There are three basic categories of forming techniques used in pottery - handwork, wheelwork, and slipcasting. It's very common for wheelworked pieces to be finished by handwork techniques. Slipcast pieces tend not to be, as that negates one of the prime advantages of casting.
Handwork methods are the most primitive and individual techniques, where pieces are constructed from hand-rolled coils, slabs, ropes and balls of clay, often joined with a liquid clay slurry. No two pieces of handwork will be exactly the same, so it is not suitable for making precisely matched sets of items e.g. dinnerware. Doing handwork enables the potter to use their imagination to create one-of-a-kind works of art.
[classic potter's wheel in Erfurt, Germany]
The potter's wheel can be used for mass production, although often it is employed to make individual pieces. A ball of clay is placed in the center of a turntable, called the wheel head, which is turned chiefly using foot power (a kick wheel or treadle wheel) or a variable speed electric motor. Oftentimes a disk of plastic, wood, or plaster is affixed to the wheel head and the ball of clay attached to the disk rather than the wheel head so as to facilitate easy removal of the finished piece. This disk is referred to as a bat. The wheel revolves rapidly while the clay is pressed, squeezed and pulled gently into shape. The process of pressuring the clay into a radial symmetry, so that it does not move from side to side as the wheel head rotates is referred to as "centering" the clay - usually the most difficult skill to master for beginning potters.
Wheel work takes a lot of technical ability, but a skilled potter can produce many virtually identical plates, vases or bowls in a day. Because of its nature, wheel work can only be used to initially create items with radial symmetry on a vertical axis. These pieces can then be altered by impressing, bulging, carving, fluting, faceting, incising, and other methods to make them more visually interesting. Often, thrown pieces are further modified by having handles, lids, feet, spouts, and other functional aspects added using the techniques of handworking. Pottery that is thrown on the wheel is often finished in a process known as trimming. The thrown piece is first allowed to dry to the leather-hard state then it is returned to the potter's wheel, usually with the rim down. The piece must be re-centered to allow trimming of the foot of the pot to create a smooth and well-defined surface.
There are two related techniques that improve repeatability of wheelwork. A jigger is a mould that is slowly brought down onto the outside of an object, whilst it is being turned on a wheel. A solid mould is used to form the inside of the piece. Similarly, a jogger is used to shape the inside of a piece, pressing the outside against a solid mold. Although these techniques have been in use since the 18th Century, they are usually considered minor "industrial" methods by modern studio potters. There is contention among potters over whether a "jigged" piece can be considered hand produced.
Slipcasting is probably the easiest technique for mass-production, especially for shapes not easily made on a wheel. A liquid clay slip is poured into plaster moulds and allowed to harden slightly. This slip can be formulated to mature at a variety of temperatures. Once the plaster has absorbed most of the liquid from the outside layer of clay the remaining slip is poured back into the storage tub, and the item is left to dry. Finally the finished item is removed from the mould, "fettled" (trimmed neatly), and allowed to air-dry. This method is commonly used for smaller decorative pieces, such as figurines, which have many intricate details. In the United States, moulds and their slipcast pieces are primarily an industrial product, and are usually called "ceramics" to distinguish them from other pottery.
[Contemporary pottery from Okinawa, Japan]
Additives can be worked into moist clay, prior to forming, to produce desired characteristics to the finished ware. Various coarse additives, such as sand and 'grog' (fired clay which has been finely ground) give the final product strength and texture, and contrasting coloured clays and grogs result in patterns. Colourants, usually metal oxides and carbonates, are added singly or in combinations to achieve a desired colour. Combustible particles can be mixed with clay or pressed into the surface to produce texture. Shredded fiberglass can be used as an additive to improve tensile strength in the finished piece. However, the resulting clay contains sharp fibers, is hard to work with and must be carefully handled.
Throughout history, potters have used a mixture of coloured clays as a distinctive decorating technique. In traditional studio pottery in Great Britain, these techniques were known as "agateware." The name is derived from the agate stone, which shows bands of colours. In Japan, various techniques for combining coloured clay on the potter's wheel are jointly known as "neriage." An analogue of marquetry can also be made, by pressing small blocks of coloured clays together, and using the resulting "mosaic" to create distinctive patterns. The Japanese term for this technique is "nerikome." Agateware and the other varieties of "mottled" ware are made by combining two or more colours or varieties of clay into one completed piece. Different colours of clay are lightly kneaded or slapped together before being formed into a vessal or decorative item. This method is most commonly used for handbuilt pieces. Coloured clay can also be added to a base clay after it is centered on the wheel. Although, in principle, any clays can be combined, differing rates of drying/shrinkage and expansion in firing create structural difficulties. It is best to select a light neutral clay body, and then add a colourant to separate portions of the same body. The different coloured clays can then be joined without significant structural problems. Members of commercial clay "families" often have a similar chemical composition and a similar shrinkage rate, and can be used together.
