Glossary of Ceramic Terms

AbrasivesVarious hard substances used for grinding, cutting or polishing softer substances, e.g. fused alumina.
AmorphousNon-crystalline, having no determinable form or crystalline structure, e.g. glass.
Apparent PorosityRelation between the volume of a mass and the volume of the water absorbed when the mass is immersed.
ArkLarge storage tank or container, e.g. Glaze Ark, Slip Ark, etc.
AutoclaveAn airtight chamber, usually of steel, used for heating articles under pressure, used for a crazing or moisture expansion test.
Ball MillA piece of machinery used in the ceramic industry for the grinding of materials. It consists of a lined cylinder rotating about its horizontal axis and charged with flint pebbles or special ceramic grinding media, plus the material to be ground. The mill may be operated dry or wet.
Batt / Kiln WashA coating of refractory material applied to saggars, kiln furniture, etc, to prevent sticking during firing.
Batt1. Plaster or wooden form used to enable the movement of ware without handling.
2. A refractory slab used to support ware during firing.
Biscuit / BisqueFired but not yet glazed ware.
BlebSmall blister or bubble on fired clay surface.
BloatingA swelling or expansion of body due to (a) over firing or irregular firing, (b) carbon trapped within a vitreous body.
It appears as a bubble formation within the body.
BlungingMechanical mixing of clay or slip with water.
BodyName given to prepared clays.
Body StainColourant used to colour clay body.
Bone ChinaA thin and translucent china - historically made from a body containing bone ash, china clay and Cornish Stone.
Bottle OvenAn intermittent kiln that is bottle shaped, usually fired with coal. Almost completely obsolete in pottery today.
BurnishingPolishing leather hard clay by rubbing with a smooth stone or back of a spoon etc.
CalcinationThe pre-firing of a ceramic material to remove all moisture and burn off any oxides and gases.
Casting Forming pottery by pouring slip into a porous mould, usually made from potters plaster.
Casting Slip A very fluid slip of high specific gravity, obtained by deflocculation and used for forming ware using the casting process.
Ceramic Derived from the Greek ‘Keramos’ meaning ‘earthen vessel’. Today it applies to a whole range of products, i.e. bricks, tiles, pipes, porcelain, china, pottery, etc.
ChitteringA series of small irregularities on the outer edge or rim of pottery ware. Mainly caused by incorrect fettling.
CMC / Carboxy Methyl CelluloseCarboxy Methyl Cellulose—See Tylose in Miscellaneous Materials.
Co-efficient of ExpansionA measure of the reversible volume or length change of a ceramic material with temperature. Used to determine correct glaze to body fit.
ColloidA suspension of extremely fine charged particles in a liquid.
Combined WaterThe water driven off when a dry substance is heated. It should be distinguished from the moisture which is driven off below 110°C and which can be variable. The combined water is present in finite proportions.
ComminutionProduction of powder by the breaking down of large particles, usually in a ball mill or mortar and pestle.
Compressive Strength The ability of an article to withstand crushing loads.
Craquelle An intentionally crazed or cracked effect on art pottery, emphasized by rubbing colouring matter into the cracks and re-firing the ware.
Crawling Movement of glaze over the body surface during the gloss firing stage, due to dust or grease on the surface or over application of the glaze or excessive colloidal material in the glaze.
Crazing A network of cracks in the glaze caused by tensile stresses greater than the glaze is able to withstand. These may result from mis-match of the glaze with the body.
Cryptocrystalline Possessing crystals so small that they cannot be distinguished by a microscope.
Cut Glaze Bare areas on glazed ware due to mechanical damage to the glazed article in the unfired state i.e. ware rubbing together.
De-Airing Removal of air from clay. Various means are used to achieve this but the most common with clay is via the de-airing chamber of a pugmill.
Deflocculation The dispersion of clay slip or glaze by the addition of an alkaline electrolyte e.g. sodium silicate, Dispex, etc.
Devitrification The crystallization of a vitreous material.
Dilatancy The property of a suspension such that when it becomes mechanically disturbed it  appears to stiffen but becomes  mobile again when the disturbing force is removed.
Dipping The application of a glaze by immersion and then allowing the excess to drain off.
