Ceramics Terminology

grog

A gritty, sandlike substance. Grog is added to claybodies for strength, texture and/or tooth. It reduces cracking and warping. It is made from grinding up fired clay.

oxidation firing

A kiln atmosphere in which the presence of oxygen is sufficient in quantity to cause combustion of carbon gases. An electric kiln has a normal oxidation atmosphere. Fuel fired kilns can also fire in oxidation providing there is adequate air intake and the atmosphere remains clear.

raku

A term in Western terminology taken from Japanese method and altered. A low temperature firing technique usually below cone 06 involving a very rapid firing cycle, removal of ware with tongs from a red-hot kiln, normally post-reducing the ware in a fireproof container with dry organic combustibles for a variable short amount of time, and removal again with tongs and quenching to prevent re-oxidation.

stamp

Any object that can be pressed into clay and create texture or design. May be made of plaster, wood, bisqued clay or found object.

earthenware

Clay bodies fired at temperatures below cone 1 (2110° F) that remain somewhat porous and open in structure. The vast majority of the world's pottery has been earthenware because of the wide prevalence of earthenware clays and the relative ease of reaching the kiln temperature necessary to mature the claybody. Two examples are terra cotta and whiteware (sometimes referred to as talc body).

shrinkage

Clay shrinks during both drying and firing process. Different types of clay shrink at different rates, usually ranging from ( 6-14%)

sgraffito

In ceramics, a technique where clay is coated with a colored slip which is carved through to expose the clay. Actually, any coating that is scratched through to expose the background.

wax/wax resist

Melted paraffin or a synthetic wax solution. It is a liquid substance painted or dipped on ceramic art to resist water based liquids such as glazes, slips or stains.

slab

The process when clay is rolled into flat sheets and made into pottery or sculpture. Hard slab is when the slab is leather hard and is stiff or board like. Soft slab is when the slab may be rolled, rounded or formed over shapes or molds.

coiling

To construct pottery or sculpture by rolling out clay in thin ropes to build a form

glaze firing

Typically the second and final firing of a clay artwork that has been bisque fired, which usually includes the coating of a raw glaze. Once fired the glaze will be fused to the clay. This includes among others: lowfire, midrange and high firings.

relief

a carved flat surface

flux

a ceramic oxide or mineral that is added to a glaze, claybody or underglaze to lower the maturing (or melting) temperature. In the proper proportion and at the appropriate temperature, fluxes enable silica and alumina to melt and form a glaze. Flux sources: include feldspars, barium oxide, bone ash, calcium carbonate, cryolite, dolomite, lead carbonate, lithium carbonate, magnesium carbonate, potassium carbonate, sodium carbonate, spodumene, talc, wollastonite, and more.

plastic

a characteristic typical of unfired clay when it is moist, soft, pliable, and capable of being formed, manipulated, or easily molded, and still maintains its shape without cracking or sagging.

claybody

a clay or clay mixture used for form objects. This can be a single clay naturally found in nature, and suitable to the needs of a specific "potter," such as a single red earthenware for bricks. But it is usually a mixture of ceramic materials based on a recipe (such as our shop clay which includes several different clays, a feldspar, and a filler). Each ingredient affects color, texture, plasticity, and firing range. The commercial clays we buy for class are manufactured formulated/blended claybodies similar to our shop clay.

oxides

a general term (non-scientific) used to casually discuss raw ceramic materials, whether oxides or carbonates. A few examples: zinc oxide, red iron oxide, lead oxide, and for carbonates like magnesium carbonate, potassium carbonate. Oxides are used in formulating glazes, clays and for coloring glazes and clays.

stilt

a high temperature metal or porcelain support used to hold glazed ware above the kiln shelf during low-temperature firings. A piece of kiln furniture.

vessel

a hollowed-out form often used as a container for liquids or dry materials.

frit

a material used in making glazes. Ground glass or glaze usually produced, frequently used, and formulated to render raw chemicals insoluble or non-toxic.

bisqueware

a piece of unglazed clay, pottery or sculpture, that has been bisque fired to Cone 010 - 04

clay

a plastic material formed by nature over eons and found in the earth, that may be used for making object for functional and sculptural purposes. In its theoretically pure state, it consists of alumina, silica, and water: Al2O32SiO2 2H2O. Clays are classified as to type, such as ball clay, fireclay, china clay, etc., based on their compositions, purity, and characteristics.

