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Ceramics Terms Glossary
(Begun largely using The Complete Guide to High-Fire Glazes: Glazing & Firing at Cone 10 (A Lark Ceramics Book) by John Britt, and Clay and Glazes for the Potter by Daniel Rhodes and Robin Hopper. Any errors are mine; if you see something inaccurate, please let me know.)
At the moment some entries are linked to pages with larger pictures and perhaps more info; hopefully this number will grow over time. Eventually I'll create a separate glossary for all those fun ceramics materials.
A measure of how much water a clay body can absorb after it has been fired into ceramic, giving an indication of how porous or vitrified the material is.
A philosophy, belief or idea of what is beautiful or pleasing. Your personal aesthetic is your taste - in art, in music, in food, in dress, etc. Aesthetics vary from culture to culture, generation to generation, and person to person, among other things.
Pottery made with colored clays swirled together in a marble-like pattern, or covered with colored slips that are swirled together to achieve a similar effect. Read more.
- Albany slip.
A chocolate brown slip made of natural clays originally found in the area around Albany, New York, but later produced throughout the Mid-west. When high-fired, it turned into a hard glossy glaze, which was used in American pottery from the early 19th century until production stopped in 1986. Substitutes include Alberta slip and other commercial blends of clays and fluxes.
- Alberta slip.
Similar to Albany slip, an iron-fluxed clay that can be fired to low temperatures or used as a glossy brown glaze starting around cone 6.
Basic, having a high pH, as opposed to acidic. Many fluxes used in pottery are alkaline in nature.
- Alkaline earths.
Magnesium, calcium, strontium and barium act as fluxes in high fire glazes. They help develop hard glaze surfaces that often have a matt finish.
Al2 O3. A very hard, heat-resistant material.
Materials such as glass and glazes, which have no long-term repeating crystalline structure.
Traditional Japanese kiln (anagama means "cave kiln"), also known as a climbing kiln, shaped like an upward-sloping tunnel, with a stoking area where wood is burned at the bottom end and a flue and chimney at the top end.
A skeleton of wood or wire used to give internal support when creating a sculpture of clay or wax.
The atmospheric environment in the kiln; either oxidation, neutral, or reduction (smoky). The atmosphere in the kiln can affect how the colors of glazes and clays turn out, among other things.
In kilns that burn fuel such as wood or natural gas, there needs to be at least some oxygen in order for the fuel to combust. If there is just enough oxygen to combust all the fuel, the atmosphere is neutral. If there is more oxygen than is needed to burn all the fuel, the kiln is said to be in oxidation, and the excess oxygen is free to chemically combine with clay and glazes. If there is not enough oxygen in the kiln to combust all the fuel, the atmosphere in the kiln becomes smoky and the kiln is said to be in reduction. In this case the fuel may strip oxygen away from glaze or clay materials, and carbon from the smoke may combine with the clay and glazes instead.
Electric kilns, heated by electric elements rather than fuel, usually have oxidation or neutral atmospheres. Electric kilns can have a reduction atmosphere, typically during a bisque firing when organic material in the clay burns out along with other combustibles, such as newspaper left in sculptural pieces, and the kiln's spy holes are plugged up, preventing smoke from escaping.
As an example of how kiln atmosphere affects glaze color, a copper red glaze may turn blood red in reduction but pale green in oxidation. Since the atmosphere in a fuel kiln can be very localized - one area may be smoky while another is less so - a pot glazed all in copper red might be red on one side and green on the other after being fired.
- Ball clay.
A very fine, plastic secondary clay with a high shrinkage rate, not suited for use by itself. Added to other clays to make them more plastic and workable. In glazes it works as a suspending agent.
- Ball mill.
A container loaded with high-fired porcelain slugs or other mechanical grinding materials that is rotated by some means, into which clay or glaze materials may be put in order to be ground finer.
- Banding wheel.
A simple turntable rotated by hand, onto which work may be set and spun around in order to apply decorative bands or work the piece evenly. On many banding wheels, including very heavy cast-iron ones, the head is not attached to the base, and if the banding wheel is picked up by the head, the base will fall off and hit the floor - or your toes - with a embarrassing crash. Don't say I didn't tell you. Read more.
- Bat, throwing bat.
A slab of plaster or fired clay used for drying out clay or as a platform for work in clay.
Throwing bats are disks of wood, plaster or plastic that are held on the wheel head by a pair of pins or suction. Work thrown on bats can quickly be removed from the wheel by lifting off the bat, without damaging the work, allowing the potter to work on multiple objects in rapid succession. Read more.
A very fine clay, usually formed from the weathering of volcanic ash. It is characterized by high plasticity and swelling properties, and used to suspend glazes or make clay bodies plastic.
- Bisqueware, biscuit ware.
Fired unglazed pottery; ceramic ware that is durable yet porous. Read more.
- Bisque-firing, biscuit firing.
The first kiln firing, before the glaze firing. During this firing the chemical water and organic material in the clay are burned off; the clay sinters, and the pot is transformed from clay into more durable ceramic.
A glaze defect in which materials in the clay or glaze are still volatilizing and producing bubbles in the glaze at the end of the firing, where they are frozen in place as the kiln cools. The bubbles often break, leaving sharp-edged craters.
Blistering of the clay body, caused by trapped gases. Associated with not burning off all the organic material during bisque firing, and with carbon/black coring.
- BLOW UP.
