is used in the Japanese tea ceremony.
Raw glaze  A glaze in which no frit is present-normally a high fire glaze over 1100C
Reducing atmosphere   A kiln atmosphere which is deficient in free oxygen(more gas less air)-used to produce specific glaze colours or effects
Refractory   Ceramics that are able to withstand high temperatures without distortion.
Rib  A shaped tool to facilitate forming of a pot on a wheel.
Saggars  A fireclay or other refractory box used originally to protect the ware during firing
Salt glaze   Thin orange peel glaze produced onthe surface of pottery-originally produced by throwing salt into the kiln and allowing it to vapourise.
Sang-de- Boeuf  Ox-blood colour used to describe deep red colour of some glazes
Sedimentation  The settling out over time of any clay or glaze slip. More noticeable in glazes low in clay content.
Sgraffito  The technique of scratching through an applied slip layer to reveal the cour of the body beneath.
Shelling Same as peeling: A defect in glazed and fired ware.The glaze flakes off in pieces particularly on edges.
Shivering Same as peeling: A defect in glazed and fired ware.The glaze flakes off in pieces particularly on edges.
Sintering  The adhesion and densificationof particles on heating
Slip  Suspension of body or glaze in water.
Slip decoration  Application of contrasting coloured slip to a body as a form of decoration.
Slabbing  A buliding technique where slabs of clay are joined at edges to form an article.
Silk screen  A form of decoration where the colour paste is forced thro a decorative patterned screen either directly onto the ware or a transfer paper.
Soak  Allowing the pottery to remain at the peak temperature for a time period. This allows all positions in the kiln to reach approx the same temperature
Spalling  Flaking.cracking or disintegration of ceramics when subjected to rapid temperature changes.
Specific surface area  The total surface area of a all particles per unit weght of material
Spitout  Rapid desorption of moisture during the onglaze firing process causing a severe fault of small craters or pinholes.
Spray drying  A means of drying clay slips or other ceramic slurries by spraying the slurry into a large chamber of hot air
Sprig  Decorative pieces applied to leather hard pots
Spur marks  The marks left on pottery from refractory supports used to fire the piece.
Stoneware  A vitreous opaque pottery containing naturally vitrifying clay. Favoured by many studio potters
Surface tension  The tendency of a fluid to wet a surface.E.g. low surface tension= high wetting
Tailings  The portion of a material which does not pass thro a sieve or is returned for further processing
Tenmoku  A stoneware glaze deeply coloured by iron oxide. Colours can vary from yellow,green.red.blue or black
Tensile strength  The resistance of a materal to being stretched by tension or pulling
Terracotta  Red eathenware body usually made with naturally red clays
Thermal conductivity  The rate at which heat passes through a material as measured by its rise in temperature.
Thermal shock  The failure of a ceramic article due to stresses created by rapid temperature change. -either hot to cold or cold to hot
Thermocouple   A device for measurement of temperature in a kiln. Usually in the form of a special metallic wires encased in a ceramic sheath.
Thread  Twisted wire sometimes used by potters for curting pots of a wheel
Throwing  Making a pot on a potters wheel
Tin glaze  White opaque glaze produced with tin oxide in the glaze recipe.
Titanium oxide  Used in glazes to produce decorative crystalline effects
Towing  The smoothing of the outer edge of flatware in its green state.
Transfer printing  The method of applying a decoration to a pot. Slide off transfers using ceramic colours are applied to the glost ware and the refired to bond the decoration to the glaze.
True porosity  The sum of open and closed pores in a ceramic body.
Tube lining   A decorating technique where coloured slips are piped thro a small nozzle onto clay ware. This is often used as a border for further decoration.
Tunnel kiln  A kiln where the pottery moves progressively thro a heated zone to fire the pottery.
Underglaze  A decoration applied either directly to the clay or biscuit prior to glaze application and firing
Viscosity  The resistance of a fluid to flow. the reciprocal of fluidity
Vitreous  Glassy-having very low or zero porosity
Vitrification  The change from a porous to vitreous state of a ceramic
Volatility  The process of evaporation of materials on firing
Wall pocket  A vase usually with a flat back for attachment to a wall
Water absorption  A measure of the open porosity of a ceramic-determined by soaking the item in water for a set time
Wax resist  Wax applied to a pot to stop glaze or decoration sticking to these areas during application.
Wedging  A method of de-airing and dispersing moisture evenly in clay. The clay is worked by hand by repeated throwing of the clay onto the bench and folding over.
Wetting agent  A chemical used to help the application of glaze or colour to allow it to better cover uneven areas.
Wheel  Also potters wheel- a horizontal disc on a foot operated or motor driven shaft. Used for throwing of pots.
Wreathing  Ripples or waves on the inside of a cast pot after draining. Caused by variation in casting rate at different parts of the pot.

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