Inert Material

Inert material: Material which is
passively resistant to any change. In
ceramics, we particularly refer to
materials which are relatively unaffected
by the action of heat or water.

Refractory

Refractory: Resistant to high temperatures.

Bentonite

Bentonite: A highly plastic collodial
clay-like material which is a useful
plasticiser and suspender

Ball clay

Glossery of Pottery Terms

Ball clay: A plastic secondary clay,
usually quite refractory, and firing to a
fairly pale, off-white colour.

Ceramic Change

Ceramic change: The point at which
chemically combined water is driven off
from clay molecules, so that the clay
becomes pot, and can no longer be
slaked down. This change is permanent
and irreversible, and is widely said to
take place at 573degrees C..although it
is, in fact, an ongoing process which may
begin as low as 350C., and may still be
taking place a temperatures as high as
700C

Scumming

Scumming: Unwanted deposits of soluble salts which sometimes appear on the surface of a raw dry pot, or more significantly, on bisc-fired ware. After bisc firing, areas where scum is apparent may be slightly fused and therefore resistant to glaze take-up. Scum is unlikely to resist glaze take-up on a raw pot.

Slipcasting

Slipcasting: A method of making objects by pouring a dense, deflocculated slip into a mould.

Feldspar

feldspar

One of a group of rock-forming minerals, the most abundant group in the Earth´s crust. They are the chief constituents of igneous rock and are present in most metamorphic and sedimentary rocks. All feldspars contain silicon, aluminum, oxygen, linked together to form a framework; spaces within this structure are occupied by sodium, potassium, calcium, or occasionally barium, in various proportions.
Feldspars form white, gray, or pink crystals and rank 6 on the Mohs´ scale of hardness.
The four extreme compositions of are orthoclase, KAlSi3O8; albite, NaAlSi3O8; anorthite, CaAl2Si2O8; and celsian, BaAl2Si2O8. These are grouped into plagioclase feldspars, which range from pure sodium feldspar (albite) through pure calcium feldspar (anorthite) with a negligible potassium content; and alkali feldspars (includingorthoclase and microcline), which have a high potassium content, less sodium, and little calcium.
The type known as moonstone has a pearl-like effect and is used in jewelry. Approximately 4,000 metric tons of feldspar are used in the ceramics industry annually.

Pyrometric Cones

Pyrometric cones: These are used to
accurately measure the amount of heat
work that has taken place in the kiln.
They are made from precisely measured
combinations of clay and flux materials
which reliably begin to ruse and bend at
a predetermined heat.

Dunting

Dunt: A crack caused by firing and /or
cooling

stress in a pot.

What is a Binder?

Binder: A polymetic substance(e.g.
starch-based adhesives) used to
strengthen unfired glaze surfaces

Raku

Raku: A Japanese word which loosely translates as ´enjoyment´, and which strictly only refers to pottery by the potter who holds the Raku title. In the
West, however, the work has become associated with a particular technique which generally involves placing pots into the already hot kiln, and often then carbonising the pots by removing them directly from the hot kiln to a bin of combustible material such as leaves or sawdust.

Suspender

Suspender: A material added to a glaze slop in order to facilitate the suspension of heavy particles in water. Suspenders usually act by the creation of a
macrostructure which hold particles in dispersal.

Pyroplastic

Pyroplastic: The condition of a clay
body in the kiln when heated to
vitrification. At this point, any impact
upon the pot may alter its shape, and,
ultimately, it may begin to sag under its
own weight.

Engobe

Engobe: The word engobe is often used
to refer to decorative (as oppossed to
casting) slips, but may also refer to any
material which is neither a pure clay slip
nor a glaze but which is used to cover
clay.

What is Siccative

Siccative: A drying agent; any material which tends to take up moisture from a slop or body.

Crazing

Crazing: a glaze fault caused by
excessive shrinkage of the glaze in
relationship to the body, and which is
characterised by a network of fine
cracks all over the fired surface.

