Ceramics Exam

KILN WASH

A coating of refractory materials (half flint and half kaolin) painted onto the kiln floor and the top side of shelves to keep the melting glaze from fusing the ware onto the shelves.

POTTERS WHEEL

A device with either a manual (foot powered) or an electric rotating wheel head used to sit at and make pottery forms.

REDUCTION (REDUCING FIRING, REDUCTION ATMOSPHERE)

A firing in which insufficient air is supplied to the kiln for complete combustion. Under these conditions, the carbon monoxide in the kiln combines with the oxygen in the oxides of the clay body and glaze, causing the oxides to change color. Commonly associated with high-fire stoneware, porcelain, raku, and lusters.

KILN

A furnace or oven built of heat-resistant materials for firing pottery

TERRA-COTTA

A low-fire, iron rich, porous, reddish clay body, frequently containing grog and other temper. Used throughout history for common, utilitarian ware; also used for sculpture.

PUG MILL

A machine used to blend clay into a moist, workable consistency. Also used to recycle clay scraps and, when equipped with a vacuum pump system, to de-air clay.

PORCELAIN

A translucent, nonabsorbent body fired at high temperature. White and hard, it was first developed in China. Traditionally fired in the 2370° to 2640°F/1300° to 1450°C range, some porcelain bodes have been developed that mature in the 2230° to 2340°F/1220° to 1280°C range.

STONEWARE

A type of clay body fired to a temperature at which the body becomes vitrified, dense, and non-absorptive, but not translucent. This is usually brownish in color because of the presence of iron, but there are formulated white stoneware bodies. Usually matures at temperatures above 2192°F/1200°C.

CLAY BODY

Any blend of clays and nonplastic ceramic materials that is workable and that has certain firing properties. Clay bodies are formulated to serve particular purposes and to achieve maturity at various firing temperatures. See earthenware, stoneware, and porcelain.

WEDGING

Any one of various methods of kneading a mass of clay to expel the air, get rid of lumps, and prepare a homogenous material.

TRIMMING

At the leather-hard stage, removal of excess clay from a piece, using any of a variety of sharp cutting tools.

SLIP

Clay suspended in water, usually the consistency of thick cream. May be colored and used to decorate surfaces, or may be cast into plaster molds to create ceramic forms

LEATHER-HARD

Condition of clay where it has stiffened but is still damp. Point at which pieces are joined and most surface modification and trimming are done. Soft-leather-hard ideal for forming, joining, thick slip-decoration. Medium-leather-hard good for thin slip-decorating, joining, incising, carving, piercing. Hard-leather-hard good for thin slip-decorating, carving, scraping.

THROWING

Forming objects on the potter's wheel using a clay body with plastic qualities

HANDBUILDING

Forming plastic clay by hand without the wheel, using pinching, coiling, and/or slab construction.

CRAWLING

Glaze fault where glaze recedes away from an area in the firing, leaving bare clay. Usually caused by dusty, dirty, or oily surface beneath glaze or by excessively powdery glaze. In some cases results from very high L.O.I. in glaze materials, causing high glaze-shrinkage and resulting cracking during firing. Used intentionally in controlled crawl and beading glazes.

POTTERY

Originally a term for earthenware, now loosely used to refer to any type of ceramic ware, as well as to the workshop where it is made.

EARTHENWARE

Pottery that has been fired at low temperature (below cone 2) and is porous and relatively soft. Usually red or brown in color. Used worldwide for domestic ware, glazed or unglazed.

BURNISHING

Rubbing leather-hard or dry clay with any smooth tool to polish it, tighten the clay surface, and compress the clay particles.

PYROMETRIC CONES

Small slender pyramidal-shaped indicators made of ceramic material formulated to bend at a specific temperature - standard method for determining maturing temperature of firing. Like clay and glazes, cones respond to temperature, duration, and atmosphere of firing, far more accurately than mechanical measurement.

PLASTICITY (PLASTIC CLAY)

The ability of a damp clay body to yield under pressure without cracking and to retain the formed shape after the pressure is released.

OXIDATION (OXIDIZING FIRING)

The firing of a kiln or open fire with complete combustion so that the firing atmosphere contains enough oxygen to allow the metals in clays and glazes to produce their oxide colors. Electric kilns always produce oxidizing firings unless reducing materials are added. Bright and clear low-fire colors are often associated with glazes and clays fired in an oxidation atmosphere.

VITREOUS / VITRIFICATION

The formation of a glass and the stage in firing at which the clay particles actually begin to form glassy melts. Thus flowing into the pores between other particles and the fluxing process continues. On cooling, it produces a less porous, harder body. Extreme vitrification results in melting of the clay to such an extent that the pot collapses. Vitrification occurs at relatively high temperatures, which vary according to the types of clay minerals present. However, it rarely occurs below 900°C and consequently is usually not seen in prehistoric pottery.

THERMAL SHOCK

The stress to which ceramic material is subjected when sudden changes occur in the heat during firing or cooling.

TERRA SIGILLATA

Ultra-refined clay slip which can give a soft sheen when applied to bone-dry wares, and if polished or burnished while still damp may give a high gloss. All ancient Greek red-black pottery and Roman red wares were finished with this technique, without the use of glaze.

GREENWARE

Unfired pottery or sculpture. This has the consistency of chalk and is extremely fragile.

BISQUE (BISQUIT) {BISQUE WARE}

Unglazed ceramic ware that has been fired at a low temperature to remove all moisture from the clay body and to make handling easier during glazing.

CRAZING

Very fine surface cracks in fired glaze surface - technically a fault in glazed wares, but often sought-after, especially in raku.

SGRAFFITO

a technique either of wall decor, produced by applying layers of plaster tinted in contrasting colors to a moistened surface, or in ceramics, by applying to an unfired ceramic body two successive layers of contrasting slip, and then in either case scratching so as to produce an outline drawing.