What your clay project will do if it is too thick, fired too wet, or has unvented hollow areas.
If the clay is too thick it takes much longer for the center to dry out thoroughly. If the clay has moisture in it when fired, the moisture will turn to steam and expand, and if the steam cannot escape from the piece fast enough it will blow the piece up. Unvented air pockets in the clay can cause similar problems. Read more.
- Boat anchor.
A piece that's far heavier than it needs to be. Most often, a piece made on the wheel by centering a mound of clay, making a hole in it, then letting it dry out and not trimming it and calling it a bowl or cup. I tend to call them ugly and unfinished and food for the slop bucket, and not fire them.
- Body reduction.
During a glaze firing, a period of reduction atmosphere induced between cone 012 and cone 08 in order to bring out the warm color and speckles of any iron in the clay body. Frequently done when high firing stoneware.
- Bone ash, calcium phosphate.
The ashes of calcined bones, usually cattle bones; high in calcium and phosphorous oxides.
- Bone china.
English soft porcelain made with calcined bones (bone ash) as a flux. Despite the name, bone china is actually stronger and less prone to chipping than hard paste porcelain.
- Bone dry.
Completely dried out unfired clay. Bone dry clay is typically lighter in color than clay that is still moist. Objects in this state are brittle and fragile, and should not be handled more than necessary.
Once a clay project is bone dry, nothing else can be joined to it or done to it safely except some careful scratching or polishing, and it is very difficult to re-wet it into a workable state. To safely re-wet it, you need to wet the piece evenly, let it absorb the moisture, then repeat the process (again and again) depending on how thick the clay is. If you soak bone dry clay in a bucket of water to try and re-wet it, the clay will quickly dissolve like the Wicked Witch of the West. If a piece gets bone dry before it is finished, it is usually dead, and easier to start the project over than try and save it.
Moral: don't let your pieces get drier than you need them to be.
In a school or community studio, one of the greatest dangers to work is people picking up others' (or even their own) dry work to look at it or show off, and breaking it in the process. Do not handle others' unfired work unless you are loading it into a kiln, or you have their permission. And don't, don't try to pull your (or even worse, someone else's) unfired masterpiece out of a half-loaded kiln to show someone.
- B.T.U., British thermal unit.
The amount of heat required to raise one pound of water 1° Fahrenheit.
To smooth and polish the surface of an object (in ceramics, usually an unfired pot) by rubbing it with a smooth object such as a river rock or the back of a spoon. With this procedure a high shine can be developed without the use of glaze. Burnished objects are fired to a lower temperature (around cone 010) than many other objects, as the surface shine is reduced or lost if fired to higher temperatures.
To fire to red heat; removes chemical water.
A pincher-like measuring device in which two pins or arms are snugged up against the edges of the part to be measured. Read more.
Metric unit of heat: amount needed to raise one gram of water one degree Centigrade. When you read a candybar wrapper they are talking about kilocalories. Even so, it would take a lot of candybars to fire a kiln.
- Carbon coring, black coring.
A gray or black layer under the surface of a fired clay piece. Carbon or black coring is undesirable, as it makes the piece weaker, can discolor glazes, and is associated with bloating. Read more.
- Carbon trap.
Carbon trapped under the surface of a glaze or clay body, giving it a black or gray appearance. Caused by putting the kiln in reduction and creating a smoky atmosphere before the clay or glaze surface seals up. Certain glazes, such as carbon-trap shinos, make use of this to achieve a desired look. Read more.
A substance produced when carbon dioxide combines with an oxide.
The European name for grog.
When a trimming tool jumps or skips rhythmically over the surface of a pot that is being turned on the wheel, leaving a rippled texture instead of a smooth clean cut. Usually caused by the pot being either too wet or too hard, or by a dull tool. Sometimes used purposefully as a decorative technique.
- China clay, kaolin.
Al2O3 • 2SiO2 • 2H2O (alumina+silica+chemically combined water). A pure, primary clay, composed mostly of the clay mineral kaolinite, with very little iron or other contaminants. Characteristics include low plasticity, a very refractory nature, the ability to be fired to high temperatures, and a pure white color when fired. A primary component of porcelain bodies.
- Chuck, or chum.
A support used to hold leather-hard pots for trimming on the wheel. Chucks or chums may be leather hard clay or bisque fired. If the chuck is leather hard, the piece to be trimmed will stick in place more, and the chuck will not damage the piece. If bisque-fired, a soft coil of clay may be wadded onto the chuck to help pieces stick and prevent the hard bisque material from gouging into them. Read more.
- Clamshell kiln.
A box-shaped updraft kiln commonly used in American raku, made of two halves that are hinged at the bottom and may be quickly opened and closed like a clamshell.
- Clay body.
A combination of clays and other materials that yields a clay with the working characteristics desired. Clay bodies are formulated to have different colors, firing temperatures, degrees of plasticity, resistances to thermal shock, and other properties. There are no set names for clay bodies beyond the broad categories of earthenware, stoneware and porcelain; clay manufacturers give their own brand names to the specific products they offer.
- Cleanup karma.
Not thoroughly cleaning up after yourself can lead to bad cleanup karma, where you will be cursed to clean up 1000x more later in life for each little thing you neglect now.
The funny thing is, you think I’m kidding.
Carboxymethylcellulose, a glaze binder and suspending agent. If you get a leaking bag of dry CMC powder and try to wipe it up with a damp sponge, you're in for some gooey fun.
