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Glossary of Terms


How much water a fired piece of clay can hold after being soaked in water. The absorption rate is a percentage from the original weight of a piece and the weight after soaking. A bar with a starting weight of 100 grams weighs 110 grams after soaking has absorbed 10 grams of water. 100 divided by 10, is 10 giving us a 10% absorption rate. See vitrification



A single open chamber kiln. Style of kiln developed in Korea taken by China. Formed from digging a tunnel into a hillside the slope adds to the draft in the firing. Traditionally fired with wood. Firings can take a little as a day but are often fired for days at a time to build up surfaces of melted ash and flame marks.


Banding Wheel

Used on the potters wheel to aid in removal of pottery from the wheel. There are many materials used and methods of securing them. Masonite or hard pressboard is a cheap easy to find commercial bat being slightly absorbent they aid in drying but are also prone to warping and wear. Plywood ones are common to studios with someone handy with a saw. Cheap durable and less prone to warping. Getting an exterior grade plywood in important for the longevity of the bat. The also are manufactured in plastic a few companies make them. The plastic bats are extremely durable but do not allow for the bottoms to dry as evenly. Plaster bats are also made for throwing on the wheel, they are very absorbent and that quality make them great for plates and wide bowls but their weight and space are a drawback. Common way to adhere a bat is through the use of bat pins. To prevent worn bats from wiggling there are rubber, and other materials used under the bat.


Bat pins

A socket cap screw 1/4 inch coarse thread. Some wheel heads like the soldner will have a threaded hole, others will require a wing nut to secure the pins. The length will be determined by the thickness of the material the wheel head is made from and the clearance under the wheel head.



Pottery that has been fired but not yet glazed. 


When a clay body has a bubble form firing a firing. Over firing is the common cause. When the clay body becomes dense enough to prevent gasses from escaping and soft enough to distort pressure from additional gases trying to leave the body causes a section to expand. Materials that release gas through the firing range later in the firing like manganese contribute greatly to the flaw. In addition in gas firings an early reduction can cause trapped carbon also leading to bloating.


Bone Dry

The final stage of greenware dried to a near or fully dry state and ready to be fired. In this state, the article is very fragile, non-plastic and porous.


Pressing on the surface of the clay to flatten the particles and make a shine. If done as the work is drying a few time a high shine can be achieved. This will demise as it completely drys and more the hotter it is fired. Often used for work that will not be glazes or in the case where terra sigilatta is used. Common in low atmospheric firings like pit, raku, horsehair, naked and barrel firings.



Removing components of a material trough heating. Done below the sintering temp of the material its used to remove plasticity and shrinkage from some material and in others to remove the chemically combined materials that may cause glaze faults



The lower temperature stage of some firing cycles used to complete the drying of the ware.

Carbon Cored

When reduction happens too early in a gas or solid fuel burning kiln. Its commonly associated Iron bearing clays but can happen with nearly any clay given the right conditions. The threshold is cone 012. Reduction starting before that can lead to carbon coring. After that nearly any reduction cycle will not produce carbon coring. The symptoms of carbon coring are the piece are very brittle. Susceptible to dunting and other thermal stress issues, bloating, sounding “off” as the will not ring. When broken open the side clay is grey to black.



Traditionally a transparent green to grey glaze meant to mimic the look of jade. Fired in a reduction the coloring was the result of small percentage of iron making the green. Now glazes are labeled celadon but no longer have same means of being colored or the same translucency.



Leather hard or fired clay tool used for trimming, shapes can vary but are often a cylinder with a widened top.



There are lots of ways to think about what clay is. Its a plastic material that when pushed maintains its new shape. Its pure theoretical chemical formula is Al2 SiO2 2H20 which doesn’t happen naturally often. Clay is formed from the chemical weathering of feldspars contained igneous rocks like granite. Its the pink or cream colored part. Carbonic acid from the decomposing plant matter higher in the water table breaks it down slowly into clay particles. When transported by water to settle down stream its considered a secondary clay. When weathered in place and not moved by water its a primary clay.


Clay Body

A mixture of clays, fluxes and other materials like grog to produce a clay that has desirable working properties like plasticity, low shrinkage and color.



One of the most important tools used to gauge firings for those in craft of clay. They are made to melt when an amount of heat has been applied to them. They measure more than temperature. Used to monitor a firing and used to confirm after a firing that an amount of heat has been applied to the work in that area of the kiln. They are great for diagnosing kiln issues early and quickly. The are also one of our best was to ensure standards with out firings. They are organized going from the lower heat 010, 09 08 through 3, 4, 5, and so on. 1 being the point where increasing numbers are an increase in heat.



Working the clay to the middle of the wheel. This can be done through a variety of techniques. The best will also homogenize the clay as well as getting it physically in the middle of the wheel.



