Ceramic Engineering Glossary: A to Z Terms for Ceramic Manufacturing

Use this ceramic engineering glossary as a practical reference for production, testing, purchasing and troubleshooting. It covers tile, refractory, tableware, sanitaryware and traditional ceramic terms, with short definitions and key values where they help shop-floor decisions.

The glossary is written for engineers, technicians, students and ceramic business teams who need a plain-English explanation before checking the detailed standard, test method or plant specification.

Quick Index

A — Abrasion class (PEI) · Additives · Apparent density · Apparent porosity · Asymmetric warping · Atomiser

B — Ball mill · BET surface area · Bigot curve · Bingham plastic · Biscuit · Black core · Black core halo · Bloating · Blunger · Body · Bone dry · Bulk density

C — Cake density · Capillary diffusion · Capillary pressure · Casting rate · Centrifugal casting · Ceramic roller · Chamfering / bevelling · Chicken-claw crack · Clayey materials · Closed porosity · Combustion-air preheat · Compacted body flake · Compaction ratio · Comminution · Conchoid cracking · Conduction drying · Continuous wet mill · Convection drying · Convex / concave planarity · Cooling cracks · Coefficient of linear thermal expansion · Crawling · Crazing · Critical moisture content · Critical speed (mill) · Cyclone separator

D — De-airing · Decal · Decarbonation · Deep abrasion resistance · Deflocculant · Deflocculation · Demould · Densification · Devitrification · Disc application · Discontinuous (batch) ball mill · Double firing · Drain (hollow) casting · Dry finishing · Dry grain applicator · Dry grinding · Dry pressing · Dry-bag isostatic pressing · Drying · Drying curve · Drying shrinkage · Dunting

E — Edge flash · Eggshell · Efflorescence · Ejection cracks · Engobe · Exaggerated grain growth · Extrusion

F — Filler box · Filling defect · Final cooling · Firing · Firing curve · Firing zone · Flat screen · Flocculation · Fluidity · Fluxing materials · Fracture toughness · Frit · Friction (slip) coefficient · Frost resistance

G — Glaze · Glaze maturing time · Glaze shrinkage · Glaze thickness · Glassy phase · Glost firing · Grain growth · Granito · Green strength / density · Grinding media

H — Hammer mill · Hard agglomerate · Hausner index · HD roller · Heat work · Horizontal / multi-layer drier · Hot pressing · Hot isostatic pressing (HIP) · Hydraulic press

I — Impact crusher · In-glaze decoration · Inert materials · Infrared (IR) drying · Initial / intermediate / final-stage sintering · Injection moulding · Isostatic die / punch · Isostatic pressing

J — Jaw crusher · Jet (fluid-energy) mill · Jiggering

K — Kiln car / batt / sagger · Kiln contamination types

L — Lamination (pressing) · Lappato · Latent crazing · Leather hard · Liquid-phase sintering

M — Magnetic separator · Mesh / sieve number · Microwave drying · Micronization · Mill lining / lifters · Mirror polishing · Modulus of Rupture (MOR) · Mohs hardness · Moisture expansion · Mullite

N — Neck formation · Newtonian fluid

O — On-glaze decoration · Opacifier · Open porosity · Orange peel · Over-firing

P — Particle / agglomerate / granule · Particle Size Distribution (PSD) · Peeling / shivering / flaking · Pendulum mill · Pinhole · Pitting · Plaster mould (model, case, working) · Plasticity · Polishing · Polishing sludge · Post-expansion · Pre-kiln zone · Preheating zone · Pressing · Pressure casting · Priest hat · Pseudoplastic / shear-thinning · Pyroplastic deformation

Q — Quartz inversion

R — Rapid cooling · Rapid double firing · Reactivity · Real (true) density · Reclaim (overspray) · Reducing / oxidising atmosphere · Refractoriness (PCE) · Refractoriness under load (RUL) · Regranulation · Rheology · Roller (rotocolor) decoration · Roller effect · Roller engobe · Roller kiln · Rotary screen

S — Screen printing · Semi-dry / dust pressing · Shuttle kiln · Sieve residue · Single firing · Sintering · Sintering atmosphere · Slip · Slip ageing · Slip pump types · Slow cooling · Soaking · Softening point · Solid casting · Solid-state sintering (SSS) · Soluble salt decoration · Solution-precipitation · Specific gravity / litre weight · Specific pressure · Sphere / semi-sphere temperature · Spray drying · Spray gun (glazing) · Spray-dried granule · Springback · Squaring (rectifying) · Stain resistance · Sulphur attack · Surface tension (glaze) · Suspension agent

T — Tape casting · Tapped density · Third / fourth firing · Thermal conductivity · Thermal shock · Thixotropy · Toggle / friction press · Total porosity · Tunnel kiln

U — Under-glaze decoration · Underfired ware · Uniaxial pressing

V — Vacuum casting · Vertical drier · Vibratory sieve · Vickers hardness (Hv) · Viscosity · Viscous composite sintering (VCS) · Vitrification · Vitrification curve

W — Water absorption (WA) · Waterfall unit · Weibull modulus · Wet finishing · Wet grinding · Wet pressing · Wet-bag isostatic pressing

Y — Yield point · Young's modulus (E)

Z — Zeta potential

Glossary A to Z

A

Abrasion class (PEI)

Visible-abrasion class for glazed floor tiles, commonly reported as PEI class 1 to 5 and tested with rotating abrasive under ISO 10545-7. Use it for glazed wear suitability, not for unglazed deep-abrasion performance. Class 1 suits light residential traffic, class 5 suits heavy commercial floors.

