CERAMIN - Energy saving concepts for the European ceramic industry

Glossary of Ceramic Terms

Abrasion Resistance - The resistance of a surface to being worn away by rubbing or friction. A measure of toughness more than of hardness.

Abrasives - Various hard substances (e.g. Fused Alumina) used for grinding, cutting or polishing softer substances.

Adhesion Test - Test methods used to determine the adequacy of ink coating adhesion to a substrate. A common test is the Cross-cut tape test.

Amorphous - Non-crystalline having no determinable form or crystalline structure, e.g. glass.

Angel Hair - Undesirable fine threads of ink caused when the ink pulls away from the printed surface. (see cobwebbing)

Apparent Porosity - Relation between the volume of a mass and the volume of the water absorbed when the mass is immersed.

Ark - Large storage vat or container, e.g. Glaze Ark, Slip Ark, etc.

Autoclave - An air tight chamber, usually of steel, used for heating articles under pressure as in the crazing test.

Ball Mill - A unit much used in the ceramic industry for the grinding of materials. It consists of a lined cylinder rotating about its horizontal axis and charged with flint pebbles or special ceramic grinding media plus the material to be ground. The mill may be operated dry or wet.

Batt Wash - A coating of refractory material applied to saggers, kiln furniture, etc., to prevent them from sticking during firing.

Bentonite - A general term for a montmorillonite clay, often added to a body to improve its plasticity. Binder - Portion of the vehicle in an ink composition that, in combination with the pigments, forms a film.

Biscuit Ware or Bisque - Pottery bodies after the first firing stage

Bleed - Migration of an ink component or dye into an area it is not wanted. Can also be the running of a pigment colour by action of the solvent.

Bloating - Expansion of a body produced by pressure exerted by gases trapped in a partially fused mass.

Blocking - An unwanted adhesion between layers of material placed in contact under moderate pressure and/or temperature in storage or use. Usually occurs when stacked prior to thorough drying.

Bloom - A milky veil, which appears on the surface of prints under adhesive conditions of heat and humidity. It is usually the result of too rapid solvent evaporation, which causes condensation of moisture on the surface.

Blunging - Mechanical mixing of clay or slip with water.

Body Stain - Colour used in the body

Bone China - Vitreous, translucent pottery made from a body of the following approximate composition (per cent): calcined bone, 45-50; china clay, 20-25, china stone, 25-30. The combined nomenclature (q.v.) defines this ware as `Completely vitrified, hard, impermeable (even before glazing), white or artificially coloured, translucent and resonant. Bone China contains calcium phosphate in the form of bone ash; a translucent body is thus obtained at a lower firing temperature than with hard porcelain. The glaze is normally applied by further firing at a lower temperature, thus permitting a greater range of underglaze decoration.' The British Pottery Manufacturers' Federation defined this material as` Ware with a translucent body containing a minimum of 30% of phosphate derived from animal bone and calculated as calcium phosphate. B.S.5416 `China Tableware' now specifies that bone china shall contain `at least 35% by mass of the fired body of tricalcium phosphate'.In the USA, ASTM-C242 permits the term `bone china' to be applied to any translucent whiteware made from a body containing as little as 25% bone ash. Bone china, though delicate in appearance, is very strong. It was first made by Josiah Spode, in Stoke-on-Trent, where by far the largest quantity of this type of high-class pottery is still made. Breakaway - In decal transfer, a decal intentionally designed to break off at the edge of a part.

Calcination - Firing of a ceramic material or mixture.

Casting - Forming pottery by pouring slip into a porous mould.

Casting Slip - A very fluid slip of high specific gravity, obtained by deflocculation and used for shaping articles by the casting process.

Catalyst - A substance which has the capability of initiating or accelerating the speed of a reaction between two or more substances when introduced into their presence, normally in small quantities.

Ceramic - Derived from the Greek 'Keramos' meaning 'earthen vessel'. Now applied to a whole field of products, i.e. Bricks, Tiles, Pipes, Porcelain, China, etc.

