Glossary of pottery and ceramic terms
Abrasives
Hard materials used for grinding, cutting or polishing e.g. alumina
Absorption
The amount of water that a body can draw into its surface at any stage of
processing
Agate ware
Pottery made by incomplete mixing of 2 different coloured bodies. The fired
result is a streaked effect resembling agate.
Alumina
Aluminium oxide. A hard non fusing powder used to stop ware sticking to the
supports in kilns.
Amorphous
Non crystalline structure e.g. glass
Antimony oxide
Used to produce yellow colours in glaze or pigments. Now considered toxic.
Apparent porosity
Relation between the volume of item and the volume of water absorbed on
immersion.
Ark
Large storage vessel or container e.g. slip ark or glaze ark
Ash glaze
A glaze using wood ash as the flux . Sometimes mixed with other materials.
Autoclave
An air tight container used to heat articles under water pressure . e.g to test
the craze resistance of a piece
Bagwall
The inner wall of kiln . Sometimes referred to as muffle it protects ware from
direct flames on gas firing.
Ball clay
Sedimentary clay usually of fine particle size. This clay is usually highly plastic
Ball mill
A ceramic lined steel cylinder filled with ceramic balls used to grind glaze or
clay body.
Baluster
The name given to a vessel that is slender at the top and bulbous at the
bottom.
Bamboo knife
A piece of bamboo that is sharpened to a chisel edge. Used in throwing.
Banding wheel
A turntable, usually made of metal, for applying decorative colour bands to
vesssels or plates
Batt
A flat disc which attaches to the potters wheel for throwing large articles. Also
a refractory shelf.
Batt wash
A coating of refractory powder applied as a slurry to kiln furniture or saggars to
stop sticking of ware.
Bentonite
A general term for montmorrllinite clay. Often added to glaze to stop
sedimentation or to clay body to add plasticity
Biscuit
Pottery that has undergone its first firing. This gives the pottery sufficient
strength to allow decoration and glazing.
Bloating
Deformation of the body on firing . Produced by gas trapped in the partially
fused mass.
Blunging
Mechanical mixing of clay body with water to produce a slip.
Body stain
Ceramic pigment used to colour the body on firing
Bone ash
Animal bones that have been processed and calcined to produce a ceramic
powder,
Bone china
A thin, white translucent pottery made using bone ash as a body component.
Borax
Sodium borate-usually as the dehydrate form. Used in the manufacture of frits.
Bottle kiln
A huge bottle shaped brick kiln. Originally used in Stoke onTrent for firing of
pottery-now museum exhibits exist only.
Bullers ring
Special ceramic discs used to assess the firing in kilns. The measured
contraction of the disc indicates the heat applied.
Burnish
Achieving a high sheen by polishing surface of piece with smooth tool marble or
spoon when semi dry.
Calcium carbonate
Used a a glaze or frit component-also called limestone
Calcination
A high temperature heating process usually employed to change the crystalline
form, e.g. flint
Caliper
A device for measuring the internal or external diameter of an object.
Casting
Forming pottery by pouring a slip into a porous mould
Casting slip
A very fluid slip of high solids content made of clay body, deflocculant and
water. It is used in the casting process to form pottery shapes.
Celadon
A transparent green glaze pale in colour of Chinese origin
Ceramic
Derived from the Greek word 'Keramos' meaning earthen vessel. Used
nowadays to describe many high temperature formed produced using ceramic
materials.
Cheesehard
Also called leatherhard-a clay body that is dry enough to retain its shape.
Chemically
combined water
Water that is chemically attached to materials e.g. clay and is only removed by
high temperature firing
China clay
Also known as kaolin. Made from pegmatite and mined in Cornwall and other
parts of the world.
Chinastone
Pegmatite. A feldspar and in its impure form the source china clay in the UK.
Chrome oxide
Used to produce yellow or green pigments or glazes
Chuck
Tube like former to hold a pot in position on the wheel while the foot is being
trimmed.
Chun
A pale blue glaze used on stoneware
Clay body
The main pottery body composed of intimately mixed clays,feldspar,silica and
other materials
Cobalt oxide
Used as a blue colorant in body glaze or decoration.
Coefficient of
expansion
A measure of the reversible change of volume with temperature. can be
quoted as linear or cubic expansion
Coiling
Rolled cylinders of clay build up to make a solid pottery form.
Colloid
A stable suspension of ultra fine particles.
Cutting parallel lines in the clay or through slip.
Combing
Comminution
Size reduction by breaking, crushing or grinding
Compressive
strength
The ability of an article to withstand crushing loads
Cones
Sometimes called pyrometric cones, these ceramic pieces change shape and
slump on exposure to heat. Used by potters as an indicator of a repeatable
firing cycle.