Additives

Inorganic or organic ingredients used at less than 1% in a slip to control rheology, suspension, binding or anti-foaming. Examples are deflocculants, binders like CMC, suspension agents and biocides.

Apparent density

Bulk density of a powder measured loose in a known volume, in g/cc. Used along with tapped density to calculate the Hausner index.

Apparent porosity

Volume of water-absorbed open pores divided by bulk volume of a fired piece, measured under standard boiling conditions. Used directly in tile water-absorption classification.

Asymmetric warping

Irregular tile warping perpendicular to the kiln rollers, caused by heat-displaced or worn rollers. One side of the tile sits higher than the other after firing.

Atomiser

Spray-dryer component that breaks slip into droplets before drying. Common designs include pressure nozzles and rotary turbine discs; the right choice depends on slurry solids, required granule size and dryer capacity.

B

Ball mill

Rotary cylindrical mill that grinds material by tumbling ceramic media (silica, alumina or porcelain pebbles) against the charge. Used for both body and glaze grinding, in batch (Alsing) or continuous form.

BET surface area

Specific surface area of a powder measured by gas adsorption (Brunauer, Emmett, Teller method). Reported in m²/g and indicates how reactive the powder will be in sintering.

Bigot curve

Plot of drying shrinkage and weight loss against drying time at constant temperature. The inflection point on the curve marks the critical moisture content.

Bingham plastic

Fluid that needs a minimum shear stress (yield point) to start flowing, then flows linearly with rising shear rate. Many ceramic extrusion mixes behave this way.

Biscuit

Body fired once and not yet glazed, used in the double-firing route. After biscuit firing the piece is glazed and re-fired.

Black core

Dark grey or black layer at the inside of a fired body, caused by trapped organic matter or iron compounds that did not oxidise during preheating. Most common in fast-fired thick wall tiles.

Black core halo

Yellow-green to grey-black ring inside the fired body around an unoxidised core, caused by partial oxidation in the preheat zone of the kiln.

Bloating

Bulge or swelling of the fired body caused by gas bubbles forming inside during firing. Usually due to organic content, trapped air, or over-firing past the body's optimum.

Blunger

High-speed mixer used to disperse clay and non-plastic raw materials into water and form a slip. Used at the start of body and glaze preparation.

Body

Formulated mix of clayey, fluxing and inert raw materials that becomes the ceramic article after forming and firing. The body is distinct from the glaze applied on top.

Bone dry

Final state of greenware with moisture below 0.5 to 1%, ready for glazing or for entering the kiln. The piece feels dry to touch and shows no further drying shrinkage.

Bulk density (powder)

Mass per total volume of loose powder including all inter-particle voids, in g/cc. Measured by filling a calibrated cylinder without tapping.

C

Cake density

Bulk density of the pressed tile, measured immediately after pressing. Reflects compaction quality and is checked across the cavity to detect filling problems.

Capillary diffusion

Movement of internal water from the wet interior to the drying surface through capillary channels in the body. Dominant mechanism in the second stage of drying.

Capillary pressure

Liquid-mediated suction that pulls particles into closer packing during liquid-phase sintering. Above 7 MPa for many silicate liquids.

Casting rate

Rate at which a solid layer of cast forms inside a porous mould, expressed as thickness per hour. Controlled by slip composition, mould absorption and applied pressure.

Centrifugal casting

Slip casting in a spinning mould that uses centrifugal force to push slip into all features of the cavity. Used for shapes that gravity casting cannot fill cleanly.

Ceramic roller

Mullite-corundum-cordierite roller used in roller kilns above 1160 to 1170 °C to convey tiles directly through the kiln. Selected by phase composition (cordierite, mullite or technical type) to suit the firing temperature.

Chamfering / bevelling

Angled edge grinding that removes the sharp face corner of a tile, typically at 45° on porcelain. Done on squaring lines after firing.

Chicken-claw crack

Through-thickness crack in a fired tile that branches out like a claw, caused by an impact on the back face during pressing or by a de-airing fault.

Clayey materials

Plastic body class made of kaolinitic, illitic or smectitic clays. Provides shape-holding plasticity and contributes Al, Si and minor Ca, Fe, Ti to the fired body.

Closed porosity

Volume of isolated internal pores not connected to the outside, divided by the bulk volume. Closed pores reduce strength but do not absorb water.

Combustion-air preheat

Heat-exchanger arrangement where exhaust gases from the kiln cooling zone preheat the burner combustion air. Saves around 5 to 14% of the kiln fuel.

Compacted body flake

Thin tile-body fragment from the side of the die-box that gets trapped on the press and causes a small lifting or micro-peeling defect on the next pressed tile.

