Glossary
Below is our glossary on all things clay and ceramic. The glossary is under construction, so be sure to check back later.
A
Earthenware is a type of clay and is easy to work with, but not as strong as other types of clay. It is also permeable unless you add a layer of glaze. And even with that layer of glaze, an earthenware pot is quite fragile: the edges break off easily. Earthenware is fired to a maximum of 1100°C.
Trimming is the finishing of a thrown piece when that piece is leatherhard.
See Trimming tool
A lead-based transparent glaze becomes yellow and opaque by adding antimony oxide. But other colors may appear, depending on the composition of the base glaze.
The main component of ash glaze is, as it says in its name, ash. Ashes of various types of wood or straw are used. You can find more info and inspiration here.
B
Ball clay is a type of clay. It is related to kaolin, but its fine particles make it more plastic than kaolin. It contains iron, magnesium and calcium oxides in addition to small parts of potassium and sodium oxide. Like bentonite, it is used in glazes as a suspending agent.
Batwash is a product you use to protect the kiln shelves of your ceramic kiln. As a result, dripping glaze will cause less damage. You can buy batwash at the pottery supply store, or you can make it yourself.
Bone china is the strongest and highest quality porcelain. It contains a high percentage of bone ash, mostly derived from bovine bones, which contributes to the distinctive strength and lightness of the resulting porcelain. So this porcelain clay is very strong, can be fired at 1250°C (which is why it is called soft porcelain) and is also translucent.
This is a sticky primary clay. Adding this to other clays makes the clay more plastic. It can be added to glazes to optimize suspension.
see Bisque fired
After drying, clay is fired a first time in the ceramic kiln, called the bisque fire. Your piece will be lighter in colour afterwards and will have already shrunk a bit. The clay is now rock hard and permanent, but not yet waterproof. Bisque fired pieces still absorb water; they are ready to glaze.
A body stain is a ceramic pigment usually purchased in powder form. Before, it was mainly used to add colour to clay, hence the name body stain. Nowadays, there is little difference between a body stain and a glaze stain or underglaze. They are both used to colour clay, clay slip and base glazes.
Bubble glaze is a decorating technique where you mix (under) glaze with a few drops of dish soap and water in a jar. Then you blow bubbles into the jar until the bubbles rise to the top. You transfer the bubbles to the part of the pot you want to glaze, and the texture of the soap bubbles leaves an interesting circular pattern on the surface of your piece. If you want, you can add a transparent glaze on top.
C
Centring is one of the basic steps of throwing clay on the potter’s wheel. This step aims to get all the clay particles centred on the potter’s wheel. Also read 8 common mistakes and how to fix them about centring.
Grog is a granular or powdery material made from fired clay. It is added to clay to make it stronger, to add texture, and to ensure that the clay shrinks less when drying. You will often see on the label the size of the grain in millimeters, and the ratio of grog to clay.
See Kaolin
See kaolin
This is a colour-adding metal oxide used to give glazes a green colour. But it can also create other colours, depending on the composition of the base glaze.
See cobalt oxide.
The colombin technique consists of building a pot out of rolls of clay.
Cones are small cone-shaped objects composed of a mixture of specific ceramic materials. They bend and melt at a certain temperature and after a certain length of time. They are mostly used these days to check whether your ceramic kiln is actually reaching the right temperature. Read more about it here.
A crawl glaze is a type of glaze that, when applied to ceramics, creates a pattern of cracks, fractures or flakes. These cracks can range from small cracks to more pronounced and larger patterns. The result can resemble reptilian scales or cracked earth, depending on the specific effect the potter wants to achieve. Crawl glaze shrinks during firing, creating small islands of glaze. The bare patches between these islands reveal the underlying clay.
D
A damp box is a sealed container used to keep clay or ceramic components damp during the creation process. It is a useful tool to prevent clay from drying out while working on a project. The damp box helps to keep the clay in an optimal damp condition, ensuring it’s still workable and doesn’t dry out or crack during the shaping or sculpting. A damp box contains a layer of plaster. By adding water to the plaster and sealing the container, you create a humid environment that allows your pieces to dry very very slowly, ensuring they stay in the same condition for a very long time.
