Field Analysis:Burning Features

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Colour changes
Reddening of the soil upon heating is one of the key indicators of burning. Red soil colours, however, may also develop for a range of other reasons; for example they may be inherited from the parent material, or they may be the result of iron movement such as podzolisation or waterlogging (gleying). The presence of iron nodules and pans, or a characteristic sequence of soil horizons (dark humic, pale grey sand, and reddened soil) may indicate a cause other than burning.

Colour changes associated with burning are also complex. The temperature to which the soil is heated - reddening is rare below 500 oC (Canti and Linford, 2000) - the mineralogy and chemistry of the soil, and the presence of organic matter can all affect colour changes. The result is that while many soils redden when heated, others change not all, and some may become yellower, grey, or even white.

Charcoal versus manganese
Larger, well preserved fragments of charcoal may contain preserved cellular structure, giving it a porous appearance. Manganese oxides may form as pseudomorphs of organic matter or as concentrations and nodules but these tend to be less porous and have a purplish black colouration. Charcoal is easily crushed between the fingers to give a black powder. Manganese will also react with hydrogen peroxide whereas charcoal will not. For more information on the field test for manganese click here.

Burnt soil crust versus iron pan
A burning crust is a cemented layer of soil material that occurs at the former landsurface after intense burning. A burnt crust will usually be redenned or blackened and care must be taken not to confuse a burning crust with a post-burial iron pan. A burning crust will occur only at the old landsurface and will follow this surface faithfully, whilst an iron pan may occur anywhere within a buried deposit - though often they are associated with organic matter, voids, aggregate surfaces, or changes in soil texture or compaction - and may traverse through several different contexts. A burning crust will also have a sharp upper edge and a more gradual lower edge, whilst an iron pan will tend to be more symmetrical. An iron pan may also be surrounded by a greyish halo formed by redox processes. Look for the other features of burning such as vesicles, ash, burnt bone and charcoal to confirm the presence of a burning crust.

Soil vesicles
Soil vesicles are spherical or ovoid voids or pores in the soil. They are formed by bubbles of gas (including carbon dixide and nitrogen oxides) which are given off during the burning processes and which become trapped in the soil.

Grittiness
In intense fires where the temperatures exceed 800oC soil particles can fuse together to give the soil a distinctly gritty feel.