Ceramic Colour Standards - A set of 12 ceramic tiles provides standard colours for the calibration of colour measuring instruments used in all industries. Specially developed colours and glazes and rigorously precise firing schedules give stable and reproducible colours. There are 3 neutral grey standards with flat spectral curves to check linearity of response; 7 strongly coloured standards with steep spectral curves to check responsivity or wavelength and waveband errors; 2 colour-difference standards which `pair' with the mid-grey and the green standards, giving a direct check on colour difference measurements. A black and a white standard are available, plus `specials' to individual requirements. (The development of these tiles, in collaboration with the National Physical Laboratory, earned CERAM Research, Stoke-on-Trent, its second Queen's Award for Technological Achievement).

China - BS 5416 specifies this to be pottery with water absorption 0.2% and translucency 0.75% (assessed by comparison with standard test pieces). In the USA, however, ASTM-C242 defines the word as any glazed or unglazed vitreous ceramic whiteware used for non-technical purposes, e.g. dinnerware, sanitary-ware, and art-ware, provided that they are vitreous. The combined nomenclature (q.v.) equates this term with porcelain (q.v.).

Chromophore - Atoms or ions whose arrangement of electrons leads to selective absorption of particular wavelengths of light, and so imparts colour to their compounds, which may be used as (q.v.). Typical chromophores are multivalent metals such as Co, V. In lattice colours the chromophore forms part of the crystal lattice. Cliché - Used in pad printing. The "inkable" steel or nylon plate that is etched with an image to be transferred on to the silicone pad and then to the part.

Clogging - Premature drying of ink in the screen, causing bad print by blocking part of the open stencil. Cobwebbing - Fine filaments produced by the ink between the screen and the printing surface resulting in a cobweb-like appearance on the finished print. Sometimes called angel hair.

Coefficient of Expansion - A measure of the reversible change of volume or length of a ceramic material with temperature.

Colloid - A stable suspension of very small particles.

Colour - Colour is important in the ceramics industry because of its aesthetic value. Most ceramic `colours' are solid coloured particles (pigments) suspended in a glass. Such decoration must be durable, non-toxic and stable. It is applied by a wide range of processes, as designs or coloured glazes. Colour Retention - Colour stability after exposure to the elements.

Combustible Liquid - Liquid having a flash point at or above 37.8OC but less 93.3OC

Comminution - Size-reduction by breaking, crushing or grinding.

Compressive Strength -The ability of an article to withstand crushing loads. Corona Discharge - An electrical surface treatment that encourages oxidation of a surface to reduce surface tension and improve ink adhesion.

Covercoat - A polymeric coating which may incorporate a low-melting glass, used to overprint decals to facilitate either water slide or heat release application to the substrate.

Covercoating - The application of the covercoat by screen-printing.

Crackle - Cracks in a glaze due to crazing which are sometimes produced intentionally.

Cratering - Undesirable depressions in the dried ink film that may be large enough to expose the substrate.

Crawling - Movement of glaze over the body surface during the glost firing stage, due either to dust or grease on the surface or to the production of colloidal material, or the cohesive effect of ink into drops after printing onto a surface which the ink does not wet completely.

Crazing - A random pattern of tiny intersecting cracks caused by excessive contraction of a glaze.

Crocking - Image rub-off that occurs when a pigment or dye is not adequately fixed in the substrate.

Cryptocrystalline - Possessing crystals so small that they cannot be distinguished by a microscope.

Cut Glaze - Bare areas on glazed ware due to mechanical damage to the glazed article in the unfired state.

De-airing - Removal of air from clay, plaster, etc. Various means are used but the most common with clay is via the de-airing chamber of the pugmill.

Decal - The term for a ceramic transfer or litho. Decals are used to apply designs to ceramic tableware, ornamental ware and tiles, and to glass containers. The decal comprises three layers: the colour, or image, layer which comprises the decorative design; the covercoat, a clear protective layer, which may incorporate a low-melting glass; the backing paper on which the design is printed (by screen printing or lithography q.v.). There are various methods of transferring the design while removing the backing-paper, some of which are suited to machine application. (See heat-release decal, heat-activated decal, slide-off decal). Deflocculation - The dispersion of clay slip or glaze by the addition of an electrolyte, e.g. sodium silicate, soda ash, etc.