An inorganic, crystalline non-metallic solid formulated from metal or non-metal compounds whose irreversible formation occurred during heating to high temperatures.


A hand method of forming pottery by building up the walls with coils of rope-like rolls of clay.


A parting and contraction of the glaze on the surface of ceramic ware during drying or firing, resulting in unglazed areas bordered by coalesced glaze. May be caused by uneven glazing, excessive glaze thickness or a greasy substrate.


The best description is in the video below. The short answer is when a glaze contacts in the cooling more than the clay does or at a different time than the clay does. The act of crazing makes a sound called a ping. If you hear a ping you have a craze. It is not caused by cooling quickly, its just that you hear it happening. Opaque and dark glazes make them harder to see, it requires staining them which can be done with a contrasting slip.


Crystalline glazes

Many exist, and vary greatly. The all have in common a material that has melted into the glaze and while cooling becomes over saturated and creates a crystal around a nucleus. Macro crystalline glazes require controlled coolings to bring about the larger crystal growth. Microcrystalline can be completely matte from an abundance of small crystals.


Damp Box

A control device in a fuel burning kiln to control the draft into and through the chimney. There are to major styles an active damper that restrict the flow by impeding the flow of gasses. The other is passive that allows the draft from the kiln to pull air throw part of the wall of the chimney. This also cools the chimney or allows the fresh air to burn up the left over fuel that made it out of the kiln.



Related to pug mills that have a vacuum pump that can pull air from a clay body.



Decals have changed a lot some can be printed at home with certain laser printers and special paper, others are manufactured. The range now from being able to be applied and fired to higher temps with durable colors like black blue and sepia, or full color decals at low temps.



Adding a material like soda ash or sodium silicate to reduce the amount of water it takes to produce a liquid slip.



Glazing pottery by immersion in a glaze suspension.


Thermal change, firing stress or a clay and glaze mismatch that fractures a piece.



Porous pottery fired at comparatively low temperatures. Compositions vary considerably, and include both prepared and 'as dug'; the former being by far the dominant type for studio and industry. Always oxidation fired. Fired colours range from white to red, depending on the raw materials. 


Simple machine that presses clay through a die by using a lever and a plunger. They make more complicated one that are

Fire Clay

Secondary clay, normally coarse and larger in particle size, less plastic than ball clay.



Heating clay enough to drive off the chemical water in clay and cause its parts to sinter. Once clay is fired it is no longer plastic and cannot be turned back into clay.



Using a material like vinegar or epson salt to reduce a glazes fluidness. Helps keep some glazes in suspension to prevent hard panning.



The part of a piece that is in contact with the surface it sets on.



A material with a comparatively low melting point, and used to promote fusion in a given mixture of raw materials. Examples used in ceramics bodies include feldspar and nepheline syenite.


Like a glass but less fluid, glaze has the addition of Alumina from the addition of clay to prevent running off the piece when melted. Used for many reasons like color, texture, and to produce a surface that is easier to clean.



Work that is yet to fired, can be leather hard to bone dry. It doesn’t have too be green though


Iron Oxide

A common oxide in glazes and some clays. The fired colour depends on a number of factors, such as concentration and firing atmosphere.


A method of forming using a spinning wheel a mold and profile rib on an arm. Used mostly in a manufacturing setting it produces consistent ware and takes less time to learn that throwing.



Primary clay, white in color and used to produce white clay bodies like porcelain


Kick wheel

A style of wheel where the inertia of the wheel allows a potter to throw. Korean kick wheels are constantly kicked to maintain speed while others have a large heavy flywheel and are kicked intermittently.


The tool we use to fire clay. Many ways of heating them exist and many styles are used around the world.


Kiln Brick
Kiln furniture

The posts and shelves used to hold work in a firing. Made from materials that are able to withstand high temperatures and not warp or crack. Tile setters, plate cranks, and stilts can be included in this.


Kiln wash/batwash

A coating applied to kiln shelves to make removing glaze runs easier, to protect a shelf from atmospheric conditions in a kiln like ash or salt, and reduce the chances of plucking.


Leather hard

As clay dries it reaches a stage where its lost some of its plasticity and is stiff enough to hold a more extreme shape. There are variations used to describe leather hard like hard or soft.



A type of glaze used at very low temps (020-018) that leaves a layer of metal on the surface. While normally done over a gloss glaze to get the most shine it can be done on any surface. There are many kinds from the solid metallic look of gold, platinum, palladium and copper, to iridescent coating like Mother Of Pearl.



Adding a colorant over an unfired opaque glaze. Often done as a low fire technique with a red terracotta clay body and a tin glaze.


Matte Glaze

A dull-surfaced glaze with no gloss. 


Carving or denting a surface that will be filled with a slip engobe or underglaze. After the inlay has set the extra is scraped off. It can be further cleaned up after a bisque.