To make clay objects by building with ropes or coils of clay. Coiling is simple to learn, allows for building very even and also very organic forms, and has no limitations as far as how large things can be made. Read more.
Oxides and carbonates added to the base glaze recipe to give color; includes oxides of copper, cobalt, chrome, and others. Read more.
- Cone, pyrometric cone.
Small triangular cones of ceramic material, designed to melt after absorbing specific amounts of heat. Used to determine how a kiln firing is progressing and when it should be ended. Cones are designated by numbers according to how much heat work it takes to melt them, and so these cones and their numbers form a scale used to measure the heat work that takes place in a kiln. Numbers on the cone scale are associated with the peak temperatures reached if the kiln temperature is raised at a certain rate. Relying on pyrometer readings to fire a kiln to a temperature listed as the equivalent of a certain cone is not as reliable as using cones to measure the heat work. As many kilns do not heat perfectly evenly (especially fuel kilns), a lot can be learned about how a kiln fires by placing cones at various places in the kiln and, after the kiln has fired, seeing how the heat work varies from place to place. Read more.
- Cone pack.
A series of pyrometric cones, set into a wad of clay, for the purpose of monitoring the kiln's heat work. Usually at least one cone pack is placed near a spy hole in the kiln so it may be observed during firing. Read more.
A reversible physical contraction of a material during cooling; also called thermal contraction.
When a glaze pulls away from areas of the pot during firing, leaving areas of bare clay body. Caused by getting dirt, dust or grease on the pot before glazing, or by a reaction between glazes.
Cracks in the glaze surface caused by a glaze that contracts more than the clay body. The cracks may be emphasized for decorative effect by rubbing ink or some other stain into them (do not do this to the inside of a food vessel. Just after firing the web of cracks will be larger and more open; as the piece continues to cool more cracks will form, creating a finer mesh. You can hear the pieces "ting", "ping" and "tzing" as they cool and the glaze cracks.
Crystalline form of silica that has a high expansion and contraction rate, making ware susceptible to thermal shock. Forms above 2200 degrees F; crystal growth is encouraged by ware remaining at this temperature due to slow heating or cooling, heat soaking, or repeat firing, and also by clays or glazes having an excess of free silica. Quick heating or cooling of high-cristobalite clay bodies at approximately 500 degrees F can cause dunting.
- Cross section.
A view of something cut along a plane. In pottery, it can be useful to visualize a pot cut in half to understand the relationship between the inside and outside of a form, and the wall thickness in between. In general, the wall thickness of a well-made piece will be even and nuanced, and proportional to the size and function of the pot. A beginning pot will often be thicker on the bottom sides and/or bottom, and thicker in general than it needs to be. Cross-sectional views are useful when designing new or complicated structures.
- Damp box.
A cabinet or container that can be sealed vapor-tight so that unfinished clay projects will not dry out. If people remember to keep it closed.
A device for adjusting or for closing the opening from the kiln to the chimney.
To remove the water that has collected at the top when a material settles in a liquid.
A base (soluble alkali) that is added to a glaze slurry, causing the particles to repel one another. This can keep particles in suspension for a time and makes it possible to use less water to achieve the same fluidity. Has the effect of thinning out the glaze slurry, but over time can cause glaze to settle into a rock-hard layer at the bottom of the bucket.
To disperse the particles in a slip so that less water is required for fluidity.
- Downdraft kiln.
A kiln that vents its exhaust through a flue set at floor level (to the interior of the kiln) that feeds into a chimney. The chimney is required to help create the convection currents necessary to draw out the waste gases. Since heat and gases from the burners enter in from the bottom of the kiln, rise through it and then must be drawn down to exit, the heat flow through the kiln is more even and efficient than that in an updraft kiln. Downdraft kilns are also preferable for reduction firing. Read more.
The weight of a material in comparison to water; also known as specific gravity (sp gr) or relative density.
- Dry foot.
The foot of a pot which has been cleared of glaze.
Cracking of the clay body often caused by rapid or uneven changes in temperature in a piece; can occur during cooling (cooling dunts, which are more common) or heating (heating dunts). The stresses that cause dunting happen around 439 degrees F and 1063 degrees F. At these points, silica (quartz) goes through a phase change and rapidly expands or contracts; if the whole piece is not at the same temperature and some parts are expanding/contracting more rapidly than others, dunting cracks can occur. Pieces that have been overfired and/or overfluxed, making them less resistant to thermal shock, are more likely to dunt. If you pull your masterpiece too hot out of the glaze kiln to ooh and ah over, you may hear it go "ping ting ping PING! ting ting POP!" - and then you will probably find some dunting cracks and be very sad you were in such a hurry.
Dunting can also be caused by extreme glaze compression, when the glaze wants to shrink more than the clay body and is stronger than the clay.
Dunting cracks are identifiable because the ware is warped, and the cracked edges will not fit exactly back together. Cooling dunts will have sharp glaze edges; with heating dunts, the glaze will have a chance to melt and smooth at the crack edges.
Pottery or other objects made from clay fired below cone 03; the ware is porous and permeable. Earthenware may be glazed or unglazed, and is usually but not always buff, red, or brown in color. Characterized by its low firing temperature, porous nature, and usual iron content. The commonest naturally found clay.
To give off gas, as in the form of bubbles rising in liquid.
- Electric kiln.
A kiln that is heated by coiled wire heating elements rather than combustion. Read more.