Degreasing - Removing dirt and oils from the stretched screen before applying the stencil to insure adhesion to the screen surface.

Densitometer - Photoelectric device that measures either the reflected or transmitted density of photographic film or printed colour.

Devitrification - The crystallization of a vitreous material.

Diazo - A photosensitive chemical added to emulsion used in making screens (stencil).

Dilatancy - The property of a suspension to thicken when stirred and to thin again when stirring is stopped.

Dipping - The application of a glaze by immersion, allowing the excess to drain off.

Direct Emulsion - A liquid photo-polymer emulsion used as a screen printing stencil which is coated onto a stretched screen, dried, exposed through a film positive to actinic and developed.

Dispersion - The breaking up of a particle aggregate into separate particles without changing the particle size.

Dispersion - The separation of clusters of particles into separate particles.

Dot Gain - The tendency of a printed halftone dot to change in size at the moment of ink imprinting changing the overall visual quality of the print.

Dragging - A flaw in the print by an obstruction coming into contact of the wet ink.

Draize Test - A test to measure the eye and skin irritability of a chemical substance.

Draw - Absorption of glaze by an unglazed surface nearby during firing.

Drawdown - A film of ink deposited uniformly on a substrate by means of a smooth edged blade or cylindrical rod to evaluate undertone, mass tone, adhesion, etc. of the ink.

Dropper - Globules of glaze found on open glost ware, usually from glaze accumulations on the roof of the kiln.

Dunting - Splitting of ceramic ware due to silica inversion.

Earthenware - Non-vitreous, opaque, ceramic whiteware. The combined nomenclature (q.v.) defines earthenware products as those made from selected clays, sometimes mixed with feldspars and with varying amounts of chalk, characterised by a white or light-coloured fragment (slightly greyish, cream or ivory). The fragment, which has a fine grain, is homogeneous; the diameter of the non-homogeneous elements (particles, inclusions, and pores) representative of the structure of the general mass should be less than 0.15 mm: these elements are therefore not visible to the naked eye. In addition their porosity as measured by the method specified in the Nomenclature, (6912 00 10) (coefficient of water absorption) is 5% or more by weight. The general body composition is (per cent): china clay, 25; ball clay, 25; calcined flint, 35; china stone, 15. The biscuit firing temperature is 1100 to 1150C.

Edge Runner Mill - A type of mill used for grinding or mixing materials in which the treatment is carried out by the pressure of large rollers squeezing the material between themselves and the base of the pan. Efflorescence - A growth on the surface of bricks and other articles due to the presence in them of soluble salts. Electrical Porcelain - Made for use as an electrical insulating material. Typical composition per cent is ball clay 28, china clay 22, quartz 25, feldspar 25.

Elutriation - The separation of particles according to their size or specific gravity by a controlled velocity water stream. The basis of a method for the determination of fineness of materials.

Enamel - A fusible vitreous coating for metals or claywares at low temperatures.

Enamel Colour - A ceramic colour for the on-glaze decoration of pottery.

Enamel Firing - A further firing to convert ceramic colouring materials applied on top of the glaze to a permanent form.

Encapsulated Colours - Ceramic colours in which the chromophore is physically trapped or encapsulated in the crystal matrix, not forming a part of the crystal lattice itself (cf. lattice colours). The pink cadmium sulphoselenide inclusion pigments (Degussa, Br. Pat. 1403470, 1975) were of this type, with the pigment trapped in the zircon lattice. Later developments have led to means to entrap much greater concentrations of chromophore to enhance colour saturation, producing bright yellow, orange and red cross colours. (British Ceramic Research Ltd UK Pat. 2106530A, 1983). The technique can also be used to impart stability to pigments which would be difficult or impossible to use, either by reducing flow problems caused by reaction with glazes (cobalt blue colours) or increasing thermal stability of the pigment - grey colours based on carbon black pigment encapsulated in zircon.

Engobe - A white or coloured coating of slip applied to a ceramic body before glazing to improve its appearance.

Eutectic - A mixture of two or more substances which has the lowest melting point in the whole series of possible compositions.

Exempt Solvents - Evaporative solvents not currently subject to air pollution standards.

