Clay Glossary

Clay Glossary

Pottery jargon can be confusing to the beginner. Use our glossary to discover the meaning of any terms you may come across.

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Alumina   Aluminium oxide. It is used to bed bone china in the biscuit firing. A fine white powder with a high fusing temperature.  Top

Antiquing   A method of applying colour, then wiping it back to highlight or accentuate the surface detail.  Top

Ash Glaze   A glaze made with ground ash, usually wood ash and sometimes mixed with other materials.  Top


Banding Wheel   A turntable used for applying lateral decoration to a pot.  Top

Bat   A flat disk that sits on the wheel. Used for heavy pieces that would be difficult to separate from the wheel.  Top

Bisque ware (or biscuit)   Pottery that has been given a preliminary firing to render them hard enough for further work, such as decoration and glazing. The higher the temperature of the bisque firing, the harder the pot will be, resulting in reduced reaction between glaze and body in the final firing.  Top

Bisque firing   The first firing of pottery, to mature the clay and give it permanence. In this firing, pots can be stacked or touching, as there is no glaze to stick them to each other.  Top

Blunge   Mix water with clay.  Top

Blunger   Machine for mixing water with clay to create slip. The clay is fed into a hopper and goes through rotating blades to mix it with the water.  Top

Body   The substance from which a pot is made. A particular mix of clay, such as stoneware or earthenware body.  Top

Bone China   A china made white and translucent by the addition of calcined animal bone (bone ash) to the body.  Top

Burnish   To smooth the surface of a pot to a polish by rubbing with a hard object.  Top

C

Calcine   To purify a substance by subjecting it to high temperatures to drive out water or carbon gases.  Top

Caliper   A measuring device resembling a pair of compasses. Internal calipers measure internal diameters and external calipers measure outside diameters.  Top

Casting   To produce shapes by pouring liquid clay into porous moulds to build up a layer of clay.  Top

China   Porcelain.  Top

Chuck   Hollow, tube-like form for holding a pot upside-down on the wheel during trimming.  Top

Coiling   Making pots or other objects using coils of clay. Roll out coils of clay with the palms of your hands before coiling the clay round into a pot shape. The coils can then be smoothed out using a scraper or rib tool to produce a flat surface to the pot.  Top

Cone   Cones of ceramic material inserted in the kiln which melt at a known point. Used to indicate to the potter when a certain temperature has been reached.  Modern kilns have programmers which detect when certain temperatures are reached, negating the use of these cones, however many potters still refer to the cone system when choosing the firing temperature.  Top

Crawling   Movement of the glaze during firing, causing exposed areas of clay body. Usually due to dust or grease on the surface.  Top

Crazing   A fine network of cracks in the glaze usually caused by contraction of the glaze during firing. In some pottery such as Raku this is a design feature, but in earthenware pots the cracks can allow unwanted moisture to penetrate to the porous body.  Top

D

Damper   A crude device - usually a refractory clay brick - used to block the flue of a kiln.  Top

Decal   Imagery or text, printed onto a transfer paper, used to decorate pottery or glass.  Top

Dipping   Applying glaze by immersion.  Top

E

Earthenware   Pottery fired to a relatively low temperature. Earthenware is porous if left unglazed so must be glazed if it is to hold food or liquid. Usually fired at low temperatures.  Top

Enamel   Low temperature fired (low-fired) coloured glazes.  Top

Engobe   Fluid clay used for decoration. This term is usually used to describe coloured slips.  Top

F

Fettling   The removal of unwanted blemishes or seams with a sponge or knife from nearly dry pottery, prior to glazing and firing.  Top

Firing   To heat pots in a kiln to a set temperature to mature the glaze or clay.  Top

Firing Range   The temperatures between which a clay or glaze matures or fuses.  Top

Flatware   Plates, saucers, trays, etc.  Top

Flux    A substance mixed with a body or glaze to help it melt readily to promote fusion.  Top

Foot   The base of a pot.  Top

Frit   A vitreous composition used in glazes and enamels.  Top

G

Glaze   A glass-like substance used to decorate the surface of pottery.  Top

Glaze Firing   The second firing of a pot after the glaze is applied.  Top

Greenware   Unfired pottery.  Top

Grog   Gritty material added to clay to add texture, lessen warping and improve resistance to thermal shock. It is usually ground, fired clay.  Top

H

Handbuilding   To make pottery without a potter’s wheel. Using clay slabs, moulded clay, clay coils or otherwise manipulating the clay by hand.  Top

I

Impress   To stamp or roll a pattern into leather-hard clay with a tool.  Top

Incise   To carve a pattern into the raw clay with a sharp tool.  Top

K

Kidney   See Rib  Top

Kiln   The oven in which pottery is fired. Kilns can be fuelled by wood, oil, gas or electricity.  Top

