Glossary of Ceramic Terms
Apparent Porosity - Relation between the volume of a mass and the volume of the water absorbed when the mass is immersed.
Kiln/Batt Wash - A coating of refractory material applied to saggers, kiln furniture etc, to prevent sticking during firing.
Batt - A refractory kiln shelf used to support ware during firing.
Biscuit - Fired but not yet glazed ware.
Bloating - A swelling or expansion of body due to (a) over firing or irregular firing, (b) carbon trapped within a vitreous body, it's appearance is of a bubble formation within the body.
Body - Name given to prepared clay.
Burnishing - Polishing leatherhard clay by rubbing with a smooth stone or back of a spoon etc.
Casting - Forming pottery by pouring slip into a porous mould.
Chittering - A series of small irregularities on the outer edge or rim of pottery ware. Mainly caused by incorrect fettling.
Co-efficient of Expansion - A measure of the reversible volume or length change of a ceramic material with temperature.
Combined Water - The water driven off when a dry substance is heated. It should be distinguished from the moisture which is driven off below 110c, and which can be variable. The combined water is present in finite proportions.
Co-efficient of expansion - A measure of the reversible volume or length change of a ceramic material with temperature.
Craquelle - An intentionally crazed or cracked effect on art pottery, emphasised by rubbing colouring matter into the cracks and refiring the ware.
Crawling - Movement of glaze over the body surface during the glost firing stage, due either to dust or grease on the surface or over thick glazing 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, and may result from mis-match of the glaze with the body.
Dunting - Cracks or cracking 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 which is fired to a temperature somewhat below that required to produce a vitreous article.
Engobe - A white or coloured coating of slip applied to the clay before glazing for decoration.
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.
Flux - A substance that lowers the melting point of material in which it is present, either naturally or to which it has been added.
Frit - A ceramic glass-like composition, melted or fused together. Used to render soluble constituents of glazes insoluble.
Glaze/Body Fit - The relationship between thermal expansion of body and glaze. Ideally the glaze should have a lower thermal expansion than the body, so that on contraction the body puts the glaze into compression. This avoids crazing of the glaze due to tensile stresses in it.
Glost - Meaning glazed.
Greenware - Unfired glazed clayware.
Grog - Ceramic material which has heated to a high temperature before use and predominantly inert.
Heat Work - Energy input during firing, normally represented in terms of temperature and time. Cones indicate this.
Incised Decoration - Marking leatherhard 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 at a fixed distance from the rotating surface of a hollow plaster mould.
Kiln Furniture - General term used to describe refractory pieces used to separate and support pottery during firing.
Lead Solubility - The solubility of lead glazes in particular in diluted hydrochloric acid.
Leather Hard - Particularly dried clay ware. The ideal stage for turning, fettling, sponging, etc.
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 it's 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 c - 1050 c. Originally named from the island of Majorca where it was first made in the 16th Century. Basically similar to Delft.
Moisture Expansion - The extent to which a porous ceramic material will expand in size when it absorbs water or water vapour.
Once-Fired - The making, glazing and firing of ware in one operation.
Soak - To maintain ceramic ware at 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.
Spit Out - Rapid desorption of absorbed moisture during the enamel firing resulting in small craters or bubbles being blown in the glaze.
Stoneware - An opaque ceramic 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.
Thermal Shock - The disruption of a ceramic article 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 partially or over-flocculated slips.
True Porosity - The sum of open pores as determined by water absorptions plus the volume of those pores which are sealed by vitreous matter and therefore closed to water.
Turning - Trimming thrown pots in the leatherhard state.
Viscosity - The resistance to flow offered by a liquid. The opposite of fluidity.
Vitreous - As applied to ceramics means glassy, having 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.
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.
Wreathing - Ripples or waves on the outside surface of a cast body caused by variations in the casting rate. Also referred to as hesitation lines.