Glossary ceramics
Below is our glossary on all things clay and ceramic. The glossary is under construction, so be sure to check back later.
A
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.
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.
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.
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.
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.
Burnishing is a technique where you rub a hard object, such as a spoon or a smooth stone, over the surface of your workpiece to make it mirror-smooth and shiny. You can see what that looks like in this article.
C
A pottery caliper is an instrument that allows you to measure the inside and outside dimensions of your pots.
Carving is a decoration technique where you carve away (thick) slices from your leather-hard piece to create an interesting relief or fascinating texture. Here you can see some examples.
Casting clay is liquid clay. But it’s more than just clay powder and water alone; it also contains sodium silicate 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.
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 solve them about centring.
This is a coloring metal oxide used to give glazes a green color. But it can also create other colours, depending on the composition of the base glaze.
If you add a lot of water to clay, you get clay slip. You can use it to fill a casting mold, join different parts of a piece together, or apply several specific decoration techniques. See also Coloured slip
Cobalt oxide is a coloring metal oxide used to give enamel a blue color.
This technique involves building a piece of pottery by rolling and then assembling coils of clay. Read more here.
A coloured slip is actually nothing more or less than liquid clay, or clay slip. When ceramists talk about ‘engobes‘, they usually mean clay slip that has been colored with oxides or pigments. But if you use the natural color of different types of clay, that is equally a coloured slip. Read more about it here.
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 know if your ceramic kiln is reaching its 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. A damp box contains a plaster board. By adding a little water to the plaster board 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. We have a free guide about how to make a damp box, you can find it here.
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. Read about more about glazing your pots here.
During the drying process, the clay acquires the same humidity level as the air in the room. Therefore, your piece might not be completely dry. To eliminate that last bit of moisture, you fire your piece at a low temperature in a ceramic kiln before the bisque firing. The goal is to completely evaporate the moisture in the clay, so that the object is sturdy enough to withstand the high temperatures of the firing process without cracking or deforming. This process is called the dry firing. It is often integrated into the bisque firing.
The process where you dry your pieces at a low temperature in the ceramic kiln before the first firing, the dry fire, to remove the last moisture that is still in your clay.
E
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.
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 miracle worker in bonding two pieces of dry clay together, when, for example, a handle has fallen off.
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 leather-hard clay to create consistent, intricate patterns You can read more information and get inspired in this article.
Also called refractory clay. It is a type of clay that can be fired up to 1,500 °C and is used, for example, to make kiln shelves and stacking material.
‘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.
Flax porcelain paperclay is a special type of porcelain. It is porcelain paperclay to which flax fibers have been added. The flax fibers provide extra strength, allowing you to build or turn thin walls while still maintaining the translucent character of porcelain. Here are some examples.
A flux is one of three main ingredients in a glaze, in addition to the flux and stabiliser. A flux lowers the melting point of the flux so your glaze will melt evenly. You can learn more about the composition of glazes here.
A workpiece 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 stated on the packaging. We have an article on this here.
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
Giffin Grip is a brand name. The Giffin Grip is a handy tool for trimming 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.
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.
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 ceramic 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.
After applying glaze, your piece is fired a second time. This is called the glaze firing or glaze kiln. For stoneware, this involves a firing of 1,200 to 1,400°C. The glazed clay has acquired a glass-like appearance and is waterproof. When you tap a piece, you hear a high-pitched sound.
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 ceramic kiln. Read more in this blogpost. And here are 3 tips for beginners.
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. Often you will see the size of the grain in millimeters on the label, and the ratio in relation to the clay. Also read our blogpost about grog here.
I
Iron oxide is a common oxide in clay and glazes. It is often added to give glazes a red color, but other colors may result depending on the composition of the base glaze.
K
Lace 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 kick wheel is a manual pottery wheel for clay where you drive the concrete flywheel with your foot. This requires quite a bit of physical effort and a bit more coordination from your body than an electric wheel. You have complete control over the spinning speed. A kick wheel makes no noise, but it is large and very heavy to move.
A ceramic kiln is a special oven for firing clay. It is an oven that can fire at temperatures above 1,000° C. Read more about it here.
Kiln wash or bat wash is a product you use to protect the kiln shelves of your kiln. As a result, dripping glaze will cause less damage. You can buy kiln wash at the pottery supply store, or you can also make it yourself. Read more here.
Kintsugi is a Japanese technique. The shards of a broken piece are glued back together with an adhesive based on urushilacquer. A thin layer of precious metal is then sprinkled on the fracture lines in gold, silver or platinum. You can read more about the philosophy behind kintsugi in this article.
L
Lead glaze is a type of transparent glaze that contains lead oxide as flux. Due to the toxicity of lead, lead glazes are being used less and less, and certainly not for tableware.
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.
Lusters are metallic, oily liquids and come in a variety of colors. With some lusters, you can achieve beautiful colors, such as nacre or shiny gold. Read more about lusters and their applications here.
