Carbon dating of archaeological pottery

Pottery are artefacts closely associated with archaeological events, making them as one of the most suitable materials for dating. Of course, in order to date pottery with carbon dating, the presence of carbon inside the object itself or in the organic matter within it is mandatory. Since the organic matter from the objects (earths, seeds, wooden structures) is not always synchronous with the pottery, if possible, the measure should be focused directly to the carbon contained in the pottery matrix.

In the case of pottery, carbon could either be included with the temper during manufacture and have survived oxidation during firing, or it could have been absorbed from the smoke or soot. Indeed, the main source of carbon in pottery is oozy clay, peat, shells, plant remains and other organic materials that were used as temper to improve the technological characteristics of the pottery, such as strength and plasticity. Carbon could be also absorbed during the use of pottery, from soot deposition, residues from the internal deposit of food, or stable compounds absorbed from food storage (lipids, proteins, …). It can also be absorbed from the burial context (humic acid), etc. Depending on where the carbon is, the measured date can be related either to the manufacture or to the use of the pot. External residues are not always in appreciable quantities: food residues are often dated since directly related to the use of pottery, but this material is not always preserved, and it is usually removed during the cleaning of the object. The material composing the potsherd may contain carbon of geological origin before manufacture, which may or not be oxidized in firing, but its age can be reasonably expected to be much older than that of the pottery.

The archeological context may help to evaluate the date. If not, various sources should be evaluated such as different sherds or fractions, or other techniques should be applied, such as stratigraphy.
Radiocarbon dating strictly requires the use of a clean room environment to avoid contamination in order to provide accurate dating estimations. One of the methods used to identify the organic chemical compounds in the food residue and in the pottery matrix is Nuclear Magnetic Resonance (NMR). Then, the different fractions containing carbon (such as coating, temper, humics and residues) are isolated , and each one follows a specific pretreatment to extract the carbon fraction. For example, the coating is mechanically removed, and then undergoes diluted acid/alkali/acid washing. Humic acids are crushed, extracted, treated with HCl to remove carbonates and extracted with NaOH.
The carbon fraction has to be converted in a suitable form for the measuring technology to be used. The decay rate of ¹⁴C contained in the samples is measured using appropriate liquid or gas counters. For gas counters, CO₂ is widely used, while for liquid scintillation, carbon is converted into liquid form by most commonly converting it to benzene. The increasing demand to preserve the material at most imposed the application of accelerator mass spectrometry (AMS) ¹⁴C dating. Indeed, this technique requires a very limited amount of mass, about 1 mg, a much lower amount with respect to the other conventional radiocarbon dating techniques (minimum 4 or 8 g). For AMS, solid graphite targets are the most common, although gaseous CO₂ can also be used. AMS ¹⁴C dating can potentially date things 100000 years old.