Lars Husby Art 260 Basic Ceramics
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.
Clay…a product of decomposition of the mineral feldspar which is found over the entire surface of the earth. In its theoretically pure state, it consists of alumina, silica, and water: Al2O3, SiO2, H2O. Clays are classified as to type, such as ball clay, fireclay, china clay, etc.
Pottery…a term 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.
Clay Body…loosely, the clay from which a form is made. 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. However, for many of our demands, a single clay cannot be easily found, and therefore the potter blends a selection of materials which achieve specific qualities…such as temperature range, color, workability, etc. Most of the commercial clays we buy for pottery use (which bear brand names) are formulated/blended clay bodies.
Ware…generally used to describe any clay object in the green, bisque, or glazed state.
Earthenware…bodies fired at temperatures below cone 1 (2110oF) 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 necessary kiln temperature. Two examples are terra cotta and whiteware (sometimes referred to as talc body).
Terracotta…literally “cooked earth,” usually indicates or refers to red earthenware clay or sometimes to architectural ceramic decoration.
Whiteware…usually refers to a low firing clay or china, in the earthenware range, that fires to a white or light cream color and which usually contains a large amount of the mineral compound “talc.”
Stoneware…vitreous gray, buff to brown firing clay bodies above cone 4 to cone 10 (2381oF). The surface is hard, dense, and impermeable –rocklike.
Porcelain…vitreous (glass-like) pale gray to white clay body consisting of primary clay (kaolin/china clay) and fired up to cone 14 or higher and are non-absorbent and have a high ring. Porcelains are extremely dense and when very thin, i.e. wall thickness, usually demonstrate a degree of translucency.
Glaze…a continuous layer of glass or glassy crystals fused to the surface of fired clay. The chemicals in a glaze formula, available in powdered form, are mixed together and combined with water. This “liquid glaze” is usually applied to bisqued clay by brushing, pouring, dipping, or spraying, and when fired to its maturing temperature/cone provides a surface impervious to liquids and easy to clean. Glaze can be smooth or textured, shiny or dull finished, and may be colored by a variety of oxides/carbonates.
Greenware…refers to any state of raw/unfired clay. Inclusive stages are: wet/plastic, leatherhard, and dry/bone dry.
Wet or Plastic…refers to the unfired state of clay (green) where it is soft, pliable, and capable of being formed or easily molded and still maintains its shape without cracking or sagging.
Leatherhard…a stage in the drying process of clay when it becomes stiff (almost rigid), 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.
Dry or Bone dry…the final state of unfired clay preparatory to bisque firing whereby the physical water has evaporated, the surface color has lightened, and in terms of touch the clay surface feels warm, dry, and room temperature.
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. SEE the “Firing Temperatures and Cone Equivalents Chart” handout.
Kiln…a fireproof box, usually brick lined, or high temperature 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.
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.
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 clay body and glazes. Reduction may greatly darken the clay body 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.
Raku…in Western terminology, 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 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.
Primitive Firing…an extremely low temperature firing, usually below 900oF, 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.
Lowfire…refers to clay, usually porous and soft, and glazes that are fired above cone 016 up to cone1.
Mid-range or medium fire…refers to clay and glaze fired above cone 1 up to cone 6.
High Fire…refers to clay and glazes that are fired above cone 6 up to cone 14.
Kiln Wash…a thin coating of refractory material (usually silica 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.
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.
Firing…heating clay or glazed ware in a kiln or open fire to bring the clay or glaze to maturity.
Bisque or Biscuit Ware…unglazed dryware which has been fired once, usually at a low temperature, whereby the chemical water is driven off. Soft or normal gisque refers to low temperature (above cone 022) and is porous while hard bisque refers to high temperature.
Glazed, Glost, or Mature…refers to the optimum fired condition/potential or temperature of a fusion glaze and/or clay when it achieves maxium hardness and nonporosity.
Shrinkage…the progressive lessening or contraction of clay in measureable dimensions and volume during both drying and firing. Different types of clay shrink at different rates, usually ranging from 6-14%.
Slaking…the breaking down of clay or other ceramic materials in water (soaking) with the end product usually being slurry or slip.
Slurry…a thick, half-mixed slip often deflocculated with vinegar (acetic acid) and used to join slabs and coils, attach handles, etc., in the plastic to leatherhard states.
Slip…any liquid clay or water thinned clay body. Types of slips include casting slips (for use in molds), decorative slips such as engobes, and slurry.
Engobe…a type of prepared vitreous slip halfway between a glaze and a clay, white or colored, that is usually applied to high firing stonewares and porcelains at the green or bisque state.
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.
In-Glaze…decoration applied on top of a (raw) glaze before firing and usually covered with another layer of glaze whereby the colors ink into the glaze during firing.
Overglaze…usually refers to very low temperature (up to cone 013) paints, pigments, or metallic salts applied on top of a fired glaze such as china paints, enamels, decals, and lusters/lustres. May also refer to the technique of applying pigment to a raw glaze, better known as “ON GLAZE” technique, specifically one example of this technique is majolica.
Stain…any oxide, carbonate, or prepared pigment (sometimes fritted) used for coloring clay bodies, slips, glazes, underglazes, or overglazes. Also, sometimes used interchangeable with the term “WASH.”
Frit…a ground glass or glaze usually produced, frequently used, and formulated to render raw chemicals insoluble or non-toxic.
Wedge…loosely, to hand mix plastic clay for the purpose of removing pockets of air, dispersing lumps, and making the clay mass smooth, consistent, and homogenous. Two processes are actually involved, slicing, blending, or layering and by manipulating the clay using the hands “Ram’s Head” method, or “Spiral” method. NOT kneading! This is what you do with bread dough to introduce air bubbles.
Handbuilding…forming clay shapes by hand (without a wheel) by pinching, coiling, slabbing, molding, or combinations of these techniques.
Throwing…the hand forming of hollow shapes out of plastic clay on a revolving pottery wheel head.
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.
Trimming or Turning…describes the final action in the throwing process and occurs in the wet to leatherhard stage. The term “footing” is used when a wheel thrown form is inverted and a foot ring (foot rim) is tooled into the bottom or base of the form utilizing specialized tools and also to remove excess clay.
Dryfoot…to clean the bottom of any piece of glazed ware before it is fired so that it is free of glaze. Wax, either heated or liquid resist, is frequently used.
Scoring…making scratches, usually in a cross-hatch pattern, with a knife, needle or serrated tool to help make two pieces (coils, slabs, handles, etc.) of clay adhere to each other.
Functional…describes pottery that has been designed and produced with a use in mind. The term FOLK POTTERY is sometimes used to describe “everyday ware” or the term UTILITARIAN might be used to describe the same. Usually very simple or basic.
Palaceware…is a term to describe highly decorated or unusual ware, sometimes only semi-functional and used for special occasions. The term derives from the 16th century, French court potter, Bernard Palissy.
Sculpture of Sculptural Ceramics…describes 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.
Vessel…is a term used frequently in ceramics to describe a hollowed-out form that is often times used as a container for liquids or dry materials. FLATWARE, in ceramic terms, describes basically horizontal works such as plates, dishes, tiles, etc.