Pottery Glossary
Absorption
Soaking up of water by a clay body.
Air bubbles
Small pockets of air trapped in a clay body. These can make working difficult but it is a complete myth that air causes clay to explode during firing. When clay with air bubbles do explode what occurs is that the water evaporates from all surfaces and in the enclosed air pockets cannot escape. If the firing is fast this water, which condenses in the pockets, can cause explosions. But if the firing is slow there is no problem–it is possible to safely fire completely enclosed hollow forms.
Air bubbles can also form in glaze if insufficient time is allowed for the glaze to mature and for gases to escape.
Armenian bole
An iron earth which can be used as a painting material. It is found on Persian and Turkish pottery.
Ball clay
Highly plastic, fine-grained, sedimentary clay, usually added to less plastic clays.
Banding wheel or ‘bench whirler’.
An unpowered tabletop wheel on a spindle, used for painting horizontal bands on to pots, or to support hand-built pots in the making.
Basalt ware
Pottery made black by the addition of oxides and/or calcined colours. Basalt ware was used extensively for objects in classical revival styles. There is no natural black basalt clay.
Bat
The term may be used to refer to both refractory kiln shelves and discs of plaster, asbestos, wood or fired clay used for throwing on, which are portable.
Bat wash
A slip of alumina and water painted on to kiln shelves and props to minimise adhesion and to facilitate removal of glaze specks.
Beehive kiln
See Bottle kiln.
Bellarmines
Salt glazed bottles originally made in Germany. The bottles are ornamented with a face mask originally representing Cardinal Bellarmine. Bellarmines are a common type of vessel and form part of an extensive tradition of bottle forms decorated with faces.
Biscuit ware or bisque
Unglazed porous pottery which has been fired once to a low temperature, but usually 1000°C
Blisters
Bubbles formed in the body of pottery and projecting from it. Blisters are formed by gases which are unable to escape because of too fast a firing. Reduction atmospheres can aggravate blistering.
The term is also, but less commonly, applied to bubbling in glaze.
Bloating
Ugly blisters in the body of the pot which usually occur in the glaze firing. This is often due to carelessly mixed clays.
Block mould
See A/faster mould.
Blunger
A machine for mixing pottery bodies and other materials in a fluid state. Blungers are containers in which paddles revolve. They are filled through the top and the fluid mixture can be drained off through taps in the bottom. Hand blungers are portable mixers which can be used in appropriate containers.
Body
The term refers to the clay mixture of which any pottery is made as, for example: porcelain body; coarse body; dark body; terracotta body, etc, etc. The term is also used to refer to the main form of a pot–other related terms are obviously: neck; foot; shoulder.
Bone china
An English type of china or porcelain which is something of a hybrid between true hard porcelain and, the now obsolete, softpaste porcelain. It contains bone ash (calcined animal bones) and is very white and translucent. It has a high biscuit firing and a lower temperature glaze firing.
Bottle kiln
Sometimes also called beehive kiln. A traditional type of large kiln circular in plan, tapering, bottle shaped, towards the top and fired with coal or coke. As a production kiln these have now been totally superceded by tunnel kilns.
Bung
A vertical stack of loaded kiln shelves (or saggars).
Calcine
To reduce to a powder or to a friable state by an appropriate heat. The actual temperature depends on the material being calcined.
Calcareous clay
Clay containing lime, often called a marl. If present in clay lime must be finely subdivided.
Celadon
The name given to a range of green stoneware and porcelain glaze colours derived from iron. The colours were first evolved in the Far East
Chattering
Also, called juddering. A surface irregularity caused by vibration which can occur during turning. It is caused by anyone of various factors including blunt turning tools, coarsely grogged clay, and holding turning tools loosely.
Chemical water
Part of the clay that is driven off by heat, causing the clay to shrink in the firing.
Clamming
The sealing up of kiln doors with a mixture of wet clay and sand or grog.
Clay bodies
Prepared clay. The most common classifications are earthenware, stoneware and porcelain.
Collaring
A process used in throwing to narrow a pot or part of a pot. Both hands encircle the form and move steadily upwards pushing inwards.
Colourants
Various oxides and carbonates which are able to withstand high temperatures and give colour to glaze or clay body.
Combined water
The water which is chemically combined with clay. Once driven off by the heat of firing it cannot be replaced.
Combing
The working of wet slip into patterns using a comb made of card or wood or leather. A similar process to feathering but it can be done through slip to a body of contrasting tone or colour as well as across the surface of different coloured slips.
