Glossary of Ceramic Terms
Bisque or Biscuit:
The first firing a pot undergoes to prepare it for glazing. In industry this is done at a
higher temperature than the subsequent glaze firing, the reverse is usually true ofthe
studio potter.
Burnishing:
A technique where the Leather hard clay is polished with a hard instrument to force the
smallest clay particles to the surface creating a soft sheen. This surface remains after
the pot is fired so long as the firing temperature is kept below 11 OO°C.
Calcine:
To purify a material through the action of heating to red heat 700-750 °c (1292-13 82
OF)
Ceramic Change:
The slow process of clay becoming ceramic. Clay which is exposed to heat 600°C /
1112°F, losses its chemically bound water molecules and can no longer be broken
down by water. Once this change has occurred it cannot be reversed. (See Firing and
Water)
Clay:
AL203 2Si02 2H20. The decomposition of Granite through the process of Kaolinization
creates clay (see Kaolinization). Clay is a mineral with a plate (platelet) like structure; it
is these plates, (about 0.5 microns across) when lubricated with water, slide against
each other to form the plastic mass we know as clay (see Water). 'Primary I clays are
those found close to the area of Kaolinization and hence the purest (Kaolin or China
Clays). Secondary clays are those moved by water away form the site of Kaolinization
and get progressively more plastic and less pure (Ball Clays, Fire clays, Earthenware's
and Marls).
Clay Body:
A clay designed for a special purpose. It is created by blending different clays of by
adding to clays other materials, such as feldspar and flint in order to produce a desired
workability, maturing temperature, or finished result. A clay body is the result of mans
technology.
Cobalt:
One of the strongest coloring oxides used by the potter. Cobalt creates a dark dense
royal blue in most cases. Historically used in China as a painting pigment on Blue and
White wares.
Coiling:
A method of hand building a form using long rolled out, or extruded, snakeelike lengths
of clay. Each coil of clay is integrated with the previous one to build the work up. The
coils may be completely obliterated in the construction process or retained for their
decorative qualities.
Cones:
Pyrometric cones are composed of clay and glaze material, designed to melt and bend
at specific temperatures. By observing them through a small 'Peep Hole' in the kiln it is
possible to ascertain the exact conditions in the kiln. Cones are a better indicator than
temperature alone as the degree of glaze melt is a combination of time and temperature
("heat work"), thus a fast firing needs to go to a higher temperature to get the same
results as a slow firing to a lower temperature.
Crystalline glazes:
Most glazes have no easily visible crystal structure. Crystalline glazes have large and
dramatic crystals up to about three inches across. A high alkaline low alummina glaze is
vital for crystals to develop. Additions of zinc and titanium also help seed the crystals.
An extremely slow cool of the kiln is necessary, to allow the crystals to grow. Because
of the low alumina content in crystalline glazes they are very runny, often pots are
supported in the kiln on stilts to avoid them adhering to the kiln shelves, the stilts can be
broken off after the firing.
Deflocculate:
To disperse the particles in a slip so that less water is required to make the slip fluid.
Dunting:
Cracks, which occur on pottery during the heating or cooling cycle of the firing. They
are usually caused by the silica inversion at 573°C, 1063°p (Alpha to Beta phase) or the
Crystobalite (one of the 'phases' of silica) inversion at 226°C, 428°p in both cases there
is an expansion and contraction of around 2-3% in the heating and cooling cycles.
Earthenware / Terracotta:
A lowfired form of pottery or objects (below 11 OO°C, 2012°p) made from fire clay,
which is porous and permeable. The clay can be any color although iron red is usually
associated with Terracotta. The low temperature vastly expands the range of glaze
colors available these are often alkaline or lead based.
Enamel:
A form of low temperature glaze that is applied on top of an already fired higher
temperature glaze. Enamels are often lead based, as it is a flux, which works at a low
temperature.
Feldspar:
One of the predominant naturally occurring fluxes used primarily in stoneware glazes.
The three most commonly used feldspars are Potash feldspars K20 Ah03 6Si02, Soda
Feldspars Na20 Ah03 6Si02 and Lithium Feldspars Li20 Ah03 8Si02. Feldspars can be
the only flux present in a stoneware glaze although this is uncommon and additions of
calcium usually supplement it.
Firing:
The process, which changes clay into ceramic. Up to 600°C I 1112 OF the chemically
bonded water in clay is driven off (AL203 2Si02 2H20 - A~03 2Si02). This is an
irreversible change know as the "Ceramic Change", (See Clay and Ceramic Change)
Frit:
A combination of materials that have been melted into a glass, cooled, and reground
into a powder prior to being added to a glaze recipe.
Grog:
Clay that has been fired and then ground into granules of more or less fineness. Grog is
considered a filler, and added to clay bodies for several reasons; it helps open a tight or
dense body, promotes even drying, which reduces warping and cracking, and reduces
overall shrinkage. Grog also adds tooth and texture to a clay body aiding in the ability of
the body to maintain its form during construction.