Flocculent

Flocculent/flocculant: Acids (or salts
which in solution behave as acids) which
alter the electrostatic charges of fine
particles in a suspension, reducing their
mutual repulsion, and thus increase their
forces of attraction. This causes
particles to floc together, increasing
friction and viscosity, thus apparently
thickening the suspension without
removing any water from it.

‘pipe'

A ‘pipe' is the metal rod that runs through the center of lamps through which the cord passes and it is threaded on each end.

What is Slip

Slip: A suspension of clay in water, usually quite creamy in consistency, which may be used either in conjunction with colouring oxides for decorative effect, or, with deflocculants in casting.

Resist

Resist: Any material which is used to create a barrier between the surface of a pot and an applied treatment, such as a slip or glaze coat. Resists may be either wax (hot paraffin wax or cold wax emulsion), latex (e.g. carpet glue, copydex etc.), paper (florists paper is good, but newspaper will do for making templates or stencils), or found materials
such as lace or leaves. Salt/vapour glazing: The glazing of pots in the kiln by introducing damp salt (or other soda compound) into the kiln at high temperatures. The salt decomposes and volatilises, and the soda thus released into the firing chamber the kiln and furniture too), thus forming a glaze.

Glost

Glost: Originally a second firing of ware
to a lower temperature than the
preliminary firing, but now used (most
certainly amoung studio potters) to refer
to any firing of glazed ware.

Dunting Points

Dunting points: These are the points at
which silica inversion takes place during
firing and cooling. The significant
points are quartz inversion (around 573C)
and cristobalite inversion (around 227C).

Maturity in ceramics

Maturity in ceramics when referring to glazes is the point at which they reach their maximum hardness during the firing process.

What is Vitrification

Vitrification: The process by which silica is converted to glass by the action of heat and fluxes. A vitrified body is one in which some of the silica in the body has been converted from its crystalline to its glassy phase. If all of the silica in a body has been vitrified, it may slump in firing, and the finished pot will be very brittle.

Oxidation

Oxidation firing: The firing of ware in
a kiln where there is an ample supply of
oxygen in the chamber, so that
combustion is complete, facilitating the
release of volatile gasses, and causing
metals in the clay and glaze to give their
oxide colours.

Kaolin

Kaolin

Group of clay minerals, such as kaolinite, Al2Si2O5(OH)4, derived from the alteration of aluminum silicate minerals, such as feldspars and mica. It is used in medicine to treat digestive upsets, and in poultices.
Kaolinite is economically important in the ceramic and paper industries. It is mined in the UK, the US, France, and the Czech Republic.

Slake

Slake: To rehydrate. Clay will usually slake down´ if it is dried completely and then covered with water; as the water penetrates the pores, it causes the particles to move apart, and forms to disintegrate.

Colloidal Particles

Colloidal: Colloidal particles are
particles so fine that, even though they
may be of a material much heavier than
water, they remain in suspension and will
not settle. In a clay body, such
particles improve plasticity, and in a
glaze they aid suspension.

Anvil and Beater

An anvil is a pebble or piece of wood used to beat pot walls. The anvil is used inside the pot, a beater or paddle is used outside. The process compresses the clay, thins the pot wall and enlarges the pot. To assist the clay to stretch without cracking, the anvil is often ´pecked´ that is, it has its surface covered with small chipped holes.

Burnishing

Burnishing for a Blackfire

The technique of burnishing pottery can be traced back to ancient times.
Burnishing involves no more than rubbing the clay surface with a smooth tool to produce a mirror-smooth surface. In reality it has a compressing effect on the clay particles. It can be done when the clay surface is leather hard and up untilit is almost completely dry. Most clays are suitable for burnishing although the finer the clay the smoother the burnished surface.
Suitable tools for burnishing include: Smooth rounded beach pebbles, The convex side of metal spoons and smooth knife handles. After the pot is smooth, draw your design with lead pencil then scratch around design with a knife. Designs can either be geometric or organic. Once the pot has been blackfired it can be left without further treatment or polished with oils to enhance the shine.

Pottery

Pottery, clay that is chemically altered and permanently hardened by firing in a kiln. The nature and type of pottery, or ceramics (Greek keramos,”potter´s clay”), is determined by the composition of the clay and the way it is prepared; the temperature at which it is fired; and the glazes used.

