Analytical Methods:X Radiography

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Questions
X-Radiography is often used in archaeological conservation and metallurgy, but has applications in geoarchaeology also. X radiography has been used to study small scale stratigraphic units, such as tephra layers (Dugmore and Newton, 1992), and rooting patterns in sedimentary sequences. This can help interpretation of sedimentary processes and aid identification of stabilisation horizons within sedimentary sequences. More commonly intact soil blocks are analysed using X-radiography to study and identify fragile artefacts they may contain, and soils may also be analysed to identify the presence of hammerscale (Fell et al. 2006).

Case studies of the use of X-radiography in geoarchaeology include:
 * Early brass in the ancient Near East.
 * Allochthonous and autochthonous mire deposits, slope instability and palaeoenvironmental investigations in the Borve Valley, Barra, Outer Hebrides, Scotland.

Sampling
Because of the costs involved in X-rdiography samples should only be taken where specific questions linked to the fine stratigraphy have been identified.

As it is the internal structure of a deposit that is of interest, undisturbed blocks of soil are required for X-radiography. Is the deposits are chesive enough, these may be cut directly from a section and then wrapped and boxed to protect them during trasport. In less cohesive soils monolith tins or kubiena tins should be used to support the sample during sampling. Usually samples will be cut from section walls after they have been drawn and photographed. The sample position should be recorded on a section drawing to show its relationship to the observed stratigraphy and archaeological contexts. Make sure the sample number and orientation of the sample are marked clearly on the sample and you may also wish to photograph it before it is removed from the section. Undisturbed soil cores may also be used for X-radiography.

The size of sample depends on the extent of the features you are interested in and on the capabilities of the equipment you are using. Consult the instrument operator before sampling. Samples should be analysed as soon as possible after sampling and samples should not be allowed to dry out as this can affect their structure.

Analysis
X-ray systems vary in size from bench-top to large industrial units. They essentially consist of an X-ray source underneath which the soil block sits on X-ray film. This detects x-rays penetrating the sample, creating a picture of the internal structure of the block based on its density and composition. The quality of the image can be controlled by adjusting the intensity and energy of the x-rays and the length of time of exposure (Fell et al, 2006).

Data interpretation
The result of X-ray analysis is an X-radiograph film showing an image of the soil block, a skilled analyst will be able to identify more detail in the image than a non-specialist. The film should be stored at less than 21oC and 20-50% relative humidity (British Standards Institute, 2000). Images may be digitised but image quality will be lost in the process so the original film should also be kept.

Related techniques

 * Coring
 * Micromorphology