Glaze:
A super cooled liquid, with a random molecular structure and high viscosity at normal
temperatures, super cooling is relative to geological cooling. A random molecular
structure is the result of fast cooling so crystals cannot develop (the exception being
crystalline glazes). Granite cools slowly (geologically speaking) so we can easily see
the crystals in polished granite, glaze cools quickly so the molecules do not have a
chance to crystallize. A high viscosity means it does not run off the pot. (Well it may in
the kiln but not in the kitchen!)
Inlay:
A decorative technique where a pattern is carved into the clay at the leather hard stage
and contrastingly colored soft clay is forced into the decoration. When the clay is a little
drier the excess is scraped of to reveal the pattern.
Iron Oxide:
Fe203 is one ofthe potters favorite colorants, when combined with the right glaze and
firing iron oxide can produce greens, browns, blacks, yellows, oranges, subtle blues and
grays. Most of the best color responses for Iron in a glaze need a reduction firing. Iron is
also a useful colorant in clay bodies and is best introduced by adding high iron clays to
the clay recipe.
Kiln:
Basically an insulated box, which is heated to fire pots in. They can be either, cross
draft, down draft, or up draft. The draught refers to the direction the combustion gasses
have to travel from input to exit flues, since no combustion takes place in an electric kiln
there are no input or exit flues and they are genuinely heated boxes. The fuels used to
heat a kiln are gas, oil, wood, coal (now almost obsolete) and electricity. Each fuel
source used to fire a kiln offers different possible outcomes for the pots fired in them.
The maximum operating temperature for most pottery kilns is about 1300°C, 2372°p,
although many woodfired kilns may be fired up to 1350°C, 2462°P.
Kiln wash:
A refractory mixture, usually kaolin or flint, which is painted on kiln shelves and saggers
to prevent glaze from adhering.
Leather Hard:
A stage in the drying process of clay when the clay is pliable but strong enough to
handle. It is ideal for trimming and the addition of appendages such as handles and
spouts. Relatively wet clay can be attached to the pot at this stage and the resulting
bond will not form cracks.
Once Fired:
A pot that has undergone a single glaze firing. The glaze is applied directly on to the
dry or leather hard pot thus avoiding the bisque firing. This approach, although offering
certain economic and aesthetic advantages, can create technical problems for the
potter.
Oxidation:
A firing where there is either no combustion occurring (electric kiln) or where there is
sufficient oxygen in the kiln to allow the fuel to bum cleanly. The atmosphere of the kiln
(oxidation, or reduction) dramatically affects the resulting clay and glaze colors, for
example; copper in oxidation is green (as is copper oxide) in reduction it becomes red
(more like copper metal).
Plaster:
2CaS04 2H20. An invaluable mold-making tool for the potter, also used extensively in
industry. It can be poured or carved into virtually any shape. When it is dry it can be
used to press clay into or to slipcast with.
Plasticity:
The properties of a material that allow it to be shaped and to retain its shape. The
plastic properties of clay are principally determined by the size of the platelets. The
smaller the platelets the more plastic the clay is. Aging or souring is also relevant to a
clays plasticity; with time bacterial action creates a colloidal gel, which aids the
lubrication of the platelets.
Porcelain:
A white highly vitrified clay body that is translucent where thin (often fired up to 13500C,
2462°F). The translucency is a result of silica glass fused into the fired clay. To achieve
this a high amount of flux is added to a kaolin based clay body. The flux to clay ratio is
often flux> clay, indeed some ofthe original Chinese porcelains had as little as 20% clay
like minerals. The low clay content makes porcelain very difficult to throw and trimming
wares is almost unavoidable. At the home of porcelain, Jingdezhen (China), all the pots
are throw in small thick sections, joined and trimmed. Accurate trimming is regarded as
more of a skillful art than throwing. The plasticity of porcelain can be improved by small
additions (2%) of white bentonite.
Raku:
Originally a Japanese seal given to a prominent family of potters (1598) who developed
the technique. The term describes a lowfire form of pottery where the pots are removed
from the kiln as soon as the glaze has melted and then left to cool or doused with water.
In the mid 20th century Paul Soldner introduced the now popular process of post firing
reduction. In this case the red hot pot is placed in a lidded bin filled with straw or
sawdust. The glazes are dramatically altered by the reduction particularly noteworthy
are the colors achieved with Copper.
Reduction:
Also see Oxidation; A situation where too much fuel is introduced into the kiln to be able
to burn with the available oxygen, consequently oxygen is 'stolen' from the pots in the
kiln, it affects the clay and the glaze color. A good example is iron, which changes from
Fe203 to FeO, even the tiny amount of iron present in porcelain changes it hue from a
creamy color in oxidation to a slight gray blue in reduction.
Sgraffito:
A decorative technique, where by the surface of the clay is scratched, often to expose
another layer of colored clay.
Shelling:
Flaking, Peeling. A glaze or glaze and slip defect in which the glaze falls from
the body in flakes. It is caused by insufficient bond between glaze and body, usually the
result of under firing.