Dispersion The separation of clusters of particles into their original separate units i.e. grinding.
Draw Absorption of glaze by an unglazed surface situated near to a kiln during firing.
Dropper Globules of glaze found on open gloss ware, usually from glaze accumulations on the roof of the kiln.
Dunting Cracks or cracking in the clay caused by the too rapid cooling or heating of ware, and due to thermal stresses set up in the body.
Earthenware A moderately porous pottery body that is fired to a temperature somewhat below that required to produce a vitreous article. Typically 1060 to 1100 C
Edge Runner Mill A type of mill used for grinding or mixing materials in which the treatment is carried out by the pressure of large rollers squeezing the material between themselves and the base of the pan.
Efflorescence A growth on the surface of ware due to the presence in the body of soluble salts.
Electrical  PorcelainMade for the use as an electrical insulating material. Typical composition is ball clay 28%, china clay 22%, quartz 25% and feldspar 25%.
Elutriation The separation of particles according to their size or specific gravity by a controlled velocity water stream. The basis of a method for the determination of the fineness of a material.
Enamel A fusible vitreous coating, fired at low temperatures, for clay articles.
Enamel Firing A further firing to convert ceramic colouring materials, applied on top of a glaze, to a permanent form.
Engobe A white or coloured coating of slip applied to the clay, for decoration, before glazing.
Eutectic A mixture of two substances which has the lowest melting point in a whole series of possible compositions.
ExtrusionThe process of forcing clay through an aperture or die. e.g. through a pugmill.
FeatheringDrawing a feather across slip-trailed ware for decorative purposes.
Fettling The removal of the seam left by the mould in greenware, by fettling knife and/or sponge.
Fillers These are materials added to a system to give it rigidity or increase its volume. Flint or quartz  are often referred to as fillers in ceramic bodies.
Filter Cloth A nylon or terylene cloth used in filter presses to filter the clay.
FlocculationThe aggregation of suspended particles by the addition of electrolytes to give a proper consistency for casting, dipping, etc.. A flocculant is a thickener.
FluxA substance that lowers the melting point of material in which it is present.
Frit A ceramic glass-like composition, melted or fused together. Used to render soluble constituents of glazes insoluble.
FrizzlingMainly a decorating fault, typified by the curling over or frizzling of parts of a decoration. Caused mainly by temperature being too high in the initial stages of firing thereby causing the boiling off or eruption of organic media, i.e. size, oils, etc.
Fusion Any treatment which converts a solid substance to a liquid state with temperature.
Glaze A thin glassy layer formed on the surface of a ceramic article by the exposure of the glaze coated  article to a high temperature. Glaze is usually applied in the form of a suspension of ground glaze in a liquid medium, to the clay or biscuit surface of the article.
Glaze/Body FitThe relationship between thermal expansion of body and glaze. Ideally the glaze should have a lower thermal expansion than  the  clay  body,  so  that  on  contraction  the  body puts  the  glaze  into  compression.   This  avoids  crazing  of  the  glaze  due  to  tensile stresses in it.
Glaze StainColouring agent added to a glaze.
GlostA surface that has been glazed. A body that has gone through both a biscuit and glaze firing.
GreenwareUnfired clay ware.
GrogCeramic material which has been heated to a high temperature to burn off moisture and other gasses therefore is predominantly inert. This is then ground to a required grain size to add to a body formulation.
Hardening OnA process of heating decorated bisque ware to a temperature of approximately 650-700°C in order to burn out the organic media of the decoration and fix the colour prior to glazing.
Heat WorkEnergy input during firing, normally represented  in terms of temperature and time. Pyrometric cones indicate the amount of heat work that has occurred during a firing.
Hot PressingDensification of particles by the simultaneous application of heat and pressure. i.e. pressure sintering.
Incised DecorationMarking leather hard clay for decorative purposes.
JiggeringShaping of flatware by means of a profiled tool at a fixed distance from the rotating surface of a plaster mould.
JolleyingShaping of hollow ware by means of a profiled tool at a fixed distance from the rotating surface of a hollow plaster mould.
KaolinFrom the Chinese ‘Kao-Lin’, meaning a high ridge. This is where white clay was first discovered.