An agent which causes deflocculation. A substance in solution, conducting electricity.
- Engobe, underglaze.
A slip-like material with a lower clay content than a pure liquid clay slip, applied to ware to change the color of the body. Engobes and commercial underglazes are formulated to have a low drying shrinkage, so they may be applied to bone dry and/or bisque fired pieces. They may be used with or without a clear glaze over the top.
- EPK, Edgar Plastic Kaolin.
A pure white kaolin mined in Edgar, Florida, where Charles Edgar found and began mining the deposit around 1888. EPK is a secondary kaolin that was transported from its parent rock source in the Carolina mountains in the age before cellphones. Since it is a secondary clay, its particle size is smaller than that of primary kaolins, making it more plastic (but still not particularly plastic).
The reversible physical expansion of a particle material as it's heated; expressed as a coefficient of expansion/contraction; also called thermal expansion. See also contraction.
- Extrusion, extruder.
The process of making shapes by forcing clay through dies. Extruders commonly consist of a wall-mounted tube that dies can be attached to, and some sort of plunger to force the clay through - like a big cookie-dough gun or pastry bag (which can be used for smaller work.) A pain to clean. Some modern pug mills can also be fitted with dies to extrude shapes.
A mixture of two or more materials that, when combined in a certain proportion, will melt at the lowest possible temperature.
Glazed earthenware. Originally, the tin-glazed earthenware made at Faenza, Italy.
- Fat clay.
Highly plastic clay.
A white or pale mineral (the white or pale part of granite) made up of alumina and silica, in combination with potassium, sodium, calcium, and/or lithium (those containing lithium are not true feldspars, but are similar in use for potters). When feldspars undergo chemical weathering over geologic spans of time they are broken down and changed into the clay mineral, kaolinite.
- Fettle, fettling knife.
To finish or smooth the surface of leather hard clay. To trim off the spare material around the seams of cast or jiggered pieces. The short single-edged knife commonly used to do this.
- Filler, tempering materials, temper.
A material of little or no plasticity added to clay in order to promote drying, control shrinkage, open up the clay body, increase resistance to thermal shock, and provide physical structure in throwing and wet-working in clay bodies or engobes. Typical fillers include sand, grog, and volcanic ash.
- Fire clay.
A high-firing refractory clay used to make bricks for furnaces, kiln shelves and props and is often used in clay bodies. A secondary clay, it has few fluxes, course particle size, and is commonly buff-colored with low shrinkage and plasticity.
- Fire color.
The color of a clay body caused by the action of the kiln atmosphere or direct flame impingement. See also flashing.
A match of the properties of a clay body to a glaze, or vice versa. A glaze that has an expansion and contraction rate similar to the clay body is said to fit the body, and will not craze or shiver. A dried glaze coat that cracks off greenware (before the firing) does not fit because the glaze and clay body shrinkage rates are different.
Coloration caused by volatile materials present during firing.
An acidic thickening agent added to the glaze slurry which causes the particles to stay in suspension but can cause cracking of the dried glaze layer. The agent causes the particles to attract to one another; they're said to "flock" together.
An oxide (generally alkaline) which causes ceramic fusion when combined with other oxides and heated. A substance that causes or promotes melting, that helps lower the melting point of a glaze.
- Force dry.
Using a heat gun, torch or other method to heat and dry soft clay in order to quickly stiffen it to a desired workability. When throwing or coiling pieces, the softness of the clay may not be able to support continued shaping or the addition of more material; a wait period may be necessary to let the work air dry until it has firmed up, or the potter may attempt to force-dry the piece somewhat in order to continue working. Force-drying clay can cause stresses in it that lead to cracking, and should be done sparingly, evenly and with care. Read more.
A precise mixture of ceramic oxides which are melted together into a glassy state, quickly cooled in water, then finely ground; used in glaze recipes as generally insoluble sources of oxides. Since the colorants in frits have already been fired to their maturation temperature, the color of raw underglazes or other products made with them closely matches the what the final color will look like. Frits are costly to produce, however, and products made with them, such as underglazes and lowfire glazes, can be more expensive.
To melt together.
An acidic oxide that retains its amorphous structure when cooled to a solid state rather than re-forming into a crystalline structure. Silica is the primary glass-former in ceramics, followed by boron and phosphorus oxides.
A glass layer or coating that fuses to a clay/ceramic piece during firing. Glazes are mixed up of various minerals, oxides and other additives into a slurry that is applied to bisqueware or greenware in a number of ways. All glazes must contain a glass-forming component (usually silica), a fluxing component to help the glaze melt at a certain temperature, and a stiffener (usually alumina) to help keep the glaze from running off the pot. Glazes may also contain colorants to give them color, and opacifiers to make them opaque.
- Glaze base.
The glaze recipe without any colorants, opacifiers, or suspenders.
- Glaze fault or flaw.
An unwanted feature in the glaze, such as pinholing, crawling, or crazing.
An igneous rock composed of feldspar, quartz and mica. Granite makes up the majority of the earth's continental crust and is the parent rock of most clays.
Clay which has been fired/calcined and then ground into granules of more or less fineness. Often added to clay bodies along with sand to reduce shrinkage, create a more open material that dries better and is more resistant to heat shock, and make the clay more suitable for building large pieces.
A Korean and Japanese technique of applying thickened slip with a course brush to a piece of greenware; the slip retains its texture after firing.
- Hare's fur.
