by Ken Turner
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 of the studio potter.
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 1100oC.
Clay: AL2O3 2SiO2 2H2O. 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 ' 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).
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.
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.
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
Cracks, which occur on pottery during the heating or cooling cycle of the
firing. They are usually caused by the silica inversion at 573oC, 1063oF
(Alpha to Beta phase) or the Crystobalite (one of the 'phases' of silica)
inversion at 226oC, 428oF in both cases there is an
expansion and contraction of around 2-3% in the heating and cooling cycles.
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
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
Feldspar: One of the predominant naturally occurring fluxes used primarily in stoneware glazes. The three most commonly used feldspars are Potash feldspars K2O Al2O3 6SiO2, Soda Feldspars Na2O Al2O3 6SiO2 and Lithium Feldspars Li2O Al2O3 8SiO2. Feldspars can be the only flux present in a stoneware glaze although this is uncommon and additions of calcium usually supplement it.
The process, which changes clay into ceramic. Up to 600oC / 1112
oF the chemically bonded water in clay
is driven off (AL2O3 2SiO2 2H2O
- AL2O3 2SiO2). This is an irreversible change
know as the “Ceramic Change”, (See Clay and Ceramic Change)
Flocculate: To thicken.
*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.
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!)
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.
Fe2O3 is one of the 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.
A china clay in its purest form Al2O3 2SiO2 2H2O.
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 1300oC, 2372oF,
although many woodfired kilns
may be fired up to 1350oC, 2462oF.
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.
Open: To make a clay more open or porous in structure by adding fillers or grog.
Oxide: Any element combined with oxygen.
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 burn 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).
A method of forming clay, which is well described by its name.
Plaster: 2CaSO4 2H2O. 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.
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.
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 Fe2O3 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.
*Refractory: Resistant to heat.
*Sgraffito: A decorative technique, where by the surface of the clay is scratched, often to expose another layer of colored clay.
To moisten dry clay with water.
*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 modern potters can compete with.
Soak: To hold a kiln at one temperature for an extended period of time.
Soluble: Capable of being dissolved in water.
Stains: A suspension of metallic oxides, clays and other materials with water, used to add color to the surface of clay and glaze.
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%.
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.
* Designates terms you will be tested on.