Terminology
Banding wheel: |
A round, horizontally rotating disc that is useful for spinning a piece around while working on it. Specialised ceramic banding wheels generally cost around £40 from pottery suppliers, or I’ve found a $10 plastic cake decorating turntable works just as well.
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Bisque: |
The first stage of firing, partially vitrifying the clay so that it can be handled more easily than raw clay but is still porous enough to absorb a liquid glaze. Bisque firings often reach temperatures ranging between 800C (1,472F) and 1,100C (2,012F) depending on the type of clay used – my white stoneware clay bisques well at 1,000C (1,832F). Please check with your supplier to check which temperature you should be taking your bisque up to. See The Basics of Firing, What Happens in a Kiln, and Firing Schedule - Temperatures and Time articles for further information.
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Bisqueware: |
Bisqueware refers to the state of the clay after it has been through a bisque firing. It is pale in colour (excluding tinted or iron-heavy clays such as raku and terracotta), rough in texture, stronger than greenware but more brittle than fully fired pottery and very porous. It has a distinctive sound when tapped with a fingernail or tool, easily distinguished from fully fired ware but difficult to describe.
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Bone dry:
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Unfired clay that has fully dried out. The bone dry stage is when the clay is at its weakest and most brittle. It will feel quite dusty to touch and must be handled extremely carefully. All pieces should be bone dry before going into the kiln.
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Clay memory: |
Once bent in a certain direction, the clay forms a ‘memory’ of that bend and is inclined to return to it during drying or firing. Kneeding and re-balling the clay can help reset any unwanted memories.
See this time-delay video for a demonstration: |
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Cones:
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Small mineral pyramidal do-dahs that are designed to melt at very specific temperatures. Used in both bisque and gloss firings, they can be very precise gauges of how high the temperature has reached, and are often used to double-check the accuracy of the thermocouple. Orton cones seem to be the most popular, and I would recommend the self-supporting pyrometric type because they don't require a separate cone holder. See The Basics of Firing for more information
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Crazing:
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Thick, glassy glazes are prone to developing hairline cracks, particularly when cooling in the kiln (you can hear them ‘ping’ each time a crack forms). This cracked-yet-whole appearance is referred to as ‘crazing’. Some glazes are specifically designed to encourage this as an aesthetic feature. Crazed glazes are suitable for decorative purposes but not recommended for functionalware such as dinner plates or bowls due to their inherent fragility and the possibility of glass slivers, although there are some other variants that are semi-gloss and more stable.
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Earthenware:
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Earthenware is a type of clay that is fired at comparatively low temperatures. It is more porous and brittle than stoneware, but takes less time and power to fire. See Choosing your Clay for more information on which clay is right for you.
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Firing range:
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This is the span of temperature within which you can fire your clay/glaze/etc. for optimal results. A recommended firing range should always be adhered to, because deviation from it can produce unexpected, unwanted or damaging results. For examples of firing ranges and schedules, the following articles will be of interest: The Basics of Firing, What Happens in a Kiln, and Firing Schedule - Temperature and Time.
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Flux:
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Fluxes are minerals used in glaze recipes that often increase the glassy appearance of a fired glaze and are designed to reduce the melting point of other glaze components.
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Frit:
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A frit is a glaze of specific mineral composition that has been pre-fired and then ground to powder, to be used in further glazes. They are used to help stabilise volatile glazes and to minimise toxic leaching of other glaze components.
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Gloss:
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The second and final firing, in the normal ceramic process. Following a bisque firing the ceramics will have glaze or colourants applied and then be fired in a gloss firing. Gloss firings are fired to a higher temperature than bisques, and have different requirements in terms of temperature ramps. See The Basics of Firing and What Happens in a Kiln for more details.
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Greenware:
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Similar in a sense to bone dry, greenware is clay that has not yet been fired. However, whereas bone dry refers to fully dried clay, greenware is more generic and can refer to both wet or dry pieces.
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Grog/Grit:
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Found mostly in handbuilding clay, grog and grit are coarse additives that provide structural strength to the clay. Common grogs include sand, crushed brick or ground fired clay. The inclusion of these hard particulates obviously makes the clay completely unsuitable for throwing or fine detail carving.
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Incising:
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The practice of carving small, intricate details in clay. Incising is most effective when the clay is leather hard or bone dry, as the detail is too easily damaged if carved when the clay is wet. Watch my Carving a Stargate video to see fine incising in detail.