A streaked brown slip glaze. Also known as temmoku (Japanese).
- Heat work.
The temperature that a ceramic piece has been heated to, coupled with the time it has to absorb that heat. Measured using pyrometric cones.
- High fire.
Pertaining to a kiln firing range of cone 8 and above.
A device for quickly measuring slip density, or specific gravity; less accurate than weighing.
Formed by cooling from a molten state.
Also known as mishima. Technique of filling a carved design on the surface of a piece with colored slip, then scraping away the excess after the slip has stiffened.
Metallic patches on a glaze surface, caused by saturation of cobalt oxide.
Forming a pot between a revolving mold which shapes the inside and a template which shapes the outside and foot.
Forming a pot between a revolving mold which shapes the outside and a template which shapes the inside.
A pure, primary clay, or a secondary clay like EPK that has undergone some movement but remains minimally contaminated by iron or other impurities, that is composed mostly of the clay mineral kaolinite. Characteristics include low plasticity, a very refractory nature, the ability to be fired to high temperatures, and a pure white color when fired. A primary component of porcelain bodies. Also known as china clay. The name kaolin comes to us from the writings of a French Jesuit priest, Francois Xavier d'Entrecolles, who in the early 1700's studied the Chinese methods of porcelain production in a village in the Jiangxi province in China, Kao-ling or Gaolin.
- Kaolinite. Al2O3 • 2SiO2 • 2H2O or Al2Si2O5(OH)4 (alumina+silica+chemically combined water).
The pure clay mineral. Kaolinite crystals take on the form of microscopic (from ≈ .2 microns to 50 microns), flat, roughly hexagonal plates that agglomerate into the similarly shaped particles of clay dust that are large enough to see. The small size and flat shape of the clay particles are what give moist clay its plastic nature. For a magic trick (and an explanation), read more.
A device for controlling the length of firing of an electric kiln, and for automatically shutting off the kiln once the desired heatwork has been done. Read more.
- Kiln wash.
A refractory mixture, usually kaolin or flint, which is painted on kiln shelves and saggars to prevent glaze from adhering. The flaky white substance sometimes stuck to the bottom of your pot.
- Leather-hard, cheese-hard.
Clay dried sufficiently to be stiff and no longer plastic, but still damp enough to be joined to other pieces with slip. Color has not yet begun to lighten due to drying.
- Local atmosphere.
Atmosphere in one area of a kiln. In a downdraft kiln, for example, the top of the kiln may be in more of a smoky reduction atmosphere than some lower portions. The type of kiln, how the shelves and ware are stacked within it, and how it is fired all can create variations in atmosphere from place to place within the kiln.
- Low fire.
Pertaining to a kiln firing range from cone 010 to cone 04.
- Luting, slipping and scoring, scoring and slipping.
Joining leather-hard clay using slip. The surfaces to be joined together are scored (scratched up) with a sharp tool in order for the slip to create a stronger bond between the pieces. Read more on joining clay.
Large crystals formed on the surface of a glaze by slow cooling.
Fired to a tight, hard, serviceable structure.
- Medium fire.
Pertaining to a kiln firing range from cone 03 to cone 6.
Clay that has been fired and become ceramic material; cannot revert to clay.
Very small crystals on the surface of a glaze formed by slow cooling.
A Korean and Japanese pottery style using inlaid slip decoration. The technique of filling a carved design on the surface of a leather hard piece with a contrasting colored slip or clay, then scraping away the excess after the slip has stiffened. For more information see "The Mystery and the Mastery" by Robert Yellin.
The inner lining of a kiln that protects the ware from the direct impingement of the flame.
- Neutral atmosphere.
The atmosphere in a kiln that is said to have a perfect balance of oxygen and fuel for complete combustion.
A material such as stiff leather-hard clay that cannot be bent or molded without rupturing.
A material that makes the glaze non-transparent or opaque, such as titanium dioxide, tin oxide, or zirconium silicate.
To make clay more open or porous in structure by adding fillers or grog. Also, when throwing on the potter's wheel, the act of creating a depression in the middle of a centered piece of clay and then pulling outward to make the depression wider and "open up" the clay.
- Open firing.
Firing in which the flames may come in contact with the ware.
A style of Japanese pottery, named after Furuda Oribe; refers to modern-day, transparent, copper-green glazes.
An accent colorant used in brushwork and small areas, applied over an already glazed piece. In some cases, the glazed piece has already been fired, and the piece is refired to a lower temperature (cone 016 or lower) to bond the overglaze on. These underglazes are well suited for exact silk-screen, stencil and decal applications, but may be less permanent and durable due to the low temperature bond. In other cases, overglazes are applied to glazed but unfired work, then typically high-fired. These overglazes become integral with the glaze, and decorations may bleed and flow with the glaze during firing.
A chemical reaction caused by heat in which oxygen atoms combine with other molecules or atoms.
- Oxidation atmosphere.
The state of the atmosphere in a kiln in which there is more oxygen present than needed to burn the fuel.
A substance made when an element combines with oxygen.
- Oxygen probe.
A device placed in the kiln during firing to provide an indication of atmospheric conditions of oxygen, neutral, and reduction atmospheres.
Small bubbles in a fired glaze coat, caused by gasses released from the clay body or the glaze itself toward the end of the firing, when the molten glaze cools before it has a chance to smooth over.
- Plastic, plasticity.
The characteristic of clay that can be formed without rupture or cracking, and be able to retain that form.