A decal or transfer is used to transfer an image to a ceramic object. It’s a drawing printed with ceramic pigments on specially treated paper. Here you can see examples.
A deflocculant is a substance that prevents clumping, also called “deflocculating”. This creates a lower viscosity without adding any extra water. Deflocculants are frequently used for casting clay.
Dipping is one method to apply glaze to a pot. The piece is partially or fully immersed in glaze by hand or with glazing tongs.
A potter’s wheel or pottery wheel is a tool to throw clay. You can easily create round shapes with a wheel.
You use a press mould by pressing tender clay into it to obtain a certain shape. You can push the clay into the mould, or you can take slices of clay and place them over the mould. A press mould is usually made of plaster, but you can use all kinds of other objects as a press mould. Just make sure that your clay does not end up sticking to the mould. Read more about it here.
The process by which you let pots dry at low temperature in the ceramic kiln before the first firing to get rid of the last moisture left in your clay.
As clay dries, it takes on the same moisture level as the air in the room. So your pot may not be completely dry. To get rid of that last bit of moisture, you can fire your pot at low temperature in a ceramic kiln before bisque firing. The goal is to allow the moisture in the clay to evaporate completely so that the object is sturdy enough to withstand the high temperatures of the firing process without cracking or deforming. That process is called dry firing. It’s often integrated into the bisque firing.
A thumb pot is made by pinching a ball of clay into the right shape. It is a handbuilding method.
E
An engobe is actually nothing more or less than liquid clay, i.e. clay slip. When ceramists talk about engobes, they usually mean clay slip that has been coloured with oxides or pigments. But even if you only use the natural colour of a type of clay, it is just as much an engobe. For example, consider a layer of red clay over a piece thrown out of white clay. Read more about it here.
Epsom salt, also know as bittersalt or magnesium sulfate, is a flocculant, i.e. a thickener. It can be added to a saturated solution to give the glaze the right consistency, among other things. And it is also a wonderful glue for attaching two pieces of dry clay, when a handle has fallen off, for example.
F
Faceting is a decoration technique that involves removing slices of clay from a leather-hard pot with a knife, trimming tool or cutting wire in order to create flattened, relatively small, areas on the surface of the pot. Find inspiration here.
Filigree is a decoration technique that involves cutting out slices of leatherhard clay to create consistent, intricate patterns You can read more information and get inspired in this article.
See Refractory clay
‘Fired-On images transfer paper’ is a type of printing paper you can use with a laser printer. With these prints you can transfer detailed drawings, photos or text to your ceramics. The black toner from the printer contains iron oxide, and turns sepia brown after firing. So your drawing, photo or text will turn sepia brown after firing.
See flux.
A frit is a powdered glaze ingredient created by melting water-soluble or toxic materials (such as lead) with silicate to form a silicate compound. The mixture cools down into a lump and is then ground into an insoluble powder that is less dangerous than the original raw materials.
G
When talking about tableware in ceramics, we refer to objects and pots that will be used for eating and drinking from. When making tableware, it’s important to use food-safe glazes.
Casting clay is liquid clay. But it’s more than just clay powder and water alone; it also contains waterglass and (calcined) soda. You can make pottery with it by pouring the clay into a plaster mould.
A casting mould is a plaster mould made of one or more parts. You can fill the mould with casting clay and make a series of the same shapes. You can see how that works in this article.
A plaster slab is a flat surface made from plaster. It is a very useful object to have in your pottery studio. You can use it to make clay dry faster because the plaster draws the moisture out of the clay and absorbs it. It can be good to do your wedging on plaster and it’s also indispensable in the process of recycling clay.
A glass former is one of three main ingredients of a glaze, in addition to the flux and stabiliser. Glass formers melt only at very high temperatures and create a glass-like layer on your ceramics. You can learn more about the composition of glazes here.
Glazing is the application of a layer of glaze to a ceramic piece after the initial firing or bisque firing. This is followed by the glaze firing in the kiln.
Glaze is a powder coating that you apply to your pot after the first firing or bisque firing. This is followed by the glaze firing in the kiln. Glaze makes your pot waterproof, forms an extra layer of protection and is often decorative as well. A good glaze consists of three groups of ingredients : glass formers,fluxes and stabilisers. Read more here.
