CERAMICS+TERMS

=** Glossary of Ceramic Terms **=

** *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). ***Clay Body:** A clay designed for a special purpose. It is created by blending different clays of by adding to clays other materials, such as feldspar and flint in order to produce a desired workability, maturing temperature, or finished result. A clay body is the result of mans technology. ***Coiling:** A method of handbuilding a form using long rolled out, or extruded, snake-like lengths of clay. Each coil of clay is integrated with the previous one to build the work up. The coils may be completely obliterated in the construction process or retained for their decorative qualities. ** *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. ***Crackle:** Decorative craze lines in a glaze. ** *Craze: ** The formation of a network of cracks in a glaze. **Crystalline glazes:** Most 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 firing. ** *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. ***Firing:** 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) ***Greenware:** Unfired pottery. ***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. ** *Glaze: ** 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!) ** *Kiln: ** 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. ***Kiln wash:** A refractory mixture, usually kaolin or flint, which is painted on kiln shelves and saggers to prevent glaze from adhering. ** *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. ** *Pinching: ** A method of forming clay, which is well described by its name. ***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. ***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. ***Refractory:** Resistant to heat. ***Shrinkage**: The decrease in the size of a clay object due to drying and firing. Dry shrinkage is reversible with the return of water, but firing shrinkage is permanent due to chemical and physical changes clay undergoes when exposed to heat (see ceramic change). ***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. Also see ***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. ***Soluble:** Capable of being dissolved in water ***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. * ** 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. ***Wax Resist:** A decorative technique where a wax based medium is used to create a pattern, which is then covered, in another coat of glaze or slip. The wax resists the subsequent coating creating the pattern. Paper stencils or tape can create a similar effect. Latex is another effective resist with other advantages. ***Wedging:** To kneed or mix plastic clay by hand. A hand process used to homogenize the clay and remove air bubbles, thus making it workable.
 * *BISQUE: **The first firing a pot undergoes to prepare it for glazing.
 * *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. (See Firing and Water)