From the foot of a piece, the pulling away of slivers of material that have stuck to the kiln shelf during glaze firing, due to the the clay body (usually porcelain) becoming vitreous/sticky at the maturing temperature. Seriously annoying. There are various strategies to prevent this, discussed in this clayart thread.
Pottery or other objects made from a white, vitrified, and translucent clay body. When tapped, it makes a pinging or ringing sound. Originally developed in China, hard paste porcelain is a combination of kaolin, quartz or some other form of silica, and feldspar; it may also contain some ball clay or other ingredients to enhance its workability or other properties.
Soft-past porcelain began as a European attempt to copy Chinese porcelain, using clay mixed with ground-up glass or frit. This type of porcelain tended to slump and warp more in the kiln, and was not as hard. Later compositions using kaolin and feldspathic materials improved the quality and are still in use.
Bone china began in England in the mid/late 18th century, when calcined ox bones began to be added to porcelain. Josiah Spode is attributed with perfecting bone china sometime between 1789-93, using 6 parts bone ash, 4 parts Cornwall Stone or china stone, an iron-free feldspathic rock, and 3.5 parts kaolin. Bone china fires to a lower temperature than hard paste porcelain, but is whiter, more translucent, stronger and less prone to chipping.
- Press mold.
A shallow open mold of plaster or other material that clay is pressed into to take the mold's shape.
- Primary clay, residual clay.
Al2O3 ∙ 2SiO2 ∙ 2H2O (alumina+silica+chemically combined water). Clays found at the place of the parent rock, that have not been transported by water or weather action and are therefor uncontaminated by other materials. Because of their coarse particle size, primary clays have low plasticity and are difficult to work with unless mixed with other clays. Kaolins are primary clays.
- Pug mill.
A mean, unclean, man-eating machine for mixing, compressing and extruding clay, useful for recycling scrap clay and destroying anything else mistakenly fed into it, which you will then find chips of in your clay exactly when and where you don't want them. Make sure not to mix up and recycle your tools with your clay. Or your hands, or arms, etc. Modern pug mills have small openings to feed the clay into, in an attempt to prevent careless and tragic accidents. I have an old-school monster with a big, beautiful, wide-open toothy maw that will swallow anything, including bucketfulls of clay; it's like having my very own pet great white shark. I love it. But I don't let the students feed it. At all. Read more.
A gauge for measuring temperature in a kiln.
- Quartz inversion.
Approximately 1 percent expansion or contraction of quartz at 1063 degrees F. If heating or cooling occurs too fast at this point, it can cause cracking of the ware, called dunting.
In Western terms, raku is a process in which pieces are rapidly heated to 1800 degrees F, then pulled red-hot from the kiln and placed into a container of combustible material, such as newspaper, wood chips or straw, to create a reduction atmosphere for the glaze and to blacken any unglazed areas with carbon.
The term raku originated in Japan as a family name given to a potter, Chojiro, in honor for his making tea bowls that fit the ideal of the wabi style of tea ceremony proposed by the tea master Sen Rikyu. Chojiro (and his family, which continue today) hand-made the bowls, and removed them hot from the kiln to either quench in water or allow to cool in the open air.
- Rate of firing.
How fast a kiln is heating.
Chemical reaction in which oxygen is removed from oxide molecules. May occur in either a glaze or a clay body.
- Reduction atmosphere.
Atmosphere in a kiln where there is a deficiency in the oxygen necessary for complete combustion of the fuel. Which means, it's smoky in the kiln.
- Reduction cooling.
Controlled cooling of a fuel kiln in a reduction atmosphere; the fuel is kept on in order to burn the excess oxygen in the kiln until the glazes are set.
The quality of being resistant to heat, or a material that is heat resistant.
Wax, tape, or any substance used to prevent glaze from adhering to certain areas of a piece during glazing.
A breathing device used to prevent the inhalation of harmful materials. Can be fitted with hazard-specific canisters to filter various fumes and/or dusts.
- Rib, potters' rib.
A tool of wood, metal or other material that has a curved edge similar to a rib's in shape. Used to help shape and smooth pieces during throwing or building with clay. Read more.
An s-shaped crack that can occur in the bottom of wheel-thrown vessels if the clay at the bottom is not as compressed as the rest of the piece, or if water is let to sit in the bottom too long when throwing.
A refractory container designed to protect ware from ash and flames, or to create local atmosphere for ware by adding combustibles and fuming agents in with the ware; also called a bung.
- Salt glaze.
Glazing by the vapors from salt in the kiln. When the kiln is at 2350 degrees F or above, at the end of a firing, salt is added into the kiln, where it volatilizes. Sodium vapors from the salt combine with alumina oxide and silica oxide on the surface of the unglazed ceramic ware, acting as a flux and forming a glaze coat that has a characteristic orange peel texture. The chlorine from the salt meanwhile combines with hydrogen to form a dilute acid gas that deteriorates the kiln (at the very least). Soda glazing, where sodium carbonate or sodium bicarbonate are used instead of salt, is a recent alternative the produces similar results without the lovely hydrochloric acid gas aftertaste.
The point at which no more material will dissolve in molten glaze; instead, it will remain suspended in the liquid.
- Secondary clays, deposited clays, sedimentary clays.
