Field Analysis:Has this deposit been affected by in-situ burning
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[edit] Has there been burning in-situ?
Because of the almost ubiquitous nature of fire in archaeological contexts charcoal is one of the commonest residues of anthropogenic activity. The relative resistance of charcoal to chemical and biological breakdown means that it often persists in the soil for far longer than ash. The presence of charcoal, ash, and burnt stone, bone and earth, are clear indicators of fire, but immediately beg the questions was this a natural or anthropogenic fire, and was the fire in-situ. Whether it was a natural or anthropogenically set fire is generally a question of context, but geoarchaeology is often helpful in determining whether burning occurred in-situ, i.e. whether this is a hearth area or whether it is redeposited burnt material.
The residues of burning charcoal, ash, burnt earth, stone and bone, and burnt soil crusts must themselves be accurately identified (more information on this can be found here). The decision as too whether these residues arise from in-situ burning or whether they have been redeposited is based on the matrix of the soil in which these residues are found as well as on the archaeological context.
[edit] Indicators of in-situ burning
If burning has been in-situ we can expect the soil surrounding the artefacts to have been altered relative to the adjacent unaffected deposits. Such alterations may include:
- Change in colour – reddening of the soil is commonly considered to result from burning as iron in the soil is oxidised, however, colour change is actually very complicated as the strength of change and resulting colour is affected by many factors including iron content of the soil, organic matter content of the soil, temperature of burn, length and/or frequency of burn, and oxygen levels. Soils may become redder or blacker or develop greenish or bluish hues compared to unburnt material.
- Soil crust – a strongly oxidised (usually red or black) cemented crust may form in the surface of the burnt soil.
- Organic matter content – the organic matter content in the upper surface of the soil where burning has occurred is likely to be very low.
- Texture – at high enough temperatures (greater than 800oC) soil particles may fuse creating a coarser grittier feel in the soil.
- Structure – With the loss of organic matter, burning can destroy the soil structure. Of course ,later soil development or compaction may produce restructuring.
- Porosity – Burning and destructuring tend to result in the loss of soil porosity and can produce spherical vesicular pores produced by gas bubbles.
- Consistency – Burning, the fusing of soil particles and the break down of soil structure can result in a dense, almost crunchy feeling deposit when trowelled, though conversely very ashy deposits are non cohesive and may feel very soft and yielding.
- Vertical and lateral changes – These changes will be most noticeable at the soil surface beneath the hottest part of the fire. There may be gradual lateral and vertical changes away from this point as the soil properties become more similar to those of the unaffected surrounding material. In constrained features such as hearths, however, the lateral changes may be more abrupt.
[edit] Uncertainties in the identification of in-situ burning
Uncertainties in the identification of in-situ burning arise because of:
- Variations in colour changes associated with burning due to the iron content of the parent material, the temperature of burn, and oxygen availability during burning.
- Confusion between colour changes caused by burning and those caused by seasonal waterlogging redoximorphic colour patterns.
- Potential misidentification of manganese as charcoal, iron pan as burnt soil crust and also in the identification of ash, burnt stone and earth as opposed to waterlogging (gleying) features.
- Disturbance of the burnt area for example by cleaning, trampling, or bioturbation.
Magnetic susceptibility is a key analysis in determining the presence of burnt materials. The use of field probes can be very helpful for field-level interpretation, whilst laboratory based magnetic susceptibility analyses can provide additional information. Micromorphology may provide information about whether burning was in-situ or whether the material is redeposited. Electron Spin Resonance has also been used to study hearth sites.
[edit] How does the SASSA Field Tool makes this interpretation?
A printable recording sheet for SASSA's 'Has this deposit been affected by burning in-situ?' can be found here
This query aims to distinguish between redeposited residues of burning and the site of burning itself such as a hearth or camp fire. To score highly the deposit should have:
- Clear, gradual or diffuse lower and lateral boundaries. This question accounts for 15% of the maximum total score.
- A similar mineralogy to a comparable ‘unburnt’ context, if one is available. This refers explicitly to stone and sand grain type rather than grain size or texture as this can be affected by burning as soil particles weld together. This question accounts for 20% of the maximum total score.
- The presence of vesicles – these will only form where the soil temperature is sufficiently high to cause nitrogen and sulphur in soil organic matter to be released as gases. This question accounts for 15% of the maximum total score.
- Gradual vertical and lateral cahnges in the soil colour. n.b. ths soil should show colours expected from burning. This question accounts for 20% of the maximum total score.
- Any charcoal present should have a predominantly horizontal orientation, except where disturbed by bioturbation, and is likely to be most frequent at or towards the top of the deposit. This question accounts for 20% of the maximum total score.
- A soil crust that is intact or at least not highly disturbed except where bioturbation has been high. This question accounts for 10% of the maximum total score.
There are other changes brought about by burning, which have not been considered here but which the user should be aware of. These include a coarsening of texture, a reduction in overall porosity, a reduction in organic matter content, and an increase in density. These were not used in this case because the potential heterogeneity of anthropogenically redeposited material could not be sufficiently taken into account. Changes in soil colour have not been relied on here, because they can be confused with the effects of seasonal waterlogging.
[edit] Follow-on Analyses
Field interpretations often have a high degree of uncertainty associated with them, this can be reduced if appropriate field and post-excavation analyses are carried out. Magnetic susceptibility is a key analysis in determining the presence of burnt materials. The use of field probes can be very helpful for field-level interpretation.
- Magnetic susceptibility
- Manganese determination can help distinguish between charcoal and manganese staining.
Post-excavation laboratory analysis may also help to confirm the presence of a buried soil and add to the archaeological interpretation. Laboratory based magnetic susceptibility analyses can provide additional information. Micromorphology may provide information about whether burning was in-situ or whether the material is redeposited. Electron Spin Resonance has also been used to study hearth sites.
