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Ceramics / Pottery Terms

Clays:
The earliest ceramics were pottery objects made from clay, either by itself or mixed with other materials. Pottery is one of the oldest human technologies and art forms, and remains a major industry today. The clay is typically fired in a kiln, glazed, and re-fired to create a colored, smooth surface.

Earthenware (low fire)
Earthenware is commonly bisque fired to temperatures between 1000 and 1,150 °C (1800 and 2100 °F) and glaze- fired from 950 to 1,050°C (1,742 to 1,922 °F). The exact temperature will be influenced by the raw materials used and the desired characteristics of the finished ware. The higher firing temperatures are likely to cause earthenware to bloat. After firing, the body is porous and opaque with colors ranging from white to red depending on the raw materials used. Earthenware may sometimes be as thin as bone china and other porcelains, though it is not translucent and is more easily chipped. It is also less strong, less tough, and more porous than stoneware, but is less expensive and easier to work. Due to its higher porosity, earthenware must be glazed in order to be watertight.

Stoneware (high fire)
Essentially man-made stone, stoneware is a vitreous or semi-vitreous clay body of fine texture made primarily from non-refractory fire clay. Stoneware fires to a gray, brown, buff, or light brown color when at high temperatures from about 1200°C to 1315°C (2192°F to 2399°F). It is a dense, opaque, and impermeable clay with a very resistant surface.

Porcelain (high fire)
Porcelain is a very fine ceramic clay body that is fired in a kiln at temperatures between 1,200°C (2,192 °F) and 1,400 °C (2,552 °F). Porcelain is an ideal material due to its toughness, strength, and translucence from these high temperatures. It is also known as "china" in some English-speaking countries, as porcelain manufacture originates in China. Properties associated with porcelain include low permeability and elasticity; considerable strength, hardness, glassiness, brittleness, whiteness, translucence, and resonance; and a high resistance to chemical attack and thermal shock.

Leather-hard
This drying stage must be reached before bisque firing, otherwise clay will warp and crack upon drying too rapidly at too high of a temperature in the kiln. This condition is called "leather-hard" because the clay body has been partially air-dried and fully shrunk to 15% moisture content. The clay body becomes hard and dry, though it is still visibly damp clay, and it can easily (but carefully) be handled or carved without breaking.


Kilns:
A kiln is similar to an oven, as it is a thermally insulated chamber specifically designed to allow for high atmospheric temperatures sufficient enough to complete the firing processes (drying, hardening, chemical, and physical changes). Firing clay permanently alters the composition of the clay body, and ceramicists manipulate kiln temperatures and firing processes to achieve their desired clay hardness and aesthetic surfacing.

Electric kiln
Using electric kilns is one of the most common methods of firing ceramic works. This type of kiln utilizes electricity for fuel and is usually fitted with electronic controls that allow the user to fine-tune the temperature (rates of heating and cooling). Electric kilns are smaller than industrial kilns, but are used to fire smaller-scale production work such as handmade and industrial pieces.

Gas kiln
Gas kilns are used almost as often as electric but they use natural gas or propane as fuel, and these types of kilns are easy to control, efficient, and generally clean sources of energy. These kilns can also be fitted with electronic controls for fine temperature adjustments and are used for smaller-scale production work, both in industry and craft.

Bisque (biscuit) firing
Earthenware pottery is initially fired at 1000C (1832F) and is considered the first firing because the ware is unglazed upon entry into the kiln. As the nature of this clay body is very porous, bisquing allows for a more resilient and hard clay body that can easily accept the application of glazes.

Wood fire (wood kiln fired)
Generally using an anagama kiln, pieces are fired in a kiln fueled entirely with wood and charcoal. Several cords of firewood are used and the process can last for several days; this method is therefore quite expensive. Wood ash settles on the pottery during the firing process, and in combination with the flame and minerals a natural ash glaze develops on the clay body. Other visual and aesthetic effects created by wood firing include oxidation/ reduction, glaze variation, and temperature fluctuation.

Anagama kiln
An anagama is a traditional Japanese pottery kiln specific to wood firing. It is a single- chamber kiln built in a long, sloping tunnel shape with a firebox at one end and a chimney/ flue at the other. Firewood is constantly fed to the kiln and the fire stoked for controlling the flame temperature, which can reach up to 2500F. At these high temperatures volatile salt and ash are produced, allowing the piece to achieve a wide range of surface appearances.

 

Firing and glazing techniques:
Glazing is functionally important for earthenware vessels, which would otherwise be unsuitable for holding liquids due to their porous nature. Glaze is also used on functional and decorative stoneware and porcelain. In addition to the functional aspect of glazes, aesthetic concerns include a variety of surface finishes, including degrees of gloss and matte, variegation and finished color. Glazes may also enhance an underlying design or texture, which may be either the "natural" texture of the clay or an inscribed, carved or painted design.

Pyrometric cones (firing, 1-10)
Ceramic kilns fire to a specific cone level that accounts for both atmospheric temperature and the amount of time spent at that temperature. Cone levels translate to heat absorption during a firing and as a tool to measure the heating rate and temperature combination. Cones are measured on a scale of 1- 10, with level 10 being the hottest temperature reached. The coolest cone temperatures include 01- 022 to allow for glass and glaze firing.

Reduction atmosphere
A reducing atmosphere is created by the removal of oxygen and other oxidizing gases from the interior of a kiln. This process prevents oxidization of glaze on the surface of a clay body. Reducing gases include hydrogen and carbon monoxide for complete combustion during a high- temperature firing. When introduced to other chemicals and elements, certain metals react with others, offering a wide variety of colors during the glazing process which results in mottled, rich, earthy colors.

