Glossary of Ceramic Terms

Introduction to Ceramic Terms and Definitions

Compiled by Ken Turner

*Bisque or Biscuit:  The first firing a pot undergoes to prepare it for glazing. In industry this is done at a higher temperature than the subsequent glaze firing, the reverse is usually true of the studio potter.

Burnishing:  A technique where the Leather hard clay is polished with a hard instrument to force the smallest clay particles to the surface creating a soft sheen. This surface remains after the pot is fired so long as the firing temperature is kept below 1100^oC.

Calcine:   To purify a material through the action of heating to red heat 700–750 ^oC (1292-1382 ^oF)

*Ceramic Change:   The slow process of clay becoming ceramic.  Clay which is exposed to heat 600^oC / 1112^oF, losses its chemically bound water molecules and can no longer be broken down by water. Once this change has occurred it cannot be reversed. (See Firing and Water)

Clay:  AL₂O₃ 2SiO₂ 2H₂O. The decomposition of Granite through the process of Kaolinization creates clay (see Kaolinization). Clay is a mineral with a plate (platelet) like structure; it is these plates, (about 0.5 microns across) when lubricated with water, slide against each other to form the plastic mass we know as clay (see Water). 'Primary ' clays are those found close to the area of Kaolinization and hence the purest (Kaolin or China Clays). Secondary clays are those moved by water away form the site of Kaolinization and get progressively more plastic and less pure (Ball Clays, Fire clays, Earthenware's and Marls).

*Clay Body:   A clay designed for a special purpose.  It is created by blending different clays of by adding to clays other materials, such as feldspar and flint in order to produce a desired workability, maturing temperature, or finished result.  A clay body is the result of mans technology.

Cobalt:  One of the strongest coloring oxides used by the potter. Cobalt creates a dark dense royal blue in most cases. Historically used in China as a painting pigment on Blue and White wares.

*Coiling:   A method of handbuilding a form using long rolled out, or extruded, snake-like lengths of clay.  Each coil of clay is integrated with the previous one to build the work up.  The coils may be completely obliterated in the construction process or retained for their decorative qualities.

*Cones:  Pyrometric cones are composed of clay and glaze material, designed to melt and bend at specific temperatures. By observing them through a small 'Peep Hole' in the kiln it is possible to ascertain the exact conditions in the kiln. Cones are a better indicator than temperature alone as the degree of glaze melt is a combination of time and temperature (“heat work”), thus a fast firing needs to go to a higher temperature to get the same results as a slow firing to a lower temperature.

Crackle:   Decorative craze lines in a glaze.

*Craze:   The formation of a network of cracks in a glaze.

Crystalline glazes:  Most glazes have no easily visible crystal structure.  Crystalline glazes have large and dramatic crystals up to about three inches across. A high alkaline low alummina glaze is vital for crystals to develop. Additions of zinc and titanium also help seed the crystals. An extremely slow cool of the kiln is necessary, to allow the crystals to grow. Because of the low alumina content in crystalline glazes they are very runny, often pots are supported in the kiln on stilts to avoid them adhering to the kiln shelves, the stilts can be broken off after the firing.

Deflocculate:   To disperse the particles in a slip so that less water is required to make the slip fluid.

*Dunting:  Cracks, which occur on pottery during the heating or cooling cycle of the firing. They are usually caused by the silica inversion at 573^oC, 1063^oF (Alpha to Beta phase) or the Crystobalite (one of the 'phases' of silica) inversion at 226^oC, 428^oF in both cases there is an expansion and contraction of around 2-3% in the heating and cooling cycles.

*Earthenware / Terracotta:  A lowfired form of pottery or objects (below 1100^oC, 2012^oF) made from fire clay, which is porous and permeable. The clay can be any color although iron red is usually associated with Terracotta. The low temperature vastly expands the range of glaze colors available these are often alkaline or lead based.

Enamel:  A form of low temperature glaze that is applied on top of an already fired higher temperature glaze. Enamels are often lead based, as it is a flux, which works at a low temperature.