Compaction ratio

Ratio of the loose-fill thickness of powder to the pressed thickness of the tile, typically 2:1 to 3:1. Higher ratio means more powder is needed per tile.

Comminution

Mechanical reduction of solid raw materials from quarry lumps to fine powder, by crushing, grinding and milling.

Conchoid cracking

Smooth, shiny, sharp-edged fracture in a vitrified body caused by tensions during rapid cooling. The fracture surface looks like broken glass.

Conduction drying

Drying by direct contact heat transfer from a heated surface to the body. Less common than convection drying in tile manufacturing.

Continuous wet mill

Tubular mill that runs continuously with material fed at one end and slip leaving the other. The standard modern technology for grinding porcelain bodies.

Convection drying

Drying by hot-air heat transfer to the body. Dominant industrial drying mechanism for tiles and sanitaryware.

Convex / concave planarity

Tile bowed downward in the centre (convex face) from glaze-body expansion mismatch, or bowed upward (concave face), which is rare and usually caused by accidental kiln shutdown.

Cooling cracks

Curving cracks with glassy edges in fired ware, caused by tensions around the quartz inversion during cooling. Glass-edged appearance distinguishes them from impact cracks.

Coefficient of linear thermal expansion (α)

Fractional change in length per degree Celsius (ΔL ÷ L₀ ÷ ΔT). Body and glaze CTE must be matched within a narrow window to avoid crazing or peeling.

Crawling

Glaze pulls away during firing leaving rounded thick borders and exposed body underneath. Caused by glaze surface tension, dust on the body, or excess binder.

Crazing

Fine network of cracks in a fired glaze, formed when the glaze CTE is higher than the body CTE so the glaze is left under tension after cooling.

Critical moisture content

Moisture level at which all free water has been removed from the body. Below this point the body is rigid enough to enter the dryer without collapsing.

Critical speed (mill)

Peripheral speed of a ball mill at which centrifugal force on the media equals gravity, so they centrifuge instead of cascade. Operating speed is set at around 70 to 75% of critical speed.

Cyclone separator

Centrifugal dust separator that removes fine particles from the spray-dryer exhaust before the main fan. Recovered fines are returned to the slip.

D

De-airing

Vacuum step (or initial gentle press) that removes entrapped air from a plastic mix or pressing powder. Prevents lamination and trapped-air defects.

Decal

Transfer paper printed with ceramic colour, applied on the glaze surface and re-fired. Lets complex multi-colour artwork be reproduced quickly.

Decarbonation

Thermal decomposition of carbonates (calcite, dolomite, magnesite) during firing. Releases CO₂ between 800 and 950 °C and must finish before glaze melt to avoid pinholes.

Deep abrasion resistance

Wear test for unglazed tiles per ISO 10545.6 that measures the volume of body eroded by a controlled abrasive wheel. Lower volume means a more durable floor.

Deflocculant

Additive that lowers slip viscosity at a given water content by raising the surface charge of clay particles. Common types include sodium silicate, sodium tripolyphosphate, sodium polyacrylate and sodium humate, dosed at 0.2 to 0.5% of the dry body.

Deflocculation

State of a slip in which clay particles repel each other due to high zeta potential, so the slip stays low-viscosity and free of agglomerates.

Demould

Removal of the cast piece from its porous mould after the cast has hardened enough to hold its shape.

Densification

Conversion of a porous powder compact into a dense ceramic by sintering, with or without applied pressure.

Devitrification

Crystallisation inside a glass or glaze, either uncontrolled (a defect) or controlled (a desired matt or speckled finish). Caused by slow cooling or by the right oxide combination.

Disc application

Glaze applicator that uses one or more horizontal discs spinning at 3000 to 4000 rpm to throw glaze outward as a curtain of droplets.

Discontinuous (batch) ball mill

Cycle-loaded batch ball mill (also known as Alsing mill), filled with raw materials and media, run for a set time, then unloaded. Still used for body and glaze grinding in many plants.

Double firing

Firing route where the body is fired first as biscuit, then glazed and fired a second time. Allows recovery of body defects before glazing, but uses more energy than single firing.

Drain (hollow) casting

Slip casting variant where slip is drained out after the wall has built up to the target thickness, leaving a hollow piece. Standard for sanitaryware basins and many tableware shapes.

Dry finishing

Smoothing of dried greenware with a scouring pad and sandpaper before glazing.

Dry grain applicator

Hopper-and-roller machine that deposits granular glaze materials onto the tile, used for textured and rustic finishes.

Dry grinding

Grinding of body or raw materials without water, suitable for homogeneous clay mixes. Common for double-fire biscuit and majolica bodies.

Dry pressing

Uniaxial pressing of granulated powder at 0 to 4% moisture, the dominant forming method for ceramic floor and wall tiles.

Dry-bag isostatic pressing

Variant of isostatic pressing where the rubber bag is fixed inside the press body, faster and more automatable than wet-bag. Common for spark plugs and small high-precision parts.

Drying

Removal of water from the formed body by evaporation, before firing. Carried out in chamber, tunnel, vertical or horizontal continuous dryers.

Drying curve

Three-stage curve plotting drying rate against time or moisture content. Stages are constant rate (free water), falling rate (capillary water) and final low rate (pore water).