Copper oxide
Used to produce green transparent colours in glaze. Under reducing fire can
give bright red colours.
Crackle
Intentionally produced decorative crazing of the glaze.
Crawling
Shrinkage of the glaze leaving exposed body or pinholes after glaze firing-has
many causes.
Crazing
Cracking of the glaze after firing. Caused by mismatch of body and glaze
expansion.
Cryptocrystalline
Possessing crystals so small that they cannot be distinguished by optical
microscope
Cut glaze
Bare areas on glazed ware caused by mechanical damage of the glaze prior to
firing.
Damper
A crude device or small hinged refractory door to control the flow of gases
from the kiln.
De-airing
Removal of air from clay either by wedging by hand or using a special chamber
in the pug mill.
Deflocculation
The dispersion of clay slip or glaze by the addition of an electrolyte e.g. sodium
silicate or soda ash
Delft
Opaque lead glazed earthenware often decorated by metal oxides prior to
firing
Devitrification
Crystallisation of a vitreous glaze after firing. Often at the surface of the glaze.
Dilatancy
The property of a suspension whereby it gets thicker as it is stirred and thinner
as it stands.
Dipping
The application of glaze or decoration slip by immersion and ten allowing
excess to drain off.
Dispersion
The separation of the clusters of fine particles in water into individual particles.
Draw
Absorption of glaze by an unglazed surface during firing.
Dropper
Globules of glaze that are found on glazed pieces after firing. These are caused
by drops from the kiln roof.
Dunting
Cracking pottery on cooling. Often caused by silica inversions in the body or
too fast firing.
Earthenware
Originally a moderately porous pottery formed by firing a mixture of ball clay,
flint , china clay and feldspar.
Edge runner mill
At ype of mill for mixing or grinding materials using 2 large stone rollers and a
stone pan.
Efforescence
A growth on the surface of bricks or other articles due to the the presence of
soluble salts.
Electrical porcelain
Type of porcelain used for producing electrical insulators. Typical formula 28
ball clay,22 china clay.quartz25,feldspar 25
Elutriation
The separation of particles according to their size or density by a controlled
velocity water stream.
Enamel
A fusible low temperature coating for metal or decoration for pottery.
Engobe
A white or coloured coating of clay slip applied to the body prior to glazing.
Often used for technical or decorative reasons.
Eutectic
A set mixture of 2 components having a lowest melting point of any ratio
of the individual components.
Extrusion
The process of forcing a clay body through an aperture or die to achieve a
specific cross section or size.
Faceting
Cutting away the clay surface.
Faience
Style of tin glazed earthenware produced in Europe.
Feathering
Effect obtained by trailing a feather thro still wet slip decoration
Feldspar
A crystalline mineral caused by weathering of granite. Used as a high
temperature flux in many bodies.
Fettling
The removal of the seams edges and blemishes from dried pots prior to glazing
Fillers
Minerals added to a body to give it rigidity or strength. Quartz and flint are
termed as fillers.
Filter cloth
Used to filter press clay slip to produce solid filter cake
Filter pressing
Process used to change slip into solid clay. Clay slip is forced thro a special
cloth to remove water.
Fireclay
Clay that is usedd for refractory items. some fireclays contain natural grogs to
improve shock resistance.
Firing
The increase in temperature used to produce a ceramic item.
Flocculation
The aggregation of ceramic particles in a suspension. A divalent electrolyte
such as CaCl2 is commonly used to create a thickening effect.
Flambe
A strong red glaze produced by a reduced copper glaze.
Flatware
Plates, saucers, trays etc
Flint
Calcined powdered silica often used in earthenware bodies.
Fluting
Cutting decorative grooves into clay in parallel lines, often on thrown ware.
Flux
A material added to a glaze body or colour to improve its melting properties
Foot
The base of the pot on which the piece stands.
Frit
A special glass ground to a powder and used to reduce the melting point of a
glaze.
Frizzling
A decorating fault where the decoration curls or moves on firing. There are
many causes.
Fusibility
The melting behaviour of a ceramic glaze, frit, flux or decoration with increase
in temperature.
Glaze
a thin glaze coating formed on the surface of pottery to render it impermeable
and provide decoration.
Glaze-body fit
The relationship between the thermal expansion of rhe body and glaze. Ideally
the glaze should have a lower expansion than the body.
Glaze stain
A colouring pigment added to a glaze to produce a coloured glaze.
Glost
A glazed and fired piece is referred to as 'glost'.
Greenware
Clay pieces that have been formed but not fired.
Grog
Ceramic material that is relatively coarse compared to the rest of body
components. Generally used to give texture or improved thermal shock
properties.
Gypsum
Plaster of paris used to make moulds
Hakeme
Oriental technique of applying white slip with a brush made of straw-allows
slip to be applied thickly.