Feldspar:  One of the predominant naturally occurring fluxes used primarily in stoneware glazes. The three most commonly used feldspars are Potash feldspars K₂O Al₂O₃ 6SiO₂, Soda Feldspars Na₂O Al₂O₃ 6SiO₂ and Lithium Feldspars Li₂O Al₂O₃ 8SiO₂. Feldspars can be the only flux present in a stoneware glaze although this is uncommon and additions of calcium usually supplement it.

*Firing:  The process, which changes clay into ceramic. Up to 600^oC / 1112 ^oF the chemically bonded water in clay is driven off (AL₂O₃ 2SiO₂ 2H₂O - AL₂O₃ 2SiO₂). This is an irreversible change know as the “Ceramic Change”, (See Clay and Ceramic Change)

Flocculate:  To thicken.

Flux:   A substance, which causes or promotes melting.

Frit:   A combination of materials that have been melted into a glass, cooled, and reground into a powder prior to being added to a glaze recipe.

Fuse:   To melt.

*Greenware:   Unfired pottery.

*Grog:  Clay that has been fired and then ground into granules of more or less fineness.  Grog is considered a filler, and added to clay bodies for several reasons; it helps open a tight or dense body, promotes even drying, which reduces warping and cracking, and reduces overall shrinkage.  Grog also adds tooth and texture to a clay body aiding in the ability of the body to maintain its form during construction.

*Glaze:  A super cooled liquid, with a random molecular structure and high viscosity at normal temperatures, super cooling is relative to geological cooling. A random molecular structure is the result of fast cooling so crystals cannot develop (the exception being crystalline glazes). Granite cools slowly (geologically speaking) so we can easily see the crystals in polished granite, glaze cools quickly so the molecules do not have a chance to crystallize. A high viscosity means it does not run off the pot. (Well it may in the kiln but not in the kitchen!)

Inlay:  A decorative technique where a pattern is carved into the clay at the leather hard stage and contrastingly colored soft clay is forced into the decoration. When the clay is a little drier the excess is scraped of to reveal the pattern.

Iron Oxide:  Fe₂O₃ is one of the potters favorite colorants, when combined with the right glaze and firing iron oxide can produce greens, browns, blacks, yellows, oranges, subtle blues and grays. Most of the best color responses for Iron in a glaze need a reduction firing. Iron is also a useful colorant in clay bodies and is best introduced by adding high iron clays to the clay recipe.

Kaolin:  A china clay in its purest form Al₂O₃ 2SiO₂ 2H₂O. Primary clay.

Kaolinization:   The natural formation of kaolin from the decomposition of feldspar.

*Kiln:  Basically an insulated box, which is heated to fire pots in. They can be either, cross draft, down draft, or up draft. The draught refers to the direction the combustion gasses have to travel from input to exit flues, since no combustion takes place in an electric kiln there are no input or exit flues and they are genuinely heated boxes. The fuels used to heat a kiln are gas, oil, wood, coal (now almost obsolete) and electricity. Each fuel source used to fire a kiln offers different possible outcomes for the pots fired in them. The maximum operating temperature for most pottery kilns is about 1300^oC, 2372^oF, although many woodfired kilns may be fired up to 1350^oC, 2462^oF.

*Kiln wash:   A refractory mixture, usually kaolin or flint, which is painted on kiln shelves and saggers to prevent glaze from adhering.

*Leather Hard:  A stage in the drying process of clay when the clay is pliable but strong enough to handle. It is ideal for trimming and the addition of appendages such as handles and spouts. Relatively wet clay can be attached to the pot at this stage and the resulting bond will not form cracks.

*Mature:   Clay that has been fired to a tight, hard, serviceable structure.

Once Fired:  A pot that has undergone a single glaze firing. The glaze is applied directly on to the dry or leather hard pot thus avoiding the bisque firing. This approach, although offering certain economic and aesthetic advantages, can create technical problems for the potter.

Open:  To make a clay more open or porous in structure by adding fillers or grog.

Oxide:  Any element combined with oxygen.