Drying shrinkage

Linear length reduction of a body between cast or wet state and dry state. Typically 5 to 8% for tile bodies and around 5 to 7% for sanitaryware.

Dunting

Hairline crack in a fired ceramic caused by thermal shock during cooling, especially around the 573 °C quartz inversion. Delayed dunting appears hours or days after firing from residual stress.

E

Edge flash

Pressing residue (thin compacted layer) trapped between the punch liner and the die wall, caused by die wear. Visible as a raised line along the tile edge.

Eggshell

Bubbled glaze edge defect with a wrinkled or pitted texture, caused by lack of oxidation in fume-passage zones of the kiln.

Efflorescence

Surface deposit of soluble salts (sulphates, chlorides) crystallising on the dried body during drying. Causes glaze defects after firing if not removed.

Ejection cracks

Cracks running from the side toward the centre of a pressed tile, caused by too-fast upper-punch ejection that lets the tile expand abruptly.

Engobe

Semi-glaze undercoat applied between body and glaze, or on the back face of a tile. Function is to limit body-glaze interaction, mask body colour, or protect kiln rollers.

Exaggerated grain growth

Sintering anomaly where a few grains grow much larger than their neighbours and trap pores inside themselves. Usually caused by impurities or local liquid pockets.

Extrusion

Forcing a plastic mix through a shaped die to form constant-cross-section parts such as wall tiles, klinker, refractory shapes and honeycomb supports.

F

Filler box

Powder feed device on the press that loads the die cavity. Modern filler boxes use floating gratings supported on anti-friction inserts for fast size changeover.

Filling defect

Density variation across the pressed tile caused by non-uniform regulation of the filler box. Appears as differences in compaction, dimension or shade after firing.

Final cooling

Last cooling zone of the kiln, which removes residual heat with cold-air blowers. Tile leaves the kiln at handling temperature.

Firing

Thermal treatment that creates the ceramic by vitrification, sintering and gas evolution. Peak temperature, soaking time and atmosphere together fix the final properties.

Firing curve

Programmed time-temperature profile of the kiln, divided into preheat, firing and cooling segments.

Firing zone

Maximum-temperature section of the kiln, above 1000 °C, where vitrification finishes and final dimensions are fixed.

Flat screen

Screen-printing decoration where the scraper moves over a flat stationary screen and the tile sits below. Used for short-run patterns and sample work.

Flocculation

Aggregation of suspended particles in a slip into denser, faster-settling clusters. Opposite of deflocculation, undesirable in a casting or grinding slip.

Fluidity

Initial flow reading of slip in a torsion viscometer or Ford cup. Higher reading means a more freely flowing slip.

Fluxing materials

Body class that forms the vitreous (glassy) phase during firing. Feldspars and feldspathoids supply the alkali oxides Na₂O and K₂O that drive vitrification.

Fracture toughness (K_IC)

Resistance of a brittle material to crack propagation, reported in MPa·m^½. Tougher ceramics like zirconia have higher K_IC.

Frit

Pre-melted and quenched glassy ingredient, ground into glaze recipes. Frits carry soluble or hazardous oxides (lead, boron, alkalies) in a chemically locked form.

Friction (slip) coefficient

Wet or dry slip-resistance of a glazed surface, measured per ISO 10545.17 or DIN 51130. Specified for floor tiles to prevent slipping accidents.

Frost resistance

Ability of a fired tile to survive freeze-thaw cycles per ISO 10545.12. Required for outdoor wall and floor tiles.

G

Glaze

Glassy coating fused on the fired ceramic surface. Provides colour, gloss, hardness and chemical resistance, and seals body porosity.

Glaze maturing time

Duration the glaze remains soft enough at peak temperature to flatten, seal and gather over the body. Must be long enough to remove pinholes and seal cracks.

Glaze shrinkage

Fine-line fractures in the glaze caused by body-glaze tensions that the viscous melt cannot seal. Different from crazing in pattern and origin.

Glaze thickness

Coating thickness of glaze on dried ware, typically 0.8 to 1.4 mm before firing. Measured non-destructively with a digital depth gauge or destructively with a scale loupe.

Glassy phase

Amorphous matrix formed by molten feldspar and other fluxes during firing, that bonds clay and quartz grains into a dense ceramic.

Glost firing

Firing of the glaze in the second pass of a double-firing route.

Grain growth

Coarsening of the ceramic grain structure during sintering, driven by reduction of grain-boundary energy. Excessive grain growth weakens the body and traps pores.

Granito

Porcelain tile pattern made by mixing two or more coloured spray-dried powders before pressing. Salt-and-pepper visual effect.

Green strength / green density

Mechanical strength and density of the unfired pressed compact. Both must be high enough to survive handling, transport and pre-firing trimming.

Grinding media

Pebbles or balls (silica, alumina, sintered alumina, porcelain) loaded into a ball mill to crush and grind the charge.

H

Hammer mill

Coarse impact-reduction mill with spinning hammer bars that shatter feed against a striking plate. Used for primary or secondary crushing.

