Ceramic Dictionary

• Raku : Originally a name used by a Japanese family that made tea ceremony ware. Now refers to the process of raku firing and to ware glazed in such a firing. Raku ware is often reduced after firing by burying it in combustible material, then covering it with an airtight lid. This reduction atmosphere aids in producing luster or opalescent colors.
• Reduction : 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-fired stoneware, porcelain, raku, and lusters.
• Refractory : Resistance to heat and melting. Refractory materials are used in porcelain and stoneware. Also used for building kilns and kiln furniture and in combustion with other materials, as kiln insulation.
• Relief : The projection of forms from a flat background in sculpture and decoration. The terms high relief and low relief describe the amount of projection above the background.
• Resist : A methods of applying a covering material such as wax, latex, stencils, or masking tape to bisque or glazed ware, then coating the piece with a slip, a glaze, or a second glaze. The resist material will not accept the added layer so that on firing, the color of the covered area will remain intact.
• Rib : A curved tool made of wood, metal, or plastic, used for shaping, scraping, or smoothing clay objects.
• Ribbing : use of a rib to shape moist clay, or scrape damp or dry clay.
• Rim : The edge of the opening of a pot.
• Salt glaze : A glaze formed by introducing salt into a hot kiln. The vaporized salt combines with the silica in the clay body, forming a sodium silicate glaze on the surface. Salt glazing releases noxious and toxic fumes, so many potters now use alternate forms.
• Scoring : Scratching or otherwise roughening the edges of damp or leather-hard clay before joining them.
• Secondary clay : Natural clay that has been moved by water or wind from its source and settled elsewhere in deposits.
• Sgraffito : Decoration of pottery made by scratching through a layer of colored slip to the differently colored clay body underneath.
• Shrinkage : The loss of volume in clay as it shrinks in drying and firing. Shrinkage varies from 7 percent to 20 percent from wet clay to fired clay, depending on the clay.
• Sieve : A utensil of wire mesh used to strain liquids or powder materials.
• Silica : Oxide of silicon. Found in nature as quartz or flint sand, it is the most common of all ceramic materials.
• Silicate of soda : A solution of sodium silicate that is used as a deflocculant to help in the suspension of clay materials in slip.
• Slab roller : A mechanical device for rolling out slabs to a set, consistent thickness.
• Slip : A suspension of clay in water used for casting pottery or sculpture in molds. Slip can also be used for painted decoration or for the sgraffito technique.
• Slip casting : Forming objects by pouring slip into a plaster mold. The mold absorbs the water in the slip so that the solid clay walls are formed to create a positive of the original.
• Slip glaze : A glaze that contains a large proportion of clay. Generally one that contains enough flux to form a glaze with few or no additives.
• Slip trailer : A rubber syringe used to apply decorations of slip on ware.
• Slurry : A thick, creamy mixture of clay and water.
• Spray booth : A ventilated booth that removes chemicals and fumes from the air so that the person spraying does not inhale them while spraying glazes, underglazes, or overglazes.
• Spray gun : A gun-like device through which compressed air passes, forcing the air into a fine mist for application. Used for spraying glazes.
• Spraying : In ceramics, a method of applying glazes using a spray gun.
• Sprigging : The process of applying low-relief decorations of damp clay onto already formed greenware.
• Stains : Commercially processed and refined raw chemicals which yield ceramic stains and offer a wide range of shades for coloring and clay and glazes. They are generally more color stable than oxides.
• Stilts : Triangular supports with either clay or heat-resistant metal points, used to support pieces of glazed pottery above the shelves during glaze firing to keep the glaze from sticking the ware to the shelf. Small stilt marks can be filled, sanded, or ground smooth.
• Stoneware : A type of clay body fired to a temperature at which the body becomes vitrified, dense, and nonabsorptive, but not translucent. Usually matures at temperatures above 2192 degrees F.
• Temper : Any material, such as sand, mica, or grog, added to a clay body to make it more porous and less likely to shrink and warp.
• Tenmoku : High-fired, saturated iron glaze; black, brown, and yellowish. Used by the Chinese and Japanese, especially on tea ware. Still a popular glaze.
• Terra-cotta : A low-fire, porous, reddish clay body, frequently containing grog and other temper. Used throughout history for common, utilitarian ware; also used for sculpture.
• Terra sigillata : A fine slip glaze. Used by the Greeks, Etruscans, and Romans to coat their pottery. Now formulated into a wide variety of colors.
• Test tiles : Small tiles made of clay used to test clay bodies in the kiln or to test glazes on a specific clay body.
• Thermal shock : The stress to which ceramic material is subjected to when sudden changes occur in the heat during firing or cooling.
• Throwing : Forming objects on the potters' wheel using a clay body with plastic qualities.
• Trailing : A method of throwing in which slip or glaze is applied using a syringe.
• Trimming : In ceramics, this refers to trimming excess clay off the foot and sometimes the body of a pot to refine the shape.
• Undercut : A negative space in a solid form, creating an overhang. Casting a form with undercutting requires a multi-part mold to release the mold from the cast.
• Underfire : To fire clay or glaze-accidentally or deliberately-to a point below its maturing point. Underfiring can turn a normally glossy glaze into a matte finish.
• Underglaze : Any coloring material used under a glaze. The color can be provided by commercially prepared glaze and clay body stains.
• Updraft kiln : A kiln in which the heat goes up through the chamber and is vented through the top of the iln.
• Vitreous : Having the nature of glass. In ceramics, a glaze or clay body that has been fired to a dense, hard, and nonabsorbent condition. High-fire glazes vitrify and combine with the glassy particles that form in the high-fire clay body as it approaches vitrification. This results in a glaze that is united with the clay body as compared to a low-fire glaze that merely coats the glaze.
• Warping : Changes in the form of a clay body. Warping of ware can occur during drying or firing if the walls are built unevenly or if drying or firing is too rapid and uneven.
• Wax resist : A method of decoration in which melted wax or oil emulsion is painted onto the clay body or onto a glazed piece.
• Wedging : Anyone of various methods of kneading a mass of clay to expel the air, get rid of lumps, and prepare for a homogenous material.
• Wedging table : A table of plaster, wood, or concrete, often covered with canvas, on which clay can be wedged. A stretched wire attached to the table allows one to cut the clay to check for air bubbles, lumps, or lack of homogeneity.