You are here:  Crafts > Clay > Clay Glossary
Register   |  Login
lossary of Terms

lossary of Terms

Minimize

This is a Glossary of Terms that David prepared a while back in preparation for teaching a medieval pottery course.

 

Banding wheel
A revolving turntable used to a work piece. It is useful for accessing different sides of the piece without touching it and for marking bands on the piece.
Bisque (Biscuit) Firing
The initial, lower temperature firing that hardens the clay and prepares it for glazing.
Blistering
This is a fault caused by gases escaping during firing that leave bubbles and craters in the glaze. Lowering the firing temperature and/or more slowly reaching/leaving the top firing temperature can help avoid this.
Celadon
A Chinese grey-green glaze. The glaze includes iron oxide and is reduction fired.
Centering
The act of positioning the clay on the wheel so that it is balanced and placed in the center of the wheel.
Chuck
A lump of centered clay that is used to hold a leatherhard work piece on the wheel.
Clay
Clay is not the same as dirt. It has the characteristic of plasticity until it is heated in a kiln.
Cone
A way of determining temperature that takes into account both the actual temperature reading in the kiln plus the time the piece has been at that temperature. Small cones of clay, known to melt at a given temperature, are placed in the kilns at an 8˚ angle. The cones are observed during firing thru the vent hole. The kiln has reached the maximum desirable temperature when the cones melts to a 90˚ angle. Different clays need to be fired to different “cones” of temperature. Standard cone temperatures are rated (lowest to highest) from O1 to O6, the from 1 to 10. (Yes, that’s right, cone O1 is not the same as cone 1.)
Crawling
This is a fault in the glaze caused by grease or dirt on the bisque ware, or by shrinkage of the glaze during firing. It appears as flaky or bald patches. If caused by the latter, the glaze formulation needs to be adjusted, otherwise, it can be refired to apply new glaze over the bald spots.
Crazing
A fault in the glazing that appears as a network of fine cracks. Intentionally doing this is called Crackle. It is caused by the glaze contracting more than the work piece during cooling.
Earthenware
Red earthenware is the most common naturally occurring clay. It fires around 1832 to 2156˚F. It is porous when fired. White earthenware fires in the range of 1940 to 2156˚F.
Engobe
A clay fluid used to decorate the work piece. The fluid includes a proportion of flux that is halfway between the fired properties of a slip and a glaze.
Footring
This is the bottom of the piece, typically raising the piece off the surface by a bit. It is usually left unglazed so the piece does not stick to the kiln during firing.
Gallery
The shelf on a pot rim that keeps a lid from tipping.
Glaze
Ground or powdered glass that is melted over the clay during firing in order to seal the clay and decorate it.
Greenware
Unfired clay items that are not dry enough to be fired, but otherwise ready to do so.
Grog
Crushed or ground, fired clay. It is added to unfired clay in order to give it more strength or texture.
Kneading
A process to evenly distribute the moisture and plasticity of a piece of clay so that it’s characteristics are uniform throughout. This is an essential step to master – and thankfully rather easy.
Leatherhard
The description of clay that is dry enough to pick up without changing shape, but wet enough to join with slip, cut, burnish or glaze.
Lustre
A glaze that has metallic surface on it that comes from precious metals.
Maiolica
Historically, a lead-glazed, low-fired item with tin added to the glaze to make it opaque.
Overfire
Glaze that was fired higher than it’s firing temperature range. Visible symptoms include warped clay items and blistered glaze.
Oxidation
A firing that includes enough oxygen for the item to reach its oxidized colors. Electric kilns do this by default.
Oxide
The combination of an element with oxygen.
Pin Holing
This is a glaze defect. Bubbles are formed in the fired glaze. Holding the kiln at the firing temperature longer may solve this problem.
Plasticity
This is the workability or malleability of unfired clay. Additional water may increase plasticity (up to a point). Grog alters the plasticity also, as can the basic formulation of the clay.
Porcelain
This is a man-made, white clay with a high firing temperature of 2264 to 2462˚F. It is non-porous when fired to maturity and can be used to construct very thin, translucent pieces. It is hard to throw as it does not maintain its shape well when wetted during the throwing process.
Pouncing
Copying a design by pricking holes in a stencil to act as the outline of the design. Graphite powder is pushed thru the holes.
Pyrometric cone
A cone-shaped piece of clay known to melt at a specific temperature. Several of them, chosen to melt at 20˚C (68˚F) intervals, can be used to determine when a kiln has reached a given temperature.
Raku
A low-temperature firing technique that removes fired work while it is still red-hot and reduces it in a combustible material like sawdust.
Reduction
The opposite of an oxidation firing, in which the oxygen supply is reduced to affect the colors of the finished piece.
Rib
A smooth, shaped tool used when throwing an item on the wheel to apply even pressure.
Roulette
A wheel that is used to place a repeating design pattern onto the ware.
Saggar
A protective box around a work being fired, made of a refractive material, that prevents the work from contacting flames or kiln gasses.
Sgraffito
Scratching thru one color of clay to reveal another color below. The topmost layer is often formed by using a different colored slip.
Slip
Slip is liquid clay, created by adding sufficient water to the solid clay and mixing it thoroughly.
Slip Trailer
A tool for decorating a surface with lines or dots of slip. It has a container to hold the slip which can be squeezed to expel the slip.
Slurry
A mixture of clay and water that is used to glue other pieces of clay together.
Sprig
An ornamental piece of clay applied to another piece of clay. Sprigs are often created by pressing clay into a mold.
Stamp
An tool that is used to make a textured impression on the surface of the clay by pressing into it.
Stilt
An item used to raise ware off the kiln shelf while firing.
Stoneware
Stoneware clay becomes very dense and hard when fired. The clay is non-porous when fired to maturity at 2192 to 2372˚F.
Terra Sigillata
A siliceous decorating slip applied to the ware. Once leatherhard, it is burnished and the ware is fired.
Thermal Shock
Cracks formed because of sudden, large changes of temperature in the piece being fired.
Underfired
Clay or glaze that did not reach its optimum firing temperature. Underfired glaze is often milky or is porous.
Underglaze
Some form of decorated color that is applied underneath a transparent glaze. More accurately, the transparent glaze is applied over the color. These might be oxides or glaze and clay body stains.
Vent
A small hole in the kiln that allows one to see into the kiln during firing.
Viscosity
A term used to describe how glaze moves when fired. While fired and fluid, it has low viscosity. When matt or dry it has a high viscosity.
Vitrified
When clay becomes a dense, hard, non-porous material. After this point, if subjected to continued firing at a higher temperature, the piece will deform.
Wedging
A process to combine two different clays into a homogenous single piece. The two pieces are cut into slices, then interleaved together. The clay is then cut in half, with one half lifted and thrown down hard onto the other, many times, until the layers mix together.
Wheel
A device used to spin the clay while forming it.

 

Most of the above definitions were adapted from the following excellent books:



Chappelhow, Mary. Thrown Pottery Techniques Revealed. Krause Publications, 2001.



Warshaw, Josie. The Practical Potter: A Step-by-Step Handbook. Hermes House, 2009.