See glaze firing
Griffin Grip is a brand name. The Griffin Grip is a handy tool for throwing pots. The tool snaps onto the potter’s wheel and has sliders that help you easily centre your clay perfectly. It holds your work in place, so you don’t have to secure it with clay rolls or a cloth.
H
After applying glaze, your piece is fired a second time. This is called the glaze firing. For stoneware, the clay is fired at 1,200 to 1,400°C. The glazed clay takes on a glassy appearance and becomes waterproof. When you tap a workpiece, you hear a high-pitched sound.
See glaze firing
I
Iron oxide is a common oxide in clay and glazes. It is often added to give a glaze a red colour, but it can also create other colours depending on the composition of the base glaze.
See Mishima
See bone china
J
K
Kanthal wire is silver-plated copper wire and can withstand the extreme temperatures in a ceramic kiln. The wire can be used to reinforce clay objects, to decorate sculptures, when making jewelry out of clay and when adding materials to objects that will be raku fired.
Kaolin is a type of clay that occurs naturally in the Earth’s crust. It is also known as porcelain clay or china clay. It has a fine texture and light colour, and is the main component of porcelain. The special properties of kaolin can make ceramics stronger and more resistant to heat.
A kiln is a special oven for firing clay. It is an oven that can fire at temperatures over 1,000° C. Read more about it here.
A ceramic pencil has a core of ceramic pigment. You use it to draw on bisque fired pieces.
Carving is a decoration technique that involves carving out (thick) slices of clay from a leather-hard pot to create an interesting relief or texture. Here you can see some examples.
Kintsugi is a technique of Japanese origin. The shards of a broken pot are glued back together using an adhesive based on urushi-lacquer. A thin layer of precious metal is then sprinkled on the fracture lines, usually gold, silver or platinum. You can read more about the philosophy behind kintsugi in this article.
If you add a lot of water to clay, you get clay slip. You can use slip to fill a mould, to glue different parts of a piece together, or to apply certain decoration techniques.
A clay roller is a device you use to roll out clay into clean and even slabs.
Cobalt oxide is a metal oxide used to give glaze a blue colour.
Clay contains a lot of water. As that water evaporates during drying and during firing, the clay particles move closer together, and that means your pot shrinks. Something to consider when deciding on the size of your pieces. The percentage of shrinkage depends on the type of clay, often it is also listed on the packaging or can be found on the supplier’s website.
A calliper is a tool that allows you to measure the inside and outside dimensions of your pots.
Quartz is a mineral and a component of clay. It is a form of silica. More than 12% of the Earth’s crust consists of quartz. This makes it one of the most common minerals. Sand and granite consist mainly of quartz.
When clay is fired, first the water evaporates. Then the various organic substances in the clay also begin to oxidise. When the temperature reaches about 573°C, the quartz jump takes place. The quartz crystals in the clay rearrange, causing the clay to expand about 1% in volume. If this happens too quickly, it can lead to cracks. It is important to increase the temperature gradually and let the quartz jump happen slowly. If you were to remove your pot from the kiln at this point, it will shrink again by 1% while cooling down.
L
Clay that has been drying out for a while is called leather-hard. The clay will retain its shape, but you can still manipulate the surface. This is the best time to carve, polish and finish the clay.
A rib is a tool that allows you to make clay surfaces nice and smooth. There are rib tools made from wood, plastic or metal. Rib tools come in different shapes (angular, rounded…) and sometimes they also have a special edge that allows you to add texture to the clay.
Lusters are metallic, oily liquids that come in many different colours. With certaing lusters you can achieve beautiful colour effects, such as pearl or shiny gold. Read more about lusters and their applications in this article.
M
Majolica is a decoration technique that involves applying a layer of tin glaze, which creates a white background, and then adding colour to that unfired layer of glaze. You can use metal oxides or frits to do this. Read more here.
A mould is a tool that allows you to create the same shape over and over again. For ceramics mainly plaster moulds are used, but you can basically use any object as a mould. There are press moulds, where you press clay into the mould and casting mouldswhere you pour casting clay into the mould. You can buy moulds or make your own.
Manganese dioxide is a metal oxide that is used to give glazes a purple or brown colour, but it can also create other colours depending on the composition of the base glaze.
