Ceramics Glossary of Terms

Absorbency- the ability of a material to soak up water.

Ball clay-  an extremely fine-grained, plastic, sedimentary clay. It is usually added to clay bodies to increase plasticity.

Bisque- unglazed ware fired to a temperature sufficient to harden but not mature the body.

Bisque fire-  preliminary firing to harden the body, usually at about cone 010, prior to glazing and subsequent glaze firing.

Clay-  a decomposed granite-type rock. To be classified as a clay, the decomposed rock must have fine particles so that it will be plastic. Pure clay is expressed chemically as Al2O3:2SiO2:2H2O.

Coiling-  a hand method of forming pottery by building up the walls with rope like rolls of clay and then smoothing over the joints.

Crawling- separation of a glaze coating from the clay body during firing resulting in exposed areas of unglazed clay.

Crazing-  an undesirable and excessive crackle in the glaze, which penetrates through the glaze to the clay body. (see crackle glaze)

Dipping- glazing pottery by immersing it in a large vat of glaze.

Dryfoot-  to clean the bottom of a glazed piece before firing.

Earthenware- low-fire pottery (below cone 03), usually red or tan in color with an absorbency to from 5 to 20 percent.

Flux- lowest melting compound in a glaze, such as lead, borax, soda ash, or lime, and including the potash or soda contained in the feldspar. The flux combines easily  with silica and thereby helps higher-melting alumina-silica compounds to form a glass.

Foot-  the ring like base of a ceramic piece, usually formed by tooling the excess clay.

Glaze-  a liquid suspension of finely ground minerals, that after being applied to the bisque-fired clay form and heated to the proper temperature, melt to form a glassy coating on the clay surface.

Glaze fire-  a firing cycle to the temperature at which the glaze materials will melt to form a glasslike surface coating. This is usually at the point of maximum body maturity, and is usually considerably higher than the bisque fire.

Greenware- pottery that has not been bisque fired.

Grog- hard-fired clay that has been crushed or ground to various particle sizes. It is used to open up a clay body and to reduce shrinkage. it is added to throwing bodies to help the clay stand up.

Kaolin-  pure clay, also known as china clay. It is used in glazes and clay bodies and fires pure white.

Kiln-  a furnace made of refractory clay materials for firing ceramic products.

Kiln furniture- refractory shelves and posts upon which ceramic ware is placed while being fired in the kiln.

Leather hard-  the condition of the clay when most of the moisture has left the body but when it is still plastic enough to be carved or joined.

Oxidation- a firing which takes place with sufficient oxygen for complete combustion.

Plasticity-  the quality of clay that allows it to be manipulated and still maintain its shape without cracking or sagging.

Porcelain-  a hard, non-absorbent clay body, white or gray in color, that rings when struck.

Pyrometer-  an instrument for measuring heat at high temperatures.

Pyrometric cones- small triangular cones made of ceramic materials that are compounded to bend and melt at specific temperatures, thus enabling the potter to determine when the firing is complete.

Reduction- a firing using insufficient oxygen which results in color changes in the clay body and glazes.

Refractory-  the quality of resisting the effects of high temperatures; also materials, high in alumina and silica, used for making kiln insulation and furniture.

Sgraffito- decoration achieved by scratching through a colored slip or a glaze to show the contrasting body color beneath.

Short- descriptive of a clay or clay body lacking in plasticity.

Shrinkage- contraction of the clay in either drying or firing.

Slab construction- a hand-building method in which forms are created by joining flat pieces of clay which have been rolled with a rolling pin or slab roller.

Slip-  a clay in a liquid suspension.

Stoneware-  a high-fire ware (above cone 6) with slight or no absorbency.

Terra-cotta-  an earthenware body, generally red in color and containing grog. It is the common body type for ceramic sculpture.

Throwing- forming plastic clay on the potter's wheel.

Vitreous- pertaining to the hard, glassy, non-absorbent quality of a body or glaze.

Warping- distortion of the form in drying or firing.

Wedging- kneading plastic clay with the hands in a rocking spiral motion which forces out trapped air and develops a uniform texture.