Field Analysis:Why Colour
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[edit] Why record soil colour?
Soil colour is directly influenced by soil composition. The main factors are geology, iron content, organic matter content (imparts brown or black colouration), and the presence of inclusions such as charcoal (black) or ash (grey). Colour is also indirectly affected by moisture content and soil structure. The colour of the ‘natural’ sediment may be altered by soil formation and anthropogenic processes such as manuring and burning.
Colour is probably the most visible indicator of boundaries between contexts, and hence is often heavily relied on when drawing stratigraphic sections, and there is also evidence that colour had cultural significance for many societies (Jones and Macgregor, 2002). However, soil colour should be interpreted with care as it can change rapidly in response to post-depositional soil forming and burial environment processes.
[edit] Parent materials
The natural and anthropogenic parent materials of a deposit impart an inherited hue to the deposit. Old red sandstone derived deposits tend to have reddish/purplish hue, whilst large quantities of lime mortar provide a creamy grey tinge.
[edit] Organic matter
Soil humus is black in colour, together with iron these pigments are what give most soils there brown colour. Where the organic matter content is high, for example in intensively manured soils and rubbish pits, the soil colour is usually darker brown or black in colour as the organic matter coats the redder iron particles.
[edit] Burning
Burning oxidises organic matter and alters the form of iron minerals in the soils directly altering the soil colour. However, the effects of these changes are complex and as well as reddening the soil, blue, black grey and yellow colours can also result from burning. Indirectly, burning also adds ash (grey) and charcoal (black) to the soil.
[edit] Iron and waterlogging
Soils typically contain between 0.5-5% iron. Iron can exist in many different forms depending on the environment. Details of some of the commonest iron minerals can be found here. In a wet, deoxygenated burial environment oxidised iron in the soil is reduced (converted from Fe3+ to Fe2+), this is accompanied by a change in colour from red to grey. In areas where there is a fluctuating watertable reddish mottles and nodules can form along the wetting front, in the centre of peds and around root channels. This produces mottling and redoximorphic colour patterns characteristic of soil gleying (seasonal waterlogging), which overprint the archaeology. More information on redoximorphic colour patterns can be found here.
[edit] When should I record soil colour?
Because of the importance of colour in the identification of stratigraphic boundaries soil colour should always be recorded. Colour changes over the boundary and the presence, frequency and colour of mottles should also be recorded, particularly when there are potential issues of post-depositional changes.
[edit] Systematic colour recording
Systematic schemes for recording soil colour are essential. Munsell® colour charts are routinely used in archaeological context recording. The comparison charts allow colour to be recorded in a systematic way by splitting colour into Hue (colour family, e.g. red, or yellow) Value (lightness or darkness) and Chroma (strength of colour) and assigning a numerical value to each. However, the result is code such as 10YR 5/4; which often fails to convey the original colour to readers; hence many recording schemes also include a text based colour description.
| Advantages | Disadvantages |
|---|---|
| Accounts for differences in colour perception between individuals | Cost of charts |
| Widely used | Numerical codes do not conjure a mental image of soil colour |
| Systematic nomenclature | To record rapidly and accurately requires practice |
Where Munsell charts are not available there are numerous systems, for example that of the Museum of London Archaeological Service, of colour lists and modifiers. However, it should be noted that no one standard scheme exists. More information on how to record soil colour can be found here.
