Glossary of Ceramic Terms

Batt wash A wash of refractory material painted onto kiln furniture to protect against glaze runs.

Biscuit/Bisque The first firing, converting clay to ceramic which cannot be reconstituted in water. Bisque is used to describe a first firing that is higher than the subsequent glaze firing. Biscuit describes the reverse where the glaze firing is the higher, also the ware produced by these firings.

Bloating Lumps on the surface of a fired piece caused by expansion of gasses within the piece generated by overfiring.

Blunger Machine for mixing clay and water in the production of casting slips

Body A manufactured clay, where raw clays and minerals are blended to produce specific qualities.

Calcination/Calcining Heating to purify and prepare materials for use.

Ceramic Clay that has been altered by firing to produce a hard substance that cannot be reconstituted in water. The general name given to all fired clayware.

Coiling Making pots by buiding up coils of clay

Cones Small pyramids of specially prepared clays formulated to bend at specific temperatures giving accurate measurement of heatwork generated in the firing.

Crazing A network of small cracks in the glaze surface caused by a poor glaze fit. The glaze contracts more than the clay and the resulting tension causes it to crack. If a glaze contracts after firing more than the clay, the glaze being unable to stretch, starts to crack under the tension. The glaze will continue to crack until the tension is eased as crazing covers the surface and becomes finer. Delayed crazing can occur days or even months after firing. The most common cause of crazing is underfired biscuit. If the clay does not reach its full maturing temperature either in the bisque or glaze firing, it will not contract sufficiently to fit the glaze.

Crawling A glaze fault where the glaze has rolled back in the firing leaving a bare patch, caused by dirt, dust or grease on the pot before glazing and by the glaze being stiff, or applied too thickly and not flowing easily in the firing.

Decal A transfer or lithograph used to apply repeat decoration, usually on-glaze. The decoration is printed in ceramic colours on to a backing which is removed and the design transferred to the glazed ware, which is then re-fired to a lower on-glaze temperature.

Deflocculation Separating and dispersing the clay particles in a slip making it more fluid. Sodium Carbonate and Sodium Silicate are deflocculants, when disolved in the slip they allow large proportions of clay to remain fluid giving a high density to the slip.

Dryfooting Leaving the footring of a pot unglazed so that it can be fired standing on the kiln shelf without being stilted.

Dunting Cracking of pots due to stress built up by Silica Inversion in the firing. Rapid and uneven heating and cooling of the pot in firing is the most common cause, pots made from uneven thicknesses of clay are also at risk and poor glaze fit can also be a cause

Earthenware Porous ware, lower fired ware usually under 1200°C and the clay and glazes that it is made from. Earthenware is usually characterised by brighter colours, softer glazes and separate glaze layer over the body.

*Element * A high temperature resistance wire, wound in a coil, which carries electrical current for heating a kiln.

Enamel Coloured glasses used to decorate metal and ceramics, see On-glaze.

Engobe Usually used to mean decorating slip. A covering over the base clay not firing to a glassy finish.

Eutectic Mixture The mixture of substances in proportions that produce the lowest possible melting point given the fluxing action in combinations of materials. The eutectic mixture of Alumina and Silica for instance is 10% Alumina and 90% Silica which gives a melting point of 1545°C. This is lower than the melting point of Alumina at 2050°C or Silica 1710°C.

Fettling Cleaning of cast ware, especially the removal of the seams produced by joins in the plaster mould.

Flocculation The opposite of deflocculation, the thickening of slips or slops to give a bulkier and more viscous liquid. In flocculation the particles collect together rather than repel each other as in deflocculation. Calcium Chloride and vinegar are commonly used flocculants.

Fluidity The liquidity of slips and molten glazes. Used to describe free flowing slips and glazes which melt to a runny consistency in the firing. The opposite of viscosity.

Flux Substances that encourage fusion in the firing to produce ceramic. An essential ingredient in both clays and glazes, fluxes interact with Silica glass formers producing the solid, fused ceramic of the pot and fused glass of the glaze. Some of the commonest fluxes are: Boric Oxide, Calcia, Lead Oxide, Potash, Soda.

Frit A fusion of soluble or harmful glaze materials in a glass that renders them insoluble and safe so that they can easily be incorporated in the glaze.

Glaze The glassy layer covering the surface of pots to provide a non-porous, decorative and usually smooth surface giving increased strength to the pot.