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Inclusions:
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Inclusions are unwanted and unexpected pieces of grog or grit that are found in smooth-body clay. Fine bodied clay is usually sieved multiple times, but hard pieces of detritus can be accidentally added in afterwards. These can be highly detrimental to detailed carving. This video, 'What are inclusions and why do we hate them?' will explain more about why inclusions are problematic when carving.
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Kneading:
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The practice of pushing/massaging a sizeable chunk of clay in a particular manner that is designed to force out air bubbles so the clay can be safely used and fired, as well as equalising the moisture content. The technique uses the palms of the hands and is similar to the kneading of bread dough.
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Kiln furniture:
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Anything used within a kiln that isn’t meant to be changed by a firing. These are mostly used for propping up or spacing out the ceramics, utilising as much of the interior as possible, or holding the pieces off the shelf to prevent the glaze from fusing them to it. Common items include shelves, pillars/struts, tile stackers, bead racks or trees, prongs and many more. This video, 'Kiln furniture', will cover the basics.
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Leather hard:
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A stage of clay moisture content in between wet and bone dry, leather hard has the tough and firm feel of leather but can still be carved or incised with relative ease. Any attempts at bending or shaping will likely result in the clay tearing. This stage can be useful for the bulk removal of clay, such as leveling off backgrounds, but doesn’t hold fine detail as easily as bone dry clay does.
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Loop tool:
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A tool made from a looped piece of metal affixed to the end of a handle. Loop tools come in two types: wire and ribbon. A wire loop tool is made from a rounded piece of wire and so has a relatively blunt edge and provides a smooth finish. Ribbon loop tools consist of a flattened piece of metal, and therefore have a much sharper cutting edge. They are better at quickly removing sizeable portions of clay but can dig deeper than intended or leave obvious cut marks behind. See The Right Tool for the Job: Everything Else for an in-depth look at useful carving tools.
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Mould/Cast:
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A reversed copy of an original item or carving used for making replicas, sometimes called a 'negative mould'. A variety of materials can be used but in ceramics the most common are made from plaster of paris or clay (usually taken no further than a bisque firing to preserve the porosity). RTV Silicone or silicon putty (such as the one shown in the photo) can also be used, but their waterproof nature may make it problematic to remove the clay copies. See the videos Taking a good mould and How to take a copy for instructions and further information.
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Off-gassing:
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Different types of gasses are released from both clay and glazes when they are fired in a process called off-gassing. With clay, this is usually involves water vapours boiling and being forced out in the early stages of bisque firing, so is therefore quite benign. But with glazes it can be the result of chemical changes of the mineral ingredients at high temperatures and the gasses expelled can be quite harmful.
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Overhang:
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In terms of taking moulds and copies, an overhand is an area of the carving with protrudes out past a 90 degree angle from the base of the clay. This means that any material used to take a mould or copy will be trapped underneath the extended edge and either tear when pulled out or prevent the mould or copy from releasing. See the videos Taking a good mould and How to take a copy for instructions and further information.
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Oxide:
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A mineral colourant, usually of metallic nature, used either within glazes or by themselves to add colour or texture to bisqueware in a gloss firing. Oxide powder can also be added to wet clay to colour the body of the piece. Oxides generally have a matt finish so there is less concern with them melting and fusing to each other or kiln furniture, but it’s still recommended they be wiped back from the base and the pieces not touch during firing. The photo to the right shows a synthetic iron oxide, which gives a smoother and more uniform finish than natural iron oxide. Further information can be found in Glaze Colourants, Choosing a Glaze, Using Oxides, and the Using iron oxide wash on bisque-fired pottery video.
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Paper clay:
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Clay with fibrous inclusions that give strength to greenware, decrease weight and help promote even drying of the clay. The fibers were originally made from paper but other variations include plant fibers such as flax and inorganic filaments like nylon. All fibers burn out during bisque firing and leave the piece lighter with miniscule channels spiderwebbing the clay that aid in the release of trapped air. Many paperclays are marketed as 'air drying clay', although they can also be fired for optimum strength. See articles on Choosing your Clay and Clay Memory and Movement for more information.
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Patina:
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An aesthetic coating, sheen or encrustation of the glaze that creates a desirable effect.
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Plasticity:
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The relative elasticity of the clay. Clays with low plasticity will tear easily and may be more challenging to handle. High plasticity is particularly useful when throwing clay on a wheel. Mold growth is said to increase plasticity, and some potters reportedly bury wrapped clay in moldy conditions for months before they intend to use it for this purpose. See the article on Choosing your Clay for further information.