Oxidation atmosphere
Alternatively to a reduction atmosphere, oxidation is obtained with an abundance of oxygen introduced into the kiln during a high or low- temperature glaze fire. Very bright and richly saturated colors are obtained from this method of glaze firing. Raku is an excellent example of the oxidation process.

Raku
Raku ware is a type of pottery that was traditionally used in the Japanese tea ceremony in Japan, most often in the form of tea bowls. The mottled and oxidized surface glaze on the clay body results from the removal of pieces, still glowing hot, from the kiln. Typically, pieces removed from the hot kiln are placed in masses of combustible material (e.g., straw, sawdust, or newspaper) to provide a reducing atmosphere for the glaze and to stain the exposed body surface with carbon. The temperature change from the kiln to the container (usually a trashcan) is where the magic of raku happens. Once the lid of the container is closed the reduction oxidation process begins, eventually reducing the glaze, leaving certain colors and patterns.

Salt glaze/ sodium firing
Salt- glazed pottery is stoneware that has had salt thrown into the kiln during the highest temperature of its firing process. As stoneware is a high-fire clay body, the high temperatures cause the sodium (from the salt) to react with the silica in the clay body, forming a glossy and translucent glaze.

Soda firing
Normally used in a wood fire kiln and more recently becoming a non-toxic alternative to salt firing, this atmospheric firing technique creates a surface similar to that of salt glazes. A mixture of soda and water is introduced to the kiln at the peak of its firing temperature (2350F). Sodas used are typically sodium bicarbonate (baking soda) and sodium carbonate (soda ash), and mixtures vary from a spreadable paste or a thin spray. The soda vaporizes upon being introduced to extreme heat and bonds with the clay body, creating a sodium-silicate glazed surface. Soda firing allows glaze colors to be very bright and the surface can be matte or even "dry", rather than the glossy and translucent finish of salt firing.

Ash glaze
Wood ash is strained through a sieve and thoroughly cleaned with water added by the potter, creating a drained and dried glaze solution. The processed ash is thrown into the kiln, is deposited on the wares, and melts on contact with the clay body to form a shiny and mottled appearance ranging from green to amber. Depending on the intended aesthetic effect, the potter will introduce "fly ash" into the kiln during a wood firing and ashes settle primarily on the rims and shoulders of pot, or the glaze is directly applied to the bare surface before a firing, which results in ash melted over the entire piece.

Building techniques:
The potter's most basic tool is the hand. However, many additional tools have been developed over the long history of pottery manufacturing, including the potter's wheel, shaping tools (paddles, anvils, ribs), rolling tools (roulette, slab roller, rolling pin), cutting/piercing tools (knives, fluting tools, wires) and finishing tools (burnishing stones, rasps, chamois).

Hand-building
Hand-building is the earliest forming method. Wares can be constructed by hand from coils of clay, flattened slabs of clay, solid balls of clay or some combination of these. Parts of hand-built pieces are often joined together with the aid of slurry or slip. Hand building is slower and more gradual than wheel-throwing, but it offers the potter a high degree of control over the size and shape of wares. While it isn't difficult for an experienced potter to make identical pieces of hand-built pottery, the speed and repetitiveness of other techniques is more suitable for making precisely matched sets of wares such as tableware.

Throwing (wheel- throwing)
In the process that is called "throwing", a ball of clay is placed in the center of a turntable, called the wheel-head, which the potter rotates with a stick, a foot-powered kick wheel, or with a variable speed electric motor. During the process of throwing the wheel rotates rapidly while the solid ball of soft clay is pressed, squeezed, and pulled gently upwards and outwards into a hollow shape. The next steps include further manipulation of the clay to achieve the final form.

Slab construction
Flattened strips of clay, varying in size and width, are used to create wares that cannot otherwise be shaped by other hand-building techniques. The clay is uniformly rolled, by hand or machine, and is typically assembled in geometric and architectural forms. Some methods consist of soft-slab construction (relatively wet clay) and others of stiff-slab construction, where the clay slowly air-dries to the leather-hard stage before assemblage.

Slip-cast
Slip-casting is often used in the mass-production of ceramics and is ideally suited to the making of wares that cannot be formed by other methods of shaping. A slip is poured into a highly absorbent plaster mould. Water from the slip is absorbed into the mould leaving a layer of clay body covering its internal surfaces and taking its internal shape. Excess slip is poured out of the mould, which is then split open and the moulded object removed.

 

Decorating techniques:

Sgraffito
A layer of slip contrasting the clay body is applied to its surface (like paint) and is then carved through to reveal the underlying clay. The contrast is most visible after the piece is fired.

Airbrushes/ sprayers
Airbrushes and paint sprayers are tools that utilize compressed air to evenly apply colors, patterns, or images to the surface of the clay body. These surfaces are usually achieved with ceramic pigments loaded into the tools, though other paint and drawing materials may be used.

Impressing
A textured or patterned material or object is pressed into a wet clay surface.

Carving/ incising
This method of decorating the clay body is usually done when the clay surface is leather-hard for sharper-edged lines. The clay surface is scratched away for either decoration or to carve a specific form from a block of clay, such as a cup. Depending on the intended effect, a variety of tools used on the surface include ribbon and loop tools for removing larger amounts of clay quickly, and small dental tools are used for fine detailing. Incising is a fine surface decorating technique that utilizes very small detailing tools that are applied to a wet clay surface for soft lines and a firmer surface for sharper lines.

Slip
A mixture of clay body and water that may be very thin and watery or thick like a paste, depending on dilution.