Glaze Fit The matching of the thermal expansion and contraction of the glaze to the clay body. If glaze and body contract at different rates the faults of crazing, shivering or dunting can occur.

Glost Glaze firing, especially where the glaze firing is lower than the bisque

Greenware Unfired clayware

Grog Fired ceramic ground to specific sizes. Added to clay bodies to give openess to aid drying without warping, 'tooth' to aid forming and to reduce shrinkage

Hardening-on The firing of underglaze colours before applying glaze to prevent crawling, glaze smudging of the colour and contamination of the glaze and to aid glaze application.

Heatwork Temperature x Time. To produce ceramic change in firing it is necessary not only to reach certain temperatures but to expose the ware to temperature for a period of time. Heatwork is measured by Pyrometric cones, Bullers rings and Minibars.

In-Glaze Decoration of colour or oxide applied to the unfired glaze surface. The decoration is therefore neither underglaze nor on-glaze but in-glaze. Majolica and Delft ar examples of in-glaze ware.

Jigger/Jolley Clay forming machine. The Jigger rotates clay over a mould which forms the inside shape of the pot, the outside shape is produced by a former being pressed against the clay as it revolves. The Jolley is th reverse, it forms clay by rotating it while being held in a mould which shapes the outside of the pot, a former in the profile of the required design is forced into the clay creating the inside shape.

Kiln Sitter A mechanical device for shutting off kilns when the firing is complete. Kiln Sitters are activated by the bending of Minibars when they reach their melting point.

Kneading The manual working of clay to produce a homogenous smooth and plastic mass ready for throwing, hand building or modelling. Kneading follows wedging in clay preparation and is often referred to as wedging.

Lawn The fine wire mesh used for sieving materials. Usually made from phosphor-bronze and measured by the number of apertures per linear inch. A 60's mesh lawn would have 60 apertures per inch.

Low Sol Low Solubility. The name given to glazes incorporating metals, especially lead, which are present in a form that is not readily dissolved by the action of acid on the fired ware. Low Sol. glazes conform to Health and Safety requirements and are therefore safe for dinnerware when fired correctly.

Lustre Precious metals in liquid suspensions that are applied to glaze fired ware and fired to low on-glaze temperatures to produce a metalic surface.

Majolica Painted decoration of oxides or decorating colour applied to a white tin glaze before the glaze is fired. See In-glaze.

Maturity The temperature, or heatwork, range at which Clays reach their optimum strength and glazes achieve the desired surface quality and colour.

Minibars A form of Pyrometric cone but in a bar shape specifically designed for use with Kiln Sitters.

Opacifier Material for making transparent glazes opaque or white. Tin and Zircon are two common opacifiers.

On-Glaze Colour decoration applied to fired glaze and fired again to between 680°C and 880°C

Once-Fired Ware that has been fired once, i.e. glaze applied to greenware with glaze and body maturing together.

Oxidising Firing in a kiln atmosphere where oxygen is present. Electric kiln firings are always considered to be in oxidising atmospheres.

Peeling See shivering

Pin Holes Small holes in the glaze surface resulting from bubbles of gas coming through the glaze in its molten state and leaving craters that have had sufficent time to heal over. Causes are; poorly deflocculated casting slips, underfired biscuit and not long enough firings.

Pint Weight A measure of density used to enable slips or glazes of matching density to be reproduced. It is the weight of a pint of the liquid expressed in ounces e.g. 30 oz pt. The metric equivalent is gramms per 100ml. 30oz pt. equals 150 g/100ml.

Porosity The quality of fired ware to absorb water. Lower fired ware is generally more porous and absorbs water readily while ware fired to vitrification will have practically no porosity. The correct level of porosity is important in glazing where the rate of water absorbtion afects the thickness of the glaze layer.

Pugmill A machine for processing and reconstituting clay. Scraps of used clay are compressed by an auger through a tapering barrel and extruded. A De-airing pugmill has the same function with the addition of a vacuum pump to remove greater amounts of air.

Pyrometer A temperature measuring instrument. Pyrometers measure kiln temperatures by interpreting the micro-volt output of a thermocouple.

Raku A method of firing where pots are removed from the kiln at maturing temperature. After removing from the kiln Raku pots are often enclosed in sawdust or other combustible material to create a reducing atmosphere where bright metallic colours can often be produced.

Raw Glaze A glaze made of un-fritted materials.

Raw Glazing Once-firing.