wedging

a process by hand where clay is mixed, cut & slammed, and "kneaded" to eliminate air pockets, made smoother, denser and homogenous.

bat

a round, flat disc used for a base to throw clay using a pottery wheel. May be made of plaster, wood, plywood,

slump mold

a rounded form, usually plaster but not always, used with clay to create a shape in that form.

leather-hard

a stage in the drying process of clay when it becomes stiff but still flexible, but is still damp enough to be joined to other pieces. The name is akin to the description of shoe leather and clay at this stage may also be carved, incised, engraved, planed, and trimmed/turned. It can also be said that its consistency is similar to hard cheese.

kiln wash

a thin coating of refractory material (usually alumina and kaolin) applied to kiln shelves, the bottoms of kilns, and other "hot face" surfaces to protect them from glaze drippings and to reflect heat.

glaze

a thin layer of glass fused to the surface of fired clay. Glaze can be smooth or textured, shiny or dull finished, and may be colored by a variety of oxides/carbonates. The raw unfired glaze never looks like what it will look like fired.

engobe

a type slip or underglaze. It is a type of slip that when fired is vitreous or in maturation is halfway between a glaze and a clay. It may be white or colored, and is usually applied to high firing stonewares and porcelains at the green or

primitive firing

an extremely low temperature firing, usually below 900°F, usually taking place in an underground pit or above ground in a fireproof container or pyre fueled by organic material such as wood, charcoal, dung, etc., that normally also produces a smoky atmosphere.

decal

an image, pattern or drawing created with ceramic oxides on a paper, (usually a commercial process) designed to be transferred onto previously glaze fired ceramics and then fired to a very low temperature around cone 020.

kiln

an insulated fireproof box, usually brick lined oven into which heat is introduced by combustion (fuel fired) or by radiant energy (usually electric) designed for firing ceramic ware. Kilns come in a wide range of shapes and sizes, some permanent and some portable.

press-mold

any form used to press clay into, to use to repeat the texture or shape.

feldspar

any of a variety of minerals mined from the earth and pulverized to fine powder and used in ceramic processes. Feldspars are nearly 60% of the earth's crust. They are found everywhere, they are used as fluxes when added to claybodies, glazes and slips. Feldspars are made of various ratios of alumina, silica and combinations of sodium, potassium, calcium, magnesium, and lithium. Examples of feldspars familiar to our studio are: Custer, Spodumene, Minspar, G-200, Nepheline Syenite, Kona f-4 etc.

slip

clay that is mixed with enough water to be as fluid as cream or as thick as yogurt. Uncolored slip is used to attach together unfired and moist clay pieces to create functional and non-functional art. Colored slip has ceramic coloring oxides added and is used to decorate. Other liquids similar to colored slip are underglaze and engobe. These can be commercially manufactured or made by hand.

slip trailing

colored slip or glaze squeezed from container in thin lines onto clay or bisque.

functional

describes an object that will be used for some activity, usually refers to physical activity. An object to be used and not as an object of contemplation.

hand-building

forming clay shapes by hand (without a wheel) by pinching, coiling, slabbing, molding, or combinations of these techniques.

ware

generally used to describe any clay object in the green, bisque, or glazed state.

pyrometric cone

heat indicators in the form of sticks/bars or elongated pyramids of ceramic materials which deform/bend at a given temperature range with time enabling the potter to determine when the firing is complete.

terracotta

literally "cooked earth," usually indicates or refers to red earthenware clay or sometimes to architectural ceramic decoration.

pottery

loosely applied to all objects (wares) made of fired clay. It includes factory-produced dinnerware as well as artistically reinterpreted industrial forms, sculpture, and hand-produced utilitarian/functional ware. As its root "pot" suggests, it is mainly applied to containers/vessels.

scoring

making scratches, usually in a cross-hatch pattern, with a knife, needle or serrated tool and using slip, to help make two pieces (coils, slabs, handles, etc.) of clay adhere to each other.

