Spot samples may be taken during the excavation, post-excavation assessment or research stages of a project. Ensure the most effective sample collection by agreeing a programme of sampling for specific purposes.

 

Have specialists on hand to take the samples during the excavation. If this is not possible, first aid for finds personnel should ensure correct sample collection according to these general guidelines:

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• Take samples only if requested and as instructed by analytical specialists.

• Consult the specialist for advice on:

  • The number and size of samples needed.

  • The correct equipment for sampling.

  • The appropriate containers for samples.

  • The required labelling and recording protocol.

• Take all precautions to prevent contamination of samples:

  • Use only clean equipment and storage containers.

  • Wear clean disposable gloves.

• Send samples to the receiving laboratories as soon as possible, by prior arrangement.

• Consult the excavation supervisor for their opinion on anything unusual observed during finds processing (e.g., possible remains of residue on a piece of pottery).

• If an artefact is suspected to have potential for residue analysis:

  • Do not wash it.

  • Handle it only with clean gloves.

  • Pack it with clean materials.

 

Spot samples are commonly collected for:

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• Radiocarbon (14C) dating.

• Optically stimulated luminescence dating.

• Dendrochronology (tree ring dating).

• Organic residue analysis (e.g., foods, beer, wine, blood, waxes, resins, pigments, dye-stuffs).

• DNA sequencing.

• Pollen.

• Soil micromorphology.

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Radiocarbon (14C) dating

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Radiocarbon dating is a sensitive method for dating organic material, especially wood, bone, textile and leather. Dates of 50,000 years ago or more are obtainable.

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Radioactive 14C forms in living things by the interaction of cosmic rays with atmospheric nitrogen. This process stops when the plant or animal dies, at which point the 14C then gradually decays. Accelerator mass spectrometry measures relative proportions of different carbon isotopes in a sample and the smaller the proportion of 14C present, the older the sample is likely to be. From this, the age of a find made of organic material can be calculated.

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When collecting samples, always follow the instructions of the 14C dating laboratory providing the service. Care to prevent contamination of find or sample is vital, especially from modern hydrocarbon-based materials such as adhesives or biocides. Sample size can be very small, just a few milligrams depending on the material source.

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Dendrochronology (tree ring dating)

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Dendrochronology can date timbers by variations in annual growth ring widths. Growth rings in a timber are cross-matched with standard sets of data curves. At least 50 rings are necessary to produce a statistically viable match. Suites of samples, rather than just one or two, are preferable. A very precise date of felling can then be obtained. Dendrochronology may also tell if all timbers in a structure came from the same source and whether repairs or additions have been made.

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In the UK, suitable samples are usually only obtained from archaeological wood if preserved by waterlogging. To obtain a sample, a cross-section is sawn from a preserved timber at the point where most rings are present. Taking a core sample using a special boring tool is also possible in some cases and is less destructive than cross section sampling. Sampling is best done by a dendro specialist.

 

Pack and store samples as for waterlogged wood.

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Organic residue analysis

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Organic residue analysis detects substances associated with the manufacture and/or use of an artefact. Types of residues preserved depends on burial conditions. The original substance is likely to have deteriorated during burial, but it can still be identifiable through bio-markers which are its break-down products. Substances detectable can include food, beer, wine, blood, waxes, resins, pigments and dye-stuffs. Residues may be carbonised or trapped within the porous structure of an artefact (e.g., in sherds of a pottery vessel or fragments of wall plaster). Spot samples for analysis are best taken by a specialist as part of a research programme. For example, samples from ceramics of different periods can be used to investigate changes in diet.

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On site and during finds processing:

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• Be vigilant for possible organic residues on finds (e.g., on the inside of a pot base or rim).

• Avoid contamination of residues:

  • Wear nitrile gloves when handling the find.

  • Do not wash the find.

  • Use clean packing materials and wrap samples in aluminium foil.

• Package the find according to its material and condition.

• Label packaging and containers such that the contents are clear without them having to be unwrapped.

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DNA Sequencing

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DNA sequencing can identify and characterise differences and similarities in animals and plants. DNA sequencing can show how populations moved or migrated. Examples of its use include investigating relationships between peoples in the past to people of today, or how the spread of grape species occurred in the production of wine.

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Human DNA sequencing can be a sensitive issue, especially for recent human populations. International codes of ethics now govern this form of research. Special paleogenomic laboratories undertake DNA sampling and analysis where rigorous conditions prevent contamination of samples and the results and records are secure.

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Follow the instructions given by DNA specialists, particularly in relation to collection, processing and labelling of finds selected for analysis.