M
Majolica is a decorative technique where you first apply a layer of tin glaze, creating a white base, and then add color to that unbaked glaze. This can be done with metal oxides or frits. See examples in this blogpost.
Manganese dioxide is a coloring metal oxide that is used to give glaze a purple or brown color, but other colors can also occur, depending on the composition of the base glaze.
The melting point of a glaze or a raw material is the temperature at which the glaze or raw material starts to melt. When creating a glaze, fluxes are used to lower the melting point of a raw material.
Mishima is a traditional slip inlay technique. The technique results in a coloured drawing or pattern of lines that sits ‘in’ the surface, without relief. Read here how to do this.
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 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 moulds where you pour casting clay into the mould. You can buy moulds or make your own.
N
Naked raku is a rather recent variation on raku firing. A special slip 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 coloring metal oxide used to give glaze a brown color, but other colors can also emerge, depending on the composition of the base glaze.
O
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.
When clay is fired in an electric kiln, an oxidation firing takes place. This is because there is oxygen in the kiln which causes the metals in the clay and glaze to oxidize. An oxidation firing results in bright colors. You can read more about this in this article.
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 melter or glass former, but they mainly add color to your glaze. They can also be added to underglaze, clay slip, or to the clay itself. Read more about it here.
P
Papier clay is clay to which cellulose fibers have been added. This ensures that your work tears or cracks less quickly while drying and is therefore stronger and more shock-resistant than regular clay. With papier clay, you can also go much thinner, so that paper porcelain, for example, can become 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. The paper fibers provide extra strength, allowing you to build or throw thin walls and still retain the translucent nature of porcelain. Here find some examples.
Ceramics pigments are a more ‘processed’ version of oxides. They are made by heating a combination of different oxides until they fuse together, then grinding them into a powder. The colors are much more predictable and there are many color variations possible. With pigments, you can color clay, slip, and glaze. The color of the pigment closely resembles the color it will have after firing (which is not at all the case with oxides). Read more in this blogpost
A pinch pot is made by pinching a ball of clay into the right shape. It is a handbuilding technique, you can read more about handbuilding with clay here.
Pitfire is a firing method. Pieces are first polished smooth and then bisque fired. After that they are then fired in a pit in the ground, filled with sawdust and wood. You can also work with metal salts to obtain certain colors. The fire is ignited from above and then burns downwards. The smoldering sawdust and the effect of oxygen on the metal salts give a natural color and surprising effect on the clay.
A plaster slab is a board cast in plaster. It is a useful object in a ceramic studio. You use it to make clay drier because the plaster draws moisture from the clay and absorbs it. Kneading is convenient on a plaster slab and the plaster slab is essential when recycling clay. With a plaster slab you can also make a damp box. Download our free guide where you learn to make your own plaster slab.
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.
A pottery wheel or potter’s wheel is a device for throwing clay. You can easily create round shapes with a wheel. Read more here.
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.
A pugmill is a device that presses reclaimed clay back into rolls. It comes in a version with or without a vacuum pump that presses the air out of the clay.
Pulling up is one of the basic steps in throwing clay on the pottery wheel. During the pulling up process, you use your fingers on the inside and outside of your pot to create height.
A pyrometer is a digital device that displays the temperature in the kiln, measured using the thermocouple inside the kiln.
Q
Quartz is a mineral and a component of clay. It is a form of silicon dioxide. More than 12% of the Earth’s crust consists of quartz. This makes it one of the most common minerals. Sand and granite are primarily composed 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 inversion 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 inversion happen slowly. If you were to remove your pot from the kiln at this point, it will shrink again by 1% while cooling down.
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.
The ram’s head method is a specific way of wedging clay.
Recycled clay is clay made from your old scraps of unbaked clay: dry pieces, trimmings, failed pots, slip. You can process it back into workable clay. So it is actually recycled clay. How to do that, you can read here.
A reducing firing usually takes place in a gas or wood kiln. Reducing means that the oxygen supply is limited. As a result, there is little or no oxygen present in the kiln, the flame seeks oxygen and will extract the available oxygen from the clay and glaze. Due to this extraction, a chemical reaction occurs in the glaze materials, causing them to sometimes take on a different color. Well-known reduction glazes include copper red and celadon. These glazes would turn green and yellow-brown in an electric kiln. Also read this article about this.
A rib is a tool used to smooth clay surfaces. Ribs are available in wood, plastic, or metal. Ribs come in different shapes (angular, rounded…) and sometimes ribs also have a special edge that can be used to create texture in the clay.
S
A saggar is an unbreakable small drum in which ceramic work is placed in the kiln. In the past, saggars were used to protect the workpieces from flames, gases, and debris, which was especially necessary in wood or coal ovens. Now, saggars are mainly used to create a ‘microclimate’ in the kiln, where smoke, vapors, and gases are trapped. These can have an effect on the workpieces. You can fill it with various materials: salt, metal oxides, sulfates… each material gives specific effects. It is best not to place a saggar in an electric kiln: the smoke, gases, and vapors are harmful to the heating elements.
