Glossary of pottery and ceramic terms

pointdepressedMechanics

Feb 22, 2014 (3 years and 8 months ago)

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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


S
edimentary clay usua
lly 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 sag
gars 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
streng
th 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 con
tent
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' mea
ning 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 c
hina 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 linea
r or cubic expansion

Coiling


Rolled cylinders of clay build up to make a solid pottery form.

Colloid


A stable suspension of ultra fine particles.

Combing

Cutting parallel lines in the clay or through slip.


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 pi
nholes 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 ea
r
thenware 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 decorati
on 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 f
iring 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 e
lectrical 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.

Eute
ctic


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 thro
ugh

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 used
d 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 electrol
yt
e
such as CaCl2 is commonly used to create a thickening effect.

Flambe


A stro
ng

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 imper
meable
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 imp
roved 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 press
ure.

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 contain
i
ng 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
witho
u
t 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 i
t 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 f
orce 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
Mould

Dropping sheet of clay ov
er or into a plaster or bisque shape.

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

glaze
d
firing (1
000
o
C). P
roduced 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 allow
ing 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 co
lo
ur
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 we
i
ght of material

Spitout


Rapid desorption of moisture during the on

glaze firing process caus
ing 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 mater
i
al 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.

T
hermal 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 encase
d in a ceramic sheath.

Thread


Twisted wire s
ometimes used by potters for cut
ting 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 th
e 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 col
our 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