Handbuilding
Constructing pottery from premade components. These components might be
made by moulding, coiling or by hand.
Hardening on
A firing process normally 650-700C to remove organic components of
underglaze decoration.
Hot pressing
Densification of a ceramic by simultaneous application of heat and pressure.
Holloware
cups, jugs, bowls etc
Impressed
A design stamped into the leatherhard clay using a die or tool
Incised
Pattern cut into the clay with a sharp tool
Inlay
Coloured clay rolled into the main body clay
Jiggering
Machine process using a mould to form the inside of the plate and the tool the
outside
Jolleying
Machine process using a mould to form the outside of the pot and the tool the
outside
Kaolin
Also called china clay. White firing clay used in many bodies.
Kiln
High temperature oven or furnace used for firing of pottery.
Kiln furniture
General term used to describe refractory pieces used to separate or support
items during firing
Kiln wash
Refractory slurry applied to kiln furniture to stop sticking during firing
Lamination
A fault structure in clay before or after firing where the clay is aligned in
layered clusters.
Lawn
A sieve
Lead solubility
The solubility of lead containing glaze or colour powders when exposed to
aqueous hydrochloric acid
Leatherhard
Or cheesehard. Clay that has dried to a point where it retains its shape.
Limestone
Calcium carbonate-often used as a glaze component
Lithograph
A method of decoration involving transfers. Allows fine detailed printing.
Loss on ignition
The loss in weight of a clay body when fired to a high temperature (usually
1000C) expressed as a %
Lustres
A metallic thin film applied to pottery either as a glaze or decorative effect
Lute
To join 2 pottery surfaces together with slip
Majolica
The technique of applying low fired tin glazes with colours to produce a multi
coloured effect.
Manganese oxide
Colouring oxide used in glaze or decoration to give brown to purple colours.
Model
The original or prototype article
Modulus of
elasticity
The term used to define the extent a material can be distorted under stress
without fracture.
Modulus of rupture
The term is used to define the strength of unfired clay body or the fired
ceramic.
Moisture expansion
The extent to which a porous ceramic expands when it absorbs moisture or
water vapour.
Mould
The shape usually made of plaster used to form the article either by casting or
pressing.
Muffle kiln
A type of kiln where the ware is protected from the flame by an inner
refractory box.
Nickel oxide
Used to produce brown, green or violet colours in glaze. Now considered toxic.
Onglaze
Decoration applied on top of the fired glaze. Overglaze or enamel terms are
sometimes used.
Opacifier
An additive to the glaze to give opacity e.g. zircon or tin oxide
Particle size
The distribution of particle sizes either as a powder or a slurry. Important
measure in manufacture of glaze or colour.
Peeling
A defect in glazed and fired ware. The glaze flakes off in pieces particularly on
edges.
Piercing
Cutting through walls of pot
Peephole
Small hole in a kiln door for the potter to observe the progress of firing.
Pinching
Indenting a pot with fingers or thumb before it is hard.
Pinholes
A common fault in the body or glaze. Small holes appear in the glaze or body as
gases erupt thro the surface.
Pitchers
Fired or broken scrap pottery (of course also a large jug)
Plaster of paris
Gypsum. Used to make moulds.
Plasticity
The property of clay which allows it to be moulded and retain its shape after
the force is removed.
Plucked ware
A fault caused by the ware sticking to the refractory support during firing
Porcelain
A vitrified and white translucent ware made predominantly in Europe and Asia.
Usually strong in use.
Porosity
The amount of pore space in a ceramic body-consists of closed and open
pores.
Pouncing
The ancient technique known as pouncing is when 'pounce', powder graphite
or charcoal, is rubbed through a series of small holes punched in a paper
pattern to transfer a design to an item to be decorated.
Press Mould/Hump
Dropping sheet of clay over or into a plaster or bisque shape.
Mould
Pugging
The intimate mixing and extrusion of plastic clay body. Also the machine used
to carry out this process.
Pulling
Shaping a handle by hand to the correct size and shape.
Pyrometer
High temperature measuring device
Pyrometric cones
Special refractory cones placed next to the ware to confirm repeatable firing.
Quenching
Method of making a frit where the molten glass is poured into cold water to
produce small granules of frit
Raku
Low fired glazed firing (1000oC). Produced normally by rapid firing. Raku is
Japanese for 'enjoyment' and this type of pottery is used in the Japanese tea
ceremony.
Raw glaze
A glaze in which no frit is present-normally a high fire glaze over 1100C
Reducing
atmosphere
A kiln atmosphere which is deficient in free oxygen(more gas less air)-used to
produce specific glaze colours or effects
Refractory
Ceramics that are able to withstand high temperatures without distortion.
Rib
A shaped tool to facilitate forming of a pot on a wheel.