KilnPottery oven or furnace in which ceramic products are fired. May be fired by wood, coal, sawdust, electricity or gas.
Kiln FurnitureGeneral term used to describe refractory pieces used to separate and support pottery during firing.
LaminationsThe structure of unfired or fired pottery in which the materials are aggregated in the form of layers or strata.
Lawn / SieveA fine mesh sieve, usually made from phosphor bronze or stainless steel, and supported by a strong frame.
Lead SolubilityThe solubility of lead glazes in particular in diluted hydrochloric acid.
Leather HardPartially dried clay ware. The ideal stage for turning, fettling, sponging, etc.
LithographyThis is a method of decoration involving manufactured transfers or ‘decals’. The printing of the transfer is done by printing the decoration in lithographic oil, the colour then being dusted onto the oiled areas. The completed transfer is applied to a clay surface that has been previously coated with a tacky size, rubbed down and the paper backing sponged off. This process has largely been replaced with slide off transfers.
Loss of Ignition (L.O.I.)The loss in weight of clay or any other material expressed as a percentage of its dry weight when it is heated under specified conditions.
Low  SolubilityL.S. or low sol glaze. Defined by the Pottery Health Regulations as a glaze which does not release more than 5% of its dry weight of soluble lead when subjected to a specified test using hydrochloric acid.
LustresAn iridescent optical appearance, due to light reflections producing diffraction patterns on a glazed surface. Produced by very thin coatings of metallic substances fired onto the glaze.
MajolicaIn modern pottery, a soft opaque coloured glaze, firing temperature approx. 900
ModelThe original or prototype of the piece to be made. Usually in clay, occasionally in plaster.
Modulus  of  ElasticityThe term defining the extent to which a material may be distorted under a given stress. Important in calculating glaze/body relationships.
Modulus  of  RuptureThe  resistance  offered  by  a  piece  of  ceramic  of  unit  cross  sectional  area  to a force. The  mechanical strength is quoted as modulus of rupture.
Moisture ExpansionThe extent to which a porous ceramic material will expand in size when it absorbs water or water vapour.
Muffle KilnA chamber or box built in a furnace and used to fire articles out of direct contact with flames or the products of combustion.
Once-FiredThe making, glazing and firing of ware in one operation.
OpacifierAn additive to a glaze that increases the reflection of light to the observer, commonly tin oxide or zircon.
Optical PyrometerThis is a form of pyrometer in which the temperature of an article or furnace is estimated by comparing the colour and intensity of its luminous surface with that of a calibrated filament.
Particle Size DistributionThe description of a powder by specifying the percentage of material within a given size range or less than a specific size. Sieve mesh sizes are only an indication of maximum particle size.
Peeling A defect in glazed ware characterised by the engobe or glaze separating from the body in flakes. This is usually due to high compressive stresses in the layer.
Pin HolesA glaze or body fault resulting from trapped air erupting through the body or glaze during maturation in the kiln.
PitchersFired or broken scrap pottery. Biscuit pitchers have various uses when crushed or ground, i.e. grog, fillers, stopping, etc.
PlasticityThe property which enables material to be deformed by a force which exceeds a certain minimum value and to retain its new shape when the deforming force is removed. The ability of a material to be modelled.
Plucked Ware A fault caused by glazed parts of the ware being in contact with kiln furniture. Or over firing of a body and it "melts" onto the shelf.
Porcelain This is a vitrified and translucent ware made from a body containing china clay, ball clay, quartz  and feldspar, which is made traditionally and fired only once.
PorosityThe amount of pore space in a ceramic material, which may consist of both open and sealed channels.
Pugging The mixing, blending, de-airing and extrusion of plastic clay bodies.
Pyrometric  ConesSmall tall "pyramids", made from various ceramic materials, which fuse and bend over at given temperatures. Used in kilns to indicate accurate firing temperatures and heat work.
QuenchingA method of cooling molten frit by allowing it to pour into a bath of water, thereby giving the frit its characteristic granular or plate form.
Raw GlazeA glaze which contains no fritted ingredients.
Reducing Atmosphere A  kiln  atmosphere  which  is  deficient  in  free  oxygen  and  causes  reduction  of  compounds  which  are  oxygen bearing.