Reduction Firing in an atmosphere where the oxygen content is reduced and oxygen is removed from metal oxides creating different colours in clay and glaze. The immersion of Raku pots in saw dust is described as post-firing reduction.

Refractory Substances that are resistant to high temperatures such as kiln furniture.

Salt Glazing Glazing with common salt which is thrown into the kiln at temperatures above 1100°C. The salt turns to vapour and the Soda content combines with Alumina and Silica in the clay to produce a glaze.

Sgraffito Decorating technique involving scratching through a slip or glaze layer.

Shelling See shivering.

Shivering A glaze fault occurring when the body contracts at a greater rate than the glaze, putting the glaze under compression and causing it to separate from the body and flake and peel off, particularly on edges of potts. Shivering is the opposite fault to crazing.

Silica Inversion The changing of the structure of Silica molecules at particular temperatures or Inversion Points, which causes expansion above the point or contraction below it. It is this change of size that creates the stresses that result in dunting. The inversion point of Silica in the Quartz phase is between 550°C and 575°C but is generally taken to be 573°C. In the Cristobalite phase it is between 220°C and 280°C and is taken to be 225°C.

Sintering The sticking together of materials prior to melting.

Slip Clay in a liquid form for casting or decorating.

Slip Casting Method of reproducing shapes in clay by moulding liquid slip in plaster moulds. The plaster absorbs water from the slip leaving a coating of clay next to the mould surface taking the shape of the mould.

Slop The name given to a glaze after it has been mixed with water ready for glazing.

Soak Holding the temperature for a given time during the firing to enable the kiln temperature to even out and to allow glazes to become smooth while molten without overfiring.

Spit Out Calcium impurities in a clay body can absorb moisture over time which causes them to swell and burst through the glaze layer leaving a crater in the pot. The bottom of the crater will typically have a small white centre confirming the presence of Calcium.

Stoneware Ceramic ware and the clays and glazes from which it is made. Stoneware is fired high enough to produce a low porosity body. Usually taken to be ware fired above 1200°C. Stoneware is also characterised by the integration of the glaze and the body.

Sprigging Embossed decoration on clay ware, usually press moulded shapes applied to the pot to give a raised decoration.

Thermal shock Sudden change of temperature in a fired pot that creates stress due to expansion and contraction. Oven ware must be made of a clay that withstands the thermal shock of repeated cooking.

Thermocouple An instrument for measuring temperature in the kiln. Thermocouples are the ceramic probes that protrude into the kiln. Wires inside the thermocouple generate a small current which is measured by a Pyrometer which displays the temperature.

Thixotropy The thickening of liquids when they are left undisturbed. A slip left for a few days will become less viscous as soon as it is stirred. Thixotropy enables the clay in a mould to keep its shape when newly cast and still wet.

Throwing The forming of round pots by rotating clay on a potters' wheel. The clay is driven through the potters' hands by the force of the wheel and the position of the hands determines the shape of the pot.

Transfer See decal.

Turning The trimming off of surplus clay on thrown pots. This is done at the leather hard stage and is usually confined to the outside and base of pots.

Underglaze Decoration applied to pots which are subsequently glazed with a transparent glaze. Underglaze colours are protected from wear by the glaze layer on top of them.

Venting Allowing the escape of gasses both from the kiln during firing and the kiln room. It is important that all kilns and kiln rooms are vented adequately to prevent the build up of harmful gasses.

Viscosity The opposite of fluidity, the stiffness of a slip or a molten glaze. Viscous slips pour slowly and viscous glazes move very little in the firing.

Vitrification The point at which clay becomes virtually non-porous and solid, almost glassy. Vitrification is the stage before melting when the pot would slump under its own weight.

Volatilisation Vaporization. Some glaze materials volatilise in the kiln atmosphere becoming gasses which can then be lost through the kiln flue or they can combine with materials on the surface of other pots giving flashes of colour. Chrome and Copper are materials prone to volatilisation and salt glazing relies on this phenomenom.

Wax Resist A decorating technique where liquid wax is used to create areas of pattern that repel a covering colour or glaze. Wax resist can be applied in several layers, each layer blocking out more of the origional colour. The wax is burned away during firing.

Wedging The process that compresses clay removes air bubbles and prepares it for use. A lump of clay is thrown down on to a work surface, cut in half with a wire and the half piece thrown down on the remainder. This process is the manual equivalent pugging and is followed by kneading.