Clays that have been transported away from their parent rock by water and weather action. As water flows away from the parent material, heavy/coarse clay particles are left behind to remain primary clay, while finer particles are carries away. If the water that is carrying this clay slows down, the larger/heavier particles of it may settle out while the finer particles go farther downstream; in this manner clay deposits of similar particle size are formed. As the clay is carried away, it is also mixed with clays from other sources, impurities such as iron, and organic material.
A decorative process which employs a scratched line through a clay slip to expose the contrasting color of the clay body beneath. Read more.
- Shelf of shame.
That special place where pieces go when they're not quite ready to be fired in the kiln they were set to go into. The usual reasons are there is glaze on or too near the bottom of the piece, the glaze is on too thick or thin, it was placed to go in the wrong kiln, or (for greenware) there was no name on it, or it was unfinished/poorly made/an ashtray or something equally special and artistic that is better placed in the slop bucket. Read more.
A glaze that contracts less than the clay body, causing the glaze to fall or pop off in sheets that can be razor-sharp. Throw the piece away.
Clay that is non-plastic and poor in working properties.
Reduction in size of a clay body as it undergoes drying or firing. Depending on the clay body and how it's fired, a piece might shrink anywhere from four to fourteen percent in size. Moist clay has physical water between the clay particles: as the clay dries, the clay particles get closer together, causing the initial shrinkage. As the clay is fired to maturity, it sinters and vitrifies (depending upon how high its fired), becoming denser and shrinking even more.
A medium which promotes the drying of oils used in underglaze or overglaze colors.
To remove impurities or large particles by putting a glaze through a metal sieve. Various mesh sizes are available.
The primary glass-former in glazes; synonymous with flint, quartz, and silicon dioxide.
A disease of the lungs caused by the inhalation of silica dust. Clay dust is one of the worst enemies of a potter - any little smear of slip or scrap of clay will dry and turn to fine dust, that can hang suspended in the air for days after it is brushed, beaten, blown or swept up. The best way to fight it is to clean up any mess while it's still wet, and to sponge, mop, or rinse away any dry clay. Make sure to rinse sponges and mops clean before and during use, otherwise you're just smearing the dust around, not removing it. (Can you tell I work in a high school classroom? Can you?) It's better to clean the table and floor with a sponge than clean the air with your lungs.
Particles that hold together through partial melting or mutual adhesion; the surface does not vitrify.
- Slab roller.
A table with a canvas-covered top and a roller that can travel across it; used for flattening clay into sheets of even thickness. Read more.
Putting dry clay into water in order to rehydrate it, breaking down dried chunks of clay with minimal effort. Dry glaze materials can also be slaked through water.
A liquid mixture of clay and varying amounts of water. Slip has many meanings: the clay and the water at the bottom of a throwing tray; a thickened clay and water mixture, like cream, to connect pieces of clay; an even more thickened mixture, like a creamy yogurt, that can be applied for texture (Hakeme). When colored, a mixture used for colored slip decorations; or deflocculated, a mixture used in slip casting. Can be a thin mixture of clay and water applied to a pot to cause flashing in salt, soda, and wood firings, called flashing slip.
The easiest way to make slip is to take bone-dry clay, break it into small chunks and slake it in water - it will quickly dissolve.
- Slip cast.
Slip is poured into a hollow plaster mold and let sit for a few minutes, allowing the plaster to suck moisture from the clay touching it and form a skin. The excess slip is then poured out and the mold opened, revealing the left-behind shell of clay.
- Slump mold.
A form that clay slabs or coils are draped over or pressed onto and then let become leather hard, thereby stiffening into the mold's shape.
A mixture of plastic clay and water. The ingredients of a glaze recipe mixed with water in the glaze bucket. Also called slop.
Holding the kiln at a desired temperature for a time to achieve certain effects. Water-soaking or candling is the process of letting the kiln run with one element on low, or just the pilot lights on, often overnight, in order to make sure all the ware inside is bone dry, thus reducing the risk of pieces blowing up.
The ability of a material to go into solution.
- Specific gravity (sp gr).
For potters, a number that represents how much heavier a material is compared to water; used as a reference point for mixing glaze slurries or slips; also referred to as density or relative density.
A relief decoration added to a clay surface. Sprigs are usually thin slabs formed in a mold.
Triangular refractory supports that hold glazed ware up and away from contact with the kiln shelves or saggars.
An atmospheric oxide that, when combined in proper proportion to a glass-former and a flux, keeps the glaze from running down the pot.
Refers to a glaze that does not exhibit glaze faults such as running or crawling; can also indicate glazes that don't leach toxic components.
To load a kiln with pottery in preparation for firing.
Coloring pigment; can be a commercially prepared coloring stain in which oxides have been fritted together and ground. May also refer to an oxide with a flux added (more accurately called a wash).
Condition where a kiln's temperature stops rising during firing. A cause of woe, hand-wringing, swearing and long nights.
Pottery or other objects made from fired clay which is dense and vitrified. Stoneware is fired to temperatures above 2192 degrees F. It may be dark or light in color, but is not translucent. Characterized by high firing temperature, density, non-porous nature, opacity, and usually at least a small amount of iron.
A firing process in which, after a fuel has reached peak temperature and is cooling, it's turned on again between 1300-1800 degrees F and adjusted to a reduction atmosphere for a period of time. A glaze fired in this way is said to have been struck.
A surface pattern in a glaze high in calcium oxide, characterized by thicker lines of glaze that are similar to an aerial view of a river tributary system; also called rivulets.
- Temmoku, Temnoku.
