The Official Seattle Pottery Supply Company Pottery Glossary
As with every area of specialization, pottery has it's own terminology. In the interest of opening up the joy of working with clay to everybody, we want to make it as simple as possible - so we've put together a glossary of terms.
Are we missing something you want to know about? Email us to let us know and we'll add it!
Bisque/ Biscuit Firing: A firing given to greenware when it is dry and before it is glazed. Bisque firing temperature is lower than the temperature of the second (glaze) firing. Bisque is fired only to between cones 08 and 06 (between 1720 and 1835 degrees F or 945 and 1005 degrees C).
Burnishing: A process where the surface of a pot is polished before firing until the clay is compacted and is smooth and glossy. A hard object like a smooth stone or the back of a spoon is usually used.
Bone Dry: Dried clay that is ready to be bisque fired; it is the most fragile stage of clay. When held, bone dry greenware should feel at room temperature, not cool to the touch. Coolness indicates that evaporation is still taking place. It can also be underglazed at this stage.
Celadon: A reduction-fired stoneware glaze, in varying shades of green-grey, depending on the amount of iron used in the glaze. Celadon glazes originated in China.
Chun: A chinese stoneware glaze that fires to a pale blue in reduction. When used over an iron-bearing glaze such as a tenmoku, the blue colour is far richer and stronger. The opalescent blue color of the glaze is caused by the way the light breaks up in the glaze and not by the addition of cobalt oxide.
Cone: A three-sided, pyramid-shaped object that is made of materials similar in composition to a glaze. Cones are used inside the kiln to measure the heat of the kiln, and will soften and bend over when the heat work has been done and a specific temperature has been reached.
Crawling: Caused by a high index of surface tension in the melting glaze. It is triggered by adhesion problems, often caused by bad application. It occurs where a glaze is excessively powdery and does not fully adhere to the surface of the clay. You’ll notice the molten glaze withdraws into 'islands' leaving bare clay patches.
Crazing: Characterized as a spider web pattern of cracks penetrating the glaze, it is caused by tensile stresses greater than the glaze is able to withstand. This tension occurs when the glaze contracts more than the body during cooling. Because glazes are a very thin coating, most will pull apart. Often times seen in Raku firing.
Earthenware: Pottery that, when fired, has a porosity of more than 5%. The maturing temperature of the clay and glazes is below 1200 C. It is relatively soft and so, for practical use, must be glazed to make it waterproof. The clay body is available in white, buff, or red.
Flameware: Pottery that can withstand sudden temperature changes without cracking
Flux: An oxide that lowers the melting or softening temperature of a mix of materials. Fluxes often melt poorly on their own but react strongly with high- melting materials. In ceramic glazes, the most common fluxing oxides include lead, sodium, potassium, lithium, calcium, magnesium, barium, zinc, strontium, and manganese.
Frits: An ingredient found in, primarily, low-fired glazes. They are made up of glaze materials that are melted, solidified and then ground into a fine powder. Frits act as a flux in the glaze recipe (to help the glaze melt). When two materials such as lead and silica are melted together they form a lead frit, which renders the lead non-toxic and safe to use. When a soluble material is fritted it becomes insoluble when used in the glaze.
Glaze Fire: Firing of the clay and glaze to make a glass like surface
Greenware: Greenware is unfired pottery. It is very fragile. Greenware may be in any of the stages of drying: wet, damp, soft leather-hard, leather-hard, stiff leather-hard, dry, and bone dry. This term is given to clay objects when they have been shaped but have not yet been bisque fired, which converts them from clay to ceramic.
Grog: Ground pottery or coarse sand which is added in varying percentages to the clay body. Grog helps “open” the clay so that it dries more evenly and reduces the shrinkage rate of the clay during the drying and firing - which helps keep the clay from warping. It also provides a texture to the clay.
Jiggering: A technique of using a die (shaping tool) to mechanically shape the clay when wheel throwing. The die, usually attached to a jigger arm, is used for mass production or duplication of the same shape. Jiggering specifically refers to shaping the outside of a plate.
Jollying: A similar technique to jiggering, but in this case refers to shaping the inside of the plate. A die (or shaping tool) is used to mechanically shape the clay when wheel throwing. The die, usually attached to a jigger arm, is used for mass production or duplication of the same shape.
