Pottery - Definition


Pottery - Definition

A man shapes pottery as it turns on a wheel. (Cappadocia, Turkey)

Pottery is a form of ceramic technology, where wet clays are shaped and dried, then fired to harden them and make them waterproof. Unglazed pottery that is fired at temperatures in the 800 to 1200 degree C range, which does not vitrify in the kiln but remains slightly porous is often called earthenware or terra cotta. Clay formulated to be fired at higher temperatures, which is partially vitrified is called "stoneware". Fine earthenware with a white tin glaze is known as faience.

The terms "pottery," "earthenware" and "stoneware" are generally used only for relatively easily constructed utensils such as pots, cups, bowls, etc., and for some decorative items. Similar types of ware made from porcelain clays are simply referred to as "porcelain." Complex extremely high-fired ceramics, where the glaze and body fuse completely, are generally referred to as "products of ceramic technology." Ceramic technology is used for items such as electronic parts and Space Shuttle tiles. Pottery is both an ancient and modern technology, in that it uses materials and techniques that are thousands of years old but also takes advantage of more modern innovations in the fields of chemistry and electronics.

A person who makes pottery is generally known as a potter. The potter's most basic tool is the potter's wheel.

Broken pottery in archaeological sites, called potsherds, help identify the resident culture and date the stratum, by the formation style and decoration. The relative chronologies based on pottery are essential for dating the remains of non-literate cultures and help in the dating of some historic cultures as well.

Contents

1 Techniques

1.1 Forming Techniques
1.2 Decorative and finishing techniques

2 Production stages

3 History

3.1 Palaeolithic Pottery
3.2 Neolithic pottery

4 See also

Techniques

Forming Techniques

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.

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, slicing, 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. There are two related techniques that improve repeatability of wheelwork. A jigger is a mold that is slowly brought down onto the outside of an object, whilst it is being turned on a wheel. A solid mold 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.

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. 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, trimmed neatly and allowed to air-dry.

Decorative and finishing techniques

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. Colorants, usually metal oxides and carbonates, are added singly or in combinations to achieve a desired color. 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.

Agateware refers to techniques that use a mixture of coloured clays. The name is derived from agate stone, which shows bands of colours. Agateware can be made with any sort of clay. Two different colours of clay are lightly kneaded together, before being formed into a shape. Colored clay can also be added to a base clay after it is centered on the wheel. In Japan, various techniques for combining colored clay on the potter's wheel are jointly known as "neriage." 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. The different coloured clays can then be joined without structural problems. Members of commercial clay "families" often have a similar composition, and can be used together because they have a similar shrinkage rate. An analogue of marquetry can also be made, by pressing small blocks of coloured clays together. The Japanese term for this technique is "nerikome."

Burnishing, like the metalwork technique of the same name, involves rubbing the surface of the piece with a polished surface (typically 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. This is particularly useful if the base clay is not of the desired colour or texture.

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). 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, or dipping 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.

Production stages

All pottery items go through a series of stages during construction.

The raw clay is wedged to make its moisture and other particle distribution homogeneous and to remove air bubbles. It is then shaped either by hand or using tools such as a potter's wheel, an extruder, or a slab roller. Water is used to keep the clay flexible during construction and to keep it from cracking.
Work that is thrown on the wheel often needs to be trimmed or turned to make its thickness uniform and/or to form a foot on the piece. This process is done when the piece has dried enough to survive this manipulation.
The piece is allowed to air dry until it is hard and dry to the touch. At this stage it is known as greenware. Items of greenware are very brittle but they can be handled with care. Greenware items are often sanded with fine grade sandpaper to ensure a smooth finish in the completed item.
Sometimes the greenware is given a coating of a liquid clay slip. This is most often done to give a coloured base for decoration, other than the colour of the main clay.
The greenware is often given a preliminary firing in a kiln. Once it has been fired once it is known as 'biscuit' ware or bisque.
Biscuit ware is normally a plain red, white or brown colour depending on which type of clay is used. This is decorated with glaze and then fired again to a higher temperature.
Some pieces are not bisque-fired before being glazed. These pieces are called once-fired.

History

Incipient pottery (10,000-8,000 BC), Tokyo National Museum, .

Incipient Jomon pottery (10,000-8,000 BC), Tokyo National Museum, Japan.

Palaeolithic Pottery

Pottery found in the Japanese islands has been dated, by uncalibrated radiocarbon dating, to around the 11th millennium BC, in the Japanese Palaeolithic at the beginning of the Jomon period. This is the oldest known pottery. In Europe, burnt clay was already known in the late Palaeolithic (Magdalenian) and was used for female figurines, like the "Venus" of Dolni Vestonice.

Neolithic pottery

In Palestine, Syria and south-eastern Turkey, the earliest finds of clay pots date from Neolithic times, around the 8th millennium BC (black burnished ware). Before that, clay had been used to make statuettes of humans and animals that were sometimes burned as well. In the preceding Pre-Pottery Neolithic, vessels made of stone, gypsum and burnt lime (vaiselles blanches or white ware) had been used. Sometimes a mixture of clay and lime was used, not very successfully, in the earliest pottery.

see history of pottery in Palestine
Nevala Cori for figurines