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Polymer clay:
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An artificial modelling material made largely from PVC with only tenuous clay-like properties. To begin with it is relatively brittle and crumbles easily, but after working it for a minute the plasticity increases and it becomes extremely pliable. It is hardened in an oven rather than fired in a kiln, is not glazed and retains a plastic feel. Thin pieces will remain flexible even after hardening. Polymer clay comes in a wide variety of colours.
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Potter's wheel / Kick wheel:
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A metal plate, usually at least a foot in diameter, used in the creation of bowls, vases, mugs and other symmetrical vessels. Such vessels are called ‘thrown’. The clay is affixed to the plate (usually by wetting the surface and throwing a ball of clay with some force to the middle) which then rotates at speed so the clay can be centred and then shaped as desired. Kick wheels have been around for several thousand years and are powered by the potter repeatedly ‘kicking’ the base of the wheel to spin the plate. Most modern wheels are electric with variable speed controlled by a foot pedal. A potter's wheel is a sizeable investment, but one necessary for producing thrown vases and vessels.
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Pugmill:
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A machine designed to remove air bubbles from bulk amounts of clay. This is particularly useful if you intend on using reclaimed clay. The machine works by alternatively slicing and wedging the clay into itself, after which it is churned out as a wide cylinder. Putting clay through a pugmill is an alternative to kneading or wedging. See the How to deal with air bubbles video if you don't have a pugmill and you're still getting bubbles after kneading.
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Raw:
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Unfired clay, very similar to ‘green’ or ‘greenware’, but the connotations are more closely associated with the base clay rather than a worked piece. For example, you may talk about adding a colourant to raw clay and then make a vase which would then be referred to as greenware. Less often, 'Raw' can also refer to an unfired glaze, most usually in the instance of the glaze having being applied to the bisqueware but not yet fired.
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Reclaimed clay:
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Most ceramic processes create offcuts and waste clay which can be ‘reclaimed’ and recycled. It usually involves in thoroughly saturating the clay and then removing the water over a period of days or weeks. It’s a lengthy process that invariably introduces problematic air bubbles into the raw clay, so it will be necessary to knead, wedge or put the clay through a pugmill before use.
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Reduction firing / Reducing:
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This is a type of firing that requires the presence of a naked flame, so is considered almost exclusively the domain of gas-fired kilns (some people alter their electric kilns to include a flame for this purpose, but I have concerns that this could be adversely affecting the kiln’s electric elements). Once the kiln has reached temperature it is sealed and the flame is allowed to burn out, consuming all available oxygen. Oxygen molecules are drawn from the molten glazes which results in a chemical change, usually a change of colour. One of the most common reasons for a reduction firing is to turn a copper-based green glaze red.
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Saggar firing:
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A method of 'safeguarding' (where the word "saggar" comes from) pottery during firing by physically enclosing it within a high-temperature container. These saggar containers are usually made from the same or similar material as kiln shelves and pillars, and are themselves considered items of kiln furniture, so will easily withstand normal firing temperatures. Potters may want to saggar fire particularly delicate or touchy pieces that otherwise may be affected by the offgassing of nearby pottery, stray debris within the kiln, or - for non-electric kilns - from the direct heat of a naked flame.
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Scoring:
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The practice of roughly cutting or scraping gouges into wet clay. Sometimes this is used for decorative purposes, but most often it is to facilitate the bonding of two separate pieces of clay. Both pieces are scored at the point of their joining and a suitable amount of slip is added to one side before the two are joined and worked into each other.
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Shrinkage:
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 The original carving next to the moulded copy
As water evaporates the clay loses mass and will shrink a noticeable and predictable degree. This continues into firing, as the high temperatures will force out more water than would be released naturally. Each clay is different, but on average many clays lose between 10-13% size from wet to fully fired. See the article on Clay Memory and Movement, as well as the Clay memory and movement: Shrinkage video for more information.
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Silica:
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 Kiln shelf showing silica sand bedding
A mineral found in clay and used extensively in glazes to product a glassy finish. Silica on its own has a high melting temperature, so silica sand or other derived products such as silica fiber paper can be used on kiln shelves to prevent damage in case of glazes running, dripping or spitting. Silica dust and fibers, particularly after heating, have been implicated in several lung-related disorders, so dust masks are strongly recommended. See The Basics of Firing and What Happens in a Kiln for more information on how silica sand is used.
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Slabroller:
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A large device similar in appearance and function to the old clothes wringers. It suspends a hand-turned roller at a variable height above a moveable base. Rolling a lump of clay through this will forcefully squish it down to a uniform height, producing a flat slab of wet clay. It's essentially a glorified rolling pin. Some slabrollers are standalone units and others can be mounted on a bench or tabletop.