Saggars
A fireclay or other refractory box used originally to protect the ware during
firing
Salt glaze
Thin orange peel glaze produced on the surface of pottery-originally produced
by throwing salt into the kiln and allowing it to vaporise.
Sang-de- Boeuf
Ox-blood colour used to describe deep red colour of some glazes
Sedimentation
The settling out over time of any clay or glaze slip. More noticeable in glazes
low in clay content.
Sgraffito
The technique of scratching through an applied slip layer to reveal the colour
of the body beneath.
Shelling
Same as peeling: A defect in glazed and fired ware. The glaze flakes off in pieces
particularly on edges.
Shivering
Same as peeling: A defect in glazed and fired ware. The glaze flakes off in pieces
particularly on edges.
Sintering
The adhesion and densification of particles on heating
Slip
A Liquid clay to which colour can be added in the form of oxides or body stains,
slip can be marbled – feathering- trailer application- paper/wax/ latex- resists
Slip decoration
Application of contrasting coloured slip to a body as a form of decoration.
Slabbing
A building technique where slabs of clay are joined at edges to form an article.
Slumping
Soft clay shaped and altered by using gravity, dropping, tapping and folding
Slurry/Slop
Soft clay used to join leather hard, cross hatched clay together
Silk screen
A form of decoration where the colour paste is forced thro a decorative
patterned screen either directly onto the ware or a transfer paper.
Smoke Firing/Pit
Firing
Firing in newspaper
Soak
Allowing the pottery to remain at the peak temperature for a time period. This
allows all positions in the kiln to reach approx the same temperature
Spalling
Flaking, cracking or disintegration of ceramics when subjected to rapid
temperature changes.
Specific surface
area
The total surface area of a all particles per unit weight of material
Spitout
Rapid desorption of moisture during the on glaze firing process causing a
severe fault of small craters or pinholes.
Spray drying
A means of drying clay slips or other ceramic slurries by spraying the slurry into
a large chamber of hot air
Sprig
Decorative pieces applied to leather hard pots
Spur marks
The marks left on pottery from refractory supports used to fire the piece.
Stoneware
A vitreous opaque pottery containing naturally vitrifying clay. Favoured by
many studio potters
Surface tension
The tendency of a fluid to wet a surface. E.g. low surface tension= high wetting
Tailings
The portion of a material which does not pass thro a sieve or is returned for
further processing
Tenmoku
A stoneware glaze deeply coloured by iron oxide. Colours can vary from yellow,
green. red. blue or black
Tensile strength
The resistance of a material to being stretched by tension or pulling
Terracotta
Red eathenware body usually made with naturally red clays
Thermal
conductivity
The rate at which heat passes through a material as measured by its rise in
temperature.
Thermal shock
The failure of a ceramic article due to stresses created by rapid temperature
change. -either hot to cold or cold to hot
Thermocouple
A device for measurement of temperature in a kiln. Usually in the form of a
special metallic wires encased in a ceramic sheath.
Thread
Twisted wire sometimes used by potters for cutting pots of a wheel
Throwing
Making a pot on a potters wheel
Tin glaze
White opaque glaze produced with tin oxide in the glaze recipe.
Titanium oxide
Used in glazes to produce decorative crystalline effects
Towing
The smoothing of the outer edge of flatware in its green state.
Transfer printing
The method of applying a decoration to a pot. Slide off transfers using ceramic
colours are applied to the glost ware and the refired to bond the decoration to
the glaze.
True porosity
The sum of open and closed pores in a ceramic body.
Tube lining
A decorating technique where coloured slips are piped thro a small nozzle
onto clay ware. This is often used as a border for further decoration.
Tunnel kiln
A kiln where the pottery moves progressively thro a heated zone to fire the
pottery.
Underglaze
A decoration applied either directly to the clay or biscuit prior to glaze
application and firing
Viscosity
The resistance of a fluid to flow. the reciprocal of fluidity
Vitreous
Glassy-having very low or zero porosity
Vitrification
The change from a porous to vitreous state of a ceramic
Volatility
The process of evaporation of materials on firing
Wall pocket
A vase usually with a flat back for attachment to a wall
Water absorption
A measure of the open porosity of a ceramic-determined by soaking the item
in water for a set time
Wax resist
Wax applied to a pot to stop glaze or decoration sticking to these areas during
application.
Wedging
A method of de-airing and dispersing moisture evenly in clay. The clay is
worked by hand by repeated throwing of the clay onto the bench and folding
over.
Wetting agent
A chemical used to help the application of glaze or colour to allow it to better
cover uneven areas.
Wheel
Also potters wheel- a horizontal disc on a foot operated or motor driven shaft.
Used for throwing of pots.
Wreathing
Ripples or waves on the inside of a cast pot after draining. Caused by variation
in casting rate at different parts of the pot