Ceramics are produced by firing objects made out of clay. Because clays are ubiquitous in the earth’s crust, ceramics are found in many areas of the world. The only areas of the world where clays are absent or inaccessible is on newly formed islands (especially, coral and recent volcanic islands) and in areas where they are buried under ice and snow such as the Arctic.
Clays are defined by their distinct chemical, miner-alogical, and particle-size attributes. Clays are the product of weathering and are hydrous alumina silicates with the basic chemical formula of AlSiOi. There are several different kinds of clay minerals, which are defined by their crystalline structures, including kao-linite (the clay mineral used for porcelain), illite, and montmorillonite (also called smectite). Particles of clay are platy in form and are defined by particle sizes smaller than silt, or less than 0.00005 mm. Clays can also be defined by their refractory properties, or how well they withstand high temperatures without fusing. Kaolinitic clays have the highest refractory properties of the commonly occurring clay minerals.
When water is mixed with clay, the platy structure of the particles allows them to slide against each other producing a plastic substance that can be manipulated by hand. The right mix of water and clay within what has been termed the ‘clay-water system’ produces substances with different workability. Too little water will produce a mixture that is sticky and cannot be manipulated. Too much water will produce a liquid slurry or ‘slip’ that is useful as a coating, but is not able to be formed by hand. Each clay type has a different absorption rate and therefore will require different ratios of water to solids. Kaolinites, because of their two-layer crystalline structure, absorb less water than do montmorillonites, which have a three-layer structure. Kaolinites are therefore less susceptible to shrinkage as the water evaporates and do not crack as easily during drying and firing.
Clays can be prepared in a number of different ways prior to the forming of the vessel. Depending on their origin, they may need to be cleaned, winnowed, or levigated to remove impurities and nonclay particles. Levigation basins allow larger particles to settle and have been archaeologically documented in many areas of the world. Sedimentary clays, often found interbedded with sandstones, may need to be ground up to disaggregate the clay minerals and allow even mixing with water. This was accomplished by grinding the clay on ground stone slabs.
Potters around the world discovered that adding some material to the clay after it has been mixed with water enhances the performance characteristics of the object. These materials can be other clays, but more often they are aplastic materials whose properties do not change with the addition of water. Aplastics added to clays are referred to as ‘temper’ by archaeologists. Temper and clay together form the ‘paste’ or ‘fabric’ of ceramic objects. Tempering materials used around the world include sand, and crushed rock, shell, or ‘potsherds’. The latter are pieces of broken pottery, also referred to as sherds or shards (the latter is the British term). Organic tempers were also used in many areas of the world and include materials such as grasses, dung, fur, and rice chaff. Organic tempers largely burn out during the firing of the object and are identified by the shape of their distinctive voids in the paste.
Specific performance characteristics that are enhanced through the addition of tempering materials include increased workability during the forming process, reduced shrinkage during drying, increased strength through the chemical or mechanical bonding of temper and clay particles, and increased toughness by reducing crack propagation. Different clays may require the use of a range of temper types. Even within the same assemblage, there may be differences in the tempers because of differences in the intended use of vessels. These different paste combinations are often the basis for distinguishing what are called ‘wares’ in archaeological assemblages.
Ceramics may be formed in a number of different ways and archaeologists may use forming methods as an indicator of the intensity of production practiced by a particular potter or potting group. For example, the use of a potter’s wheel is often seen as an indicator of higher production output. The simplest way to form an object out of clay is by forming a ball and then pinching the clay out with the thumb and forefinger. Pinch pots may be made in this way, or they can be the bases of vessels, which are then added on to other building methods. The walls of a vessel may be built up successively with coils or slabs of clay. These building methods are especially common for hand-building without a wheel, but may be used with a turning plate or slow wheel. Wheel-made ceramics rely on centrifugal force and may begin with a large lump or cylinder of clay that is then shaped during the turning process. Smoothing or finishing of the walls of vessels after coiling is often accomplished with scrapers, hence the term coiled-and-scraped. In other cases the walls may be thinned and smoothed using a paddle on the exterior surface and a hard object called an anvil held against the interior surface. Another technique found in some archaeological assemblages is the use of a two-piece mold. Slabs of clay are pressed into the two sides of the mold, removed from the mold, and then pieced together before the sides have completely dried.
Many of the above building methods may be combined on a single vessel, creating objects with complex building methods. The different techniques sometimes can be distinguished based on surface characteristics, such as scrape marks from making coiled-and-scraped vessels, the dimpling that results from the use of paddle-and-anvil construction, or the concentric rings evident on wheel-made pottery. However, in cases where multiple building techniques were used and/or when the surfaces have been smoothed, slipped, and painted over, the methods used to form pottery vessels may only be determined through techniques such as X-radiography. Nonetheless, these different techniques are important to reconstruct because they can provide information on the intensity of production practiced by potters as well as on modes of transmission in learning styles that can be associated with different potting groups in the past.
After the vessel has been formed, the exterior can be altered using a variety of tools and materials. Some of these change the color of the surface and/or enhance the ability of a vessel to retain liquids or conduct heat. After the vessel has been formed, but still plastic, the surface may be decorated by tools used for texturing, including incising and stamping. It may be covered with a clay slurry or slip, an organic coating such as pitch, and/or polished. Depending on the firing temperature and the contents of the slip, some slips may turn vitreous and produce a glaze. Glazes were more commonly found in the Near East and Asia, as well as in historical period assemblages. All-over glazes, covering the exterior of the vessel, are absent from archaeological assemblages in the Americas until Spanish explorers brought the tin-glaze technique widely known as majolica.
Painted decoration may involve a variety of materials, but are generally divided into organic (usually plant) and mineral categories. In practice, many pigments used in preindustrial contexts were mixtures of organic and mineral materials because organic materials help to bind mineral particles together and minerals within an organic matrix add depth and density to the color. Commonly used mineral paints include hematite and manganese. Glaze paints are made by adding a flux to a paint mixture that also includes silica. Common fluxes are lead, copper, and tin. The fluxes lower the melting point of silica allowing the paint to vitrify at temperatures closer to what was possible using traditional firing techniques.
Firing of clay objects chemically and physically transforms the clay minerals to produce a hard object from what was plastic material. Up until the object was fired, objects made from clay will rehydrate when subjected to moisture and lose their form. Some low-fired objects, or what has been referred to as ‘soft ware’, may be highly susceptible to weathering and therefore not well preserved in the archaeological record. Earthenwares were fired at between 600 and 1000 °C. Some low refractory clays cannot be fired at higher temperatures than this. Open firing was used throughout the world and can be highly efficient forfiring ceramics in this range. Formal kilns became prominent parts of ceramic technology in many parts of the world. Kilns are necessary for high-fired ceramics, such as porcelains, which were made from kaolinitic clays fired at temperatures of between 1200 and 1400 °C.
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