Hard agglomerate

Strongly bonded cluster of primary particles that resists redistribution during pressing. Hard agglomerates create pore clusters and large pores in the fired body.

Hausner index

Ratio of tapped density to bulk density of a powder. Higher Hausner index means poorer flow.

HD (High-Definition) roller

Decoration roller that uses point-by-point image reconstruction for marble, stone and complex pattern effects on porcelain tiles.

Heat work

Combined effect of temperature and time on a fired body. Tracked with Bullers rings or pyrometric cones rather than peak temperature alone.

Horizontal / multi-layer drier

Roller-conveyor drier with one to five horizontal layers and zone-controlled atmosphere. Standard drier in modern tile plants.

Hot pressing

Densification with simultaneous uniaxial pressure (6.9 to 34.5 MPa) and heat in graphite dies. Used for high-strength technical ceramics that cannot be densified by sintering alone.

Hot isostatic pressing (HIP)

Densification under isostatic gas pressure (Ar or He) up to 200 MPa at 2000 °C. Closes residual porosity in pre-sintered ceramics.

Hydraulic press

High-tonnage oil-pressure press for tile dry-pressing, the dominant press technology in modern tile plants.

I

Impact crusher

Secondary crusher that reduces clays from around 6 cm down to 5 mm by impact against a rotor and breaker plates.

In-glaze decoration

Decoration where the colour penetrates into the glaze layer during firing, giving the look of being below the surface.

Inert materials

Non-plastic body ingredients used at small percentages to control firing behaviour, expansion or whiteness. Examples are talc, silica, pyrophyllite and calcium carbonate.

Infrared (IR) drying

Radiation drying at wavelengths 0.8 to 3 µm that penetrates only a few tenths of a millimetre, so the tile surface heats first.

Initial / intermediate / final-stage sintering

Three classical stages of sintering. Initial stage forms necks between particles, intermediate stage shrinks the pore channels into cylinders, final stage removes isolated closed pores.

Injection moulding (ceramic)

Forming method where a thermoplastic-bound ceramic mix is injected into a shaped die. Used for complex 3-D parts in technical ceramics.

Isostatic die / punch

Pressing tool fitted with a flexible polymer membrane and oil-filled compensation channels, so that filling pressure is equalised across the die cavity.

Isostatic pressing

Pressing where hydrostatic pressure is applied from all sides of the powder via a flexible bag. Gives uniform density in complex shapes.

J

Jaw crusher

Primary crusher that reduces hard quarry lumps from 15 to 20 cm down to 4 to 6 cm by compression between two opposed steel jaws.

Jet (fluid-energy) mill

Mill where particles are accelerated in high-velocity fluid jets and reduced by particle-particle impact. Produces submicron powders.

Jiggering

Profile-tool shaping of plastic clay on a rotating wheel, for bowls, plates and similar tableware shapes.

K

Kiln car / batt / sagger

Refractory-topped car that carries ware through a tunnel kiln, the supporting plate (batt) on the car deck, and the open refractory container (sagger) used in some firings.

Kiln contamination types

Five common contamination defects on fired ware: vitreous (Pb-B condensate drops), ceramic (kiln-dust dunes), ferrous (rust from doors and pipes), non-ferrous (blower deterioration) and carbon (poor combustion).

L

Lamination (pressing)

Internal foliated stratification in a pressed tile, caused by air trapped in the powder during compaction. Visible after firing as an internal gap or peeled flake.

Lappato (semi-polished)

Lapping process that gives a softer-than-mirror lustre to glazed or unglazed porcelain tiles.

Latent crazing

Hidden glaze tensions that show as crazing only later in life, after de-tensioning or after the glaze rehydrates from atmospheric humidity.

Leather hard

Greenware state where plastic water has been removed but the body still carries some moisture. Firm enough to trim, stamp or assemble without slumping.

Liquid-phase sintering (LPS)

Densification with a viscous liquid present at the grain boundaries that pulls particles together by capillary pressure. Dominates traditional ceramic firing.

M

Magnetic separator

Iron-removal device installed in slip and glaze pipelines. Bar, honeycomb or rotating-roller form, sometimes electromagnetic for higher pull.

Mesh / sieve number

Standard count of openings per inch (ASTM) or per cm² (Italian "maglie/cm²") of a sieve. Higher mesh number means a finer screen.

Microwave drying

Drying using 2.45 GHz electromagnetic energy that heats water dipoles uniformly through the whole piece.

Micronization

Fine grinding of a powder down to below 50 µm or to a few microns. Done in jet mills or attritor mills for special bodies and decoration.

Mill lining / lifters

Wear-resistant inner surface of a ball mill (rubber, silica, alumina, steel) and the protrusions that lift the media to the cascade angle.

Mirror polishing

Tangential-head polishing of porcelain tile to 1200 to 1500 mesh abrasive grade, producing a high-gloss reflective finish.

Modulus of Rupture (MOR)

Bending strength per unit cross-section, measured by 3-point or 4-point flexure. Reported in MPa or N/mm² and used as the primary tile strength specification.

Mohs hardness

1 to 10 relative scratch hardness scale. Quartz is 7 and zircon is 7.5; common tile glazes fall between Mohs 5 and 7.