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Slip:
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A mixture of clay and water that's too wet for general modelling. Slips can range in consistency depending on their uses. In modelling, slip the consistency of whipped cream is used to affix separate pieces of clay together. A more liquid slip is used with plaster moulds to form casts. Slip coloured with stains or oxides may be used in a variety of consistencies for decorative purposes, painted on to a piece to add colour and texture.
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Sprigging / Sprig mould:
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Sprig moulds are usually simple 'straight pull' moulds whereby the clay is pressed into a moulded design, cut horizontally across the mould surface so as to remove the excess background from the cast, and the moulded piece is then affixed to a vase or similar. Sprigging is the application of these moulded pieces to larger pieces of pottery, often vases or mugs. Watch the video on How to take a copy to see this mould in use.
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Stoneware:
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Clay that is fired to a comparatively higher temperature than Earthenware. Stoneware is completely vitrified and therefore is stronger and non-porous. Note: This is a specific type of clay, not simply a choice to fire at stoneware temperatures. Firing earthenware clay at stoneware temperatures will likely result in the clay sagging and becoming misshapen. See Choosing your Clay for more information on which clay is right for you.
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Throwing / Thrown clay:
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Using a potter's wheel to rotate the wet clay while shaping it with your hands or tools (think the movie 'Ghost' but with less hanky panky - usually). Thrown clay is always initially symmetrical, although the potter may choose to shape it differently after it's thrown. Throwing requires extremely fine bodied clay with little to no added grit or grog, as these can impede the throwing process, cause unwanted deformations in the surface texture, and even result in injuries in extreme cases.
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Turning / Turned clay:
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Once a piece has been thrown and allowed to dry to leather hard, it is often cleaned up by 'turning'. Turning usually involves the piece going back on to the potter's wheel (although the wheel is dry this time, not wet as when throwing the clay) where its surface is smoothed by holding a wire or ribbon loop tool against it while it rotates. Excess clay from the base is also removed at this point, and a suitable 'foot' is created. Turning can also be used to add texture or decoration, as in the case of chattering.
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Underglaze:
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Colour pigments that are held in a paint-like suspension and that survive through firing. Underglazes, as the name would suggest, are often used underneath a transparent or translucent glaze, although can also be used on their own. The colour can alter or burn out at high temperatures, but for the most part the appeal of underglazes is that what you see is what you get, unlike glazes which undergo a chemical reaction during firing that can drastically alter their colour. See Choosing a Glaze for more details.
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Vitrify:
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The fusing of silica molecules into glass, which makes the clay stronger and less porous. It begins at roughly 800C (1,472F). Earthenware ceramics are usually considered to be partially vitrified, whereas stoneware is generally fully vitrified, depending on the clay composition and firing temperatures. See The Basics of Firing, What Happens in a Kiln, and Firing Schedule - Temperature and Time articles for further information.
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Warp:
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Usually seen during the process of drying or firing, the clay can become slightly misshapen for a number of reasons. The two most common are uneven drying/heating, which will result in one area of clay shrinking faster than the other and therefore can twist it out of shape, or the clay reverting to the shape of a 'clay memory' that has been imprinted at the modelling stage. Warps are usually highly undesirable and steps should be taken to avoid them if at all possible. See the Clay Memory and Movement article and time-delay video for more information.
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Wax resist:
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Wax, usually heated until it can be applied as a liquid, is painted or drawn onto the surface of a piece that is then glazed. The glaze does not adhere to the water-resistant wax and the area beneath is protected from it. The wax will burn off within the kiln, leaving the original layer exposed and not covered by the glaze. Sometimes this is used to keep areas of bare clay, other times it can be used on top of underglazes, stains, or other glazes. It's also effectively used to protect the base of vases, mugs and similar so as to prevent glaze from fusing to the kiln shelves.
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Wedging:
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A term that is almost interchangeable with 'Kneading', it serves much the same purpose by removing air bubbles and balancing moisture content. There are various different methods of both kneading and wedging, but kneading usually applies solely to the manipulation of clay using the hands (like kneading bread dough), whereas wedging can also involve cutting the clay into chunks and forcibly slapping them together to drive out any air bubbles. This should be done only by someone who is familiar with the technique, as it may result in the creation of additional air pockets if done incorrectly.
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Wheel throwing:
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See 'Potter's wheel' and 'Throwing/Thrown clay'.
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