Throwing off the hump is a technique used on the potter’s wheel that allows you to quickly throw many small objects. You centre a large amount of clay, create a large cone, and use only the top part to make a small pot. When you’re done, you cut it off and then use the new top part ot the cone to make the next pot, until you run out of clay. Because you don’t have to re-centre the clay each time, you can make a lot in a short period of time.
A measuring gauge is a useful tool when throwing on a potter’s wheel. A measuring gauge helps you create uniform shapes by giving you a target to aim for, so each piece has the same height or diameter each time. It has one or more arms with a pointer that shows you how high or how wide the piece should be. The perfect tool for production throwing.
See Nerikomi
Trimming tools are fine instruments that are useful for many techniques. They also come in handy when trimming and cleaning the base of your pot. They usually consist of a wooden or plastic handle and a metal loop. There are wire trimming tools and ribbon loop trimming tools.
Mishima is a traditional inlay technique. The technique results in a coloured drawing or line pattern that is integrated into the surface layer, without relief. Learn how to do this here.
A saggar is an unbreakable small container in which ceramic work is placed while it’s in the kiln. Before, saggars were used to protect pieces from flames, gases and debris, which was especially necessary with wood or coal burning kilns. Nowadays saggars are primarily used to create a “microclimate” in the kiln, catching smoke, fumes and gases inside. These can have an effect on the pieces inside the saggar. You can fill a saggar with all kinds of materials: salt, metal oxides, sulfates…. each material gives specific effects. It is best not to use a sagger (too often) in an electric kiln: the smoke, gases and fumes can damage resistors.
Molochite is a specific type of grog. It is made of kaolin and is pre-fired at very high temperatures. It doesn’t expand much when heated, which means it can resist thermal shock well. If you want to make very large or complex pieces, you can add molochite to the clay. Adding molochite reduces the amount of stress your piece will be under during the firing and cooling process.
A single firing is a firing where a piece is fired only once and immediately at a high firing temperature. Unfired pots are glazed and fired only once. So the bisque firing is skipped.
See mould
See making a mould.
N
Naked raku is a rather recent variation on raku firing. A special engobe is applied to the pot, which prevents the raku glaze from adhering fully. The glaze shrinks and flakes off after firing. Only black lines and dots, caused by smoke migrating through the crackle in the glaze, remain on the pot. Here you can see some examples.
In the nerikomi technique, several layers of coloured (porcelain) clay are placed on top of each other and rolled. Whatever you end up throwing or handbuilding with this clay, will have special coloured patterns. Take a look here.
Nickel oxide is a metal oxide that is used to give glazes a brown colour, but it can also create other colours depending on the composition of the base glaze.
O
You apply underglaze to a pot that is leather-hard or bisque fired is. You can paint and draw with it. Finish by adding a layer of transparent glaze so the colours really stand out. Underglaze exists in powder form, in pans, in pencils or as a liquid in jars or bottles.
Overglaze is a decorative glaze with a low melting point. It is also called melt paint or enamel. Overglaze is used to decorate a high-fired piece. It is then fired one more time (at 750°C to 850°C) so that the new colours of the decoration are baked into the glaze that was already in place.
Pulling up is one of the basic steps of throwing clay on the potter’s wheel. While pulling up, use your fingers on the inside and outside of your pot to create height.
The ox head or ram’s head method is a specific way of wedging clay. Read more about it here.
see Batwash
When you fire clay in an electric kiln, this is called an oxidising firing. This is because there is oxygen in the kiln, which causes the metals in the clay and glaze to oxidise. An oxidising firing provides bright colours.
Oxides are an ingredient of glaze that consist of oxygen and another element, such as silicon or calcium. Oxides can have several functions. They can act as a flux or glass former, but mostly they add colour to your glaze. They can also be added to underglaze, clay slip, or to the clay itself. Read more about it here.
See Oxidation
P
Paper clay is clay to which cellulose fibers have been added. This makes your work less likely to tear or crack when drying and thus stronger and more shock resistant than ordinary clay. With paper clay, in addition, you can make your clay much thinner, so paper porcelain, for example, becomes translucent. The cellulose fibers burn away during firing.