Moisture expansion

Long-term swelling of a fired body caused by absorbed atmospheric water. Slow process that can cause tiles to bow or lift years after installation.

Mullite

Aluminium-silicate phase (3Al₂O₃·2SiO₂) that crystallises from feldspar-rich liquid during high-firing. Provides the mechanical strength of porcelain and porcelain stoneware.

N

Neck formation

First stage of sintering where a bridge of material forms between two contacting particles, with little overall shrinkage.

Newtonian fluid

Fluid whose viscosity is independent of shear rate. Pure water and dilute solutions are Newtonian; ceramic slips are not.

O

On-glaze decoration

Decoration applied over the fired glaze and re-fired between 700 and 950 °C. Colours sit on the surface and offer the brightest tones.

Opacifier

Glaze additive that scatters light and gives whiteness or opacity. Industry standard is zircon (zirconium silicate); SnO₂ and TiO₂ are also used.

Open porosity

Volume of pores connected to the outer surface, divided by the bulk volume. Open pores fill with water during the water-absorption test.

Orange peel

Rough, dimpled glaze surface that resembles orange skin. Caused by low-density glaze application or by under-firing.

Over-firing

Firing above the optimum peak temperature, leading to bloating, exaggerated grain growth, glaze running and dimensional drift.

P

Particle / agglomerate / granule

Three particle-population terms. A primary particle is a single grain; an agglomerate is a soft cluster held by surface forces; a granule is a deliberately formed cluster (usually water-bound) used for pressing.

Particle Size Distribution (PSD)

Statistical distribution of particle diameters in a powder. Controls packing, reactivity, sintering and final properties of the ceramic.

Peeling / shivering / flaking

Glaze release from the body, caused by the glaze CTE being lower than the body CTE so the glaze sits in compression. Different from crazing, which is the tension case.

Pendulum mill

Dry grinding mill that uses a swinging pendulum action with dynamic air-flow size classification. Used in dry body preparation lines.

Pinhole

Crater-like glaze hole formed when gas bubbles burst through the molten glaze and do not heal before cooling. Driven by trapped gas, organic matter or sulphate residues.

Pitting

Small surface depression in the fired glaze, caused by gas escape or local glaze loss. Smaller and shallower than a pinhole.

Plaster mould (model, case, working)

Three-stage mould hierarchy used in casting. The model is the master plaster shape, the case (or block) mould is taken from the model, and the working mould is taken from the case mould for production casting.

Plasticity

Property of a clay-water mix that lets it deform under force, hold the deformed shape, and fix that shape on firing. Set by clay mineralogy, particle size and water content.

Polishing

End-of-line surface treatment using diamond and abrasive tools. Includes rough flattening, coarse grinding, and fine polishing to mirror or semi-polished (lappato) finishes.

Polishing sludge

Wet residue produced during tile polishing, around 2 to 3 kg per square metre polished. Has to be filter-pressed and disposed or recycled.

Post-expansion

Dimensional growth of the fired body after firing, either from over-firing (immediate) or moisture re-expansion (long-term).

Pre-kiln zone

First kiln zone (200 to 500 °C) that drives off residual hygroscopic water before higher-temperature reactions begin.

Preheating zone

Kiln zone (500 to 1100 °C) where the body finishes degassing, organics burn out and carbonates decompose, before the glaze starts to melt.

Pressing

Compaction of a granulated or spray-dried powder under pressure into a die. Most common forming method for floor and wall tiles.

Pressure casting

Slip casting where the slip is forced into a porous-plastic mould at up to 4 MPa, much faster than gravity casting. Used for high-volume sanitaryware lines.

Priest hat

Tile-shape defect where the centre is convex and the two opposite ends turn upward by 3 to 4 cm. Caused by over-correction of convex planarity.

Pseudoplastic / shear-thinning

Fluid whose viscosity drops as shear rate rises. Most ceramic glazes and slips behave this way under spraying or pumping.

Pyroplastic deformation

Softening and sagging of a body or a kiln roller under its own weight at peak firing temperature. Limits how thin and large a piece can be fired without distortion.

Q

Quartz inversion

Reversible transformation between alpha and beta quartz at 573 °C, with a sharp volume jump. The kiln cooling curve has a controlled slow zone here to prevent dunting.

R

Rapid cooling

Cooling zone of the kiln from peak temperature down to around 600 °C using blowers above and below the rollers. Locks the glassy phase before crystallisation.

Rapid double firing

Modern variant of double firing using fast roller-kiln cycles. Replaces the long-cycle traditional double-firing route in many wall-tile plants.

Reactivity

Tendency of a powder to densify during sintering. Controlled by surface area, particle size and lattice defects.

Real (true) density

Mass per actual occupied solid volume of a powder, excluding all pore space. Reported in g/cc and used as the baseline for porosity calculations.

Reclaim (overspray)

Recovery system in a glazing booth that captures overspray and returns it to the glaze tank. Saves glaze and reduces waste.

Reducing / oxidising atmosphere

Kiln atmosphere with low or excess oxygen. Reducing atmosphere can cause black core, while oxidising atmosphere ensures complete burn-out.

