Glossary of Ceramic Terms
BISQUE WARE: Clay objects that have been fired for the first time and without any glaze applied to them.
BONE DRY: Clay that is completely dried but not yet fired.
CERAMIC: Having to do with clay or glass or the making of objects from clay or glass.
CERAMIC CHANGE: The slow process of clay becoming
ceramic. Clay which is exposed to heat 600oC / 1112oF, losses its
chemically bound water molecules and can no longer be broken down by
water. Once this change has occurred it cannot be reversed.
CLAY: The American Ceramic Society has defined clay as "a
fine-grained rock which, when suitably crushed and pulverized, becomes
plastic when wet, leather-hard when dried and on firing is converted to
a permanent rock-like mass." In general, clay is considered to be
hydrated aluminum silicate and can be represented by the general
formula Al2O32SiO22H2
COIL: rope-like form of clay.
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.
EARTHENWARE / TERRACOTTA: A lowfired form of pottery or objects
(below 1100oC, 2012oF) 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.
FETTLING KNIFE: A special knife-like tool with a fairly flexible blade for cutting into moist and leather-hard clay.
FIRING: The process by which clay is heated in a kiln and converted to ceramic.
FLUX: Any substance that promotes fusion.
FOOTING: the bottom of a clay piece that rests upon a surface (may be
hand-built or wheel-thrown). There are two kinds of footing: raised and
flush.
GREENWARE: A term used to describe unfired clay objects in general.
GLAZE: A chemical mixture composed of silica, fluxes, and
metallic oxides, most often with added colorants, that when applied in
liquid form to bisque ware, and fired at high temperatures in a kiln,
becomes glasslike, forming an appealing, often glossy, coating to the
surface of the clay.
GLAZE WARE: bisque ware that has been glazed, then fired.
INCISING: A common decoration technique created by carving lines into
the surface of leather-hard clay or carving small areas out of the clay
but not perforating it.
KILN: A furnace for firing ceramics. A kiln is specifically
designed to heat clay to the high temperatures necessary to make it
permanently hard and stone like.
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.
LOOP TOOL: A special tool with a wooden handle and a wire loop at one
or both ends, used for carving and hollowing out clay forms.
MOLDS: Hand-building techniques using permanent forms into or over which clay is impressed to shape vessels.
OPEN FIRING: Method of firing vessels without a proper kiln
structure. The most common methods are bonfires or pits. Open firings
are rapid and economical of fuel. They are characterized by rapid rise
in temperature and frequently last less than one hour, though the
duration of the firing is largely determined by the type of fuel used.
OVER FIRING: Fired to or above the point at which defects such as
warping, bloating, and blistering occur. Either excessive temperature
or too rapid firing can cause these defects.
PINCHING: A method of forming clay, which is well described by its name.
PINCH POTTERY: The simplest method of pottery manufacture,
involving the opening out and expanding of a ball or cone of clay by
squeezing the clay between the fingers, while the shape is supported by
and turned in the potter's hand. It tends to result in small,
round-based, open shapes (such as bowls), in which the method of
manufacture can be recognized by the indentations in the vessel walls
left by the pressure of the potter's hands. It can, however, be used as
a preliminary method of manufacture, the shape so formed being added to
later by the addition of coils or rings of clay
PLASTICITY: The ability of clay to be molded and maintain its
shape. It is the result of water being present between the clay
particles, which allows them to glide over each other. The smaller the
clay minerals involved, the more water can be absorbed between the
particles in a give volume of clay. Some types of clay are more
plastic than others.
PORCELAINS: A white highly vitrified clay body that is
translucent where thin (often fired up to 1350oC, 2462oF). 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 of
the 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.
SCORE: to draw or make lines into clay.
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 (577oC, 1063 oF). A
glass forming oxide must be present in any glaze and as silica’s
melting point is 1800oC, 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.
SLAB: a flattened out piece of clay; you may use a rolling pin or slab roller to achieve a slab of clay.
SLAB BUILDING: Hand-building technique which involves forming flat slabs of clay and connecting them to form a vessel.
SLIP: clay that has been watered down; acts similar to a glue in slip/score technique.
SLIP CLAY: Liquid mixture of clay and water.
SLIP/SCORE TECHNIQUE: method used in hand building to connect two pieces of clay together.
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 modern potters can compete with.
SPRIGGING-ON: A term used to describe the technique of adding small
clay forms as decoration on the surface of pottery forms; also called
applied decoration.
STONEWARE: Highly vitrified ceramics fired to above 1200oC, 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%.
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.
VITRIFICATION: The formation of glassy material in a ceramic
body. Vitrification is considered complete when pore spaces are filled
and exterior volume is reduced to a minimum
WEDGING: The process of kneading the clay so as to remove air bubbles and create uniform consistency of clay making it workable.
WHEEL-THROWN: The term used to describe vessels that have been
made on the potter's wheel. Such vessels are usually, though not
always, flat-based.