Paper porcelain is a special kind of porcelain. It is porcelain clay to which paper fibers have been added. These paper fibers provide extra strength, allowing you to handbuild or throw thin walls while keeping the translucent nature of porcelain. Here you can find some examples. And also our online courses on paper porcelain.
Pehatine is a commercial product, it is a brand name. Diluted with water, it is used to make powdered glaze. A glaze or engobe made with Pehatine, is easier to brush on, creates fewer streaks when brushing and is smudge-proof. It ensures that the glaze does not separate in the bucket and the suspension stays. It reduces hairline cracks or crackling.
Pigments are a more “processed” version of oxides. They are made by heating a combination of different oxides so that they fuse together, and then grinding them into a powder. The colours are much more predictable and they exist in a wide colour range. Pigments can be used to add colour to clay, slip and glaze. The colour of the pigment closely resembles the colour it will have after firing (which is not the case at all with oxides ). Read more about it here.
Pit firing is a firing method. Pieces are first polished smooth and bisque fired. Then they are fired again in a pit in the ground filled with sawdust and wood. You can also add metallic salts to achieve certain colours. The fire is lit from above and then burns downwards. The smoldering sawdust and the impact of oxygen on the metal salts creates a natural colour and surprising effect on the clay.
Porcelain is the strongest, and also the most expensive type of clay. It is a lot stronger than earthenware, but also harder to work with. You have to work hard to shape the clay, and porcelain doesn’t hold its shape easily. You need a lot of water to make porcelain workable, which limits how much time you have to work with it. It also shrinks a lot in the oven, so the chance of cracks is pretty substantial. Porcelain is fired at 1300°C or even more.
See kaolin.
See Pottery wheel
A pyrometer is a digital device that displays the temperature in the ceramic kiln, which is measured by the thermocouple in the kiln.
Q
R
Raku is an ancient Japanese firing technique for ceramics. After the pieces are bisque fired, they are glazed with special raku glazes and fired again. The pieces are taken out of the kiln while still red-hot, and put into a barrel of sawdust. Once the sawdust catches fire from the heat of the clay, the barrel is closed and the pieces stay in the smoke. Rapid cooling causes the glaze to show hairline cracks or crackles. The smoke soaks into the hairline cracks and creates the typical raku effect. You can read more about raku here.
see reclaimed clay
Reclaimed clay or recycled clay is clay made from your old scraps of unfired clay: dry bits, trimmings, failed pots that were not yet fired, slip. You can change these back into workable clay. So it is actually recycled clay. How to do this recycling, you can learn here.
See reduction.
A reduction firing is generally done in a gas or wood kiln. Reduction means the supply of oxygen is limited. Because there is little or no oxygen present in the kiln, the flame seeks oxygen and will extract any oxygen that is present in the clay and glaze. Because of that extraction, a chemical reaction takes place with the ingredients of the glaze, which (sometimes) changes their colour. Well-known reduction glazes include copper red and celadon. Those glazes would turn green and yellow-brown in an electric kiln.
A precision applicator or slip trailer is a small instrument that allows you to very precisely apply engobe or clay slip to your work.
Smoke firing is a traditional firing method. You don’t use a glaze, but simply fire the piece as is in a barrel or in a saggar with combustible material. That barrel is also called “saggar”. The smoke created during firing penetrates the clay, leading to an effect of slight tonal differences up to deep shadows on your piece. Smoke firing is typically done with a layer of terra sigillata, a fine clay slip, on top of your clay surface.
S
See Saggar
See Smoke firing
Shellac is a product used by painters and restorers, either as a varnish or as a fixative for charcoal and chalk drawings. It is extracted from the shells of a species of louse. You can dissolve shellac in alcohol. In ceramics, shellac is used to decorate pieces and create a relief. After the shellac dries, you can wash away the uncoated clay around it with a damp sponge. Wherever the shellac is, the clay won’t disappear and you’ll create a relief on your work.
The material that your piece is made of, the clay itself actually, is sometimes called the shard. In particular, one refers to fired clay that is not glazed.
A kick wheel is a manual potter’s wheel for throwing clay, where you power a concrete flywheel with your foot. It requires quite a bit of physical effort, and a bit more coordination from your body than an electric potter’s wheel. But you have complete control over the rotation speed. A potter’s wheel is very silent, but it is large and very heavy to move.