Kiln: An oven which can withstand high temperature that is used by potters to fire pottery. There are many different types of kilns and different fuels are used to build up heat in the chamber where the pots stand.
Leatherhard: Clay that has dried to a stage where it is firm enough to handle but still contains enough moisture that pots can be turned, and knobs or handles can be joined without cracking off. Most of the shrinkage that occurs during the drying process has occurred by the leatherhard stage.
Oxidation: Oxidation occurs when a kiln is fired without removing the oxygen, as in an electric kiln. The oxygen attaches itself to the surface of the pots and interacts with the material in the clay and the glaze. This means that certain elements change color, often becoming brighter and more colorful. For instance, copper gives a green color.
Pinholing: A glaze defect where tiny holes present in the fired glaze surface. The holes normally go down to the body surface below the glaze; in a tile or utilitarian item, this can make it a reject.
Pit Firing: Method of “baking” clay by digging a hole in the ground, about 18”-24” deep, and covering the pots with wood shavings, sawdust, or wood chips, then setting it on fire. You can also use dried grass clippings and dried leaves.
Plucking: A firing error, in which vitreous ware (typically glaze) sticks to the kiln shelf. Removing the pottery leaves sharp fragments glued to the shelf. Applying kiln wash can help prevent plucking.
Porcelain: Porcelain is a high-fired, white vitreous ware with translucent qualities, the level of translucence depending on how thin the clay is worked. True porcelain requires a high temperature of 1300C (2372F) or more for the clay body and glaze to mature and interact. This is called a hard-paste porcelain. An imitation porcelain, or soft-paste porcelain, requires a much lower temperature for the body and glaze to mature.
Raku: A Japanese low-firing method that usually involves removing pottery from the kiln while at bright red heat, and placing it immediately into containers filled with combustible materials.
Reduction: A low-oxygen atmosphere created in the kiln when firing with wood, oil or gas. Not enough oxygen is available in the kiln to complete the combustion of the flame so oxygen is drawn out of the metal oxides present in the pots, often causing rich, organic, earthy colors. For instance, it causes iron to turn green and copper to turn a rich red.
Rutile: A mineral composed primarily of titanium dioxide but also with a small percentage of iron. When used in glazes, colors ranging from creams to oranges and yellows can be produced. The presence of rutile causes a crystallization effect which gives a broken, mottled coloring to the glaze.
Saggar Firing: A firing process where one fires a pot within another clay chamber.
Score: To score a pot or piece of clay means to scratch hatch marks on it as part of joining clay pieces together. This is done before brushing on slurry and joining the pieces together.
Sgraffito: A decorating technique where a design is scratched through a layer of colored slip, generally with pointed tools, to reveal the contrasting color of the clay underneath.
Slip: a liquefied suspension of clay particles in water. Slip is usually the consistency of heavy cream. Slip may also be used for casting clay in plaster molds.
Slip Casting: Using slip (see above) to make casts using plaster molds.
Sludge/Slurry: The thin, sloppy, wet mixture of liquid and solid components from clay.
Soda Firing: A method of atmospheric firing in which sodium (usually soda ash, soda bicarbonate, or borax) is vaporized in the kiln at cone 8 or above. The sodium combines with the silica and other elements in the clay to form a basic glaze.
Stains: These are coloring oxides that have been stabilized and are used to color clay and glazes.
Stoneware: Stoneware is a vitrified non-porous ware, making it excellent for functional ware. The clay body and glazes are fired to about 1200C (2194F); high-firing stoneware can be fired up to 1366C (2491F). Stoneware clay is available in various colors, including white, buff, or red.
Tenmoku: An old Chinese stoneware reduction-fired glaze. Tenmoku glazes contain a high percentage of iron oxide (up to 10%) and are brown to black in color, but break up rust red and orange on edges and throwing rings.
Thermocouple: The temperature sensors used to measure temperature in electric kilns.
Wedging: The act of kneading the clay to make the clay homogenous, to align the particles, and to remove air pockets.
Handbuilding: pinching, coiling, slab building; essentially building without using a potter’s wheel.
Wheel Throwing: Using a potter's wheel to shape round ceramic ware. A potter’s wheel is a disc that turns while the potter shapes the clay.