RefractoryThe ability of a material to withstand high temperatures, i.e. kiln shelves, cones, stilts, etc.
RheopexyThe action of inducing thixotropy by gentle agitation or vibration. That is making something fluid by an act of vibration.
SaggarsFire clay boxes of various shapes, in which ware is placed to protect it from contamination or flame impingement during firing.
Salt GlazeA glaze is applied to ware within the kiln by throwing some salt onto the flames during firing, the ensuing vapour then reacts with the clay surface to form a sodium alumino-silicate glaze.
SedimentationThe settling out, on standing, of particles from suspension in a liquid. This can occur when a casting slip or glaze is left overnight.
SgraffitoThe cutting or scratching through the outer colour or engobe, using a sgraffito tool, to expose the clay body underneath.
SinteringThe adhesion and densification of particles of a single compound on heating.
SlipThe suspension of a clay body in water, used for casting ware in moulds. Deflocculants are added to keep the clay in suspension.
Silk Screen A decorating technique where the colour in paste form is forced by the means of a squeegee through a fine mesh and onto a clay surface.
ScummingThe formation of dull scum on the glaze surface, caused by gases present in the kiln atmosphere, or sulphates present in the body or in the kiln gases.
SoakTo maintain a pre-arranged temperature in the kiln for a particular time.
SpallingThe flaking, cracking or other disintegration of ceramics when subjected to sudden temperature changes.
Specific Surface AreaThe total area of the surface of all the particles in unit weight of material.
Spit OutRapid desorption of absorbed moisture during the enamel / lustre firing resulting in small craters or bubbles being blown in the glaze.
Spray DryingThe process of drying clay slips by spraying them into a chamber through which hot air is passed.
StonewareA ceramic body containing a naturally vitrifying clay e.g. a stoneware clay or a suitable ball clay. Sometimes a non-plastic constituent and a flux are added. See clay chart for vitrification temperatures.
Surface TensionThe capacity of molecules of a liquid to bind together. Water has high surface tension.
TailingsThe residue of a material that will not pass through a sieve. It may be further processed and then re-sieved.
TenmokuA stoneware glaze deeply coloured by iron oxide. It produces often lustrous results that vary in colour from yellow, green, rust red, brown and black.
Tensile StrengthThe resistance of a material to being torn apart by tension or pulling.
Thermal ConductivityThe rate at which heat passes through a material as measured by its rise in temperature.
Thermal ShockThermal shock is the way in which some materials are prone to damage by stresses set up due to differences in temperature in different parts of the article.
ThermocoupleA device for the measurement of temperature based on the voltage generated when two dissimilar conductors are heated in contact e.g. copper/constantan, chrome/alumel, platinum/rhodium, etc.
ThixotropyThe ability of certain clay suspensions to thicken up on standing; characteristic of partial or over-deflocculated casting slip.
ThrowingThe technique of forming pottery on the potter’s wheel. A ball of prepared clay is thrown on the wheel and it is centred and shaped with the hands.
TowingThe process of smoothing the outer edge of greenware. It is very similar to fettling.
True PorosityThe sum of open pores as determined by water absorption plus the volume of those pores which are sealed by vitreous matter and therefore closed to water.
TurningTrimming and shaping thrown pots in the leather hard state.
UnderglazeDecorative colours applied to ware before the application of glaze.
ViscosityThe resistance to flow offered by a liquid. The opposite of fluidity.
VitreousA "glassy" like material / body that has extremely low or no porosity.
VitrificationThe progressive fusion of a material or body during the firing process. As vitrification proceeds the proportion of glassy bond increases and the apparent porosity of the fired product becomes progressively lower.
Water AbsorptionA measure of the water a material can absorb, by soaking the material under specified conditions.
Wax ResistUsed as a masking medium for application to areas on which no glaze is required.
WedgingA method of de-airing and dispersing moisture uniformly by hand in a piece of clay. The lump of clay is repeatedly thrown hard onto the work bench, turned over and occasionally cut through and re-joined.
Wetting AgentA substance which when added to a liquid reduces its surface tension and causes the liquid to wet surfaces more efficiently.
WreathingRipples or waves on the outside surface of a cast body caused by variations in the casting rate and excessive fluidity in the casting slip.