Pottery vessels are made from clay. Clay exists in the ground in two forms, pure clay (primary) and impure clay (secondary). Secondary clay has been moved around by glacial action, and has picked up impurities along the way. For prehistoric potting methods, impure clay was used most often; the impurities make the clay easier to work with.
Because of this, impurities were often deliberately added to clay to make it easier to work with. These impurities are known as inclusions, opening materials or temper. These additions range in size from around 1mm to over 10mm in size, and can often be clearly seen in the fabric of the pottery. A variety of materials was used, included broken old pottery (called grog), flint, quartz, igneous rock and even organic materials such as grass or chaff. However, whilst these make the clay easier to work with, they can cause problems when the finished pots are fired. Materials such as flint and quartz can explode when heated (known as spalling), damaging the pot or even destroying it.
Building the pot
Once clay has been prepared, it can then be used to create pots. A variety of techniques were used in prehistoric times, with wheel-turning not introduced until the Iron Age, and only then in southern England. A common method for manufacturing earlier prehistoric pots was pinching, whereby a ball of clay is pinched out to form a cup shape. This only forms a small vessel, but these were often used as the base for much bigger vessels. Pinch pots were also often used for some of the Bronze Age miniature vessels such as incense cups.
Pinched pots could be built up using a variety of methods, including ring, coil and strap building. Ring-building and coil-building are very similar techniques, with ring-building involving adding several snakes of clay to build up the pot, whereas coil-building involves coiling just one continuous snake of clay to build up the pot. Strap-building involves flattening out the snakes of clay prior to the building of the pot, allowing for the pot to have a thinner wall. Moulding was also used, whereby the clay was moulded around a template, but this was only suitable for pots with very simple profiles in order to remove the pot from its template. These construction techniques can sometimes be visible on the finished pot, but when the post was well made it can be impossible to detect which technique was used to create the pot.
Pots could be further worked with a paddle and anvil, where the walls of the vessel were beaten with a wooden paddle on the outside against an object such as a stone held on the inside pot. This can thin the wall, and also reshape the pot. It was then possible to decorate the pot if required.
Once the pot was complete, it was then necessary to fire it. During prehistory, pottery was open-fired, either in a bonfire or pit. The purpose of firing is to remove all water from the clay and any of the inclusions. Open-firing can be unpredictable and difficult to control, and if heating is too rapid, the water within the clay may not be able to find an escape route, which can cause the whole pot to explode. If the whole pot does not explode, a phenomenon called spalling can occur, whereby the escaping water causes discs of clay to blow off the wall of the pot during firing. Spalling does not necessarily make the posts unusable, although it does affect the appearance of the pot. Dunting may also occur, whereby the pot cools too quickly and cracks, but this is difficult to identify in archaeological pots. As with spalling, pots that suffer dunting are not necessarily unusable, and dunted pots may have been repaired.
Uneven firing can also result in the finished pot not being uniform in colour, with different shades of pink and brown occurring on fired pottery. There may also be black patches present on the pottery, as a result either of sooting or the presence of carbon monoxide. Glazing is not a characteristic of prehistoric pottery; it was usually left in the same state as when it was freshly fired.