Refractoriness (PCE)

Pyrometric cone equivalent — softening behaviour of a refractory raw material, expressed as the cone number that bends to the same shape as the test cone.

Refractoriness under load (RUL)

Resistance of a refractory to deformation under a fixed compressive load while heated at a controlled rate. Defines the maximum service temperature in a furnace lining.

Regranulation

Re-aggregating of dry-ground or spray-dried powder into 2 to 8 mm grains using a small amount of water. Used to make textured granito-effect porcelain tiles.

Rheology

Study of flow and deformation of fluids and suspensions. Slip rheology is the practical study of how a ceramic slip flows under shear.

Roller (rotocolor) decoration

Decoration system using a laser-engraved silicon-polymer cylinder that transfers ink to the moving tile. Industry workhorse for series production.

Roller effect

Tile deformation that looks like a porpoise — central concavity with bent-up leading and trailing strips. Caused by uneven heat transfer between rollers and tile.

Roller engobe

Anhydrous-alumina and kaolin coating applied to kiln rollers and to tile backs to limit cleaning frequency and avoid sticking and crust formation.

Roller kiln

Continuous flat-bed kiln that conveys tiles directly on rollers through preheating, firing and cooling zones. Single-layer and double-layer designs are available.

Rotary screen

Screen-printing decoration where a rotating cylinder screen prints onto a continuously moving tile. Gives constant print speed and high throughput.

S

Screen printing

Decoration method where ink is pushed through a fabric mesh by a squeegee onto the tile. Main ceramic decoration system before roller and digital printing.

Semi-dry / dust pressing

Uniaxial pressing at the lower end of the moisture range (around 4 to 6%). Used for traditional dust-pressed tiles.

Shuttle kiln

Batch kiln with one chamber and a dedicated firing cycle. Good for refire, special products and small lots.

Sieve residue

Mass of material retained on a defined sieve, expressed as a percentage. Used to characterise grinding fineness; for example "residue at 63 µm".

Single firing

Firing route where body and glaze are fired together in one cycle. Standard for most modern porcelain and floor tiles.

Sintering

Thermally activated atomic transport that joins particles and reduces interparticle pore space. Driving force is the reduction of total surface energy.

Sintering atmosphere

Gas (air, vacuum, N₂, H₂, CO/CO₂) present in the firing chamber. Controls oxidation, valence states and burn-out of organics.

Slip

Aqueous suspension of clay and non-plastic raw materials, used for grinding, casting and spray drying.

Slip ageing

24 to 48 hour storage of fresh slip with slow agitation, to develop final rheology and casting rate before use.

Slip pump types

Positive-displacement pumps (mono, peristaltic) are preferred for pumping deflocculated slip, because diaphragm and centrifugal pumps can introduce air or shear the slip.

Slow cooling

Kiln cooling zone between 600 and 450 °C, set to a low rate so that the body passes through the quartz inversion at 573 °C without dunting.

Soaking

Hold at peak firing temperature, typically 30 to 45 minutes, that lets gases escape and temperature equalise across the kiln cross-section.

Softening point

Temperature at which a frit or glaze becomes plastic and starts to flow under its own weight. Measured on a heating microscope.

Solid casting

Slip-casting variant where slip is continuously added until the part is fully solid. Wall thickness is fixed by mould geometry, not by casting rate.

Solid-state sintering (SSS)

Densification by atomic diffusion alone, without any liquid phase. Used for high-purity technical ceramics.

Soluble salt decoration

Decoration where a pigmented vehicle is absorbed into the pressed body so that colour develops within the body during firing. Resists wear because the colour is not just on the surface.

Solution-precipitation

Mass-transport mechanism in liquid-phase sintering where the solid dissolves in the liquid at high-energy contact points and precipitates back at the necks.

Specific gravity / litre weight

Slip density. Reported as g/cc (specific gravity, around 1.78 to 1.82 for body slip) or as g/L (litre weight, around 1900 to 1920 for high-pressure casting slip).

Specific pressure

Force applied per unit punch surface during pressing, in kg/cm² or MPa. Typical tile pressing values are 250 to 450 kg/cm².

Sphere / semi-sphere temperature

Reference points on a heating microscope where the test piece becomes spherical or hemispherical. Used to characterise glaze melting behaviour.

Spray drying

Continuous slip drying in a hot-air tower that produces free-flowing spheroidal pressing granules. Standard powder-preparation route in modern tile plants.

Spray gun (glazing)

Hand-held or robotic atomiser that sprays glaze through a 1.2 to 2.6 mm nozzle at 2 to 6 bar.

Spray-dried granule

Roughly spherical hollow-dimpled grain produced by spray drying, with PSD around 100 to 600 µm and residual moisture 4 to 7%.

Springback

Post-pressing dimensional expansion of the pressed tile after the punch is released. Reported as a percentage and used to size the die.

Squaring (rectifying)

Wet diamond milling that trims tile edges to uniform size and a tight squareness tolerance. Lets tiles be laid with very thin grout joints.

Stain resistance

Glazed-tile resistance to surface marking by household substances. Tested per ISO 10545.14 with reagents like olive oil, iodine and chrome green.