Short Clay

Short: A clay of poor plasticity and strentth which tends to split when worked, and which fails to stand up well in throwing.

Cristobalite inversion

Cristobalite inversion: A sudden change
in the physical structure of silica in its
cristobalite phase. This change causes
sudden expansion on heating (with
corresponding contraction on cooling
from alpha to beta form and takes pace
a temperatures between 220C and
280C(227C is widely taken as a guide).

Gloss glazes

Gloss glazes produce a shiny glass-like finish

Plasticity

Plasticity: The characteristic property
of clay which allows it to be a mouldable
solid. Plasticity in clays depends on a
number of criteria, including the purity
of the clay, uniformity and size of the
particles, and the bonding of particles
through souring.

Thermal Shock

Thermal shock: Extreme stress caused to a pot by sudden or uneven change in temperature.

What is an Opener?

Opener: Openers are used for similar
pruposes to grog(see above). Grog is
itself an opener, but other materials)
such as sand, sawdust or straw maybe
used.

Pinholing

Pinholing: a glaze fault which causes
pitting of the glaze surface. It is caused
by bubbles rising to the surface during
drying or firing, and being prevented
from healing over because of the
viscosity of the glaze melt, which may
be caused either by its composition, or
by underfiring.

Fritt/Frit

Frit/fritt: Materials which have been
prepared for use by melting (usually
together with at least one other
material-often a pure silica-to form a
fused compound) and grinding. This may
be done for a number of reasons,
including: reducing solubility, safely
containing poisons, reducing volatility

Bisque

Bisc: A general term for used to
describe a prelominary firing of unglazed
wares.
Biscuit firing: A preliminary firing of
unglazed ware beyond the point of
ceramic change, but below the point of
fusion, so that the ware remains
porous. Biscuit may refer to finished
unglazed wares, despite the fact that
these may be fired to vitrification.
Bisque: Generally refers to unglazed
ware fired to maturation (and in some
cases, to vitrification)before the
application of a glaze whose maturing
temperature is lower than that of the
body.

What is an Electrolyte

Electrolyte: A liquid or solution which is
able to conduct electrical currents.

Quartz Inversion

Quartz inversion: A sudden change in the physical structure of silica in its quartz phase. This change causes sudden expansion on heating (with corresponding
contraction on cooling) from its alpha to its beta form and takes place at temperatures between 550C and 575C (573C is the usual given temperature).

Friable

Friable: Easily crumbled.

What is Bat wash?

Bat wash/Batt wash: A mixture of
refractory material(eg.Alumina hydrate)
and water painted on to kiln furniture
and/or clay to resist fusion of touching
surfaces during firing

Boxing pots

Boxing pots: The stacking of pots rim to rim and foot to foot for drying, storage and firing.

What is a Waster?

Waster: Pots which are rejected because of some fault which has become apparent after firing.
In Cast ceramics a waster is the extra of the mold that is cut off.

Ceramic Glossary

If you are just starting out with ceramics you may find the technical jargon intimidating. Look through our Ceramic Glossary and Ceramic Terms to learn about the craft, tools and techniques of ceramics. Learn from our ceramics guru about the business, art and production of high quality ceramics.

What is Thixotropy?

Thixotropy: The property of some materials in a suspension which causes a thickening or gelling of the suspension when still.

Celadon

Celadon: A high firing glaze of Chinese
origin which is characterised by the
effects of reduction firing on small
amounts of iron in the glaze, giving a
rang of colours from pale grey or
blue-green to a deep olive green.

Egyptian Paste

Egyptian paste: a prepared body which
contains soluble alkalalis which migrate
to the surface by evaporation and thus
deposit fluxes which will fuse with the
body during firing, hence its description
as a self glazing clay.

Defocculant

Deflocculent?deflocculant: Generally
soluble alkalis added to a glaze slop
which bring about ion exchange, thus
causing clay particles in the suspension
to repel eachother. This repulsion
reduces friction between the particles,
thus increasing the fluidity of the
suspension without the addition of more
water.