casting

method of building an object by pouring a fluid material into a mold and then it sets up/becomes hard. The mold is removed and the object holds that shape. Materials used may be plaster, clay slips, bronze, waxes, rubbers, resins and other synthetics

wheel

referred to as a "potter's" wheel which is a device with a flat circular revolving head mounted on a vertical shaft propelled manually, by hand, or foot (kick), or motorized, usually electric incorporating a variety of drive mechanisms of which there are numerous types, designs, and shapes.

greenware

refers to any state of raw/unfired clay. Including wet, leatherhard, and dry/bone dry.

high-fire

refers to clay and glazes that are fired above cone 6 up to cone 14.

low-fire

refers to clay, usually porous and soft, and glazes that are fired above cone 016 up to cone1. Earthenware & terracotta are examples.

mid-range or medium fire

refers to clay-bodies and glazes fired above cone 1 up to cone 6.

reduction firing

simply put, "reduced oxygen." A firing in which oxygen in a fuel/combustion fired kiln has been cut down to produce a smoky atmosphere in the kiln chamber, thus, allowing carbon to draw oxygen from the claybody and glazes. Reduction may greatly darken the claybody and certain oxides/carbonates develop special colors. Notably, iron develops the various hues of green (known as celadon) and copper develops the various hues of red, pink, and purple under the right circumstances. Primary gases are carbon monoxide and dioxide and hydrocarbon gases. A "NEUTRAL" kiln atmosphere is between oxidation and reduction.

pinch pots

small vessels made by manipulating clay by hand. Often a first assignment for beginners.

cones

small, elongated thin pyramids of ceramic materials with different melting temperatures. They are used to determine when a kiln firing is complete. They are used inside kilns during firings to indicate the heat-work temperatures (claybody and glaze maturation) by their melting. The kiln is turned off aat that point. Form more information about the cones we use in our studio, see this Orton Cones chart.

ceramics

the art and science of objects made from earth materials containing or combined with silica with the aid of heat or the process of making these objects.

trimming or turning

the final action in the throwing process. A leather-hard wheel thrown form is inverted onto the potter's wheel head and a foot ring is carved into the bottom or base of the form utilizing specialized tools and also to remove excess clay.

throwing

the hand forming of hollow shapes out of plastic clay on a revolving pottery wheel head. a gritty texture of a claybody due to added grog or sand or other filler. It is rough before and after firing, as compared to a smooth porcelain.

vitreous

the hard glassy and nonabsorbent quality of a clay-body and/or glaze when fired to maturity

firing

the heating of clay or glazed ware in a kiln or open fire to bring the clay or glaze to maturity.

viscosity

the nonrunning quality of a glaze during firing, a highly viscous glaze is stiff and does not flow much during the firing.

mature

the optimum fired condition/potential or temperature of a fusion of glaze and/or clay when it achieves maximum hardness and non-porosity.

centering

the process for taking a ball of clay and making it perfectly round in the center of a pottery wheel before proceeding to make it into a vessel.

bisque firing

typically, the first firing of clay artwork without a glaze to Cone 010 - 04, around 1800° Fahrenheit. The second firing is usually a glaze firing.

reclaimed/recycled clay

unfired clay can be re-used by slaking down and remixed and wedged

Bone dry

unfired clay which is warm (not cool or damp), dry, and dusty/chalky in feel. Ceramic ware needs to be bone dry prior to buique firing whereby the physical (free) water has evaporated.

luster or lustre

usually an iridescent, pearly or metallic glaze surface effect created by painting an application of a metallic solution onto previously glaze fired ceramics and then firing at a very low temperature around cone 020.

underglaze

usually refers to pigments, applied to raw or bisqued clay, that are normally covered with a glaze, such as commercial liquid underglazes like AMACO, chalks/crayons, and pencils. May also describe the technique of application of pigments such as washes.

porcelain

vitreous (glass-like) pale gray to white claybody consisting of primary clay (kaolin/china clay) and fired up to cone 14 or higher and are non-absorbent and have a high-pitched ring when tapped. Porcelains are extremely dense and when very thin, i.e. wall thickness, usually demonstrate a degree of translucency.

stoneware

vitreous gray, buff to brown firing clay bodies above cone 4 to cone 10 (2381° F). The surface is hard, dense, and impermeable -rocklike.

sculptural ceramics

ware which has no utilitarian function, where the form/shape is the most important aspect of the work. The term "form follows function" is used in reference to utilitarian ware and not sculpture.