Shivering:
A defect in which the fired glaze pulls away from the ware taking some of the body with
it in the form of slivers. This generally occurs on sharp rims and edges of handles, and
is due to improper glaze fit caused by too much compression by the body.
Shrinkage:
The decrease in the size of a clay object due to drying and firing. Dry shrinkage is
reversible with the return of water, but firing shrinkage is permanent due to chemical
and physical changes clay undergoes when exposed to heat (see ceramic change).
Silica:
The primary glass forming oxide used in pottery. Boron is the other glass forming oxide
used although more commonly as a flux than as a glass former due to its low melting
point (577°C, 1063 OF). A glass forming oxide must be present in any glaze and as
silica's melting point is 1800°C, 3272 of, a flux is always present to reduce the melting
point to a workable range. Pure boron glasses are water-soluble so of little use but
Boro-sillicate glasses have a very low thermal expansion and are the main constituent
of 'Pyrex' etc. Also see Dunting.
Slip:
A fluid suspension of clay with and water, with a "cream" like consistency. Most often
colored with oxides and painted or poured onto pots for decoration.
Slipcasting:
Plaster molds are filled with a deflocculated slip; deflocculation reverses the electric
charges in the clay particles, which reduces the water content in a slip to that of most
plastic clays, around 30% of total weight. A common deflocculant is Sodium Silicate.
The plaster absorbs sediment of clay leaving the remaining moisture over the entire
interior surface of the mold. The excess slip is drained off and the cast can be removed
from the mold soon after. This approach is used widely by industry and some studio
potters.
Slipware:
A traditional English decorative technique associated with red earthenware and lead
glaze. Colored slip is piped onto the leather hard pot much like cake decoration. The
most noted exponent of slipware was the 18-century potter "Thomas Toft"; his dishes
set a standard that few modem potters can compete with.
Stains:
A suspension of metallic oxides, clays and other materials with water, used to add color
to the surface of clay and glaze.
Stoneware:
Highly vitrified ceramics fired to above 1200°C, 2192 OF. Most of the silica in a fired
stoneware body is melted into a glassy matrix and the resulting body is of high density
and usually has a water absorption rate of less than 1 %.
Terra Sigillata:
A slip comprised ofthe smallest particles of clay, which consequently resembles a
burnished surface. The technique was used to impressive effect in the GrecooRoman
period.
Throwing:
To make pottery by hand on the potters wheel. A delicate balance, which defies gravity
and centrifugal force as clay is coaxed up by hand from a spinning turntable.
Trimming, or Turning:
Certain forms made on the potter's wheel will not support themselves unless excess
clay is left at the base, alternatively, extra definition on the foot of a pot may be needed.
The solution to both these problems is turning, which is done at the leather hard stage.
The pot is inverted onto a potter's wheel and a metal cutting tool is applied to the bottom
of the pot until the desired finish is achieved.
Underglaze:
Ceramic colors combined with clay applied under a glaze, usually a clear glaze.
Although a durable method of decorating, colors can run especially if colorants, which
double as fluxes, are used, however more dependable than overglaze stains.
Viscosity:
The ability of a liquid to flow, the term is used by the potter in relation to molten glazes,
glaze suspensions, and slips. A stiff molten (liquid) glaze is one of high viscosity, while a
runny molten (liquid) glaze is one oflow viscosity.
Vitrification:
The degree of melt in a clay body as the silica forms a glass with fluxes present. See
stoneware.
Water:
H20. A most important part of clay and is also used to suspend the particles of a glaze
prior to application. The water in clay takes three forms; Water of plasticity, Pore Water
and Chemically bonded water. Water of plasticity lubricates the clay platelets. The pore
water is water of plasticity that is trapped during the drying process in between platelets
of clay. Pore water can cause problems in the bisque firing even if it appears the pot is
bone dry and caution must be taken up to around 150°C, 248 OF (Water smoking
period). The chemically bonded water is driven offup to around 600°C, 1112 OF trailing
off at 700 °c, 1292 OF henceforth it becomes ceramic. (The theoretical formula for clay
is Ah03 2Si02 2HzO and becomes AL203 2Si02 upon the ceramic change) See Clay,
Firing, and Ceramic Change.
Wax Resist:
A decorative technique where a wax based medium is used to create a pattern, which
is then covered, in another coat of glaze or slip. The wax resists the subsequent coating
creating the pattern. Paper stencils or tape can create a similar effect. Latex is another
effective resist with other advantages.
Wedging:
To kneed or mix plastic clay by hand. A hand process used to homogenize the clay and
remove air bubbles, thus making it workable. The techniques for wedging are called;
Spiral, or Chrysanthemum wedging, Rams head, or Monkey face wedging and wire/slab
wedging. Both Rams head and Spiral wedging involves the folding of the clay on itself
too build up an ever-tightening spiral of clay platelets. Wire wedging builds up
increasing layers of clay platelets and is the best for introducing other clays and fillers
into an already plastic clay body.