Glossary of Ceramic Terms

Term	Description
Abrasives	Various hard substances used for grinding, cutting or polishing softer substances, e.g. fused alumina.
Amorphous	Non-crystalline, having no determinable form or crystalline structure, e.g. glass.
Apparent Porosity	Relation between the volume of a mass and the volume of the water absorbed when the mass is immersed.
Ark	Large storage tank or container, e.g. Glaze Ark, Slip Ark, etc.
Autoclave	An airtight chamber, usually of steel, used for heating articles under pressure, used for a crazing or moisture expansion test.
Ball Mill	A 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 Wash	A coating of refractory material applied to saggars, kiln furniture, etc, to prevent sticking during firing.
Batt	1. Plaster or wooden form used to enable the movement of ware without handling. 2. A refractory slab used to support ware during firing.
Biscuit / Bisque	Fired but not yet glazed ware.
Bleb	Small blister or bubble on fired clay surface.
Bloating	A 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.
Blunging	Mechanical mixing of clay or slip with water.
Body	Name given to prepared clays.
Body Stain	Colourant used to colour clay body.
Bone China	A thin and translucent china - historically made from a body containing bone ash, china clay and Cornish Stone.
Bottle Oven	An intermittent kiln that is bottle shaped, usually fired with coal. Almost completely obsolete in pottery today.
Burnishing	Polishing leather hard clay by rubbing with a smooth stone or back of a spoon etc.
Calcination	The 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.
Chittering	A series of small irregularities on the outer edge or rim of pottery ware. Mainly caused by incorrect fettling.
CMC / Carboxy Methyl Cellulose	Carboxy Methyl Cellulose—See Tylose in Miscellaneous Materials.
Co-efficient of Expansion	A measure of the reversible volume or length change of a ceramic material with temperature. Used to determine correct glaze to body fit.
Colloid	A suspension of extremely fine charged particles in a liquid.
Combined Water	The 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.
Comminution	Production 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  Porcelain	Made 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.
Extrusion	The process of forcing clay through an aperture or die. e.g. through a pugmill.
Feathering	Drawing 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.
Flocculation	The aggregation of suspended particles by the addition of electrolytes to give a proper consistency for casting, dipping, etc.. A flocculant is a thickener.
Flux	A 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.
Frizzling	Mainly 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 Fit	The 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 Stain	Colouring agent added to a glaze.
Glost	A surface that has been glazed. A body that has gone through both a biscuit and glaze firing.
Greenware	Unfired clay ware.
Grog	Ceramic 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 On	A 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 Work	Energy 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 Pressing	Densification of particles by the simultaneous application of heat and pressure. i.e. pressure sintering.
Incised Decoration	Marking leather hard clay for decorative purposes.
Jiggering	Shaping of flatware by means of a profiled tool at a fixed distance from the rotating surface of a plaster mould.
Jolleying	Shaping of hollow ware by means of a profiled tool at a fixed distance from the rotating surface of a hollow plaster mould.
Kaolin	From the Chinese ‘Kao-Lin’, meaning a high ridge. This is where white clay was first discovered.
Kiln	Pottery oven or furnace in which ceramic products are fired. May be fired by wood, coal, sawdust, electricity or gas.
Kiln Furniture	General term used to describe refractory pieces used to separate and support pottery during firing.
Laminations	The structure of unfired or fired pottery in which the materials are aggregated in the form of layers or strata.
Lawn / Sieve	A fine mesh sieve, usually made from phosphor bronze or stainless steel, and supported by a strong frame.
Lead Solubility	The solubility of lead glazes in particular in diluted hydrochloric acid.
Leather Hard	Partially dried clay ware. The ideal stage for turning, fettling, sponging, etc.
Lithography	This 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  Solubility	L.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.
Lustres	An 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.
Majolica	In modern pottery, a soft opaque coloured glaze, firing temperature approx. 900
Model	The original or prototype of the piece to be made. Usually in clay, occasionally in plaster.
Modulus  of  Elasticity	The term defining the extent to which a material may be distorted under a given stress. Important in calculating glaze/body relationships.
Modulus  of  Rupture	The  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 Expansion	The extent to which a porous ceramic material will expand in size when it absorbs water or water vapour.
Muffle Kiln	A chamber or box built in a furnace and used to fire articles out of direct contact with flames or the products of combustion.
Once-Fired	The making, glazing and firing of ware in one operation.
Opacifier	An additive to a glaze that increases the reflection of light to the observer, commonly tin oxide or zircon.
Optical Pyrometer	This 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 Distribution	The 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 Holes	A glaze or body fault resulting from trapped air erupting through the body or glaze during maturation in the kiln.
Pitchers	Fired or broken scrap pottery. Biscuit pitchers have various uses when crushed or ground, i.e. grog, fillers, stopping, etc.
Plasticity	The 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.
Porosity	The 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  Cones	Small 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.
Quenching	A 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 Glaze	A 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.
Refractory	The ability of a material to withstand high temperatures, i.e. kiln shelves, cones, stilts, etc.
Rheopexy	The action of inducing thixotropy by gentle agitation or vibration. That is making something fluid by an act of vibration.
Saggars	Fire clay boxes of various shapes, in which ware is placed to protect it from contamination or flame impingement during firing.
Salt Glaze	A 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.
Sedimentation	The settling out, on standing, of particles from suspension in a liquid. This can occur when a casting slip or glaze is left overnight.
Sgraffito	The cutting or scratching through the outer colour or engobe, using a sgraffito tool, to expose the clay body underneath.
Sintering	The adhesion and densification of particles of a single compound on heating.
Slip	The 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.
Scumming	The 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.
Soak	To maintain a pre-arranged temperature in the kiln for a particular time.
Spalling	The flaking, cracking or other disintegration of ceramics when subjected to sudden temperature changes.
Specific Surface Area	The total area of the surface of all the particles in unit weight of material.
Spit Out	Rapid desorption of absorbed moisture during the enamel / lustre firing resulting in small craters or bubbles being blown in the glaze.
Spray Drying	The process of drying clay slips by spraying them into a chamber through which hot air is passed.
Stoneware	A 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 Tension	The capacity of molecules of a liquid to bind together. Water has high surface tension.
Tailings	The residue of a material that will not pass through a sieve. It may be further processed and then re-sieved.
Tenmoku	A 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 Strength	The resistance of a material to being torn apart by tension or pulling.
Thermal Conductivity	The rate at which heat passes through a material as measured by its rise in temperature.
Thermal Shock	Thermal 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.
Thermocouple	A 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.
Thixotropy	The ability of certain clay suspensions to thicken up on standing; characteristic of partial or over-deflocculated casting slip.
Throwing	The 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.
Towing	The process of smoothing the outer edge of greenware. It is very similar to fettling.
True Porosity	The 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.
Turning	Trimming and shaping thrown pots in the leather hard state.
Underglaze	Decorative colours applied to ware before the application of glaze.
Viscosity	The resistance to flow offered by a liquid. The opposite of fluidity.
Vitreous	A "glassy" like material / body that has extremely low or no porosity.
Vitrification	The 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 Absorption	A measure of the water a material can absorb, by soaking the material under specified conditions.
Wax Resist	Used as a masking medium for application to areas on which no glaze is required.
Wedging	A 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 Agent	A substance which when added to a liquid reduces its surface tension and causes the liquid to wet surfaces more efficiently.
Wreathing	Ripples or waves on the outside surface of a cast body caused by variations in the casting rate and excessive fluidity in the casting slip.