Cones
See Pyrometric cones
Continuous kiln
A kiln which the ware is moved through allowing continuous firing.
Cottle
A wall of clay, wood or linoleum used to surround the clay mould to prevent the liquid plaster from spilling.
Crackle
A network of fine cracks in a glaze due to a greater shrinkage in the body than the glaze. The term is generally applied to the intentional use of this phenomenon for decorative purposes.
Crawling
This is when the glaze shrinks away from the body of the pot. Dust or grease on the pot can be the cause of this.
Craze
A network of fine cracks in a glaze as described under Crackle. The term generally implies the unintended occurrence of cracks in a glaze. Crackle and crazing should not occur on vessels used for food or drink as the fine cracks can retain material. When crazing occurs on a porous body it is very unhygienic and in severe cases the body itself can become badly stained.
Creamware
A cream-coloured earthenware developed
in the mid eighteenth century and popular until the beginning of the nineteenth century when whiter bodies became more favoured .
Deflocculants
Substances that interfere with the natural cohesion of the particles of plastic clay and make it fluid. Soda ash and sodium silicate are deflocculants and introduced in the proportion of approximately 0.5% into soft plastic clay will reduce it to a thick runny slip.
Delft ware
Loosely used to describe pottery glazed with an opaque white tin glaze and usually painted. The name originated from the town of Delft in The Netherlands where such pottery was produced.
Devitrification
The formation of crystals in a glaze. This can be caused by very slow cooling as well as by the presence in glaze of certain substances which tend to crystallise out of the mixture. Zinc and titanium both promote crystallisation.
Diatomaceous earth (Kieselguhr]
A form of silica which consists of the skeletons of microscopic marine animals–diatoms. It is extremely light and makes excellent insulating material for kilns both in powder form and made into bricks.
Dod-box
See Wad-box and Extrusion .
Down-draught kiln
A kiln in which heat and flames either enter the kiln at the top, or are deflected towards the top, and are drawn downwards and out through flues at the base of the back wall or in the floor of the kiln.
Drawing a kiln
Unpacking it after firing.
Dunting
The cracking of fired pots during cooling caused by uneven or too rapid cooling. There are two critical points in a cooling cycle which must not be passed too quickly one is between 575°C and 550°C and the other is 225°C. At these points silica in the body undergoes certain changes which are accompanied by a change of volume. If cooling is fast or uneven at these points dunting will occur.
Earthenware
A low firing natural clay body usually red or brown when fired to maturity, non-vitreous and porous with comparatively coarse grain structure and low chipping resistance.oft, porous, glazed pottery fired to a temperature of 1000-1100°C.
Efflorescence
White scumming which sometimes occurs on the surface of terracottas (and other biscuit ware). It is caused by soluble salts present in the clay and can be overcome by the addition of about 2% of barium carbonate to the clay which neutralises these salts.
Egyptian paste
A self-glazing body that was used by ancient Egyptian potters.
Engobe
Loosely the term is sometimes applied to all slips. More specifically it is used for mixtures of clay and frit (and colour too sometimes) which are painted in a slip state onto clay. Engobes can be more readily built up to opaque thickness than slips.
Encaustic tiles
A nineteenth century term for inlaid tiles.
Encapsulated Stain (Inclusion Pigments)
A new generation of stable stains made by melting metallic colourants with zirconium silicate, cooling the melt, and grinding the result to a fine powder. Because zirconium silicate is refractory, stains containing it can produce brighter colours up to cone 10 using pigments that would otherwise fade at high temperatures. These colours are safe to use in the studio.
Extrusion
The forcing of clay through a die forming it into lengths of specific cross section. Extrusions can be solid as with extruded handles or hollow as with drainage pipes. Small extrusions are generally done in a manual wad-box larger ones in a mechanical extruder rather like a pugmill.
Faience
The French term for earthenware. Not a word of specific meaning. The word comes originally from the Italian town of Faenza, in the province of Ravenna, which exported large quantities of tin glazed ware to France.
Fat clays
General term for clays with plastic qualities. Clays lacking such qualities are referred to as lean clays.
Felspar
This mineral contains silica and alumina. It is the commonest ingredient in stoneware and porcelain glaze, as well as being a major constituent of clay.
Fettling
Cleaning away of mould seams from slip cast ware. Sometimes also used loosely for any general cleaning up of pots, or the smoothing or trimming the surface of a hardening pot in preparation for firing.
Filter press
Substantial machine for squeezing water out of slip to produce plastic clay.
