Ceramics: Key Terms and Stages from Greenware to Firing

Greenware

Any unnfired clay

Leatherhard

Clay which has dried enough to hold its shape when handled, still slightly flexible, and wet enough o be joined by slip. This is the best time to carve, engrave, or finish the clay to a smooth surface.

Bone dry

A stage in drying where the clay has lost most of its physical water and is ready for firing. This is the most fragile stage for a clay object, and breakage can easily occur.

Ceramic

A term used to describe clay that has been fired past a stage know as sintering, where the fine particles become chemically joined.

Bisque

A term used to describe low-fired, unglazed ceramic work which is usually porous. Also, the first firing of the work, usually a lower temperature (cone 08 ~ 1728 F) than the glaze firing.

Maturity

The temperature or time at which a clay or clay body develops the desirable characteristics of maximum nonporosity and hardness; or the point at which the glaze ingredients enter into complete fusion, developing a strong body with the body, a stable structure, maximum resistance to abrasion, and a pleasant surface texture.

Vitrify

Convert to glass or a glasslike substance using heat. Clay that has been vitrified will hold water without leaking or seeping, and is no longer porous.

Clay

Clay is a fine particle sediment that is formed through the decomposition of granite and igneous rock. When fired past a particular temperature, clay becomes impermeable to water and very duarble, making it an ideal containment material. Generally speaking, clay is a raw material that has not been altered or added to by humans. Chemical Formula: Al2O3 2SiO2 2H2O

Primary clay

is found at the site of formation, while secondary clay has been moved by wind, water, or glacial means.

Clay Body

A particular mixture of raw and/or refined clays with possible additions of silica as a filler and thermal expansion increaser and feldspar as a flux. A clay body is a specific mixture of multiple ingredients—a recipe. Many ceramists create their own custon clay bodies to suit their working needs.

Earthenware

A type of clay body that is remains slightly porous after firing, usually red but can also be white, brown, or tan. Earthenware is usually fired between cone 08 and cone 02. Can also be used to refer to pottery or other objects made from this clay.

Stoneware

A dense, vitrified, non-translucent clay body, can range widely in color. Stoenware can be fired to much hotter temperatures than earthenware. Stoneware is used in many functional applications because of it strength and durability.

Porcelain

Very smooth and white clay body that is slightly translucent and completely vitrified when fired, and can be fired to very high temperatures.

Grog

Hard fired clay that has been crushed or ground to various particle sizes; It is used to reduce shrinkage and allow even drying in ceramic products as sculpture and architectural terra cotta tiles, which have drying and shrinkage problems. Finely crushed grog is also used in throwing bodies to help the clay stand up.

Plasticity

The property of a material enabling it to be shaped and to hold its form. Clay with good plasticity is malleable and easy to form, and does not readily crack.

Short

Clay lacking in plasticity. It will crack or break and not hold its shape.

Porosity

Ability of fired clay body to absorb water. Earthenware still has some porosity after it is fired, but stoneware and porcelain do not because they are fully vitrified.

Reclaim

Clay that has been recycled. Clay is usually dried out and then submerged in a bucket of water. After the clay breaks down, it can be laid on a plaster board to dry out to a usable state.

Pottery

Functional objects made from fired clay, like cups, bowls, and plates.

Coil

A long, thin rolled or extruded piece of clay used for hand building.

Extrusion

Shapes or forms created by forcing clay through a die at the end of a narrow tube (an extruder).

Slab

A flat sheet of clay used in hand building. Slabs can be used soft to create organic forms, or at the leatherhrad stage to create rigid forms.

Pinch

Compress clay between the fingers and thumb.

Sgraffito

A drawing or design created by scratching lines through a layer of slip to expose the clay underneath.

Slip

A fluid suspension of clay in water, used as decoration (can be colored), or as glue to join pieces of clay, or as a casting material (poured into plaster molds).

Slip Casting

Forming pottery or scupture by pouring deflocculated (water reduced) clay slurry into plaster molds. In the process, the absorbent plaster pulls water from the slurry and over a period of minutes a layer builds up against the mold surface. The slurry is then poured out and within a short time the item shrinks slightly and can be removed from the mold. This process is used in many comercial applications.

Throw

To form a pot by hand on a flat, horizontal, spinning wheel. Throwing can be done on an electric wheel or a kick wheel. This is a common way to make functional pottery.

Trim

To cut a foot into a pot with a sharp tool, usually done on the wheel. Most pieces of thrown pottery are also turned upside down and trimmed on the wheel.

Wedging

Method of kneading clay to make it homogenous, ridding the clay of all air pockets.

Silica

Silicon dioxide, also known as silica (from the Latin silex), is a chemical compound that is an oxide of silicon with the chemical formula SiO2. Silica is most commonly found in nature as quartz or a major constituent of sand, as well as in various living organisms. Silica is a major component of both clay and glaze. Pure silica is very refractory, so it needs the addition of flux to melt at reasonable temperatures. Altering the amount of silica in a glaze can change the shrinkage of the glaze (glaze fit). Breathing silica dust is harmful to the lungs.

Alumina

Properly called aluminum oxide (Al2O3). Along with silica, a basic building block of clay and glaze. It doesn't melt at temperatures used for ceramics and glazes, reduces flow in glazes, and helps with strength. The most refractory of all ceramic ingredients.

Flux

On the theoretical chemistry level, a flux is an oxide that lowers the melting or softening temperature of a mix of others. Fluxing oxides interact with others, sometimes their combinations flux much more than logic would expect given their individual performance.

Glaze

Firing The firing of a ceramic object to a higher temperature than bisque in order to complete chemical transformations and completely melt glaze materials. We glaze fire to cone 04 (1945 F).

Crawling

The separation of a glaze coating from the clay body during the firing caused by too heavy application. This results in exposed areas of unglazed clay.

Crazing

A network of cracks which forms in the glaze due to improper fit of glaze and clay body.

Pinholing

The formation of tiny holes in the glaze due to escaping gas bubbles.

Shivering

A defect in which the fired glaze pulls away from the pottery 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.

Heatwork

The combined effect of time and temperature on clay and glazes during a firing. Cones are used to measure heatwork.

Cone

A unit used to measure 'heat work' during firings. Cones in normal studio use range from 022 (lowest) to 12 (highest). The zero in front of a cone number is very important- it reads similaryly to a negative sign. ^06 is much colder than ^6. 022...015...011...08...bisque...06...earthenware...01....1....6....stoneware...10...12

Pyrometric cones

Small triangular cones (1 1/8" & 2 5/8" in height) made of ceramic materials that are composed to bend and melt at specific temperatures, thus enabling the potter to determine when the firing is complete. Orton is a commonly used brand.

Dunting

Cracking that occurs in ceramic ware that is cooled too quickly. Dunting can exhibit itself as simple hairline cracks or ware can fracture into pieces.

Firebrick

A brick capable of withstanding high temperatures without deforming. 'Insulating firebricks' have the additional advantage of acting as good insulators due to the large pockets of air in the matrix of the brick.

Firing

The heat treatment of ceramics material that allows for chemical transformations and leads to a finished ceramic object.

Kiln

An enclosure, usually made of bricks, used to fire (heat) clay objects, generally powered by natural gas, propane, electricity, or wood.

Reduction

A gas firing done with a reduced amount of oxygen, so that the fuel does not burn completely. This creates depth and mystery in the glazes, and causes some to change color completely.

Refractory

The quality of resisting the effects of high temperatures; also materials, high in alumina and silica, that are used for making kiln insulation, muffles and kiln furnituree