Off the Hump

Throwing small work from a larger piece of clay.



A mineral precipitate formed when a glaze contains soluble material and is subjected to temperature changes. They are like little round balls as small as a sprinkle and up to nerd.


Once firing

Where unfired work is glazed and brought through low temps like it was a bisque but continuing on through to glaze temperatures.


Oxidized Firing

A firing in a kiln with an oxygen containing atmosphere.

Over Fired

When an article has been subject to excessive firing, either time or temperature, such that the physical properties have been adversely affected.

Peep, Peephole

A hole in a kiln used to monitor the firing. A space where cones can be viewed.


Pinch Pot

Vessel formed by pinching from inside and outside of a form to thin and shape.


Pit Firing

Low fire, atmospheric firing done in a pit or a shielded above ground fire. Colors are from carbon, iron, copper, and salts.



White smooth clay that when fired to maturity are vitrified. Porcelain can now be produced along a range of temperatures and not just high fire anymore. When made thin the ware can also be translucent.


Potter's Gauge

A measuring tool used to ensure that thrown pots are of uniform size or shape.

Potter's Wheel

A pretty loose term for anything that spins and pottery is thrown on. There are many means of powering or getting the wheel to rotate like kick wheels, electric wheels, some are spun by hand.



Fired clay things with a functional use. Cups, bowls, plates, planters are all pottery.


A machine that processes clay by pressing it through paddles and the wall of the pugmill. Some have an added feature of creating a vacuum which removes all the air from the clay which produces a wonderfully plastic clay. Brands include, Peter Pugger, Shimpo, Bluebird, Bailey, Venco. Some brands have models that mix in batches and other that mix as a continuous feed, some are twin auger, Stainless steal instead of aluminum, rate of pugging control, finding the right machine for you needs can be daunting especially with the cost of a new machine.



Used with a thermocouple a bimetallic probe produces electrical current when heat is applied, the pyrometer reads the current and translates it into temperature. As most will need to be calibrated to be truly accurate they are a great tool to determine rise and fall of temperature but not as good to determine temp at higher ranges.


Quartz Inversion

This really are two different things stuck under the same title. The first relates to the tea ceremony and the family name associated with a style of tea ware. The second is American Raku which, as the story goes was started with Paul Soldner. After removing a piece too hot to handle from a kiln it rolled into a hay bale and caught fire. Smothered the glaze finish was different. Its been developed into many different lowfire process in which a piece is heated in a kiln and removed while still hot. From that point the treatment departs greatly from using horsehair, vitreous slips or glazes meant to crackle or flash in post firing reduction.



In a kiln the uses fuel to produce heat (Natural gas, propane, wood) for example reduction is limiting the oxygen entering the kiln till there is not enough to burn the fuel in the kiln. There are many reasons to do this or avoid it. Starting reduction to early can cause clay and glaze faults. The common cone that it should be avoided until reached is 012. For shine with soluble salts its important for reduction to start soon after 012, too late and the carbon will not be trapped in the melting soda ash. Some colors like blue in iron bearing celadon, Reds in copper glazes and blues in rutile glazes are impacted by the length strength and timing of reduction during a firing.



A clay, body or article that has not been fired.

Raw glazing

Adding glaze to unfired work. It can be leather hard to bind dry, glazing for once firing.



Used over a clay surface to prevent something in the next stages to effect the whole surface. Common would be wax resist to prevent glaze from adhering but tape, oil, latex, shellac, can all be used as well.



Flexible or rigid tool used to shape a pot or smooth a surface. They are made from different materials depending on the application. They come in wood, rubber, plastic, metal, coconut shells, really anything can be made into a rib.



A type of kiln furniture that can be used to protect a piece from the firing environment, or trap it in a controlled one inside. The can be any shape but are made from durable clay and have a way to close them. Sometimes the bottom of one becomes the lid for the one below. With coal firing and early wood firings they were used to keep some glazes from being ruined by reduction, and the ash produced in a firing. Used in lower temps they can be thought of as a mini pit or barrel firing. The pot and reduction and metallic material are loaded in and fired. This really should be done in a fuel burning kiln so the exhaust gasses can be eliminated out of the kiln and space.


Salt firing

After burning dried drift wood the salts were found to leave a glassy surface on the pots. Now its done in fuel burning kilns like gas or wood, normally at a high cone like 9-11. Salt is introduced near the end of the firing. When its heated the salt separates into sodium and chlorine gas, the sodium is attracted to the silica in the clay and produces a glass on the bare clay.



Scratching at the surface of the clay to loosen it up as a way to join to pieces.


Secondary Clays

Clay that’s been moved by water and settled. The further from its source of origin and the slower the body of water the finer the clay.