Burnishing, like the metalwork technique of the same name, involves rubbing the surface of the piece with a polished surface (typically wood, steel or stone), to smooth and polish the clay. Finer clays give a smoother and shinier surface than coarser clays, as will allowing the pot to dry more before burnishing, although that risks breakages.
[An ancient Armenian Urn]
To give a finer surface, or a coloured surface, a thin slurry of clay called slip can be coated onto the leather-dry clay. Slip produced to a specific recipe is sometimes called an "engobe." Slips or engobes can be applied by painting techniques, or the piece can be dipped for a uniform coating. Many pre-historic and historic cultures used slip as the primary decorating material on their ware. Sgraffito involves scratching through a layer of coloured slip to reveal a different colour or the base clay underneath. If done carefully, one colour of slip can be fired, before a second is applied prior to the scratching or incising decoration. Often slips/engobes used in this process have a higher silica content, sometimes approaching a glaze recipe. This is particularly useful if the base clay is not of the desired colour or texture.
Glazing and Firing Techniques
Glazing is the process of coating the piece with a thin layer of a glassy material (often a mix of dolomite, frit, silica/flint, feldspar, sodium borate, clay and whiting plus metal oxides or carbonates). This is important for functional earthenware vessels, which would otherwise be unsuitable for holding liquids due to porosity. Glaze may be applied by dusting it over the clay, spraying, dipping, trailing or brushing on a thin slurry of glaze and water. Brushing tends not to give very even covering, but can be effective with a second coating of a coloured glaze as a decorative technique. With all glazed items, a small part of the item (usually on the base of the piece) must be left unglazed, else it will stick to the kiln during firing.
Glazes can be formulated to melt within the kiln at various temperatures called "cones" and denoted by a small triangle and a number, which run upwards from cone 1 at 1154 °C and backwards with a preceding 0. Cone 06, for example, is a lower temperature than cone 1 at approximately 999 °C. Glazes formulated to melt between cone 09 (~923 °C) and cone 01 (~1137 °C) are often referred to as "low fire", while glazes which melt between around cone 6 (~1222 °C) and cone 12 (~1326 °C) are called "high fire". Those which melt in the intermediate range are called "mid fire". The temperature within the kiln is often identified using small triangular "Pyrometric cones" of carefully formulated chemical mixtures which melt within a specific temperature range and begin to bend slightly - hence the term "cones" being used to denote temperature.
Some clays and glazes are oxygen-sensitive, most notably those containing iron and copper, and will change color depending on the presence of oxygen during the firing. Kilns can either be "oxidized" by opening a port to allow oxygen into the interior or "reduced" by closing off the kiln from outside air to attain colors as desired.
A number of various firing techniques can be used in addition to normal glaze-firing. Most of these involve heating the kiln to a high temperature and then delivering an amount of dry chemical into the kiln's interior. Sulfur is commonly used, as are various salts or ashes. Such substances will stick to pieces within the kiln and melt onto their surfaces, often resulting in a mottled texture which has a distinctive "orange peel" feel. Colors generally depend on what chemical is added to the kiln. These techniques can have very unusual and frequently unexpected results whether used on an unglazed piece or in combination with normal glazing.
Wood firing is another type of firing which involves using wood, rather than gas or electricity as in most modern kilns, to heat the kiln's interior. It is frequently time-consuming, as the kiln must be stoked for as long as a few days, but the pieces which emerge often have characteristic patches of orange color on the clay itself, known as "blushing".
The western adaptation of Raku firing, a traditional Japanese technique, has enjoyed a deal of popularity due to its relative ease. The kiln is heated to a low temperature, usually no higher than cone 06, and then ware is pulled out of the kiln while still hot (using tongs, of course) and smothered in ashes, paper, or woodchips. This can be done in an enclosed container, which allows the supply of oxygen to be cut off and reduction to take place. The finished products of this process are not suitable for functional use, as the clay remains porous and may have some toxic chemicals held within it as a result of burning the surrounding woodchips or paper used to smother it. However, because of the low temperature it is an extremely quick and easy technique to do, and the clay has a distinctive black color.