Fireclay
Refractory clays which withstand high temperatures, are yellow or grey in colour and are used for parts of kilns and bricks. They are too coarse for use in most domestic pottery.
Firing
Placing the pottery in a kiln and heating it to a temperature to make it hard and permanent.
Flat ware
Industrial term for plates, shallow dishes, saucers, trays, etc. Cups, bowls, jugs, teapots, etc. are termed hollow ware .
Flaking or scaling
Can occur to slips during drying or firing and is caused by a difference in shrinkage. It can also occur to glaze prior to firing. In both these cases the slip or glaze materials need changing to give more or less shrinkage as appropriate. Scaling of fired glaze can also happen at the edges of pots. To correct this adjustments have to be made to the body, usually to its silica content.
Flux
A melting agent which causes silica to form glaze or glass.
Flux (molar) Unity or Seger Formula
The chemical composition, commonly of a fired glaze, expressed as one mole of total flux to the number of moles of all other ingredients in the glaze. The term ‘unity molecular formula’ does not specify whether the flux, alumina or glass formers are in unity. Each can be at different times for different purposes, but flux unity is used almost universally by ceramic artists.
Footring
If the pot is raised from the surface on which it stands by a circle of clay at the base, this is called a footring. During the turning stage it is shaped and hollowed.
Frit
Flux and silica are melted together and reground to a fine powder, making a glaze ingredient which has the same effect as, but avoids the toxicity of, raw fluxes such as lead.
Gilding
The application of gold by various processes to pottery.
Glaze
A coating of glass applied to the surface of a pot. Silica melted with a flux, such as lead, sodium or potassium makes a glaze, and is often coloured with metal oxides.
Glaze fire
The maturing temperature at which glaze will melt to form a surface coating. Firing ranges vary with the type of glaze.
Glostfiring
Glaze firing.
Green ware
The name applied to pots before they are fired.
Gres
The French term for stoneware.
Gres de Flandres
Salt glazed ware from the Low Countries.
Grog
Ground-down fired pottery added to plastic clay gives texture and reduces shrinkage.
Hard paste (Pate dure in French)
A term used for the true porcelain which originated in China.
Hare’s fur
The name given to a stoneware iron glaze which has flowed slightly giving a soft fur like linear texture. Tea dust and oil spot are very similar glazes which have a different textual quality. All three were originally descriptions of glaze quality in Chinese stoneware.
Hollow ware
See Flat ware.
Incised
Surface designs cut into the clay for decorative purposes.
Intermittent kiln
Any kiln of any type in which the ware has to be packed, fired, cooled, and unpacked. The term has only been in use since the advent of continuously firing tunnel kilns and in fact refers to all kilns other than these.
Ironstone china
A non translucent but very hard dense body in vogue in the nineteenth century. Mason’s Ironstone China is the best known but there were many varieties. Much experiment was done with hard white earthenware bodies at the beginning of the nineteenth century and these are often difficult to categorise.
Juddering
See Chattering.
Kidney
A kidney-shaped metal or rubber tool used for finishing pots on the wheel, made in press-moulds or by hand.
Kiln
A refractory clay-lined furnace for firing ceramic ware.
Kneading
Wedging or working the clay with fingers to obtain an even texture throughout.
Lamellar
Describes the microscopic structure of clay–that of thin flat scales.
Lawns
The woven screens of sieves made of phosphor bronze wire in varying degrees of fineness. The grading of lawns into 60, 100, 200, etc. indicate the number of wires per linear inch.
Lean clays
See Fat clays.
Leather
Chamois leather often used for finishing thrown edges and for fettling.
Leather hard
Clay that has, through drying, lost its plasticity but is not yet dry and is still impressionable with a finger nail but not with a finger print. Also described as cheese-hard.
Lugs
Side projections on pots functioning as handles.
Luting
The fixing together of two clay surfaces with slip.
Majolica
A term used for Italian tin glazed ware and, loosely, for the technique of painting onto tin glazed ware. The term originates from Majorca. The style of pottery is also known as Delft ware.
Marl
Clay containing lime.
Master mould
Block mould; the mould from which working moulds can be cast.
Matt glaze
A dull-surfaced glaze, lustre less and non-reflecting.
Mature
Refers to the development of the desired properties in clays, clay bodies, and glazes.
Mishima
The name given to a type of fine inlay in black and white found on some Korean pots.
Moving hearth kiln
The term is most usually applied to intermittent trolley kilns (those where the trolley is packed, moved into the kiln, fired, cooled, moved out of the kiln and unpacked).
