Aspects of the chemical and microscopic characteristics of plant ashes found in archaeological soils

Ash from plants occurs in archaeological soils all over the world. It is particularly significant in cave deposits but is also found on open air sites where dissolution has been discouraged by aridity, deep burial or calcareous surrounding sediments. This paper describes the main microscopic characteristics of ash, as well as providing details and illustrations of their origin from plant mineral and organomineral compounds. Chemical summaries are presented which reflect the major trends in phytomineralogy and highlight some additional groupings running through plant physiology. Emphasis is placed on the importance of understanding the complete range of transformations acting on ash as a result of heating and cooling, followed by weathering and dissolution. Finally, some future directions for ash research are suggested, both from within the central topic of plant ashes and extending to other types of ash.