Extrusion - The process of forcing a plastic material through an aperture or die.

Faience - Originally the French name for the earthenware made at Faenza, Italy, in the 16th century; the ware had a tin-opacified glaze and in this resembled Maiolica and Delft ware. The meaning has now changed. In France, faience is any glazed porous ceramic ware; faience fine is equivalent to English earthenware (q.v.). In England, the term `faience' now refers to glazed architectural ware, e.g. large glazed blocks and slabs (not dust-pressed glazed tiles). In USA, faience ware signifies decorated earthenware having a transparent glaze. Fettling - The removal of the seam left by the mould in clayware, by a fettling knife and/or sponge. Fillers - Materials added to a system to give it rigidity or increase its volume. Flint and quartz are often referred to as fillers in ceramic bodies.

Filter Cloth - Used to filter clay slips. Now of nylon or terylene.

Fine China - A term used, without being precisely defined, to describe ware as high-quality china (q.v.). Fine china is not necessarily bone china, and the term may aptly be applied to ware (e.g. E.T.C.) that does not contain bone ash.

Fish Eye - An undesirable circular flaw in the printed image usually caused by bubbling of the ink.

Flame Treat - Exposure of the surface to be printed/decorated with a flame, dispersing surface solvents to improve adhesion.

Flammable Liquid - A liquid having a flash point below 37.8OC.

Flash Point - The minimum temperature of a saturated vapour of a specified liquid at which the vapour will ignite in the presence of a spark or flame.

Flocculation - 1) The aggregation of suspended particles by the addition of electrolytes to give a proper consistency for dipping, casting, etc., thickening. 2) The process by which an aggregation of pigment particles in ink forms clusters or chains changing the hue and/or chroma of that ink. Also called Livering.

Flux - A substance added to a material to enable it to fuse more readily, i.e. at a lower temperature (e.g. alkali oxides). This liquid or cooling forms a glass which binds the grains of the body together.

Four Colour Printing - Reproducing full colour by photographically separating the art into its three primary colours: yellow, magenta and cyan plus black and printing them in that configuration.

Frit - Any fused substance or mixture quenched to a glass-like form.

Frizzling - Mainly a decorating fault, typified by the curling over or frizzling of parts of the decoration. Caused mainly by too high a temperature in the initial stages of firing thereby causing the boiling off or erupting of organic media, i.e. size, fat oil, printing oil, etc.

Fusion - Any treatment which converts a solid substance to the liquid state.

Glaze - A thin glassy layer formed on the surface of a ceramic product by exposure of the coated article to a high temperature. Usually applied in the form of a suspension of ground glaze in water, to the clay or biscuit surface of the ware.

Glaze Stain - Colour added to the glaze.

Glaze-body Fit - The relationship between thermal expansion of body and glaze. Ideally the glaze should have a lower thermal expansion than the body, so that on contraction the body puts the glaze into compression. This avoids the crazing of the glaze due to tensile stresses in it.

Glost - Meaning glazed.

Green Ware - Unfired clayware.

Grog - Ceramic material which has been heated to a high temperature before use and therefore predominantly inert.

Halftone - Representing the level of a printed colour (or black) with dots of varying size and frequency. Hardening On - A process of heating decorated biscuit ware to a temperature of approx. 650-700C in order to burn out the organic media of the decoration and in order to fix the colour prior to glazing. Hard paste Porcelain - True feldspathic porcelain (continental porcelain), Hard-paste Porcelain being the older term still used by collectors. The combined nomenclature (q.v.) defines hard paste porcelain as made from kaolin or kaolinitic clays, quartz, feldspar and sometimes calcium carbonate. It is covered with a colourless transparent glaze fired with the body.

Heat Release - The process of transferring decals to the substrate by application of heat. There are different systems available for flatware and hollowware.

Heavy Metals - Toxic chemicals found in some pigments including lead, chromium, cadmium.

Hot Pressing - Densification of particles by the simultaneous application of heat and pressure, pressure sintering.

Hotelware - Tableware of extra strength (usually achieved by extra thickness) for use in hotels and restaurants; for specification see B.S. 4034, which demands no crazing after 16 hours in an autoclave at 340 10 kPa, and a water absorption less than 0.2%.