Sgraffito is a decoration technique. The term comes from the Italian “sgraffiare,” to scratch. Using a fine instrument, you scratch away the top layer of clay to create very fine lines. Read about it hereand see examples.
Clay sintering is a process that occurs during firing. At a temperature of 850°C to 900°C, the silica particles in the clay melt, causing the clay to “vitrify”. The clay becomes firmer and waterproof, and also begins to shrink a lot. This is because the sintering particles adhere more tightly together, filling in the empty spaces created by organic matter that previously burned away.
The moment in the firing process when the clay sinters, is called the sintering point. Often the maximum firing temperature of your clay is the temperature at which the clay sinters and becomes glass-like. Especially with stoneware, that point is more of a line. The line starts at the point where the clay starts to vitrify and goes up to the point where the clay begins to deform.
A flux is one of three main ingredients in a glaze, in addition to the glass former and stabiliser. A flux lowers the melting point of the glass former so your glaze will melt evenly. You can learn more about the composition of glazes here.
See Flux
See overglaze
Spiral wedging is a method used to wedge clay. Read more about it here.
A stabiliser is one of three main ingredients in a glaze, in addition to the flux and glass former. Stabilisers stabilise the process of melting and glass forming, stop the flowing glaze at the right time and ensure the glaze has the right viscosity. You can learn more about the composition of glazes here.
Stoneware is a durable type of clay, which is also quite easy to work with. It easily holds its shape, and after firing it becomes very strong and watertight. Because of these benefits, this clay type is most often used to make high volumes of ceramics like tableware, and it is the most recommended clay type for beginners. Stoneware is fired between 1200°C and 1300°C.
A pug mill is a machine that presses recycled clay into big rolls. Some pug mills come with a vacuum pump that squeezes the air out of the clay.
A suspension is a mixture of two substances where one substance, in the form of very small particles, has been mixed with another substance. The mixture does not separate easily. Examples of suspensions include paint (a suspension of dyes in water or turpentine), orange juice (a suspension of pulp in fruit juice) or clay (a suspension of water droplets in a structure of stone particles).
T
Terra sigillata is a decoration technique that involves applying clay slip from ferruginous river clay to leather-hard clay. Once this slip layer has been polished, the terra sigillata gets its characteristic shine. You can achieve different colours and shades depending on the clay powder you start with. Read more about it here.
The thermocouple is a sensor inside the ceramic kiln that measures the temperature. It is connected to the pyrometer which displays the temperature measured by the thermocouple.
Tin glaze is a white, opaque glaze containing tin oxide. Among other things, it is used for the decoration technique majolica.
See Decal.
Tripod stilts are triangular supports that you use to stack your pieces in the kiln so they don’t stick to the kiln tray. You can use ceramic stilts (which can only be used once) or metal stilts. The stilts leave a small mark on the bottom of the fired pieces, but you can remove this mark if necessary by sanding. When glaze firing stoneware at high temperatures, tripod stilts are usually not used because they would cause the piece to deform.
U
See Wax resist.
Urushi is a type of lacquer extracted from the resin of the Japanese Rhus tree. It is an organic material that hardens when it comes in contact with oxygen and water.
V
Viscosity refers to the thickness, consistency or texture of a liquid. For example, water is liquid with low viscosity, honey has a high viscosity. Fluids with high viscosity are called viscous.
See viscosity.
See flux.
A piece is food-safe if every part that comes into contact with food or drink is glazed with food-safe glaze. Whether a glaze is food-safe is always clearly mentioned on the label. You can read more about food-safety in this article.
see Refractory clay
Also called refractory clay or fire clay. It is a type of clay that can be fired up to 1,500 °C and is used, for example, to make kiln plates and stacking material.
W
Wax resist is actually liquid wax. It prevents glaze from adhering to the clay. The wax disappears completely during the firing. So you can use it to keep parts of your piece free from glaze. It is often applied to the bottom of a pot to prevent the glaze from sticking to the kiln plate. And it’s also used for decorative purposes. In this blog post on working with wax resist, you can see some examples.
See overglaze.