A layer of some colored surface like slip or underglaze that is carved through to show the clay body underneath.



As a piece dries the space that the water was taking up allows the clay to contract. In the firing as particles like feldspar and the clay itself start to melt together removing the air space in the clay causing another contraction.



Flat piece of clay, can be made all sorts of ways like a slab roller, rolling pin and table throwing.


Slab Roller

Rehydrating dry clay. Best done when the clay is completely dry. As the surface becomes wet it expands and falls off to allow the next layer to become wet. If the water is left still it works well if mixed the particles stick to the newly wetted clay and seal it up preventing further wetting.



Really wet clay, wet enough that it becomes a fluid. Can be used for casting when additions like a deflocculant are included. Decoration and attaching.


Slip Casting

A technique for shaping an article by pouring a deflocculated, high-solids content slip into a porous, often plaster, mold.

Slip Trailing

Applying slip throw a narrow tip to leave a raised area of slip.



pottery where decoration in slip is a main feature. Includes slip-painting, slip-trailing, and many other techniques


Another name for slurry.


The unwanted deformation of an article occurring at high temperature in a kiln. Also known as pyroplastic deformation.

Slump Mold

An aqueous suspension of clay and water. Generally lower solids content than slip.


A period during a firing cycle when a set temperature is maintained. The period of time at the maintained temperature is called the soak, hold or dwell.

Specific gravity

Density of a material related to water. Used to measure slips and glazes for water content. Can be done with an accurate measurement of volume and weight, or with a hydrometer.


Spiral wedging

Wedging the clay off axis to form a cone or spiral shape. Can handle more weight than straight rams head wedging.



Decoration technique whereby small moulded pieces of body are applied to an article before firing. Results in a relief decoration.


A thinned mixture of colorants used to create highlights and lowlights on a piece. A stain is applied to the surface and wiped off leaving it in the deeper textures of a piece. A stain can be just oxides and a flag or a thinned glaze or underglaze.



Used as kiln furniture to support work with a glazed bottom. Normally associated with low firing some stilts to not make it to higher temps with out failure.



The term has less meaning now that porcelain has a wide range of temperatures so does stoneware. Its a clay from mid to high fire that contains fine grog or sand up to coarse. Its not bright white or translucent but is fired to vitrification.



Its a term used to describe a kind of clay body and a type of finished work. As a clay body its a red clay body colored by natural iron present in the secondary clay. It can be tempered with sand, crushed shells or other minerals like mica to give it different properties. As a description of a finished work terracotta generally means unglazed red ware that’s low fired.


Terra Sigillata

We commonly think the translation means sealed earth or fine earth but the most direct translation is clay bearing small images. For us its a thin slip made from deflocculating a clay to get just the smallest particles. Its used cover low fire atmospheric work that’s burnished to make a shine. It is also more receptive and whiter than bare clay. It takes colors from the firing better. If fired hotter it loses its shine as the clay underneath becomes sintered and vitrified.


Thermal Shock

High stress in an article due to the sudden creation of a large thermal gradient. If the stress is sufficiently high it can result in the formation of a crack, such as a dunt.


A probe used to gauge temperature in a kiln. Works by having a pyrometer read the electrical variation of the bimetallic probe. They come in different materials and the most common for us is the K-type but a more expensive, accurate and longer lasting can be had with a S-type made from platinum.



Simply making work on a surface that spins which we refer to as a wheel.



A description of the coarseness of a clay body. Tooth can be Any granular addition but is often in reference to sand or grog. The bigger and more abundant the material the toothier it is.



When throwing on the wheel its a description of the force of drag on the thrown form.



Removing or finishing the bottom of a thrown form on the wheel. Used to make a foot, reduce weight or finish the form.


Under Fired

When an article has been subject to insufficient firing, either time or temperature, such that the physical properties have been adversely affected.


Many commercial varieties exist but they all consist of a colorant and enough flux to get it to stick. Some are nearly like a glaze while others are more like a thin slip. Nearly all of them can be applied from the wet stage and in any instance of dryness and on bisque. Some use it over a dry glaze before its fired like in a majolica. Underglazes in general are pretty stable and do not change color or move in the firing. Some glazes though can mute, eat or change some colors. Zinc free glaze is often recommended for reducing the change. Thick clear glazes can cause them to run.


Process by which ceramic raw materials bond to become non-permeable after firing.


A few styles exist for studio ceramics now, the common emulsified wax that’s just mixed with water and applied a resist for glazes or to slow drying. There is also hot wax which would need to be melted in order to use.


A procedure for preparing clay or a clay body by hand: the lump of clay is repeatedly thrown down on a work bench; between each operation the lump is turned and sometimes cut through and rejoined in a different orientation. The object is to disperse the water more uniformly, to remove lamination and to remove air.


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