Indirect Emulsion - Indirect stencils are supplied in either sheets or rolls with the sensitised emulsion already coated onto a polyester support sheet.

Ink - A mixture of a medium and colour/pigment.

Jiggering - A machine for the shaping of pottery, by means of a profiled tool at a fixed distance from the rotating surface of a plaster mould. Mainly flatware.

Jumping - The loss of colour to colour relationship in the printed image due to inaccurate printing in one or more colours. (poor registration)

Kaolin - From the Chinese Kao-lin meaning a high ridge where the whiteclay was first discovered.

Kiln - Pottery oven or furnace in which ceramic products are fired.

Klin Furniture - General term used to describe refractory pieces used to separate and support pottery during firing.

Laminations - The structure of un-fired or fired body in which the materials are aggregated in the form of layers or strata.

Lattice Colour - Ceramic colours in which the chromophore or colouring atom forms part of the crystal lattice.

Lawn - A fine mesh sieve, usually made from phosphor bronze or stainless steel, and supported by a strong frame.

Lead Solubility - The solubility of lead, particularly from glazes into dilute aqueous hydrochloric acid.

Leather Hard - Partially dried clayware. The ideal stage for turning, fettling, sponging, etc.

Lithography - A method of decoration involving manufactured transfers. The printing of the transfers is done by printing in lithographic oil, the colour then being dusted on to the oiled areas. The completed transfer is applied to ware that has been previously coated with a tacky size, rubbed down and the paper backing sponged off.

Loss on Ignition - Usually abbreviated to L.O.I. The loss in weight of a clay or other material expressed as a percentage of its dry weight when heated under specified conditions.

Low Solubility or low sol - If the solubility of lead from an inorganic lead compound, usually a glaze, which does not release more than 5% of it's dry weight of soluble lead, when subjected to a specified test using hydrochloric acid, then it can be described as low sol.

Lustres - An iridescent optical appearance, due to light reflections producing its volume.diffraction patterns on a glazed surface. Produced by very thin coatings of metallic substances fired on to the glaze.

Majolica - Originally named from the island of Majorca where it was first made in the 16th century, similar to Delft. Majolica - Originally a porous type of pottery, with a glaze opacified with tin oxide, from the island of Majorca; this pottery was first made in the 16th century under the combined influence of Hispano-Moresque and Near Eastern wares; it was essentially similar, technically, to delft-ware and the original faience. As applied to present-day pottery, the term signifies a decorated type of earthenware having an opaque glaze, usually fired at a comparatively low temperature (900-1050C). The term has also been applied to vitreous enamels that are transparent, but contain colouring oxides; they are used over ground-coats and allow the latter to show where there is any relief.

Matte - A low gloss (no gloss) finish.

Medium - A vehicle used either hot or cold to mix with the colour to create an ink

Mercury Vapour Lamp - A high intensity light use into photo screen preparation and curing of some inks.

Mesh Count - The unit of measure which indicates the number of openings in a linear measurement of screen fabric.

Mesh Marks - A fine pattern left by the mesh of a screen printing fabric.

Model - The original or prototype of the piece to be made. Usually in clay, occasionally of plaster or other material.

Modulus of Elasticity - A term defining the extent to which a material may be distorted under a given stress. Important in calculating glaze/body stress relationships.

Modulus of Rupture - The resistance offered by a piece of ceramic of unit cross sectional area to a cutting force. The mechanical strength of most ceramics is quoted as modulus of rupture.

Moiré - An undesirable optical effect that occurs with halftone printing.

Moisture Expansion - The extent to which a porous ceramic material will expand in size when it absorbs water or water vapour.

Mottle - A spotty or uneven printed surface with is most apparent in solid areas.

Muffle Kiln - A chamber or box, built in a furnace and used to fire articles out of direct contact with flame or products of combustion.

Newton - A unit of measure used in screen fabric tensioning.

Off-Contact - The preset distance between the screen and the substrate to be printed.

Opacifier - An additive to a glaze which increases the reflection of light to the observer. Commonly tin oxide or zircon.

