A Glossary of Ceramic Terms

bisque. Once-fired but unglazed clay. Derives from the French "bisquet" meaning half-baked.

bisque fire. First firing of ware usually at a low temperature (012-04) to drive off water and harden ware so as to facilitate glazing.

clay. A natural, fine-grained earthlike material, the product of the geological weathering or aging of the surface of the earth. The root of the word "clay" is "sticky": sticky soil.

clay body. Generally refers to a combination of clay ingredients calculated to mature at a desired temperature and to have desired working characteristics.

coiling. A method of hand-building pottery in which the clay is rolled out into long, narrow ropes of clay that are placed one on top of another and joined to build up the form. Either the coils are left visible or the joints are smoothed over.

cones (Pyrometric). Made of ceramic materials, these cones are placed in a kiln where they can be viewed by the potter through spy- or peepholes in the walls or door of the kiln. They are calculated to measure the heat work of the firing (temperature and duration) and are graded to soften and melt to indicate to the potter that his clay and glazes have reached their maturity. Final temperature is designated by cone number, e.g., cone 4, cone 9.

earthenware. Generally refers to a low-fired clay. More specifically, it is a non-vitreous clay with an absorbency of from 5 to 20 percent.

engobe. A slip, usually with colorants added, that is halfway between a clay and a glaze. It is usually brushed over the surface of a pot to harden the surface, change the texture, or alter the color of the clay. It can be fired as is or glazed.

fire clay. A refractory clay used in the manufacture of bricks, muffles, saggers, etc. It is often plastic enough to be used by potters as an ingredient in stoneware clay bodies.

flux. The melting agent in a glaze.

glaze. A liquid suspension of fine mineral particles that is applied to pottery and fired to its maturity to form a glassy surface that seals the clay and decorates the piece.

greenware. Unfired pottery; also called raw-ware.

grog: Clay that has been fired and crushed in a variety of mesh sizes. It is added to clay to reduce shrinkage and to add texture and/or tooth.

kiln. A furnace for firing pottery made of refractory and insulating materials.

leather-hard. Refers to that state in the drying of a raw pot when enough moisture has air-dried so that the piece can be lifted without distortion and yet is damp enough to be worked further; carved, burnished, joined, etc.

maturity. That point in a firing where the clay has reached its maximum non-perosity and hardness and when the glaze has flowed and formed a strong bond with the clay.

oxide. A compound containing oxygen and one or more elements.

oxidation fire. A firing condition when the fire has sufficient oxygen to cause complete combustion free of carbon or carbonaceous gases.

plasticity Refers to that quality in a clay that allows it to be worked and reshaped without cracking or crumbling.

porcelain. A hard, totally vitreous clay, generally fired at high temperatures. It is usually white or gray and free of impurities. In some cases, when thin, the clay will be translucent.

raw-ware. Refers to unfired and dry pottery.

reconstituted or recycled clay. Used but unfired clay that has been allowed to dry and is then reliquefied for reuse.

reduction fire. A firing condition in which the amount of oxygen mixing with the fuel is reduced so that the carbon in the fuel must seek out and combine with the oxides in the clay and glazes to combust. This causes changes in color and texture of the clay and glaze.

saggers. Boxlike containers made of a highly refractory fire clay used to house pots in firings so that they will be protected from direct contact with the flame.

salt glaze. A glaze surface that forms on pots by introducing rock salt into the kiln at a high temperature. The salt volatilizes and combines with the silica in the clay to form sodium silicate.

slab. A method of hand building with a great variety of uses in which the clay is either rolled out with a rolling pin or sliced with a wire or tossed into sheets that are then used to construct a form.

slip. A clay in liquid suspension used decoratively or as a binding agent. Clay slips often have oxides added to them for decorative purposes.

stoneware. High-firing clay with little or no rate of absorbency. Closer to porcelain than earthenware, it is more plastic and depends upon its impurities for its color and texture.

throwing. The act of forming clay on the potter's wheel.

vitreous. Refers to the non-absorbency of a clay or a glaze.