The name temmoku comes from Tien-mu-Shan, "Mountain of the Eye of Heaven," a mountain in Chechiang province, China. It was from a monastery on this mountain that Dogen, A Japanese Zen priest, was said to have brought the first temmoku bowl to Japan in 1228. Usually a shiny black or dark brown glaze colored by iron oxide.
- Terracotta, terra cotta.
(Italian: "baked earth") is a porous red-brownish-orange earthenware commonly used to make garden pots. Unglazed terra cotta pots (or pots made out of any porous earthenware) left out in a climate where freezing temperatures are reached are subject to cracking and decomposition, caused by the expansion of water as it freezes within the ceramic.
To make pottery by hand on a potter's wheel. Or to get rid of ugly stuff by tossing it in the air, pitching it in the dumpster, etc.
- Tin enamel.
A lead glaze opacified by tin.
Roughness in a clay; course grain structure caused by larger particle sizes and/or the addition of sand or grog.
- Top hat kiln.
A barrel-shaped updraft kiln commonly used in American raku. The kiln body is essentially an insulated can with a hole on the top that can be raised and lowered in a frame by pulling on a counterweighted cable.
The degree to which a material is considered to be poisonous.
- Trimming, or turning.
The process of taking a leather-hard piece thrown on the wheel and returning it to the wheel (usually upside-down) to remove excess material and weight, to give the piece an even wall-thickness and make the inside match the outside, and to finish the bottom. Some potters hate doing it, some potters will sneer and call you a dishonest son-of-a-banana if you trim anything more than the bottom of a piece, and some potters trim pieces top to bottom and inside to boot. Part of the throwing process; if you want to be good at the wheel, master the skill so you can use it as desired.
- Trimming through the bottom of your pot.
What you will do if you don't check how thick the bottom of your pot is before you start trimming, or if you continue trimming without re-checking from time to time. A cause of infinite woe and sadness amongst beginners who refuse to learn to now and then check how much they have trimmed.
- Underglaze, engobe.
A colored slip meant to be applied under a clear (or no) glaze. Underglazes are formulated to have low drying shrinkage so they may be used on greenware and/or bisqueware. Meant mostly for lowfire use, but can be high fired with varying results. Modern underglazes are made with colored frits, so the wet color is close to the fired color, and colors may be mixed like paints.
- Unity Molecular Formula.
A theoretical method of looking at the oxides that make up a glaze on a molecular level. The UMF divides oxides into three categories (flux, stabilizer, and glass-former) in order to express their relationships to each other.
- Updraft kiln.
A kiln in which the waste gases are vented through a hole in the top. Updraft kilns do not have a chimney, and do not heat as evenly as downdraft kilns. Most American raku kilns, that are rapidly fired to relatively low temperatures with small loads, are updraft kilns.
- Venturi burner.
A gas burner that employs the Venturi Principle to mix gas and air. Pressurized gas forced through a constricted pipe eliminates the need for a burner fan and electricity.
The stiffness of a liquid or glaze; resistance to flowing.
A glassy, non-porous state, much like the eyes and minds of many students at 7:10am.
- Vitreous slip.
A clay slip with sufficient flux to become almost vitreous; a mixture that is halfway between a clay slip and a glaze.
To fire to the point of glassification; to turn or melt into glass.
Describes materials that vaporize (change from a liquid to a gaseous state) during firing.
- Volatilization point.
The point or temperature that a material changes from a liquid to a gaseous state.
The process of converting from a liquid to a gas.
A mixture of oxide or stain and water that is applied to a piece of ceramic or a glaze coat in order to color it; sometimes includes an oxide or stain and a flux to help fuse it to the clay body or glaze.
To knead or mix plastic clay by cutting or rolling. The purpose of wedging is to mix the clay to an even consistency, to work out any trapped air bubbles, and perhaps to firm wet clay up, if it is wedged on a porous surface such as wood. Wedging the clay you work with is particularly important if you are throwing on the wheel, and a skill that should be developed just as much as centering or pulling up. Two types of wedging are rams-head wedging and spiral wedging.
- Witness cone.
A large pyrometric cone that is visible through the spy hole of a kiln during firing; used to determine when firing is complete. Read more.
- Wood ash.
Ashes produced from burning wood. You have to burn a LOT of wood to get a little ash. If you want to make ash glazes, make friends with someone who produces wood ash as a byproduct of their industrial process.
- Working edge.
Any edge on a piece of functional pottery, such as the foot of a pot or the rim of a lid, that comes into frequent contact with other things. Working edges should be smooth and rounded, and not too thin, otherwise they may be prone to chipping, breaking, and being uncomfortable. If you don't want your work to scream "beginning potter!", pay attention to your working edges. It's the details that'll make your work shine.
A visible disturbance in the surface of the glaze; can be caused by under-firing high-feldspathic glazes or by heavy reduction periods that lower the kiln temperature and temporarily freeze the glaze surface.
- Banding Wheel
- Bat, Throwing Bat
- Bisqueware, Biscuit Ware
- Blow Up
- Boat Anchor
- Carbon Coring, Black Coring
- Carbon Trap
- Chuck, Chum
- Downdraft Kiln
- EPK, Edgar Plastic Kaolin
- Electric Kiln
- Extrusion, Extruder
- Fettle, Fettling Knife
- Force Dry
- Pug Mill
- Pyrometric Cones
- Shelf Of Shame
- Slab Roller
- Test Tile