Optical Pyrometer - A form of pyrometer in which the temperature of an article or furnace is estimated by comparing the colour and intensity of its luminous surface with that of a calibrated filament.

Particle Size Distribution - For a powder it indicates both the variety of particle sizes and the proportion of particles of each size.

Peeling - A defect in glazed ware characterised by the engobe or glaze separating from the body in flakes, usually due to high compressive stresses in the layer.

Pigment - Pigments are solid particles of colour, which are suspended in glass to form ceramic colours. Ceramic pigments, as well as having good colouring properties, must be able to withstand firing temperatures of at least 750C, and possibly up to 1400C, in contact with fluxes and glazes. They must be chemically inert and colour stable, non-toxic (meeting metal release regulations) and alkali-stable to withstand machine dishwashing detergents.

Pin Holes - A glaze or body fault resulting from trapped air erupting through the body or glaze during firing.

Pinholes - Undesirable tiny holes in the printed image that are sometimes only visible when backlit.

Pitchers - Fired broken or scrap pottery. Biscuit pitchers have various uses when crushed or ground, i.e. grog, fillers, stopping, etc.

Plasticity - The property which enables material to be deformed by a force which exceeds a certain minimum value and to retain its new shape when the deforming force is removed.

Plucked Ware - A fault caused by glazed parts of the ware being in contact with kiln furniture.

Porcelain - One type of vitreous ceramic whiteware. The combined nomenclature (q.v.) defines this as: completely vitrified, hard, impermeable (even before glazing), white or artificially coloured, translucent (except when of considerable thickness) and resonant. In the UK the term is defined on the basis of composition: a vitreous white-ware made from a feldspathic body (typified by the porcelain tableware made in Western Europe and containing 40-50% kaolin, 15-25% quartz and 20-30% feldspar). In the USA the term is defined on the basis of use: a glazed or unglazed vitreous ceramic white-ware used for technical purposes, e.g. electrical porcelain, chemical porcelain, etc. Note, however, that the term electrical porcelain is also used in the UK. The firing of porcelain differs from that of earthenware in that the first firing is at a low temperature (900-1000C), the body and the feldspathic glaze being subsequently matured together in a second firing at about 1350-1400C. Tensile strength 140 MPa; Young's modulus 200 GPA; Poisson's Ratio c. 0.25.

Porosity - The amount of pore space in a ceramic material, which may consist of both open and sealed channels.

Pugging - The intimate mixing and extrusion of plastic clay body constituents.

Pyrometric Cones - Small tetrahedral pyramids, made of various ceramic materials, which fuse and bend over at given temperatures. Used as thermoscopes.

Quenching - Method of cooling molten frit by allowing it to pour into a 'bosch' or bath of water, thereby giving the frit its characteristic granular form.

Raw Glaze - A glaze in which none of the constituents has been fritted.

Reclaiming - The process of removing ink and stencil from a screen to enable that screen to be reused.

Reducing Atmosphere - A kiln atmosphere which is deficient in free oxygen and causes reduction of compounds rich in oxygen which may be present in it.

Refractory - The property of being able to withstand high temperatures. It is evaluated in terms of the Pyrometric Cone equivalent.

Registration - The colour to colour relationship in a printed image. The quality of the printed image depends on the accuracy of the registration.

Resin - A solid (or semi-solid) which can be dissolved to a liquid state, suspended in a vehicle to make an ink or coating and which, upon drying, forms the solid part of printed film.

Saggars - Fireclay boxes of various shapes, in which pottery is set in order to protect it from contamination during firing, and in order to stack it in the old bottle ovens.

Salt Glaze - Used in the 17th and 18th century for the glazing of domestic pottery. Now except for some studio potters, confined to some salt glazed sewer pipes. The glaze is applied by throwing salt on to the fires during firing, the ensuing vapour then reacting with the clay to form a Sodium Aluminosilicate glaze.