Muffle
A fireclay chamber or box that protects pottery from direct contact with flames and kiln gases.
Opening materials
Non-plastic materials such as sand or grog which can be added to a body to open it up; that is to facilitate drying by giving it a coarser texture. Doing this reduces shrinkage and may also increase refractoriness.
Open firing
Firing in which the pottery is exposed to kiln gases and flames, unprotected by a muffle box.
Oil spot
See Hare’s Fur.
Over-firing
This usually refers to the glaze and often means that the pot has gained ugly characteristics from exceeding the optimum temperature. Over-fired pots often stick to the kiln shelves.
Oxidation
Firing process requiring the presence of oxygen in sufficient quantities to cause combustion of the carbon gases. If not sufficient some carbon may remain in the ware. A chamber free from smoke and a clear bright flame indicate an oxidized atmosphere.
Oxidised
Fired in a kiln with adequate oxygen leaving metallic colours bright. Oxygen is only required in kilns in which combustion occurs, i.e. gas or wood kilns. Electric kilns give oxidised results since there is no combustion.
Parian body
A porcelainous body, used unglazed, popular in the nineteenth century.
Paste
Term used for porcelain bodies.
Pate dure
See Hard Paste.
Pate tendre
See Hard Paste.
Pinholing
When gas bubbles out of the glaze it can cause masses of minute holes on the surface. Usually due to mineral impurities.
Pipe clay
A white firing clay found in small seams with other dark firing secondary clays. It is relatively non plastic. It was used for white inlay of tiles in mediaeval times and until recently was used for press moulded clay pipes for smoking.
Plastic
A plastic clay has the ability to be shaped easily and to retain its shape.
Porcelain
White, often translucent stoneware made from a mixture of pure clays, with the addition of calcium for translucency.
Pottery
Term loosely applied to any ceramic form or to the place where it is made.
Pug mill
Machine for pugging clay–mixing it and de-airing it.
Pyrometer
An instrument for recording the exact temperature of the kiln.
Pyrometric cones
Slender pyramids of ceramic material made in a graded series. They do not measure temperature: they measure the amount of heat-work done. At certain temperatures they soften and bend indicating to the potter that the clay and glazes which require a certain time and temperature to mature have done so.
The best known series are Seger Cones (Germany), Orton Cones (USA) and Staffordshire Cones (UK).
Queen’s ware
A hard earthenware.
Raku
A method of making pottery using a high-fire, porous clay body. Named after the great tea master Seno-no-Rikvu, it was developed by Japanese potters for the tea ceremony. Also the specific firing method.
Raw glaze
The term has two distinct meanings: it can mean a glaze made of natural materials which have not been fritted (such as litharge or it can mean a glaze applied to an unfired pot.
Reduced
Fired in an oxygen-starved atmosphere which reduces the colour of metal oxides to their respective metal forms.Thus copper becomes coppery-red unlike the characteristic green of oxidised copper roofs.
Refractory
The capacity of a material to withstand high temperatures in pottery. Clays suitable for stoneware and porcelain are described as refractory clays.
Ribs
Throwers’ tools for finishing the surface of pots.
Saggar
A fireclay box used in oil, gas or solid fuel-burning kilns to contain the pottery, thus protecting it from the flames.
Salt glaze
Produced by tossing salt into a hot kiln. The salt vapourizes and combines with the surface of the work.
Seger cone
A pyroscope designed to indicate heat by melting and changing shape. They are small pyramids of glaze-like material which bend when the correct temperature is reached. They are placed in the kiln so that they can be seen through the spy-hole.
Sgraffito
A decorative technique in which a sharp tool is used to scratch through a slip to clay below. The same technique is applied to glazed pottery, when designs are scratched through an unfired glaze to show a fired glaze below. Comes from the Italian word meaning ‘scratched through’.
Shale
Clay found in deep seams that have been under great pressure, often found adjacent to coal. Shales are usually dark in colour and usually need grinding up before they can be used. Some fireclays occur as shales.
Shrinkage
Contraction of clay during drying or firing.
Silk-screen
The application of design to a pot by pressing colours in a carrier medium through those parts of a fine screen which have not been blocked off with wax.
Slip
Liquid clay in a non-plastic state, with the
consistency of cream.
Slip-trailing
Applying designs by passing clay through a nozzle attached to a small rubber bag, like icing a cake.
Slurry
A mixture of clay and water. An alternative name for slip , but more commonly used when the mixture cements clay joins.
Smoking
The slow heating of a biscuit kiln, to allow water vapour to escape. The ‘smoking’ is, of course, steam.
