Hi Allan,

"carbonisation" in archaeological contexts seems unlikely just by  
definition of the term, as explained in undergraduate geology  
textsbooks.
In order to get plant remains tranformed into coal without being  
charred by fire (requiring oxygen), the plant material passes through  
a series of transformations from peat (under absence of oxygen), brown  
coal to anthracite by enrichment in C through ageing (5-10 ky in the  
case of anthracite), high pressure (tectonic processes --> note the  
depth of seam ranging between 100-700 m) and geothermal effects.
As these conditions are not given in archaeological layers, carbonised  
plant remains cannot have evolved by coalification.
In relation with the Saxon graves, one may ask, whether the textiles  
are indeed charred, and not just black due to other kinds of  
transformations, similar as with the formation of peat?
In some of our Near Eastern sites we found grape seeds which are  
preserved uncharred, dark brownish in color. They have been AMS-dated  
and exactly had the age of the archaeological layer they were embedded  
in. It may be that the conditions for preservation were slightly  
anoxic due to unknown reasons.
However, these remains had an uncharred appearance, and would not have  
evolved into coal, just because the basic conditions are not given.

As a reference for your colleague, the chapter on coalification in  
"coal geology" by Larry Thomas would be worth reading, or any other  
geology textbook.

Cheers,
Simone


Zitat von Allan Hall <[log in to unmask]>:

> Could someone jog my memory concerning the debunking of the dichotomy
> between carbonised (turned to carbon but not burnt) and charred plant
> remains? Jessen and Helbaek in their monograph on British/Irish cereals from
> 1944 allude to the carbonisation explanation by early archaeologists who
> found blackened cereals in Egyptian tombs and thought they must have been
> turned to carbon simply by being 'buried' for a long time, and they refer to
> workers such as Percival who "even maintains that the carbonisation has
> generally taken place in the last-named way, rejecting the idea that the
> carbonisation of cereals occurred through the action of fire".
>
> But I am sure there is some literature challenging what is surely an 'urban
> myth' in archaeology. Can anyone shed any light? I ask, because a senior
> colleague (not in this Department, I hasten to say) clearly still thinks
> that carbonisation without charring is a real phenomenon - citing the
> example of black, apparently charred, textile remains from Saxon graves in
> contexts where exposure to fire is not considered likely. If not charred,
> how are these remains preserved?
>
> Allan
> --
>
>
> Dr Allan Hall, English Heritage Senior Research Fellow, Department of
> Archaeology, University of York, The King's Manor, York YO1 7EP, UK
> +44 1904 434950 (fax 433902)
>
> http://www.york.ac.uk/depts/arch/staff/Hall.htm
>
> Consider the environment. Please don't print this e-mail unless you really
> need to.
>
>
>



-- 
Simone Riehl (Dr. rer. nat., M.A., Archaeobotanist)

Institute for Prehistory and Quaternary Ecology
Ruemelinstrasse 23
D-72070 Tuebingen
Germany
Tel. / Fax +49 (0)7071 2978915 / 295717
http://www.urgeschichte.uni-tuebingen.de/index.php?id=132
http://www.cuminum.de/archaeobotany/
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