Sulphur attack

Glaze defect family caused by SO₃ released by sulphate impurities. Includes opalescence, pinholes, devitrification and edge wrinkling.

Surface tension (glaze)

Cohesion of the molten glaze that controls how it gathers around drops, how gas escapes and how it wets the body.

Suspension agent

Additive (NaCl, CMC, bentonite) that slows particle sedimentation in a slip or glaze.

T

Tape casting

Slurry cast through a doctor blade onto a moving carrier film, drying to a thin flexible green sheet. Used for capacitor and substrate ceramics.

Tapped density

Bulk density of a powder measured after a controlled vibration cycle. Always higher than the loose bulk density.

Third / fourth firing

Additional firing of decorated trim or accessory pieces after the main glaze firing. Used to apply on-glaze colours, decals and lustres.

Thermal conductivity

Rate at which heat is conducted through a ceramic, in W/m·K. Set by phonon transport in non-metallic crystalline ceramics.

Thermal shock

Failure caused by transient thermal stresses from rapid heating or cooling. Resistance is set by CTE, modulus, strength and thermal conductivity together.

Thixotropy

Time-dependent thickening of a slip when it stands at rest, and thinning when it is sheared. Measured as the viscosity rise after 1 minute or 6 minutes.

Toggle / friction press

Older mechanical presses driven by a knuckle joint or by rotating discs and a screw-coupled flywheel. Mostly replaced by hydraulic presses in tile plants.

Total porosity

Sum of open and closed porosity, expressed as 1 minus (bulk density divided by real density).

Tunnel kiln

Long continuous refractory-lined kiln through which kiln cars carrying ware are pushed through preheat, firing and cooling zones. Still used for sanitaryware and large-format tiles.

U

Under-glaze decoration

Decoration applied to the body before a transparent glaze is fired over it. The fired glaze seals the colour under the surface and improves wear protection.

Underfired ware

Ware that did not reach full vitrification, with low strength, high water absorption and dull appearance.

Uniaxial pressing

Compaction with force applied along one axis between two punches in a die. The dominant pressing geometry for floor and wall tiles.

V

Vacuum casting

Slip casting assisted by vacuum applied through a permeable mould. Speeds up casting and improves coverage in deep cavities.

Vertical drier

Rapid drier with chain-driven baskets that carry tiles up and down through hot-air zones. Compact footprint but high mechanical complexity.

Vibratory sieve (vibrosieve)

High-efficiency circular vibrating sieve used to remove oversize residue from slip and glaze, with apertures down to around 110 µm.

Vickers hardness (Hv)

Indentation hardness measured by pressing a diamond pyramid into the surface and measuring the impression diagonal. Use it to compare surface hardness and wear resistance; do not treat it as a direct substitute for strength testing.

Viscosity (η)

Ratio of shear stress to velocity gradient (shear rate) of a fluid. Reported in Pa·s or in centipoise (1 cP = 1 mPa·s).

Viscous composite sintering (VCS)

Densification mode where more than 20% liquid is present along with solid particles. Characteristic of porcelain, vitreous china and other whitewares.

Vitrification

Densification of the ceramic body during firing as the glassy phase fills pore space and bonds the grains. End-point is water absorption below 0.5%.

Vitrification curve

Plot of fired-body shrinkage and water absorption against firing temperature. Used to find the optimum peak temperature for a body.

W

Water absorption (WA)

Percentage weight gain of a fired piece after standard water boiling, divided by dry weight. Primary tile classification parameter (porcelain BIa is below 0.5%).

Waterfall unit

Triangular hopper glaze applicator with an adjustable bottom slot that releases a sheet of glaze onto the moving tile.

Weibull modulus (m)

Statistical shape parameter of the brittle-strength distribution. Higher m means narrower scatter; ceramic engineers aim for m above 15.

Wet finishing

Sponging, punching of vent holes, trimming and edge smoothing of freshly demoulded greenware before drying.

Wet grinding

Grinding of body or glaze with water, the standard technology for vitrified tile bodies and most porcelain.

Wet pressing

Uniaxial pressing of a clay-bearing body at 10 to 15% moisture. Used for traditional bricks and some clay-pipe processes.

Wet-bag isostatic pressing

Variant of isostatic pressing where the rubber bag is immersed in the pressurising fluid. Slow cycle, suits large or complex parts.

Y

Yield point

Minimum shear stress needed before a material starts to flow. Bingham-plastic slips and pastes show a yield point; Newtonian fluids flow as soon as shear is applied.

Young's modulus (E)

Stiffness of a solid, expressed as the ratio of stress to strain in the elastic regime. Reported in GPa.

Z

Zeta potential (ζ)

Electrical potential at the slipping plane around a charged particle in suspension. Magnitude indicates how well the particles repel each other and stay dispersed.

How to use this glossary

Use this page as a shop-floor reference when a drawing, test report, supplier quote or production note uses an unfamiliar ceramic term. For deeper troubleshooting, pair the definition with the relevant process data: body recipe, moisture, particle size, firing curve, glaze application weight or test method.

If a term affects a specification or customer claim, verify the value against the applicable ISO, ASTM, EN or plant standard before releasing the batch.