Kiln Furniture   Items made of refractory material used in the kiln to support the pots. E.g. Shelves, posts, etc.  Top

Kiln Wash   Shelf separator brushed onto kiln furniture to prevent glaze from sticking to them during firing.  Top

Kneading   See Wedging  Top

L

Leather-hard   (Sometimes known as cheese-hard). Clay that is stiff but still slightly damp. Hard enough to retain its shape but wet enough for further work to be done on it (e.g. joining or stamping).  Top

M

Maturing Temperature   The temperature at which a clay body reaches its optimum hardness or a glaze is fully fused into the clay body.  Top

Mould   A former made from plaster into which clay can be pressed or slip cast to take on the mould shape.  Top

N

Nesting   Stacking pots on a kiln for bisque firing. In this firing unglazed pots can be stacked without fear of them sticking.  Top

O

On-glaze   Enamels. A form of decoration applied on top of a fired glazed, then fired again.  Top

Overglaze Decoration   A decoration applied over an already glazed finish, such as decals or enamels.  Top

Oxidation   Firing pottery in a kiln with a sufficient supply of oxygen so that combustion is complete.  Top

P

Peephole   Small hole in the kiln for the potter to see inside during firing without the kiln losing too much heat.  Top

Pinching   A handbuilding method using your thumb and fingers to pinch a cup or pot shape from a ball of clay.  Hold the clay ball in one hand and push the thumb of your other hand into the top of the ball. Using your thumb and fingers in a pinching motion, squeeze the clay as you rotate it with the other hand to gradually make a bowl shape.  Top

Pinholing   Small dots of unglazed or depressed areas in the glaze surface. Usually unwanted, pinholing occurs when gases in the glaze and clay bubble up to the surface. The bubbles pop and a 'hole' appears, which doesn't fuse over.  Top

Porcelain    Fine, high-fired clay body. It is white and translucent when fired.  Top

Prop (or Post)   Refractory pillar shape onto which a kiln shelf is placed when firing.  Top

Pugmill   A machine for wedging clay. It mixes and extrudes clay.  Top

Pyrometer   An instrument for measuring the very high temperatures in a kiln.  Top

R

Raku   Japanese earthenware firing technique where pots are placed into a hot kiln, and removed when still red hot, creating a crazed effect in the glaze.  Top

Reducing Atmosphere   When there is insufficient oxygen in the kiln for the flame to burn out its carbon content. This affects the resulting clay and glaze colours.  Top

Refractory   Ceramic material capable of withstanding very high temperatures. Kiln bricks and furniture are usually made from refractory material.  Top

Rib   Wooden, metal or plastic shaping tools used in forming and smoothing pots. Sometimes known as a kidney.  Top

S

Saggar   A box used to protect pottery from flames and gases while it is being fired in a (non-electric) kiln.  Top

Sgraffito   The technique of cutting or scratching through a coating of slip to reveal the different coloured body beneath.  Top

Slab Building   A handbuilding technique where pottery is made using slabs of clay. Create the slabs by either pushing the clay out flat with the heel of your hand, beat them out with the side of your fist, roll them out with a rolling pin or cut slabs from a cube of clay with a wire. Slabs should be joined when leather-hard using slip and soft clay.  Top

Slip   Liquid clay used for decorating, joining and for casting.  Top

Slip Trailing   Decorating with slip by trailing it onto a pot through a fine nozzle.  Top

Slipware   Slip decorated earthenware pottery.  Top

Soak   Keeping a steady temperature in the kiln, usually at the end of a firing to enhance the glaze finish.  Top

Sprig   Press-moulded decorative piece of clay, applied to pot when it is leather hard.  Top

Stoneware   Vitrified clay, fired at high temperature, which is inherently non-porous, unlike earthenware. Stoneware is also more durable than earthenware.  Top

T

Terracotta   Red earthenware. Red due to the iron content in the clay.  Top

Thermal Shock   Sudden expansion or contraction of the clay due to sudden temperature change. Causing weakness or damage to the pottery.  Top

Throwing   Making pottery using a potters' wheel. Clay is thrown onto the centre of the revolving wheel and the potter shapes it by hand.  Top

U

Underglaze   Colour applied to greenware or bisque before covering with a transparent glaze.  Top

V

Vitrification   When clay particles fuse to a glass-like state (during firing).  Top

W

Wax-resist   Wax applied to a pot as a barrier to slip or glaze. Used to create a decorative effect.  Top

Wedging   Preparing clay body by kneading and mixing it to an even consistency.  Top

Wheel   A revolving wheel onto which clay is thrown and shaped. Usually powered by electricity or the potter’s foot.  Top

Wire   Length of cheesewire with a handle at each end. Used for cutting clay.  Top