Oxidation:  A firing where there is either no combustion occurring (electric kiln) or where there is sufficient oxygen in the kiln to allow the fuel to burn cleanly. The atmosphere of the kiln (oxidation, or reduction) dramatically affects the resulting clay and glaze colors, for example; copper in oxidation is green (as is copper oxide) in reduction it becomes red (more like copper metal).

*Pinching:  A method of forming clay, which is well described by its name.

Plaster:  2CaSO₄ 2H₂O. An invaluable mold-making tool for the potter, also used extensively in industry. It can be poured or carved into virtually any shape. When it is dry it can be used to press clay into or to slipcast with.

Plasticity:  The properties of a material that allow it to be shaped and to retain its shape.  The plastic properties of clay are principally determined by the size of the platelets.  The smaller the platelets the more plastic the clay is. Aging or souring is also relevant to a clays plasticity; with time bacterial action creates a colloidal gel, which aids the lubrication of the platelets.

Porcelain:  A white highly vitrified clay body that is translucent where thin (often fired up to 1350oC, 2462^oF). The translucency is a result of silica glass fused into the fired clay. To achieve this a high amount of flux is added to a kaolin based clay body. The flux to clay ratio is often flux > clay, indeed some of the original Chinese porcelains had as little as 20% clay like minerals. The low clay content makes porcelain very difficult to throw and trimming wares is almost unavoidable. At the home of porcelain, Jingdezhen (China), all the pots are throw in small thick sections, joined and trimmed. Accurate trimming is regarded as more of a skillful art than throwing. The plasticity of porcelain can be improved by small additions (2%) of white bentonite.

  Raku:  Originally a Japanese seal given to a prominent family of potters (1598) who developed the technique. The term describes a lowfire form of pottery where the pots are removed from the kiln as soon as the glaze has melted and then left to cool or doused with water. In the mid 20th century Paul Soldner introduced the now popular process of post firing reduction. In this case the red hot pot is placed in a lidded bin filled with straw or sawdust. The glazes are dramatically altered by the reduction particularly noteworthy are the colors achieved with Copper.

Reduction:  Also see Oxidation; A situation where too much fuel is introduced into the kiln to be able to burn with the available oxygen, consequently oxygen is 'stolen' from the pots in the kiln, it affects the clay and the glaze color. A good example is iron, which changes from Fe₂O₃ to FeO, even the tiny amount of iron present in porcelain changes it hue from a creamy color in oxidation to a slight gray blue in reduction.

*Refractory:  Resistant to heat.

*Sgraffito:  A decorative technique, where by the surface of the clay is scratched, often to expose another layer of colored clay.

  Shards:  Pieces of broken pottery.

  Shelling:   Flaking, Peeling. A glaze or glaze and slip defect in which the glaze falls from the body in flakes.  It is caused by insufficient bond between glaze and body, usually the result of underfiring.

  Shivering:   A defect in which the fired glaze pulls away from the ware taking some of the body with it in the form of slivers.  This generally occurs on sharp rims and edges of handles, and is due to improper glaze fit caused by too much compression by the body.

  Short:  Nonplastic clay poor in working qualities.

  *Shrinkage:  The decrease in the size of a clay object due to drying and firing.  Dry shrinkage is reversible with the return of water, but firing shrinkage is permanent due to chemical and physical changes clay undergoes when exposed to heat (see ceramic change).

  Silica:  The primary glass forming oxide used in pottery. Boron is the other glass forming oxide used although more commonly as a flux than as a glass former due to its low melting point (577^oC, 1063 ^oF). A glass forming oxide must be present in any glaze and as silica’s melting point is 1800^oC, 3272 ^oF, a flux is always present to reduce the melting point to a workable range. Pure boron glasses are water-soluble so of little use but Boro-sillicate glasses have a very low thermal expansion and are the main constituent of 'Pyrex' etc.  Also see Dunting.

Slake:  To moisten dry clay with water.

*Slip:  A fluid suspension of clay with and water, with a “cream” like consistency. Most often colored with oxides and painted or poured onto pots for decoration.