Sanitaryware - The various types include: sanitary earthenware (q.v.); vitreous china sanitaryware (q.v.); sanitary fireclay (q.v.); vitreous-enamel sanitaryware, e.g. baths. B.S. 1125 and B.S. 1188 specify W.C. flushing cisterns and ceramic wash basins and pedestals respectively. B.S. 5503 and B.S. 5504 specify materials, dimensions and quality of vitreous china washdown W.C.'s and wall hung W.C.'s respectively. B.S. 5627 specifies plastic connectors for vitreous china W.C. pans. B.S. 5505 is a specification for bidets; B.S. 5506 for pedestal and wall hung wash basins; B.S. 5520 for vitreous china bowl urinals; B.S. 6340 specifies shower trays (Pt. 6 - vitreous enamelled cast iron; Pt. 7 vitreous enamelled sheet steel; Pt. 8 glazed ceramic). B.S. 6465 specifies the required scale of sanitary ware installation. B.S. 3402 specifies the quality of vitreous china sanitary ware. B.S. 8000 Pt. 13 specifies site workmanship.

Sawtooth - The effect of a stencil that leaves a printed image that conforms to the mesh of the fabric rather that the desired clean contour of the image.

Sedimentation - The settling out, over time, of particles from suspension in, a fluid. Theory used for determination of fineness of ceramic materials.

Sgraffito - The cutting or scratching through an outer coating or engobe to expose the different coloured body beneath, in clayware. From the Italian 'graffito', to scratch.

Shelf Life - The length of time during storage of an ink, emulsion, etc. in a specific container under normal conditions without changing chemically and affecting usefulness.

Silk Screen - A method of decoration where the colour in paste form is forced by means of a squeegee through a fine mesh mounted on a frame, on to paper or other article to be decorated. The area of the screen not required in the pattern is blanked out. Now highly mechanised.

Sintering - The adhesion and densification of particles of a single phase on heating.

Slip - Suspension of clay or body in water.

Snap Gap - The preset distance between the screen and the substrate to be printed.

Soak - To maintain ceramic articles at a prearranged temperature in the kiln for a considerable time.

Soft paste Porcelain - A type of porcelain made from a soft body containing a glassy frit and fired at a comparatively low temperature (1100C). The most famous soft-paste ware was that produced in the 18th century at the Sèvres factory in France, and at Chelsea, Derby, Bow, Worcester and Longton Hall in England. The combined nomenclature (q.v.) states that soft porcelain contains less alumina but more silica and fluxes than hard porcelain.

Solvent - Any dissolving, thinning or reducing agent.

Spalling - The flaking, cracking, or other disintegration of ceramics when subjected to sudden temperature changes.

Specific Surface Area - The total area of the surface of all the particles in unit weight of material. Spitout - Rapid desorption of adsorbed moisture during the enamel firing resulting in small craters or bubbles being blown in the glaze.

Spray Drying - A means of drying clay slips and other suspensions by spraying them into a chamber through which hot air is passed.

Stoneware - The combined nomenclature (q.v.) is: "Stoneware, which, though dense, impermeable and hard enough to resist scratching by a steel point, differs from porcelain because it is more opaque, and normally only partially vitrified. It may be vitreous or semi-vitreous. It is usually coloured grey or brownish because of impurities in the clay used for its manufacture, and is normally glazed". A translucency (q.v.) test is specified. Water absorption is to be 3 wt %. The body consists either of a naturally vitrifying clay (a stoneware clay) or of a mixture of suitable ball clays, filler and flux. Stoneware may be once-fired or it may be biscuit fired at about 900C followed by glost-firing at 1200-1250C. At the present day stoneware is produced on a commercial scale chiefly as kitchen-ware and tableware, for which purposes its high strength and freedom from crazing are valuable; on a smaller scale, stoneware is much favoured by studio potters.

Sublimation - The process whereby certain dyes change directly from a solid to a vapour and back again to a solid by the application of heat.

Surface Tension - The capacity of a liquid to wet a solid surface or otherwise. The property (measured in dynes per centimetre) by which all liquids through contraction of the surface tend to bring the contained volume into a form having the least area. If an ink is to be compatible with a substrate, the surface tension of the ink must be close to that of the substrate.

Tailings - That portion of a material which does not pass through a given screen or sieve and is returned to the plant for further treatment.

Tenmoku - A stoneware glaze deeply coloured by iron oxide. Colours can vary from yellow, green, rust red, brown, and black, and are often lustrous.