Soaking
Leaving the kiln on at the firing’s required temperature, for half an hour or so.
Often improves the quality of the glaze. Ideally no increase or drop in temperature should occur during soaking.
Soft paste (Pate tendre in French)
is the term used for European attempts to copy this by introducing frits into the body.
Soft-paste porcelain
Soft-paste porcelain is a type of a ceramic material. The term refers to soft porcelains such as bone china, Seger porcelain, vitreous porcelain, new Sèvres porcelain, Parian porcelain and soft feldspathic porcelain, and is also used more narrowly to describe clay bodies mixed with glass frit that were used in the production of decorative figures and domestic wares in 18th century Europe. They were called “soft” because of their lower firing temperatures compared with hard-paste porcelain.
Sprigging
Taking small solid casts from moulds and fixing them to the surface of pots, usually of a different colour for contrast, as in Wedgwood Jasper ware.
Stack
To pack a kiln.
Stain
Sometimes a single colouring oxide, but usually a combination of oxides plus alumina, flint and a fluxing compound. A frit made of colorant chemicals, compatible fluxes, and possibly glass formers, which has been melted, cooled, and pulverized to a fine powder. Used in decorating and in colouring glaze.
Stoneware
Glazed pottery fired to a temperature above 1200°C, when the body is vitrified. Also a hard, vitreous, non-porous, non-translucent body with a wide range of temperature over which it will vitrify. More refined than earthenware and a very useful clay for the potter. Usually buff, tan, or grey when fired.
Tea dust
See Hare’s Fur.
Tenmoku
A Japanese word for a lustrous, dark iron stoneware glaze. The term covers those black stoneware glazes which break to a warm red brown where thin.
Terra cotta or Terracotta (Literally burnt earth):
The term is generally applied to unglazed objects made of ironbearing clays, e.g. terra cotta portraits, terra cotta plant pots. Usually a type of earthenware, a clay-based unglazed or glazed ceramic, where the fired body is porous. Terracotta is the term normally used for sculpture made in earthenware, and also for various practical uses including vessels, water and waste water pipes, roofing tiles, bricks, and surface embellishment in building construction.
Tests
Are small samples of clay, slip, colour, or glaze etc. fired to discover the colour, texture, etc. of the material or to check the mixing of a newly made batch of clay, slip, colour or glaze etc. As clay is less destructible than notebooks, tests should be clearly marked with relevant details. If the clay tests are lost, notes are not much use but if a notebook is lost and tests are numbered perhaps but not marked with details the tests are frustrating as well as useless.
Throwing
Making pots with the hands from plastic clay on a wheel.
Top-hat kiln
A kiln which can be raised bodily from its base. Work is packed on the base and the kiln is lowered over it. This type of kiln always requires a hoist to raise it and is perhaps most sensibly used if the hoist can travel along an overhead rail to allow the kiln to be used on two or more bases.
Trolley kiln
A kiln into which work may be placed on a movable trolley. Trolley kilns can be either intermittent or continuous.
Tunnel kiln
A kiln through which trolleys may move allowing a continuous firing of ware to take place. Somewhere near the middle is the hottest part of a tunnel kiln; the work enters the kiln cold and is slowly moved through as fired trolleys are removed from one end and unfired are inserted at the other. A tunnel kiln is termed a continuous kiln.
Turning
Trimming thrown pots when they are leather-hard, using metal tools on a wheel.
Vitreous
Impurities in the clay which melt upon heating to form glass and act as a bond.
Water glass
Common name for sodium silicate. See deflocculent.
Wad box
A machine for extruding clay. See Extrusion
Wad mill
Another name for an extruder. The wad is simply the lump of clay which goes inside it.
Warping
Distortion in a pot caused by non-uniform drying or uneven ware thickness. or from being fired in a kiln which does not heat evenly.
Wax resist
A method of decoration using warm wax or a wax emulsion applied to either raw or bisque pots or between two layers of glaze. Wax applied to the pot will resist the glaze and will lift off after firing.
Weathering
Rendering the clay more plastic by exposing it to all types of weather.
Wedging
The cutting and re-forming of plastic clay before kneading. This ensures an even texture. Many potters apply the term wedging to the kneading process as well. The process of vigorously forces any air bubbles from the clay in order to obtain a uniform consistency.
Wheel
A smoothly revolving platform powered either by the potter’s legs or by electricity and capable of varied speeds of rotation.
Wheel head
A circular flat disc on which the pot is formed. It is attached to the revolving spindle of the potter’s wheel.