Sgraffito is a decoration technique. The term comes from the Italian ‘sgraffiare’, which means to scratch. With a fine instrument, you scratch away the top layer of clay to obtain a fine drawing. Read more about it and view examples here.
Shellac is a product used by painters and restorers as a varnish or as a fixative for charcoal and chalk drawings. It is obtained from the shells of a type of insect. You can dissolve shellac in alcohol. In ceramics, shellac is used to decorate pieces and create a relief. After the shellac has dried, you can wipe away the uncoated clay with a wet sponge. Where shellac has been applied, the clay does not disappear, and you achieve a relief on your work. You can see examples in this article.
Clay contains a lot of water. When that water evaporates during drying and firing, the clay particles move closer together and your work shrinks. Something to take into account when determining the dimensions of your pieces. The shrinkage percentage depends on the type of clay, it is often also listed on the packaging or can be found on the supplier’s website. Read more in this blogpost.
Single firing is a ceramic firing technique where pottery are glazed in their raw, unfired state and then fired directly to their final temperature in one go. This process eliminates the traditional two-step method (bisque firing followed by glaze firing)
Sinter engobe is a slib that contains a little flux and thus sinters in the oven. As a result, the engobe adheres better to the clay.
The sintering of clay is a process that occurs during firing. At a temperature of 850°C to 900°C, the silicon particles in the clay melt, causing the clay to ‘glaze’. The clay becomes stronger and waterproof, and also begins to shrink significantly. The sintering particles bond more tightly together, filling the gaps that have been created by organic materials that have previously burned away.
The moment in the firing process at which the clay is sintering is called the sintering point. Often, the maximum firing temperature of your clay is the temperature at which the clay sinters and thus vitrifies. Especially with stoneware, that point is more of a trajectory. That trajectory starts at the vitrification of the clay up to the point where the clay begins to deform.
A slab roller is a device with which you roll out clay into smooth and even slabs.
A slip trailer is a tool that allows you to apply clay slip very precisely to your piece.
Smoke firing is a traditional firing method. You do not use glaze, but simply fire the workpiece in a drum or saggar with combustible material. The smoke that is generated during the baking penetrates the clay and leads to fine tonal differences and even deep shadows on your workpiece. Smoke firing can typically be done with a layer terra sigillata, a fine slip, on your workpiece.
Sodium silicate is a water-soluble silicate that is mainly used as a deflocculant in slip casting clay. This makes the clay suspension more fluid without having to add extra water, which makes pouring in moulds easier.
The spiral method is a technique for wedging clay. Read more about it in our blogpost.
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.
Stilts are supports that you use to set your pieces in the kiln, to avoid them sticking to the kiln shelves. There are ceramic (which can only be used once) and metal stilts. The stilts leave a small mark on the bottoms of the pots, but you can potentially remove that by sanding. In a glazing firing of stoneware at high temperature, triangles are generally not used because they could cause the pot to deform.
Stoneware is a strong type of clay that is also quite easy to work with. It holds its shape well and is very sturdy and non-porous after firing. This type of clay is most often used to make large volumes of, for example, tableware, and it is the most recommended type of clay for beginners. Stoneware is fired between 1200 °C and 1300 °C.
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
In ceramics, tableware refers to objects and pieces that will be used for eating and drinking. For tableware, you use food-safe glazes. Read more about food safety here.
Terra sigillata is a decorative technique in which clay slip made from iron-rich river clay is applied to leather-hard clay. Once the slip layer is polished, the terra sigillata displays its characteristic shine. You can obtain different colors and shades by choosing the powdered clay you start with. Read more about it in this blogpost.
The thermocouple is a sensor rod in the kiln that measures the temperature. It is connected to the pyrometer on which you can read the temperature.
A throwing 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.
Throwing off the hump is a technique on the potter’s wheel that allows you to quickly shape many small objects. You center a large amount of clay, form a large cone, and then make a small piece with the top part each time. Once it’s done, you cut it off and move on to the next one until you run out of clay. Because you don’t have to center each time, you can produce a lot in a short time.
Tin glaze is a white, opaque glaze with tin oxide. It is used among other things for the decoration technique majolica.
Trimming is the finishing of a thrown piece when that piece is leather-hard.
Trimming tools are fine instruments that are useful for many techniques. They can also come in handy when trimming and cleaning the base of your workpiece. They usually consist of a wooden or plastic handle and a metal loop. There are wire trimming tools and ribbon loop trimming tools.
U
Underglaze is applied to a piece that is leather-hard or bisque fired. You can paint and draw with it. By finishing your piece with a transparant glaze the colors of the underglaze really pop. Underglazes come in powder form, in pans, as pencils or prepared in jars or bottles.
A underglaze pencil has a core made of ceramic pigment. You use it to draw on bisquefired pieces. Read more about it in this blogpost.
Urushi is a type of lacquer obtained from the resin of the Urushi tree. It is an organic material that hardens when it comes into contact with oxygen and water. In ceramics, it is used as an adhesive in the Kintsugi technique and colored with natural pigments to decorate pieces.
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.
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.