Tensile Strength - The resistance of a material to being torn apart by tension or pulling.

Thermal Conductivity - The rate at which heat passes through a material as measured by its rise in temperature.

Thermal Shock - The disruption of a ceramic article by stresses set up due to differences in temperature in different parts of the article.

Thermocouple - A device for the measurement of temperature based on the voltage generated when two dissimilar conductors are heated in contact, e.g. Copper/Constantan, Chrome/Alumel, Platinum/Rhodium, etc.

Thermoplastic - A soft and mouldable plastic when subjected to heat. Most plastics that are decorated are thermoplastics.

Thermoplastic Inks - Inks that when heated (70 - 90C) are liquid but on cooling solidify.

Thixotropy - The property of a suspension to continue thinning when stirred constantly. It is often used to describe a suspension which thickens when left unstirred for some time. e.g. partially-flocculated or over-flocculated slips. For inks the property to liquefy under vibration or shearing (by a squeegee).

Throwing - The method of shaping pottery on the potter's wheel, in which a ball of prepared clay is thrown on to the wheel where it is centred and shaped by hand.

Towing - The smoothing of the outer edge of clay (green) flatware, using tow, although sandpaper or profile scrapers are also used. (See fettling).

True Porosity - The sum of open pores as determined by water absorption volume of those pores which are sealed by vitreous matter and therefore closed to water.

Under-Glaze - Any method of decoration applied before the glazing process. See 'Hardening On'.

Viscometer - The instrument used for measuring the viscosity (thickness) of liquids.

Viscosity - The resistance to flow offered by a liquid. The reciprocal of Fluidity.

Vitreous - As applied to ceramics, means glassy. Having extremely low or no porosity.

Vitreous-china Sanitaryware - Defined in B.S. 3402 as: "A strong high-grade ceramic ware used for sanitary appliances and made from a mixture of white-burning clays and finely ground minerals. After it has been fired at a high temperature the ware will not, even when unglazed, have a mean value of water absorption greater than 0.5% of the ware when dry. It is coated on all exposed sur faces with an impervious non-crazing vitreous glaze giving a white or coloured finish." A typical batch for this type of body is: 20-30% ball clay, 20-30% china clay, 10-20% feldspar, 30-40% flint, 0-3% talc; sometimes nepheline syenite is used instead of feldspar. B. S. 3402 also includes tests for chemical resistance to various acids, alkalis and detergents, (the visual assessment of reflectivity to be unchanged by the chemical treatment); an autoclave test for crazing resistance, and definitions of various blemishes and faults affecting sanitaryware.

Vitrification - The progressive fusion of a material or body during the firing process. As vitrification proceeds the proportion of glassy bond increases and the apparent porosity of the fired product becomes progressively lower.

Volatile - Subject to evaporation at a relatively low temperature.

Water Absorption - A measure of the open pore volume of a ceramic body determined by soaking the article in water under specified conditions.

Water Based Inks - Inks containing a medium is water soluble or water dispersible.

Water Slide Decals - The cover-coat and pattern layer are attached to the backing layer with a water soluble adhesive, such as dextrin, the decals are then soaked in water allowing the decal to removed from the paper for hand application.

Weathering - The effect of atmospheric elements on a test subject when exposed out of doors in a natural environment or under environmental influences.

Web Fed - Automatic feeding system whereby substrates or carriers are printed on a continuous roll.

Wedging - A method of de-airing and dispersing moisture uniformly by hand in a piece of clay. The lump of clay is repeatedly thrown hard on to the work-bench, turned over, and occasionally cut through and re-joined. Well Side - The side of a screen where the ink is and the squeegee is dragged across.

Wet On Wet - Describes the printing of multiple Colours onto a substrate before the previously printed colours have dried.

Wetting Agent - A chemical which when added to a liquid reduces its surface tension and causes the liquid to wet solid surfaces more efficiently.

Wreathing - Ripples or waves on the drained surface of a cast body caused by variations in the casting rate over different parts of the article.

Glossary of Ceramic Terms based on Pottery & Ceramics Handbook, 9th Edition, Glossary; Walker Ceramics - Pottery Clays, Glazes & Colours, Australia, Revised December 2009