Bloating

Bloating: Blistering of a pot wall during
firing, caused by the expansion of
unreleased gases after fusion has begun.

What is Cation

Cation: A positively charged ion.

Souring

Souring: A process of organic breakdown which releases acids into the water of plasticity in a clay body. These acids bring about the flocculation of fine particles, thus improving the body´s placicity and strength.
Stoneware: A high-firing clay body. Stonewares are fired to the point of vitrification, and are therefore much harder, stronger and more brittle than lower fired earthenwares

Shop Rotten

Shop rotten: A raw pot which is shop rotten is one which has been stored for a long time, and been subjected to extreme variation of ambient conditions which have weakened it to such a point that it becomes friable.

Opacifier

Opacifier: A material added to a glaze
in order to render it opaque. Opacity is
achieved by the deflection of light,
either by the suspension of vey fine
bubbles or particles in the glaze, or by
the formation of crystals in the glaze.

Flux

Flux: Oxides (usually alkaline) which,
when combined with the acid
glassformer, silica, encourage and lower
the temperature of ceramic fusion during
firing.

What is Slipware

Slipware: Earthenware pottery decorated with slips under a transparent glaze.

What is Hakame?

Hakeme/hakame: A Japanese way of
freely brushing slips onto a pot so that
it is decorated with bold stirations
around the from. The same technique
may be used with slip glazes.

Glaze

Glaze

Transparent vitreous coating for pottery and porcelain, which gives the object a shiny, protective finish and helps to keep it from leaking and chipping. Glaze is applied by dipping a formed ceramic body into it or by painting onto the surface. It is fixed by firing in a kiln.
Different mineral glazes will combine chemically with different bodies (according to the minerals present in the clay). Glazes may be alkaline, lead, leadless, tin, salt, or feldspathic. Glazed pottery is first known from the mid-to late-Neolithic in Egypt, where glass was first made.

What is Viscosity

Viscosity: The thickness of a liquid or its resistance to flow.

Chuck

Chuck: A made or found object used to
support a pot whose shape makes it
difficult to invert for processes such as
turning or banding. Typically a chuck
will be a heavily thrown hump or bowl
which is allowed to stiffen in situ on the
wheel head before resting a pot on or in
it.

How and why Calcine

Calcine: Heat treatment of a material in
order to alter to improve its behaviour in
use. Calcining may purify a material,
for example, or it may reduce its
plasticity and shrinkage.

Ion Exchange

Ion exchange: An exchange of ions
(electrically charged atoms) which alters
the electrostatic charge and thus the
behaviour charge and thus the behaviour
of particles in a suspension.

Raw Glaze

Raw glaze: Although this term is frequently used by studio potters to refer to the practice of glazing pots when they are raw, it strictly (particularly in industry) denotes a glaze which is, itself, composed of raw, unfritted materials.Reduction firing: The firing of ware in a kiln where there is an insufficient supply of oxygen in the in the chamber, so that combustion is incomplete, causing the
flames to draw on the combined oxygen in the fabric of the ware, which has the effect of reducing oxides to their respective metal forms.

Porcelain

Porcelain: Porclains are usually prepared
white clay bodies which firm to a hard,
vitrified finish. They are particularly
admired for their translucent quality
when finely potted and appropriately
fired.

What is Earthenware?

Earthenware: A clay body which is
fired to relatively low temperatures
(below 1150C) and which, after firing
remains porous in its unglazed state.

Setters

Setters: Special refractories which are designed to support the shape of a pot during firing.
Setting: Setting is the word which potters use to mean placing ware into a kiln.

Crawling

Crawling: A glaze fault most common in
very viscous or opaque glazes which is
characterised by a rolling back of the
glaze during firing, leaving bald patches
of pot body exposed.

Galena

Galena: Lead ore: historically a
commonly used flux in earthenware
glazes, but which is poisonous, and
therefore rarely used in its raw form
today.

What is Sintering?

Sintering: The bonding of particles by heating, but without fusion, thought to be brought about by friction.