Slipcasting:  Plaster molds are filled with a deflocculated slip; deflocculation reverses the electric charges in the clay particles, which reduces the water content in a slip to that of most plastic clays, around 30% of total weight. A common deflocculant is Sodium Silicate. The plaster absorbs sediment of clay leaving the remaining moisture over the entire interior surface of the mold. The excess slip is drained off and the cast can be removed from the mold soon after. This approach is used widely by industry and some studio potters.

Slipware:  A traditional English decorative technique associated with red earthenware and lead glaze. Colored slip is piped onto the leather hard pot much like cake decoration. The most noted exponent of slipware was the 18-century potter “Thomas Toft”; his dishes set a standard that few modern potters can compete with.

Soak:  To hold a kiln at one temperature for an extended period of time.

Soluble:  Capable of being dissolved in water.

Stains: A suspension of metallic oxides, clays and other materials with water, used to add color to the surface of clay and glaze.

*Stoneware:  Highly vitrified ceramics fired to above 1200^oC, 2192 ^oF. Most of the silica in a fired stoneware body is melted into a glassy matrix and the resulting body is of high density and usually has a water absorption rate of less than 1%.

  Terra Sigillata:  A slip comprised of the smallest particles of clay, which consequently resembles a burnished surface. The technique was used to impressive effect in the Greco-Roman period.

*Throwing:  To make pottery by hand on the potters wheel.  A delicate balance, which defies gravity and centrifugal force as clay is coaxed up by hand from a spinning turntable.

*Trimming, or Turning:   Certain forms made on the potter’s wheel will not support themselves unless excess clay is left at the base, alternatively, extra definition on the foot of a pot may be needed. The solution to both these problems is turning, which is done at the leather hard stage. The pot is inverted onto a potter’s wheel and a metal cutting tool is applied to the bottom of the pot until the desired finish is achieved.

  Underglaze:  Ceramic colors combined with clay applied under a glaze, usually a clear glaze. Although a durable method of decorating, colors can run especially if colorants, which double as fluxes, are used, however more dependable than overglaze stains.

  Viscosity:  The ability of a liquid to flow, the term is used by the potter in relation to molten glazes, glaze suspensions, and slips.  A stiff molten (liquid) glaze is one of high viscosity, while a runny molten (liquid) glaze is one of low viscosity.

  Vitrification:  The degree of melt in a clay body as the silica forms a glass with fluxes present. See stoneware.

  Water:  H₂O. A most important part of clay and is also used to suspend the particles of a glaze prior to application. The water in clay takes three forms; Water of plasticity, Pore Water and Chemically bonded water. Water of plasticity lubricates the clay platelets. The pore water is water of plasticity that is trapped during the drying process in between platelets of clay. Pore water can cause problems in the bisque firing even if it appears the pot is bone dry and caution must be taken up to around 150^oC, 248 ^oF (Water smoking period). The chemically bonded water is driven off up to around 600^oC, 1112 ^oF trailing off at 700 ^oC, 1292 ^oF henceforth it becomes ceramic. (The theoretical formula for clay is Al₂O₃ 2SiO₂ 2H₂O and becomes AL₂O₃ 2SiO₂  upon the ceramic change) See Clay, Firing, and Ceramic Change.

  *Wax Resist:  A decorative technique where a wax based medium is used to create a pattern, which is then covered, in another coat of glaze or slip. The wax resists the subsequent coating creating the pattern. Paper stencils or tape can create a similar effect. Latex is another effective resist with other advantages.

  *Wedging:  To kneed or mix plastic clay by hand.  A hand process used to homogenize the clay and remove air bubbles, thus making it workable. The techniques for wedging are called; Spiral, or Chrysanthemum wedging, Rams head, or Monkey face wedging and wire/slab wedging. Both Rams head and Spiral wedging involves the folding of the clay on itself too build up an ever-tightening spiral of clay platelets. Wire wedging builds up increasing layers of clay platelets and is the best for introducing other clays and fillers into an already plastic clay body.

 * Designates terms you will be tested on.

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