Tutorial:PH Preservation

From SASSA

SASSA Home Page ⇒ Tutorial Home Page ⇒ Post-Burial Processes ⇒ Burial as Preservation⇒ Conditions of preservation

---

[edit] pH, preservation and soil processes

The following tables show the potential for preservation in different soil types (Table 1) and the preferential conditions for preservation (Table 2).

Table 1: Soil types and preservation of artefacts and ecofacts, based on Goldberg and Macphail (2006).

Conditions	Parent Materials	Soil Types	Processes	Humus	Preservation potential	Comments
Acid pH less than 6.5	Quartz sand; schist	Podzol, Ranker	Leaching, Podzolisation	Mor or Moder	Pollen; plant macros; phytoliths; soil diatoms	Bone and molluscs are lost, iron can be corroded and pottery may be damaged
Neutral pH 6.5-7.5		Brown earths, Luvisols	Bioturbation	Mull	Bone; molluscs; phytoliths; soil diatoms	Pollen is oxidised (unless waterlogged), bioturbation damages artefacts and stratigraphy
Alkaline pH greater than 7.5	Shell sand; chalk; limestone; saline sediments	Rendzina, Solonetz	Calcification; salinization; bioturbation	Mull	Bone; molluscs; phytoliths to pH8	Pollen is oxidised, deposits can be bioturbated, and secondary crystal growths on and in pottery and other artefacts.
Waterlogged	Clay, peat, estuarine soils	Gley, Peat	Gleying, Peat formation	Peat	Pollen; molluscs; diatoms; most organic materials including leather and insects	Permanent waterlogging: soft-tissue pickling can occur (bog-man). Fluctuating water table: secondary mineral formation, corrosion and oxidation

Table 2: Preferential conditions soil conditions (pH, drainage and oxidation) for preservation of different archaeological material. Based on English Heritage (2002).

Material	Likelihood of survival	Drainage and oxidation	pH	Comments
Stratigraphy	**	Waterlogged	Strongly acid or strongly alkaline	Preservation aided by deep, rapid burial below fine textured sediments.
Wood	**	Waterlogged and anaerobic, or dessicating	Acid to Alkaline
Plant remains	*	Waterlogged and anaerobic, or dessicating	Acidic
Seeds	*	Waterlogged and anaerobic, or dessicating	Acidic to Neutral
Charred organic remains	**	Waterlogged and anaerobic or dessicating	Acidic to Neutral	Preservation affected by charring conditions.
Pollen	**	Waterlogged and anaerobic	Acidic	May survive in acidic oxic environments, survives well in anaerobic, acidic environments
Molluscs	*	Waterlogged and anaerobic	Alkaline > pH7	May survive in oxic alkaline conditions, and neutral soils.
Insects	*	Waterlogged and anaerobic	Acidic to neutral
Bone	**	Waterlogged and anaerobic, or dessicating	Neutral or alkaline	Survival affected by pre-burial treatment, species and size
Skin	*	Waterlogged and anaerobic or dessicating	Acid
Leather	**	Waterlogged and anaerobic, or dessicating	Acid to moderate basic
Textiles	*	Waterlogged and anaerobic, or dessicating	Neutral or moderately alkaline
Ceramics	***	Anaerobic or dessicating	All	Neutral or alkaline conditions favour low-temperature fired materials.
Iron	**	Anaerobic	Neutral or alkaline	May be damaged by oxidising agrichemicals
Copper	***	Anaerobic	Neutral or alkaline	May be damaged by oxidising agrichemicals
Glass	***	Anaeorbic or dessicating	Neutral or alkaline	Roman glass highly resistant, Medieval glass is less resistant to decay
Plaster and Mortar	**	Anaerobic or dessicating	Neutral or alkaline

Likelihood of survival refers to moderately acid, moderately drained soil conditions. * poorly resistant - rarely survive; ** moderately resistant; *** highly resistant - usually survive.

NB. all materials are susceptible to physical damage in the ploughzone

[edit] References

• English Heritage (2002) Environmental archaeology: a guide to the theory and practise of methods, from sampling and recovery to post-excavation. Centre for Archaeology guidelines. Portsmouth, English Heritage.
• Goldberg, P. and Macphail R.I. (2006) Practical and theoretical geoarchaeology. Oxford, Blackwell Publishing. ISBN 0-632-06044-1